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Article

Contrasting Reaction of Dissolved Organic Matter with Birnessite Induced by Humic and Fulvic Acids in Flooded Paddy Soil

by
Xiangbiao Zhang
,
Xin Zhou
,
Yanyue Ma
,
Wenjin Zhang
,
Ruihua Zhang
and
Weiwei Zhai
*
Zhejiang Key Laboratory of Low-Carbon Control Technology for Industrial Pollution, College of Environment, Zhejiang University of Technology, Hangzhou 310000, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(16), 7203; https://doi.org/10.3390/su17167203
Submission received: 15 May 2025 / Revised: 28 July 2025 / Accepted: 6 August 2025 / Published: 8 August 2025
(This article belongs to the Section Soil Conservation and Sustainability)
Browse Figures
Versions Notes

Abstract

Manganese (Mn) oxides exhibit significant potential to either stabilize or destabilize soil organic carbon (SOC) through the polymerization and/or oxidation of organic molecules via organo-mineral interactions. Birnessite (MnO2) is known to strongly interact with soil dissolved organic matter (DOM), which is DOM composition-dependent. Humic acid (HA) and fulvic acid (FA) are commonly used as organic fertilizers in soils. In this study, the contrasting reaction of DOM with birnessite in flooded paddy soil with HA and FA amendment was investigated at a molecular level. The results demonstrated that HA amendment enhanced the reaction of phenolic compounds in soil DOM with birnessite, leading to the formation of condensed aromatic compounds and polymeric products (PP) with higher molecular weights and aromaticity. This suggests that HA amendment enhances the birnessite-induced polymerization of soil DOM. In contrast, FA facilitated the birnessite-induced oxidation of soil DOM, yielding dicarboxylic acids (DA), monocarboxylic acids (MA), and quinones products (QP). These findings demonstrate that the reactivity of soil DOM with birnessite is significantly influenced by the composition of DOM exogenously added. This study provides comprehensive understandings of the interactions among Mn and C and helps to predict behaviors of DOM molecules in flooded paddy soil, which is critical for optimizing sustainable soil management.

1. Introduction

Soil is the largest terrestrial reservoirs of organic carbon (OC), where the balance between the degradation and preservation of soil organic carbon (SOC) is vital for the modulation of atmospheric CO2 concentrations and further influences on global climate change [1,2]. Agricultural soils, particularly paddy soils, exhibit substantial carbon (C) sequestration potential, with average C stocks 78% higher than those in adjacent upland soils [3,4]. Much of this OC is intimately associated with soil minerals phases [5,6]. The recently proposed mineral carbon pump (MnCP) concept describes the critical role of mineral-induced processes in long-term OC storage and stabilization in soils [7,8]. Dissolved organic matter (DOM) represents the most reactive fraction of soil organic matter (SOM). The interaction between DOM and minerals plays a crucial role in regulating C cycling processes [9].
Manganese (Mn) oxides are typically an order of magnitude less abundant than iron (Fe) (oxygen) oxides; however, the interaction between Mn-C can be disproportionately important for OC preservation, because they have higher surface area and extremely reactive surfaces that facilitate dynamic interfacial reactions. For example, soil organic matter (SOM) accumulates preferentially in the zones of Fe-Mn nodules containing silicates and Mn oxides and the transitional zones between Fe and Mn oxides, while being absent in the pure Fe accumulation zones [10]. However, Mn oxides play dual roles in destabilizing and stabilizing SOM through complex coupled reactions [11]. Mn oxide-induced carbon destabilization occurs through multiple pathways, including reductive dissolution of Mn oxides [12], extracellular enzyme Mn peroxidase, and the ligand-promoted extraction of Mn(III) [13]. Carbon stabilization by Mn oxides involves various mechanisms such as cation bridging [14] and electrostatic interactions [15], physical trapping [16], coprecipitation [17], and polymerization [18]. Mn oxides not only sorb organic compounds but also facilitate their subsequent oxidation [19]. DOM undergoes significant molecular transformations through various reaction mechanisms catalyzed by Mn oxides. However, the extent of reaction of DOM and Mn oxides depends on the initial DOM composition [20,21]. Previous study has demonstrated that Mn oxides preferentially remove DOM molecules with higher molecular weight (MW) and aromaticity [14]. It was also reported that the molecular fractionation of Suwannee River fