Classification of Fish Pond Soils in Soil Classification Systems

Abstract

The classification position of substrates forming on the beds of aquaculture ponds remains a poorly resolved issue at the intersection of pedology, limnology, and aquaculture science. We examine how major international and national soil classification systems—the USDA Soil Taxonomy, the World Reference Base for Soil Resources (WRB), the German Bodenkundliche Kartieranleitung, the Australian Soil Classification (ASC), the Russian Soil Classification, and the classification systems of Brazil and China—approach the systematics of subaqueous soils and their aquaculture analogues. A systematic literature search was conducted across the Web of Science, Scopus, and Google Scholar databases covering the period from 1953 to 2025. Our analysis reveals that Soil Taxonomy provides the most developed taxonomic framework through specialized suborders (Wassents and Wassists), while the WRB offers the greatest flexibility via its qualifier system (subaquatic, limnic, and gleyic). The German classification uniquely assigns subaqueous soils to the highest taxonomic level (division) with a substantive typology that is directly applicable to pond substrates. The Australian classification contributes a three-part sulfidic material typology of practical significance for pond management. The Russian and Brazilian systems currently lack formal taxa for subaqueous soils, although recent proposals (e.g., Aquazems) may address this gap. The Chinese paddy soil model offers a conceptual bridge between subaqueous pedology and aquaculture. No existing system adequately addresses the specific anthropogenic impacts of aquaculture management on pond soil formation. Permanently inundated little-disturbed ponds fall within the subaqueous soil concept, whereas intensively managed, frequently drained or dredged ponds are better treated as anthropogenic soils with a subaqueous phase. We recommend the WRB (4th edition, 2022) as the most suitable framework for current classification of aquaculture pond soils while acknowledging that a multi-system approach may ultimately prove most effective. These findings carry particular relevance for countries of the former Soviet Union (CIS), where extensive pond aquaculture is practiced but pond substrates remain outside formal pedological classification.

1. Introduction

The systematic position of substrates forming on the beds of aquaculture ponds remains a poorly studied issue that simultaneously concerns pedology, limnology, and aquaculture science. Throughout the twentieth century, soil science was based on the premise that soils form exclusively on land, while bottom substrates of water bodies were assigned to the category of sediments studied by geologists and limnologists [1,2]. This premise traces back to the classical definition of soil by Dokuchaev, who regarded it as “a surface natural body arising from the combined action of… on surface rocks” (as cited in [3]), and was maintained in most national classification systems until the late 1990s.

A conceptual breakthrough was achieved through the work of Demas and Rabenhorst at the University of Maryland (USA), who, based on Chesapeake Bay estuaries, were the first to demonstrate that substrates of shallow permanently submerged environments undergo all four basic soil-forming processes identified by Simonson [4]: additions, losses, translocations, and transformations of matter [1,5]. In his dissertation, Demas [6] described 85 soil profiles in Sinepuxent Bay (Maryland), establishing the presence of recognizable horizon differentiation and a systematic relationship between properties of subaqueous substrates and landscape units, thus confirming the applicability of the “soil–landscape” paradigm to underwater environments. The principal results were published in two papers [1,2].

Earlier proposals to consider permanently submerged substrates as soils had been made repeatedly by v. Post (1862), Kubiëna [7], Goldschmidt (1958), and Ponnamperuma [8] (as cited in [2]). However, according to Demas and Rabenhorst [2], these proposals were “largely conceptual in nature, relying on subjective reasoning rather than on analytical data.” The field investigations of the 1990s provided the empirical foundation for expanding the concept of “soil” in leading world classification systems.

Building on Dokuchaev’s foundational concept of soil as a product of five interacting factors—climate, organisms, relief, parent material, and time (1889–1890)—which became the basis for all the subsequent factorial models, including Jenny’s [9] classical equation S = f(cl, o, r, p, t) and Folger’s (1972) sedimentogenesis concept Se = f(G, H, B) (as cited in [2]), Demas and Rabenhorst [2] proposed a modified equation for subaqueous soil-forming factors,

Ss = f(C, O, B, F, P, T, W, CE),

where C = climatic temperature regime (precipitation loses its independent significance in permanently submerged environments, its role being subsumed by water column properties); O = organisms (submerged aquatic vegetation, benthic fauna, and microorganisms); B = bathymetry (a direct analogue of Dokuchaev’s relief factor, adapted for the subaqueous setting—depth, underwater topography, slope, and aspect); F = flow regime (current energy and tidal oscillations—an expression of climatic and hydrological forcing specific to submerged landscapes); P = parent material; T = time; W = water column properties (salinity, alkalinity, sulfate content, and dissolved oxygen); CE = catastrophic events (hurricanes and storms).

The factors B, F, W, and CE may look like additions to the classical scheme. They are better understood as differentiations of Dokuchaev’s original factors—relief, climate, and time—forced by the move from terrestrial to permanently submerged conditions. In this sense, subaqueous soils are an extreme expression of hydromorphic pedogenesis, a position long held in Russian soil science (cf. Kovda’s hypothesis on the aquatic origins of soil formation). What permanent submergence demands is only that factors usually treated as secondary—water chemistry, hydrodynamics, and catastrophic disturbance—be made explicit. The model has since been applied well beyond its origin, from New England estuaries [10] to Adriatic lagoons [11] and the Brazilian coast [12].

For aquaculture ponds, the problem of classification status is compounded by several specific circumstances. Ponds are anthropogenic water bodies with a managed hydrological regime: during periodic draining (the practice of “letovanie” [summer fallowing] in Russian fish farming [13]), soils transition from a subaqueous to a subaerial phase, complicating their unambiguous assignment to subaqueous taxa. Intensive aquaculture is accompanied by massive inputs of allochthonous organic matter—feed residues, excreta of cultured organisms, and dead plankton—leading to the formation of a specific sedimentary horizon with no direct analogues in natural water bodies [14,15]. Boyd [14] indicates that, in intensively managed shrimp ponds, the rate of sediment accumulation can reach 2–5 cm per year, and organic carbon content in the upper horizon can reach 3–8%, which is several times higher than typical values for terrestrial mineral soils in the same region; these accumulation and enrichment processes have since been corroborated by Avnimelech and Ritvo [16] and quantified for managed European ponds by Schmaltz et al. [17]. Pond bottom soils inherit properties of the original soils on which the pond was constructed, creating a profile with a “buried” subaerial legacy and a newly formed subaqueous horizon [18,19]. Boyd and Bowman [18] characterized this profile duality as a superposition of aquaculture sediment on undisturbed parent soil, a feature that fundamentally distinguishes pond soils from both terrestrial soils and natural subaqueous substrates of estuaries and lakes.

The proper classification of fish pond soils carries substantial practical significance. Accurate taxonomic placement is essential for developing science-based recommendations for pond soil management—including liming, sediment removal, and aeration protocols—that directly affect aquaculture productivity [14,20]. Furthermore, the inclusion of pond soils in national soil classification systems would facilitate their integration into environmental monitoring programs and cadastral assessment of aquatic lands. From a broader perspective, aquaculture pond sediments represent a globally significant but poorly quantified pool in the carbon cycle, and their classification is a prerequisite for systematic inventory and carbon accounting [14].

Recent estimates suggest that global aquaculture pond sediments sequester approximately 16.6 million metric tons of carbon annually across roughly 11 million hectares of production ponds [21]. However, this figure may overestimate net carbon benefit as intensification of aquaculture has been associated with rapid growth in greenhouse gas emissions—particularly methane (CH₄) and nitrous oxide (N₂O)—from pond soils [22]. The 2019 Refinement to the IPCC Guidelines for National Greenhouse Gas Inventories explicitly classifies aquaculture ponds under “other Flooded Lands,” providing standardized methods for reporting CH₄ emissions from these systems [23]. Proper soil classification is therefore a prerequisite not only for agronomic management but also for integrating pond substrates into national carbon accounting frameworks under the IPCC methodology. Furthermore, biogeochemical processes in pond soils—including coupled cycling of carbon, nitrogen, phosphorus, sulfur, iron, and manganese at the water–soil interface under anthropogenic management—exemplify the “coupled processes operating within complex media that control element cycling” recognized as central to modern soil systems science [16,21].

The aim of this review is to analyze how the major international and national classification systems—the USDA Soil Taxonomy, the World Reference Base for Soil Resources (WRB), the German Bodenkundliche Kartieranleitung, the Australian Soil Classification (ASC), the Russian Soil Classification, and the classification systems of Brazil and China—approach the systematics of subaqueous soils and their aquaculture analogues, with an analysis of the available taxa, diagnostic criteria, and unresolved issues pertaining to the taxonomic accommodation of aquaculture pond substrates.

2. Materials and Methods

This review was conducted using a systematic literature search across the Web of Science Core Collection, Scopus, and Google Scholar databases. The search covered publications from 1953 to 2025 and employed the following keyword combinations: “subaqueous soils” AND “classification”; “pond soils” AND “taxonomy”; “aquaculture” AND “soil classification”; “Wassents”; “Subaquatic” AND “WRB”; “Unterwasserböden”; “subaqueous” AND “pedogenesis”. Additional searches were performed for each national classification system individually (e.g., “Australian Soil Classification” AND “subaqueous”; “Russian soil classification” AND “submerged”). Reference lists of key publications were screened for additional relevant sources (snowball method). Primary classification documents—Soil Taxonomy (8th through 13th editions of Keys to Soil Taxonomy), the WRB (1st through 4th editions), Bodenkundliche Kartieranleitung (KA5 and KA6), Australian Soil Classification (2nd and 3rd editions), Classification and Diagnostics of Soils of Russia (2004 edition), Sistema Brasileiro de Classificação de Solos (5th edition), and Chinese Soil Taxonomy—were analyzed directly. Russian-language sources were included where they provided unique information on national classification approaches not available in the English-language literature. In total, 86 sources met the inclusion criteria and were retained for analysis. Sources were excluded if they did not address classification aspects of subaqueous or pond soils or if they dealt exclusively with water chemistry without reference to bottom substrates as pedological objects.

Multiple editions of each classification document were analyzed to trace the chronological evolution of the treatment of subaqueous soils within each system as the inclusion of subaqueous taxa proceeded incrementally across successive editions (e.g., the USDA soil definition was expanded in the 8th edition of Keys (1998), while formal subaqueous suborders were introduced only in the 11th edition (2010)).

3. Classification of Subaqueous Soils in International and National Systems

3.1. Pedogenesis in Subaqueous Environments: The “Sediment or Soil” Debate

The question of whether substrates of coastal and shallow-water environments are “sediments” or “soils” remains a subject of interdisciplinary debate. Kristensen and Rabenhorst [24] established that the terms “sediment” and “soil” for substrates of coastal environments with mangrove, marsh, and seagrass communities are not mutually exclusive: hydrologists use the term “sediment,” while pedologists and some ecologists have described vegetated substrates as “soils” for over half a century. The expansion of the USDA soil definition (1999) caused “concern among many coastal scientists” [24].

Ferreira et al. [25], based on mangrove forests of southeastern Brazil, concluded that “the mangrove forest substrate should be regarded as soil rather than sediment” because colonization by vascular plants leads to radical changes in physicochemical conditions and pedogenetic processes (input of organic matter, translocation of soluble iron, gleyzation, and sulfidization) [25].

The documented soil-forming processes in subaqueous environments include: sulfidization—chemical transformation of sulfur under anaerobic conditions with pyrite formation (controlled by sulfate content in the water column); gleyzation—transformation and translocation of iron under reducing conditions; melanization—accumulation of organic matter from submerged aquatic vegetation, algae, and benthic organisms with the formation of dark A horizons; bioturbation—mixing by benthic organisms (bivalve and gastropod mollusks; polychaetes), creating a three-dimensional mosaic of oxidizing and reducing microzones; and humification—transformation of organic substances into humic compounds [1,2,26].

For aquaculture ponds, these processes are supplemented by enhanced input of allochthonous organic matter from feeds, accelerated sulfidization resulting from the decomposition of protein feeds (a source of sulfur-containing amino acids), specific formation of soil horizons during management cycles (flooding–draining), and, in some cases, chemical amelioration (liming and aeration) [14,15].

3.2. USDA Soil Taxonomy

The USDA Soil Taxonomy became the first major international classification system to formally expand the definition of soil to include subaqueous substrates. This process proceeded in two stages. First, the soil definition was revised (1998). Then, twelve years later, formal taxonomic units were introduced (2010).

The revision of the soil definition occurred in the 8th edition of Keys to Soil Taxonomy [27], where subaqueous substrates under shallow water, generally no deeper than 2.5 m, were included within the concept of “soil.” This expanded definition was codified in Soil Taxonomy, 2nd edition [28]. However, until 2010, pedologists working with subaqueous substrates were forced to use existing taxa developed for terrestrial soils, which inevitably led to unsatisfactory classification [19,29].

Formal taxa for subaqueous soils were introduced in the 11th edition of Keys to Soil Taxonomy [30] in the form of two new suborders: Wassents (in the order Entisols) and Wassists (in the order Histosols). The formative element “Wass-” derives from the German word Wasser, meaning water [29]. In subsequent editions (12th edition, 2014 [31]), these suborders were refined and supplemented.

The 13th edition of Keys to Soil Taxonomy [32], published in 2022, incorporated amendments approved by the National Cooperative Soil Survey since 2014 while retaining Wassents and Wassists as the two established subaqueous suborders. No new Wass-suborders were formally added. However, a critically important development followed: in September 2022, the NRCS posted “A proposal to revise freshwater subaqueous soils” [33], and, in January 2023, a more ambitious “Aquasol Order Proposal” [34] was made publicly available. The Aquasol proposal envisions elevating subaqueous soils from suborder level to a new independent soil order—a fundamental restructuring that would place subaqueous soils on an equal taxonomic footing with Entisols, Histosols, and other established orders. Neither proposal has been incorporated into the current keys; however, they signal growing recognition within the NCSS community that the existing suborder-level accommodation is insufficient for the diversity and pedological significance of subaqueous soils. For aquaculture ponds, the Aquasol proposal would potentially provide a more natural taxonomic home than the current placement within Entisols and Histosols, particularly for pond soils with intermediate profile development that fit neither Wassents nor Wassists. The September 2022 freshwater-revision proposal [33] is the more immediately relevant of the two for fish ponds since it explicitly acknowledges that criteria calibrated on Atlantic estuarine material perform poorly in inland fresh water—the very setting to which most fish ponds belong. The common intermediate case, a mineral pond bottom carrying a thick partly humified anthropogenic sedimentary mantle that is neither a typical Entisol nor organic enough for a Histosol, is precisely the profile that a dedicated Aquasol order could accommodate. Although neither proposal has yet entered the keys and both may be revised before adoption, their existence signals that the discipline’s own gatekeepers regard the present suborder-level accommodation as provisional.

Subaqueous soils in Soil Taxonomy are diagnosed by the following criteria [31,32]:

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Positive water potential at the soil surface for at least 90% of the day (≥21 h out of 24);

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Water column depth not exceeding 2.5 m (a threshold acknowledged as arbitrary; several states have extended it to 5 m for soil survey purposes [29]);

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Areas with extreme tidal range are included even when the soil surface is exposed for 1–2 h during neap tide.

The 2.5 m threshold was established as a compromise value approximately corresponding to the lower limit of hydatophyte (submerged aquatic vegetation, SAV) growth in estuarine environments of the U.S. Atlantic coast [1,2]. For aquaculture ponds, this threshold aligns well with typical depths: most carp, tilapia, and shrimp ponds have an operating depth of 0.5–2.0 m [13,14].

3.2.1. Suborder Wassents and Its Great Groups

Wassents (order Entisols) are subaqueous mineral soils with weakly developed profiles lacking significant development of diagnostic subsurface horizons [27]. For aquaculture ponds, this suborder is potentially most relevant as the majority of pond soils form on mineral substrates with relatively weak horizon differentiation: a surface sedimentary layer (a combination of dead plankton, uneaten feed, and feces) overlies the original pond bottom substrate [14].

At the great group level, Wassents are differentiated as follows [31,32]:

• Sulfiwassents—where sulfidic material is present within 50 cm of the soil surface. Sulfidic material contains oxidizable sulfur (predominantly as pyrite FeS₂) capable of causing a sharp pH decrease to values below 4.0 upon aeration. For aquaculture, this great group is most characteristic of coastal ponds with brackish or marine water, where active sulfidization is driven by high sulfate content in the water column.
• Frasiwassents—freshwater subaqueous soils with electrical conductivity below 10 dS/m (the prefix “Frasi-” from English “fresh”). This great group is most applicable to freshwater fish ponds.
• Hydrowassents—soils with a high n-value (n-value > 0.7 for loamy and >1.0 for clayey materials), characterized by thixotropic properties—pronounced fluidity that complicates field investigation.
• Psammowassents—sandy subaqueous soils.
• Fluviwassents—subaqueous soils with alluvial stratification.
• Haplowassents—typical subaqueous Entisols without specific diagnostic features.

For freshwater fish ponds, the great groups Frasiwassents and Haplowassents are generally most applicable, while Sulfiwassents are more characteristic of coastal aquaculture systems with brackish water.

3.2.2. Suborder Wassists

Wassists (order Histosols) are designated for subaqueous soils with high organic matter content (more than 12–18% organic carbon depending on clay content—the standard criterion for Histosols [30]). In the context of aquaculture ponds, this suborder applies to ponds with peat accumulation due to macrophyte overgrowth (cattail Typha spp., common reed Phragmites australis, and bulrush Schoenoplectus spp.), as well as to ponds with substantial sapropel horizon thickness. Great groups include Sulfiwassists (with sulfidic material) and Haplowassists (typical) [31].

3.2.3. Suborder Wassepts

The suborder Wassepts (order Inceptisols) is designated for subaqueous soils with a more developed profile including a cambic horizon. It should be emphasized that, in the 12th edition of Keys to Soil Taxonomy [31], this suborder has not been formally introduced into the system. Erich and Drohan [19] proposed the term “Frasiwassept” for freshwater subaqueous soils with more pronounced profile development found in freshwater reservoirs of Pennsylvania; however, this taxon has not received official recognition. Nevertheless, its potential relevance for aquaculture ponds is considerable: in ponds with a long history of exploitation (several decades or more), the formation of a cambic horizon may be expected through pedogenic processes—redistribution of iron and manganese, structure formation, and color changes.

3.2.4. The Munsiri–Boyd–Hajek Pond Soil Horizon System

Independently of academic pedology, a specialized system for describing the profiles of pond bottom soils was developed in the field of applied aquaculture. Munsiri, Boyd and Hajek [15], based on experimental ponds at Auburn University (Alabama, USA) aged 2, 23, and 52 years, proposed a system of five horizons:

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F (flocculent layer)—water at the water–soil interface with a high concentration of suspended particles, lacking a clearly defined boundary with the overlying water column;

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S (stirred/mixed sediment)—mixed sediment of aquaculture origin, characterized by high moisture content and bulk density below 0.3 g/cm³; subdivided into So (oxidized—upper oxidized part, typically several millimeters thick) and Sr (reduced—lower reduced part);

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M (mature bulk sediment)—a formed stable sediment with bulk density of 0.3–0.5 (0.7) g/cm³;

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T (transitional layer)—a transition layer with a sharp density gradient from 0.5–0.7 to 1.4 g/cm³; subdivided into MT (similar in properties to horizon M) and PT (similar to horizon P);

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P (parent/original pond bottom)—the original pond bottom: undisturbed compacted soil with bulk density of 1.4–1.7 g/cm³.

This system reflects the specifics of anthropogenic pond ecosystems, where the profile forms through accumulation of sediments and organic matter on top of the original substrate, in contrast to natural subaqueous soils with in situ pedogenesis. Although the Munsiri–Boyd–Hajek system is not part of Soil Taxonomy, it is widely cited in the aquaculture literature [14,18,20] and could potentially serve as a basis for developing diagnostic criteria for pond subaqueous soils.

3.2.5. Key Experimental Studies

The development of subaqueous pedology within the Soil Taxonomy framework is associated with the work of several research groups. Bradley and Stolt [10] established soil–landscape relationships in a Rhode Island estuary (Ninigret Pond, Greenwich Bay), demonstrating that subaqueous soil properties explain the distribution of Zostera marina better than water depth. Stolt and Rabenhorst [26] prepared a review chapter on subaqueous soils for the Handbook of Soil Science, and later Stolt et al. [35] contributed a chapter on subaqueous soil survey to the official USDA Soil Survey Manual.

Erich and Drohan [19] demonstrated that freshwater reservoirs, typologically similar to fish ponds, contain soils classifiable as Haplowassents. The study was conducted at Black Moshannon Lake reservoir (91 ha, Pennsylvania), where formerly terrestrial soils had become submerged. The authors described the formation of newly formed subaqueous A horizons atop buried Btg and C horizons of the original subaerial landscape and established that freshwater subaqueous soils do not fully meet the existing diagnostic criteria, which were developed primarily for estuarine environments [19].

Bakken and Stolt [36] demonstrated the feasibility of mapping subaqueous soils of freshwater bodies (lakes and ponds of southern New England) by landscape units, analogous to approaches previously tested for estuaries [10]. Wessel, Rabenhorst and Needelman [37] developed the first subaqueous soil–landscape conceptual model for the geomorphological setting of a “flooded river valley” based on the Rhode River subestuary (western shore of Chesapeake Bay) and identified seven proposed soil series.

More recently, Park et al. [38] extended subaqueous soil–landscape modeling to the South River subestuary of Chesapeake Bay, further validating the applicability of the conceptual model developed by Wessel et al. [37] to smaller tributary systems. Rabenhorst and Stolt [39] provided a comprehensive synthesis of subaqueous soil pedogenesis, mapping methods, and practical applications, consolidating the field’s progress since the foundational work of Demas [6]. Of particular relevance to the present review, Duball et al. [40] investigated the impacts of oyster aquaculture on subaqueous soils in Ninigret Pond (Rhode Island), demonstrating that aquaculture activities alter soil physical properties, including bulk density and organic matter content, making it the first study to directly link aquaculture practice to measurable changes in subaqueous soil characteristics. Manetta and Stolt [41] developed predictive models for particle size distribution in subaqueous soils, advancing the methodological toolkit for subaqueous soil survey. These methodological developments are directly transferable to pond soil characterization, although no study to date has formally classified aquaculture pond soils using Soil Taxonomy or the WRB, a gap that underscores the need for the present review.

Wessel et al. [42] documented formal proposals for acid sulfate soil and subaqueous soil classification changes submitted to the NCSS at the 8th International Acid Sulfate Soils Conference, reflecting ongoing efforts to refine the taxonomic treatment of these soils at the international level.

3.3. World Reference Base for Soil Resources (WRB)

The WRB is an international correlation classification system coordinated by a working group of the International Union of Soil Sciences (IUSS). The evolution of the WRB’s approach to subaqueous soils passed through several stages: the first (1998) and second (2006) editions did not provide for the classification of subaqueous soils (as noted by Erich and Drohan [19]); the third edition (2014, update 2015) introduced for the first time the qualifiers subaquatic and tidalic [43,44]; the fourth edition [43], presented by P. Schad (Technical University of Munich) at the 22nd World Congress of Soil Science in Glasgow (22 July 2022), retained and refined these qualifiers.

WRB 2022 defines the object of classification as “any material within 2 m of the Earth’s surface that is in contact with the atmosphere, excluding living organisms, areas of continuous ice without covering material, and water bodies deeper than 2 m” [43]. This definition includes subaqueous soils. The upper limit of the water column for subaqueous soils is 2 m (at mean spring low tides in tidal zones), which is 0.5 m less than in Soil Taxonomy.

Unlike Soil Taxonomy, the WRB does not designate subaqueous soils as independent reference soil groups (RSGs) but uses a system of qualifiers—principal qualifiers and supplementary qualifiers—that allows subaqueous formation conditions to be reflected within existing RSGs.

Key qualifiers applicable to aquaculture pond soils [43,44]:

Subaquatic—a principal qualifier indicating soil formation under permanent shallow water inundation (no more than 2 m at mean spring low tides). It serves as a principal qualifier for several RSGs: Histosols, Technosols, Cryosols, Leptosols, Solonchaks, Gleysols, Arenosols, and Fluvisols [43].

Tidalic—a qualifier for soils inundated by tidal waters at mean high tide but not inundated at mean low tide. Unlike subaquatic (permanent inundation), tidalic reflects periodic inundation. For aquaculture ponds in tidal zones (e.g., shrimp ponds of mangrove coasts of Southeast Asia and Latin America), this qualifier may be applicable.

Limnic—a qualifier associated with diagnostic limnic material: material deposited in water by precipitation or the activity of aquatic organisms (algae and diatoms) or formed from submerged and floating aquatic plants [43]. Includes coprogenous earth (sedimentary peat), diatomaceous earth, and marl. This qualifier is directly relevant for ponds with a long history of exploitation and significant accumulation of organic–mineral sediments.

Sapric, hemic, and fibric—qualifiers for the degree of organic matter decomposition in soils with high organic content (Histosols). For pond soils with peat accumulation or a thick sapropel horizon, these qualifiers allow differentiation of the degree of humification: fibric—slightly decomposed material (plant remains retain macrostructure); hemic—moderately decomposed; sapric—highly decomposed (amorphous mass that has lost cellular structure).

Gleyic—a qualifier reflecting the presence of reducing conditions due to water saturation. In the 4th edition of the WRB, it was transferred from supplementary to principal qualifier status for several RSGs, reflecting an updated understanding of the diagnostic significance of redoximorphic features [43,44].

Stagnic—a qualifier for soils with stagnant moisture. May apply to non-flowing ponds with prolonged water stagnation.

Fluvic—a qualifier for soils containing stratified alluvial (or lacustrine) material, characteristic of ponds fed by watercourses.

Hypersulfidic and hyposulfidic—qualifiers for sulfidic materials, introduced in the 3rd edition of the WRB (2014). Hypersulfidic material upon aerobic incubation yields pH < 4.0 (analogous to sulfidic material in Soil Taxonomy), while hyposulfidic material upon incubation decreases pH by ≥0.5 units but not to values <4.0. This distinction, absent in Soil Taxonomy (which recognizes only sulfidic = hypersulfidic material), has practical significance for the management of aquaculture ponds with acid sulfate soils [43].

Gleysols—soils forming under conditions of groundwater saturation with characteristic redoximorphic features. For subaqueous pond soils of mineral composition, classification as Gleysols with qualifiers subaquatic and/or limnic is most typical. Ferronato et al. [11,45], studying subaqueous soils of San Vitale Park (1222 ha, part of the Po Delta Regional Park, Northern Italy)—specifically the transition from subaqueous to hydromorphic soils along a hydrosequence—classified them as Subaquatic Stagnic Gleysols under the WRB, which correlates with Aeric Haplowassents under Soil Taxonomy [11]. It was established that sulfidization and carbonate leaching control transitions in the hydrosequence. Gleysols have the most extensive list of permissible qualifiers—79 in the 4th edition of the WRB [44]—providing high flexibility in describing specific pond soils.

Histosols—soils dominated by organic material (more than 40 cm of organic material within the upper 80 cm). Applicable to ponds with thick peat or sapropel horizons, with the qualifiers subaquatic, limnic, and the corresponding decomposition degree qualifier.

Fluvisols—soils on alluvial deposits with characteristic stratification. Ponds located in river floodplains may be assigned to Fluvisols with qualifiers subaquatic and gleyic; however, under prolonged aquaculture exploitation, alluvial stratification becomes overlain by a newly formed sedimentary horizon.

Anthrosols—soils substantially modified by prolonged economic activity. The qualifier hydragric indicates paddy (rice-growing) use with a characteristic anthraquic horizon [42]. For aquaculture ponds that are systematically drained and reflooded, the formal application of this qualifier is debatable; however, the conceptual similarity of rice paddy and fish pond regimes has been noted repeatedly [14].

Nóbrega et al. [12], studying subaqueous soils of seagrass meadows along the Brazilian coast, were the first to identify the processes of gleyzation, sulfidization, salinization, peat formation, and solonetzicity in subaqueous substrates of Brazil and classified them within the WRB as Fluvic Subaquatic Gleysol and Subaquatic Solonchak. In a subsequent study, Nóbrega et al. [46] performed detailed classification with a full set of WRB qualifiers: Eutric Fluvic Reductigleyic Subaquatic Gleysol (Loamic, Hypersalic, Sodic, and Hypersulfidic) (northeastern coast) and Eutric Fluvic Reductigleyic Subaquatic Gleysol (Loamic, Protosalic, Sodic, and Hypersulfidic) (southern coast). Vittori Antisari et al. [47] conducted the first study of transitions from hydromorphic to subaqueous soils in the Grado Lagoon of the Adriatic, one of the largest in the Mediterranean. These examples illustrate how the extended WRB nomenclature (long descriptive names with multiple qualifiers) conveys a significant volume of diagnostic information.

Methodological Advantages and Limitations of the WRB

An advantage of the WRB is its flexibility: any RSG can receive the subaquatic qualifier while retaining information about the primary pedogenetic characteristics of the soil. A limitation is the absence of a specialized RSG for subaqueous soils. As Schad [44] notes, subaqueous soils can be classified within existing RSGs with appropriate qualifiers; however, this area of classification remains insufficiently developed.

It should be acknowledged that the applied evidence base for the WRB and the German classification is smaller than that available for Soil Taxonomy. This asymmetry reflects the longer history of subaqueous soil survey in the United States rather than a deficiency of the systems themselves. A recent direct application to a managed production pond is provided by Schmaltz et al. [17], who characterized a Central European carp pond in situ and described its subaqueous accumulation as a gyttja-to-sapropel sequence—a real-pond demonstration that bridges the WRB qualifier system and the German subhydric typology (Section 3.4) and supplies profile data relevant to both.

The WRB 2022 text has undergone two rounds of post-publication corrections (update with corrections: 18 December 2022; errata corrections: 24 September 2024), indicating that the system remains under active refinement [43]. Gerasimova and Smirnova [48], analyzing the qualifier system of WRB 2022 from a Russian pedological perspective, noted that the expanded depth-specificity afforded by the subqualifier/specifier system (Epi-, Endo-, Amphi-, Ano-, Kato-, Panto-, Poly-, and Bathy-) enables increasingly precise description of subaqueous soil layers, a capability that could prove valuable for characterizing the vertically differentiated profiles of aquaculture pond soils described by Munsiri et al. [15].

3.4. German Classification (Bodenkundliche Kartieranleitung)

The German soil classification, established in the Bodenkundliche Kartieranleitung (Soil Mapping Manual), occupies a unique position among world classification systems: subaqueous soils (Unterwasserböden) are assigned to the highest taxonomic level—the division (Abteilung)—since the 5th edition [49]. No other national system assigns subaqueous soils to such a high hierarchical level.

KA5 (5th edition, 2005) [49] contains four divisions (Abteilungen): terrestrial (Terrestrische Böden), semi-terrestrial (Semiterrestrische Böden), semi-subhydric and subhydric (Semisubhydrische und Subhydrische Böden), and mires (Moore). The division of semi-subhydric and subhydric soils includes two classes: semi-subhydric soils (Semisubhydrische Böden—periodically inundated, e.g., tidal flat soils) and subhydric soils (Subhydrische Böden, synonym—Unterwasserböden), forming on the beds of inland water bodies under permanent water saturation [49].

Within the class of subhydric mineral soils (Subhydrische Mineralische Böden), four types are distinguished [49]:

Protopedon—a primitive underwater soil forming in zones of strong water movement or currents, where active hydrodynamics prevents the accumulation of fine-grained and organic material. The aquaculture analogue is the bottom substrate of water supply channels and pond areas near water inlets.

Gyttja—a sediment forming in eutrophic water bodies under aerobic to suboxic conditions at the water–bottom interface. Characterized by relatively high organic matter content, good distribution, and brownish-olive coloration. Gyttja is a typical product of biogenic and abiogenic sedimentation in productive pond ecosystems.

Sapropel (Faulschlamm)—a sludge forming under strictly anaerobic conditions with H₂S release. Characterized by dark gray to black coloration due to metal sulfides (predominantly FeS and FeS₂), a characteristic hydrogen sulfide odor, and high content of reduced forms of sulfur, iron, and manganese.

Dy—a brown sludge forming in acidic oligotrophic waters with high dissolved organic matter content (humic and fulvic acids). Less characteristic of aquaculture ponds, which are typically eutrophic.

The 6th edition (KA6, 2024) [50], published in two volumes, additionally distinguishes subhydric organic soils (Organische Unterwasserböden and Subhydrische Organische Böden) as a separate class within the organic soils division [50]. This innovation reflects recognition that underwater organic substrates (peat and sapropel deposits) require independent classification status.

The KA6 was published on 30 July 2024 in a new two-volume format: Band 1 covers fundamentals, parameters, and methods, while Band 2 presents field survey guidelines and the complete soil systematics [50]. Major innovations include completely revised water balance tables, expanded description of anthropogenic soils for urban mapping, and incorporation of the periglacial layer model. For a comprehensive English-language overview of the German soil classification system and its relationship to international classifications, see Schad [51]. The elevation of subhydric soils to divisional level in the German system—unique among all the national classifications examined—reflects the influence of the Central European limnological tradition, particularly the work of Kubiëna [7], who recognized underwater substrates as soils as early as 1953.

The German typology of subhydric soils (Protopedon–Gyttja–Sapropel–Dy) essentially constitutes a classification based on formation conditions and material composition of bottom sediments. For aquaculture ponds, it offers a conceptual framework linking bottom substrate types to the trophic status of the water body and redox conditions: Gyttja as a typical product of meso- and eutrophic ponds with relatively favorable oxygen conditions near the bottom; Sapropel as an indicator of sustained anaerobiosis, often associated with overfeeding and excessive organic matter accumulation. Notably, the German classification does not establish an upper depth limit for subhydric soils, unlike Soil Taxonomy (2.5 m) and the WRB (2 m). The operational relevance of this typology to aquaculture has recently been demonstrated directly: Schmaltz et al. [17] characterized a managed Central European carp pond in situ and described its subaqueous accumulation as a gyttja-to-sapropel sequence, applying the German subhydric materials to a real production pond and providing profile data that also inform the WRB classification of the same substrate (Methodological Advantages and Limitations of WRB).

3.5. Australian Soil Classification (ASC)

The Australian Soil Classification (ASC) included subaqueous soils in its 2nd edition [52]. The main innovations concerned the expansion of sulfidic material classification and the introduction of suborders for permanently inundated soils.

Within the Hydrosols order, two suborders were introduced [52]:

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Subtidal Hydrosols—soils of tidal estuaries, bays, and river deltas permanently inundated between Mean Low Water Springs (MLWS) and 2.5 m below MLWS;

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Subaqueous Hydrosols—soils of inland (non-tidal) water bodies located between the water surface and 2.5 m below the surface.

The 3rd edition [53] retained these suborders and added a new Arenosols order for deep sandy soils.

Australia possesses some of the most extensive areas of acid sulfate soils (ASSs) in the world, approximately 215,000 km², of which about 17,000 km² comprise permanently subaqueous soils of the coastal zone [54]. Fitzpatrick [54] proposed a specialized Acid Sulfate Soil Identification Key with five types, including Subaqueous Soils, and 18 subtypes.

A significant contribution of the 2nd edition of the ASC was the redefinition of sulfidic materials into three types [52]:

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Hypersulfidic—upon aerobic incubation, pH decreases to <4.0;

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Hyposulfidic—upon incubation, pH decreases by ≥0.5 units but does not reach 4.0;

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Monosulfidic—containing monosulfidic black ooze (MBO).

The Australian approach to acid sulfate soil classification has been further elaborated through the National Acid Sulfate Soils Guidance framework [55], which provides standardized sampling, identification, and laboratory methods for ASS characterization across all Australian jurisdictions. Rabenhorst, Fitzpatrick, and Boman [56] provided a comprehensive cross-system review of how Soil Taxonomy, the WRB, and the ASC have evolved to accommodate acid sulfate soils, highlighting both convergences (adoption of hypersulfidic/hyposulfidic terminology by the WRB and ASC) and persistent gaps (Soil Taxonomy’s recognition of only sulfidic = hypersulfidic material). Of direct relevance to aquaculture pond management, Fitzpatrick et al. [57] documented the behavior of acid sulfate soils in freshwater wetlands during drying–wetting cycles on Norfolk Island, demonstrating that repeated exposure and re-submersion—processes directly analogous to pond draining and refilling (“letovanie”)—produce characteristic transformation sequences in sulfidic materials. The European Journal of Soil Science published a major 28-paper special issue entitled “New Horizons for Acid Sulfate Soils Research” [58], representing the most comprehensive current treatment of ASS classification, characterization, and management. Stirling et al. [59] reviewed the broader effects of drought on wet soils in inland wetlands and peatlands, providing context for understanding how periodic water level fluctuations affect the pedogenesis of pond substrates containing sulfidic materials.

For aquaculture, the practical implications of the three-part sulfidic material classification are considerable: Fitrani et al. [60] demonstrated that the choice of liming material (agricultural lime, calcium hydroxide, or calcium oxide) for acid sulfate pond bottom soils significantly affects neutralization efficiency and water quality recovery. Tarunamulia et al. [61] proposed soil remediation with nano-biosilica as a management strategy for brackish-water aquaculture ponds affected by acid sulfate soils in Indonesian coastal zones. These studies confirm that the classification of sulfidic material type (hyper-, hypo-, or monosulfidic) has direct implications for the selection of pond management protocols.

3.6. Russian Soil Classification

The Classification and Diagnostics of Soils of Russia [3] represents a substantive-genetic system built on the principle of diagnosis based on the combination of profile horizons. The system was developed by a team at the Dokuchaev Soil Science Institute (Shishov, Tonkonogov, Lebedeva, Gerasimova) under the editorship of Academician Dobrovolsky and represents the second expanded edition of the 1997/2000 system. The classification includes eight taxonomic levels (trunk, division, type, subtype, genus, species, variety, and category) and three trunks: post-lithogenic, syn-lithogenic, and organogenic [3].

Unlike Soil Taxonomy, the WRB, the German, and the Australian classifications, the Russian system contains no specialized taxa for subaqueous soils. The central conceptual barrier is the definition of soil contained in the 2004 classification: “soils proper are natural bodies forming on the land surface” [3], which formally excludes subaqueous formations from the object of classification.

Nevertheless, according to Gerasimova [62], the definition of soil, formally introduced into the Russian classification for the first time (preceding systems did not contain one), “is important for the perception and recognition of new objects studied by pedologists and ecologists, in particular, artificial-urban and other non-agricultural soils, subaqueous soils.” In other words, the definition is conceptually open to subaqueous objects, although formal taxa have not been developed.

The absence of subaqueous soils in the Russian classification is attributable to several factors:

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The system is based on soil profile morphology as a result of pedogenetic processes; subaqueous soils lack traditional diagnostic horizons identifiable by standard field methods;

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Factor-ecological and regime parameters are excluded from diagnostics in the 2004 system [3]; subaqueous soils would require fundamentally different diagnostic criteria (bathymetry, hydrodynamic regime, and water column properties);

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In the Russian tradition, a fundamental distinction is maintained between “soils” (pochvy) and “bottom substrates” (grunty): underwater substrates are historically classified as bottom substrates in the geological and engineering sense, not as soils;

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The Dokuchaev school defines soil as a surface natural body forming on land; the concept of pedogenesis under permanent water cover was long perceived as contradicting this paradigm.

Several taxa of the current classification are potentially applicable to aquaculture pond soils, with significant caveats, depending on profile properties and formation conditions.

Eutrophic peat soils (trunk of organogenic soils and division of peat soils) are diagnosed by the presence of a eutrophic peat horizon (TE) forming under conditions of sustained waterlogging with eutrophic mineral nutrition [3]. They are applicable to ponds undergoing intensive overgrowth by eutrophic vegetation (cattail, reed, and bulrush) with peat horizon formation exceeding 50 cm in thickness. They represent the closest analogue of Histosols (the WRB)/Wassists (Soil Taxonomy).

Alluvial soils (trunk of syn-lithogenic soils and division of alluvial soils). Ponds located in floodplains and constructed on alluvial deposits inherit the properties of alluvial soils. Subtypes of alluvial gray-humus (designated AY in the 2004 diagnostic system [3]; English translations follow Gerasimova [62]) (soddy) gleyic and alluvial humus-gleyic soils are most similar to the mineral substrates of floodplain fish ponds. The presence of gleyic subtypes in all divisions allows reflection of the reducing conditions characteristic of permanently inundated ponds.

Humus-gleyic soils (trunk of post-lithogenic soils and division of gleyic soils) are characterized by a thick humus horizon and signs of sustained gleyzation, which may be observed in ponds with high organic matter inputs and prolonged waterlogging.

In Russian fisheries practice, pond substrates are characterized primarily by their agrochemical properties rather than pedological classification. Privezentsev [13] recommended substrates derived from humus-rich loamy soils for carp ponds and sandy/stony substrates for trout ponds—reflecting a functional agronomic approach focused on productivity rather than taxonomic placement. It should be noted that the older term “meadow soil”, used by Privezentsev, belongs to the superseded Soviet classification (1977); in the current Russian soil classification [3], these soils correspond to gray-humus or dark-humus types, which lack any ecosystem-specific designation. In some works [13,63], bottom substrates of fish ponds are characterized from the perspective of agrochemical properties (organic matter content, pH, and liming requirements); however, the classification aspect remains outside the scope of these works.

For practical purposes, the Russian-language literature widely employs a simplified typology of pond bottom substrates by textural and chemical composition: sandy, loamy, clayey, silty, peaty, and sapropelic [13,64]. Although this typology does not constitute a classification in the strict pedological sense, it has practical significance for fish farming and corresponds to the agronomic assessment of substrate properties.

Bottom sediments of water bodies, including sapropels, have historically been treated in Russian soil science as subjects of limnology or peat science rather than pedology proper. The fundamental monograph by Kordé [65] laid the groundwork for biostratigraphic typology of Russian sapropels, distinguishing types by the biological composition of organic matter (diatomaceous, chrysomonad, peaty, zoogenic, etc.). Parallel classifications exist based on ash content (organic < 30%, mixed 30–65%, and mineralized 65–85%) and an industrial–genetic classification developed in Belarus by Kurzo [66]. Sapropels are essentially freshwater lacustrine bottom sediments formed through biogenic and abiogenic sedimentation—protoforms of freshwater subaqueous soils. The classification of sapropels provides a material-compositional basis that could inform diagnostic criteria for freshwater subaqueous soils if they were to be included in the Russian classification.

Conceptual Premises in Russian Pedology

Kovda built his classification concept on the hypothesis of the youthfulness of Earth’s soil cover and its predominantly aquatic origin. According to this hypothesis, the majority of soils on accumulative plains passed through a hydromorphic stage [67]. Kovda effectively regarded hydromorphic conditions as primary and automorphic conditions as secondary (arising only after tectonic uplift and lowering of the groundwater table), which creates a theoretical basis for recognizing subaqueous pedogenesis.

Dobrovolsky, within the geochemical landscape tradition (Polynov–Glazovskaya), identified subaqueous landscapes as accumulation zones receiving material from watersheds. However, Dobrovolsky did not define subaqueous deposits as “soils” in the strict sense (as cited in [3]).

Karavaeva [68], in her work on the evolution of waterlogged soils, addressed the issue of semi-hydromorphic and hydromorphic soils as a continuum, the extreme link of which may be represented by subaqueous soils; however, a special systematics of subaqueous soils was not developed by her. Khitrov and Gerasimova [69] proposed 20 new diagnostic features and a new section on parent materials and substrates (including anthropogenic ones) but did not address subaqueous soils directly.

Gerasimova [62], in comparing the Russian and international classifications, noted a significant gap regarding subaqueous soils: international systems (Soil Taxonomy and the WRB) actively include these objects, while the Russian classification retains a conceptual limitation associated with the definition of soil as a body forming on land.

The only formal Russian-language proposal for subaqueous soil taxonomy was advanced by Tkachenko et al. [70], who studied bottom sediments in the shallow deltaic areas of the Volga, Don, and Kuban Rivers and proposed the term “Aquazems” for subaquatic soils. The proposed system introduces an AQ-prefix horizon nomenclature and argues that deltaic shallow-water substrates meet the criteria for pedological objects, exhibiting recognizable horizon differentiation and systematic relationships with geomorphological landscape units. While the Aquazems proposal has not been incorporated into the formal Russian classification, it represents a significant conceptual step toward bridging the gap between the Russian tradition and international systems that already accommodate subaqueous soils.

Gerasimova [71], in a subsequent review, outlined the trajectory “towards the next approximation” of the Russian soil classification system, identifying several areas requiring expansion, although subaqueous soils were not among the priorities addressed. Khitrov and Gerasimova [72] further refined the diagnostic horizon system, proposing 20 new diagnostic features and updated descriptions of parent materials; however, these refinements likewise did not extend to subaqueous environments. The continuing absence of subaqueous soils from the Russian classification stands in growing contrast to international trends: the Aquasol Order Proposal in Soil Taxonomy [34], the qualifier-based accommodation in the WRB [43], and the divisional status in the German system [49,50] all reflect a progressive integration of subaqueous objects into formal pedological frameworks.

3.7. Brazilian Soil Classification System (SiBCS)

The Brazilian Soil Classification System (Sistema Brasileiro de Classificação de Solos, SiBCS) [73] does not formally include subaqueous soils. The sole barrier to their inclusion is the definition of the soil upper boundary, which stipulates, “only the atmosphere can be considered the upper limit of soils” [73]. Critically, Nóbrega et al. [12] demonstrated that subaqueous soils of Brazilian seagrass meadows satisfy all the other SiBCS criteria for soil classification; the atmospheric contact requirement is thus the only definitional obstacle. In the absence of a dedicated taxon, subaqueous soils in Brazil are currently assigned to Gleissolos Tiomórficos (thiomorphic Gleysols)—a workaround that captures the sulfidic (thiomorphic) character of these substrates but fails to reflect their subaqueous genesis. A revision of the upper boundary definition would therefore be the sole prerequisite for the formal incorporation of subaqueous soils into SiBCS.

In a complementary study, Queiroz et al. [74] bridged soil biogeochemistry and microbial communities in the subaqueous soils of tropical seagrass meadows along the Brazilian coast, demonstrating that archaeal and bacterial community composition is systematically linked to pedogenetic processes (sulfidization and gleyzation) in these substrates, further supporting their recognition as pedological objects rather than mere sediments.

3.8. Chinese Classification: Paddy Soils as a Conceptual Bridge

The Chinese Soil Taxonomy (CST) also does not distinguish subaqueous soils as an independent category. However, paddy soils, formally classified as Stagnic Anthrosols, represent a valuable conceptual bridge to understanding pond soils.

China possesses approximately 29–30 million hectares of paddy soils, constituting about 25% of China’s arable land and supporting approximately 65% of China’s rice production, which in turn accounts for roughly 28% of the total national grain output [75,76]. The diagnostic horizons include the anthrostagnic epipedon (≥20 cm) and the hydragric horizon (≥10 cm, accumulation of Fe and Mn) [75]. Paddy soil profile development proceeds through three phases: (1) desalinization and formation of the plow pan (decades); (2) decarbonation and accumulation of organic carbon (centuries); (3) redistribution of oxides and formation of hydromorphic features (≥700 years) [75].

The analogy with pond soils lies in the fact that both paddy soils and aquaculture pond soils are anthropogenically managed inundated soils. Paddy soils experience seasonal flooding, while pond soils experience permanent or semi-permanent flooding; the diagnostic horizons of paddy soils—products of repeated flooding–draining cycles—are analogous to processes in ponds with the practice of summer fallowing [13]. It should be noted that Gong (1986) [75] remains the primary English-language source on Chinese paddy soil classification; more recent Chinese-language publications may contain updates that were not accessible for this review. The Chinese paddy soil model may thus serve as a starting point for developing diagnostic criteria for pond subaqueous soils, in which anthropogenic management of the water regime is considered a pedogenetic factor.

Kögel-Knabner et al. [77], who documented the distinctive processes operating in these anthropogenically managed flooded systems, have comprehensively reviewed the biogeochemistry of paddy soils: Fe (III) reduction and re-oxidation cycles, methane production and oxidation, and the selective preservation of organic matter under alternating redox conditions. These processes are directly analogous to those occurring in aquaculture pond soils, where similar redox fluctuations are driven by management cycles rather than seasonal rice cultivation. Lee et al. [78] provided a methodological template for cross-system classification comparison by applying both Soil Taxonomy (2022) and the WRB (2022) to Korean paddy soils with different drainage grades, an approach that could be directly adopted for aquaculture pond soils.

Recent developments in the Chinese Soil Taxonomy merit attention: Hao et al. [79] documented the “Tentative Soil Classification System for the 3rd National Soil Survey” (January 2023), which added an artificial engineering soil suborder under Anthrosols, reflecting China’s evolving approach to anthropogenic soils. While this revision does not address aquaculture pond soils specifically, it demonstrates institutional willingness to expand the classification to accommodate human-modified substrates. Given that China hosts the world’s largest aquaculture pond area—approximately 2.6 million hectares of freshwater ponds as of 2022 [76]—the absence of pond soil classification in the Chinese Soil Taxonomy represents a particularly significant gap.

The growing practice of integrated rice–fish coculture systems in China [80,81] creates hybrid substrates that are simultaneously managed as paddy soils and aquaculture substrates. Studies of these systems have shown that coculture modifies soil bacterial community composition, enhances paddy soil fertility, and increases bacterial network stability compared to rice monoculture [82]. These rice–fish system soils occupy an intermediate position between the well-classified paddy soils and the unclassified pond soils, providing a natural gradient along which diagnostic criteria for aquaculture pond substrates might be developed.

A limitation of the present review must be stated explicitly for the Chinese material. Our treatment of Chinese Soil Taxonomy rests substantially on Gong’s foundational English-language account of paddy soil classification [75] and on a small number of recent English-language papers [78,79]; the extensive Chinese-language pedological literature, including any revisions issued in the course of the Third National Soil Survey, was not directly accessible to the present authors. The Chinese section should therefore be read as a conceptual bridge—paddy Anthrosols as the analogue for managed inundated soils—rather than as a complete account of how the Chinese system does or could treat aquaculture pond soils. Given that China holds the world’s largest area of freshwater aquaculture ponds [76], a dedicated review of the Chinese-language sources would be a valuable complement to this work and is, in our view, the most consequential gap in the present coverage.

4. Discussion

A comparison of the classification systems examined reveals significant differences in approaches to subaqueous soils.

Table 1. Proposed correlation of classification approaches to aquaculture pond soils.

Pond Soil Type	Soil Taxonomy (USDA)	WRB (IUSS, 2022)	German (KA5/KA6)	ASC (Australia)	Russian Classification
Mineral, weakly differentiated	Haplowassents (Entisols)	Subaquatic Gleysol	Protopedon	Subaqueous Hydrosol	Alluvial gray-humus gleyic (with reservations)
With sulfidic materials	Sulfiwassents (Entisols)	Subaquatic Gleysol (Thionic)	–	Subaqueous Hydrosol (hypersulfidic)	Not provided
Organic (sapropelic, gyttja)	Haplowassists (Histosols)	Subaquatic Limnic Histosol (Sapric)	Gyttja/Sapropel	–	Sapropel (limnological nomenclature)
Peaty (macrophyte overgrowth)	Wassists (Histosols)	Subaquatic Histosol (Fibric/Hemic)	Subhydrische Organische Böden	–	Eutrophic peat soil
Floodplain, stratified	Fluviwassents (Entisols)	Subaquatic Fluvic Gleysol	–	Subtidal Hydrosol	Alluvial humus-gleyic
With anthropogenic sedimentary horizon	pending Aquasol proposal)	Subaquatic Gleysol (Technic)	–	–	Not provided

Note. The correlations in Table 1 are proposed rather than empirically validated. To our knowledge, no published study has formally classified an aquaculture pond soil in more than one system; the entries therefore represent the most defensible cross-system reading of each pond-soil type given current diagnostic criteria and should be read as hypotheses for field testing. Where a system provides no applicable taxon, this is shown as “not provided” rather than left blank. Source citations for individual taxa are given in the corresponding sections of the text (Section 3.2, Section 3.3, Section 3.4, Section 3.5 and Section 3.6) and are omitted from the table to reduce density.

presents a proposed correlation of the main types of pond soils across the classification systems.

To make the correlations concrete and to demonstrate how they would be applied, the following worked example classifies the documented managed carp pond characterized in situ by Schmaltz et al. [17] (

Table 2. Hypothetical multi-system classification of a documented managed freshwater carp pond profile [17].

System	Reasoning	Classification
Soil Taxonomy	Mineral bottom, weak in situ horizonation, freshwater (EC < 10 dS m−1), permanently inundated < 2.5 m	Frasiwassent (Entisols); Haplowassent where the freshwater prefix is not applied
WRB 2022	Reducing mineral profile with limnic surface material under permanent shallow water	Subaquatic Gleysol (Limnic; Hyposulfidic tentative, pending incubation-pH data)
German KA5/KA6	Nutrient-rich partly oxic surface accumulation grading to anoxic sludge—the gyttja–sapropel sequence reported for the pond	Subhydric soil: Gyttja over Sapropel
ASC (Australia)	Inland, non-tidal, permanently inundated between surface and 2.5 m	Subaqueous Hydrosol
Russian (2004)	No subaqueous taxon; closest reading is a gleyic mineral type, applicable only with reservation	Not formally provided (cf. Aquazem proposal [70])

)—a permanently inundated infrequently drained freshwater production pond on acidic crystalline bedrock in Central Europe with a nutrient-rich gyttja-to-sapropel surface accumulation over the original pond bottom. We deliberately take a permanently inundated infrequently drained pond as the worked example—the case that, on the criteria set out in Section 4.1, falls within the subaqueous soil concept; a frequently drained or dredged pond would instead be treated as an anthropogenic soil with a subaqueous phase.

Further development of the classification of pond soils will likely require an interdisciplinary approach combining the methodology of pedology (horizon diagnostics and a factorial model of soil formation), applied knowledge of aquaculture (bottom substrate management and soil quality monitoring), and limnology (sedimentology and geochemistry of bottom sediments). The model of subaqueous soil-forming factors by Demas and Rabenhorst [2] may serve as a theoretical basis for such integration; however, it requires adaptation to the specifics of managed anthropogenic water bodies that fish ponds represent.

Table 3. Comparative characterization of approaches to subaqueous soils in classification systems.

Characteristic	Soil Taxonomy	WRB	KA5/KA6 (Germany)	ASC (Australia)	Russian Classification
Year of inclusion of subaqueous soils	1998 (definition)/2010 (taxa)/2023 (Aquasol proposal, pending)	2014	2005	2016	Not included
Mechanism	Suborders (Wassents and Wassists)	Qualifiers (Subaquatic and Tidalic)	Division (Abteilung), classes, and types	Suborders (Subtidal and Subaqueous) within Hydrosols	–
Classification level	Suborder (high)	Secondary (qualifier)	Separate division (highest)	Suborder	–
Maximum water depth	≤2.5 m	≤2 m	Not established	≤2.5 m	–
Sulfidic material types	Only sulfidic (=hypersulfidic)	Hypersulfidic and hyposulfidic	–	Hyper-, hypo-, and monosulfidic	–

presents a comparative characterization of approaches to subaqueous soils in the classification systems.

The Chinese paddy soil model [74], in which anthropogenic management of the water regime is considered a diagnostically significant factor of pedogenesis, offers a promising conceptual bridge between subaqueous pedology and aquaculture.

4.1. Do Aquaculture Pond Substrates Satisfy the Criteria for Subaqueous Soils?

Before pond bottoms can be placed within any of the systems reviewed here, it is worth confronting a prior question that has not been settled in the literature: do aquaculture pond substrates actually meet the diagnostic concept of a subaqueous soil (SAS)? The SAS concept of Demas and Rabenhorst rests on three propositions—that the substrate undergoes the four Simonson process groups (additions, losses, translocations, and transformations); that pedogenesis produces recognizable horizonation; and, in its original estuarine formulation, that the substrate is shallow enough to support rooted submerged aquatic vegetation (SAV) [1,2,5]. Aquaculture ponds satisfy the first proposition unambiguously but sit awkwardly against the second and third.

On the process criterion, the four process groups operate in ponds with, if anything, greater intensity than in natural shallow water: additions are dominated by allochthonous organic matter from feed and excreta; transformations include sulfidization driven by the decomposition of sulfur-bearing feed proteins and sulfate reduction; translocations include the reductive mobilization of iron and manganese under the anoxic horizon; and losses occur through periodic draining and dredging. On this criterion alone, the case for treating pond bottoms as soils is at least as strong as for estuarine SAS.

The more searching objections concern horizon development and vegetation support. Intensively managed hypereutrophic ponds rarely carry rooted SAV: organic accumulation there proceeds through planktonic and allochthonous inputs rather than vascular macrophytes, so the SAV-support criterion—framed around the seagrass and eelgrass habitats that motivated the early United States work—is frequently not met. Two considerations temper this. First, the SAV criterion is an estuarine convenience rather than a universal requirement of SAS: Erich and Drohan [19] showed that freshwater subaqueous soils in Pennsylvania reservoirs depart from the estuarine diagnostic template yet remain classifiable as Haplowassents, and the NRCS freshwater-SAS revision proposal of 2022 [33] exists precisely because the estuarine criteria transfer imperfectly to inland waters. Second, recognizable horizonation is in fact present: the five-member sequence of Munsiri, Boyd and Hajek [15] is a reproducible vertical differentiation, even where it arises by deposition over a buried subaerial soil rather than by in situ profile development. Whether that constitutes pedogenic horizonation in the strict sense is debatable; that it constitutes diagnostic horizonation that is usable for classification is not.

Two further objections concern spatial scale and disturbance. Many ponds are too small to express the catenary soil–landscape relationships on which estuarine SAS mapping was built; however, the soil–landscape model is a tool for predicting and mapping soil distribution, not a precondition for assigning a given profile to a taxon. A two-hectare carp pond may be too small to carry an internal catena and still contain a profile that is unambiguously a Subaquatic Gleysol; engineered ponds also possess a designed micro-bathymetry that imposes a predictable internal structure, and Bakken and Stolt [35] mapped freshwater ponds of southern New England by landscape units at exactly this scale. Regular dredging and bottom tillage do reset profile development, but, rather than disqualifying these substrates from soil status, this places them in the company of other repeatedly disturbed soils that classification systems already accommodate. Cultivated soils are ploughed annually, and paddy soils are puddled each season, yet both are classified, the latter as Anthrosols precisely because the disturbance regime is itself diagnostic.

Taken together, these points support a qualified position. Pond bottoms that are permanently inundated, infrequently disturbed, and developed on a stable substrate sit comfortably within the SAS concept and the existing subaqueous taxa. Intensively managed, frequently drained or dredged ponds strain that concept and are most honestly handled as anthropogenic soils with a subaqueous phase. The classification recommendations that follow are framed with this distinction in mind.

4.2. Anthropogenic Pedogenesis in Aquaculture Ponds

Four management practices give aquaculture pond soils a pedological signature with no close natural analogue: feeding, liming, draining, and mechanical bottom treatment. Each acts on the profile in a way that existing subaqueous taxa, derived from undisturbed estuaries, were never designed to record.

Feed loading delivers a sustained flux of labile organic matter, nitrogen, phosphorus, and sulfur-bearing amino acids to the bottom, producing the soft organic-rich surface mantle described by Munsiri, Boyd and Hajek [15]—the flocculent and stirred layers—in which sediment accumulates at the rates of 2–5 cm yr⁻¹ reported by Boyd [14], with surface organic carbon of 3–8%. This mantle is autochthonous to the management system rather than to the natural water body, and its accelerated feed-driven sulfidization distinguishes it from the more slowly forming organic horizons of natural lakes [16]. Liming and amendment impose a chemical overprint on the upper profile: in acid sulfate pond soils the choice of liming material measurably changes neutralization and recovery [60], and remediation with reactive silica has been proposed for affected brackish-water ponds [61]; from a classification standpoint, repeated liming produces an artificially elevated pH in the surface horizon that can mask the underlying sulfidic character.

The periodic summer fallow of pond beds—letovanie in the Russian fish-farming tradition [13]—is the single feature that most resists the static logic of subaqueous classification. During fallow the bed is exposed, the surface mantle oxidizes and partly mineralizes, sulfides oxidize to sulfate with attendant acidification, iron and manganese re-oxidize, and the substrate cracks and consolidates; on reflooding, the sequence reverses. The pond soil thus oscillates between a subaqueous and a subaerial regime on a managed schedule, accumulating the redoximorphic legacy of repeated wetting and drying. The closest natural and managed parallels are instructive: Fitzpatrick et al. [57] documented characteristic transformation sequences in sulfidic wetland materials through drying and rewetting, and Stirling et al. [59] reviewed how water-level fluctuation reshapes the pedogenesis of wet soils—both directly analogous to the engineered fallow of a fish pond—while the paddy flood–drain cycle, which produces the diagnostic hydragric horizon, is the agronomic analogue [75,77]. The classificatory consequence is that letovanie ponds are not simply subaqueous soils observed under water; they are soils whose defining horizons form through a forced subaqueous–subaerial alternation.

Mechanical bottom treatment—drying, disking, and dredging of the exposed bed—removes or inverts the surface mantle. Where frequent, profile development is truncated; where occasional, it produces buried truncated mantles that are themselves a record of management history. Duball et al. [40] provided the first direct measurement of aquaculture’s imprint on subaqueous soils—altered bulk density and organic matter under oyster culture—and their approach is the obvious template for quantifying the bottom-tillage signature in finfish and shrimp ponds. The common thread is that the pond profile is built downward by anthropogenic deposition and reworked by an engineered hydrological regime rather than developed in place under natural forcing—which is why aquaculture pond soils warrant explicit recognition as a distinct anthropogenically governed case within subaqueous pedology.

4.3. Cross-Cutting Observations and the Empirical Gap

Several cross-cutting observations emerge from the comparative analysis. There is a clear temporal gradient in the recognition of subaqueous soils: the German classification incorporated them at the divisional level as early as 2005 [49], followed by Soil Taxonomy (suborder level, 2010) [30], the WRB (qualifier level, 2014) [43], and the Australian classification (suborder level, 2016) [52]. The Russian, Brazilian, and Chinese systems have yet to make formal provisions, although proposals exist for Russia [70], and empirical justification has been demonstrated for Brazil [12]. This gradient reflects not merely institutional inertia but fundamental differences in classification philosophy: the German tradition, rooted in Kubiëna’s [7] broad concept of soil that embraced underwater substrates, was conceptually predisposed to inclusion, whereas Dokuchaev-derived systems required a more radical redefinition of their foundational concepts.

A notable finding of this review is that no published study has formally classified aquaculture pond soils using either Soil Taxonomy or the WRB. The closest parallels are the freshwater reservoir soils described by Erich and Drohan [19] and the aquaculture-impacted subaqueous soils documented by Duball et al. [40], but neither study addressed pond-specific features such as the Munsiri–Boyd–Hajek horizon sequence [15]. This represents a critical empirical gap: the formal taxonomic placement of aquaculture pond soils remains entirely hypothetical, and field-based classification studies are urgently needed.

The carbon accounting dimension adds urgency to this classification gap. Boyd et al. [21] estimated global carbon sequestration in aquaculture pond sediments at approximately 16.6 MT C yr⁻¹, while more recent Chinese national inventories indicate that intensively managed freshwater aquaculture ponds may be net greenhouse gas sources when methane and nitrous oxide emissions are accounted for [22,83]. The IPCC [23] provides reporting methodologies for aquaculture ponds under “other Flooded Lands,” but their application requires systematic characterization of pond soil properties, a task facilitated by standardized classification. The Coastal Zone Soil Survey program of the NCSS [84] demonstrates that subaqueous soil mapping is operationally feasible at scale, with surveys now completed or underway in over ten U.S. states; extending this approach to aquaculture pond systems would provide the empirical foundation for classification that is currently lacking.

4.4. Pond Soils as Landscape Elements: Carbon, Retention, and Ecosystem Services

The practical stakes of classifying pond soils extend well past pond management. Ponds and pond networks are now recognized as multifunctional landscape elements, and a classification that renders their soils legible is a precondition for accounting for the services those soils underwrite. Cuenca-Cambronero et al. [85] frame ponds and “pondscapes” as nature-based solutions delivering a wide range of contributions to people—among them carbon storage, water and nutrient retention, and biodiversity support—and argue that realizing these benefits depends on integrating biophysical, societal, and policy knowledge. Soil classification is one strand of that biophysical knowledge: without a way to identify and map the substrate that holds the carbon and binds the nutrients, the landscape-scale accounting these authors call for has no pedological foundation.

Direct measurements bear this out. Schmaltz et al. [17], working on a managed Central European carp pond, quantified the mass of sediment and its stores of organic carbon, nitrogen, and phosphorus and showed that the pond functions as a nutrient and carbon sink whose subaqueous layers are dominated by gyttja and sapropel—the very German subhydric materials discussed in Section 3.4. Their study is one of the few to characterize a managed freshwater pond bed in situ as a pedological object, and it supplies a documented profile against which the correlations of Table 1 can be tested (see the worked example in Table 2). Translating flux estimates into verifiable national inventories requires exactly this kind of standardized characterization: Millar et al. [86] developed an explicit framework for accounting subaqueous soil organic-carbon sequestration and storage, demonstrating that, once subaqueous soils are mapped to consistent taxa, their carbon pools can be inventoried with the same rigor applied to upland soils. Extending that logic to aquaculture ponds is the practical pay-off of the classification effort this review advocates.

5. Conclusions

Soil Taxonomy provides the most developed toolkit for the classification of subaqueous pond soils through specialized suborders (Wassents and Wassists), with detailed differentiation at the great group level [31,32]. However, the taxonomic system was developed primarily from estuarine and coastal lagoon material of the U.S. Atlantic coast [1,2], and its applicability to anthropogenic freshwater bodies (fish ponds) is substantially less documented [19].

The WRB offers the greatest flexibility through its qualifier system (subaquatic, limnic, gleyic, sapric, etc.), which allows detailed characterization of pond soils of various genesis [43]. Extended WRB descriptive names contain a significant volume of diagnostic information. At the same time, the absence of a specialized RSG for subaqueous soils may complicate their unambiguous identification during mass-scale mapping [44].

The German classification is unique in assigning subaqueous soils to the division level with a substantive typology (Protopedon, Gyttja, Sapropel, and Dy) that is directly applicable to pond bottom substrates [49,50]. The absence of an upper depth limit and the presence of types based on trophic status and redox conditions make this system particularly suitable for applied use in aquaculture.

The Australian classification is notable for introducing a three-part typology of sulfidic materials (hyper-, hypo-, and monosulfidic) that has practical significance for the management of ponds with acid sulfate soils [53,54].

The Russian classification currently contains no formal taxa for subaqueous pond soils [3]. Existing gleyic and peat taxa are applicable only with significant reservations, and a substantial body of pond soils (especially mineral subaqueous ones) formally lies outside the scope of the classification. A promising direction is the development of a subaqueous division within the syn-lithogenic trunk (analogous to the alluvial division) or the inclusion of subaqueous subtypes within existing types. The need for such expansion becomes increasingly urgent in the context of aquaculture development, environmental monitoring tasks, and cadastral assessment of aquatic lands.

The specificity of aquaculture anthropogenic impacts (feed application, liming, periodic draining, and mechanical bottom treatment) is not adequately reflected in any of the classification systems examined. The Munsiri–Boyd–Hajek horizon system [15] represents an attempt to create a specialized description of the pond soil profile; however, it has not been formalized in any classification. The integration of aquaculture-specific features into soil classifications—through the introduction of diagnostic horizons or qualifiers reflecting aquaculture genesis (analogous to the hydragric qualifier for paddy soils in the WRB)—remains an unresolved task.

It follows that the soil status of pond substrates is best stated as a qualified position rather than a blanket claim: permanently inundated infrequently disturbed ponds fall within the subaqueous soil concept and its existing taxa, whereas intensively managed, frequently drained or dredged ponds are more honestly treated as anthropogenic soils with a subaqueous phase. We also note a specific limitation of the present review: our coverage of the Chinese system rests on the available English-language literature, and a dedicated appraisal of Chinese-language sources—given that China holds the world’s largest area of freshwater aquaculture ponds—remains the most consequential gap in the present synthesis.

Based on the comparative analysis presented above, we recommend the WRB (4th edition, 2022) as the most suitable framework for the current classification of aquaculture pond soils. This recommendation is grounded in several considerations. The qualifier-based approach provides the necessary flexibility to capture the diverse and often transitional nature of pond soils—from mineral subaqueous substrates (Subaquatic Gleysols) to organic-rich accumulations (Subaquatic Limnic Histosols)—within a single internally consistent nomenclature. The WRB’s international scope and correlation function make it the most practical system for comparative studies across the global aquaculture sector, which spans diverse national classification traditions. The distinction between hypersulfidic and hyposulfidic materials (absent from the USDA Soil Taxonomy) provides a finer diagnostic resolution that is directly relevant to acid sulfate soil management in coastal shrimp ponds. The WRB permits the incorporation of anthropogenic influences through qualifiers such as hydragric and technic, potentially enabling the recognition of aquaculture-specific pedogenesis without structural modification of the system.

Nevertheless, the USDA Soil Taxonomy remains preferable for detailed soil survey and mapping in regions where subaqueous soil series have been established (primarily the U.S. Atlantic coast), and the German classification (KA5/KA6) offers the most operationally applicable typology for characterizing pond bottom substrates by trophic status and redox conditions. A multi-system approach—combining the WRB for international communication, Soil Taxonomy for formal survey, and German typology for applied pond management—may ultimately prove most effective.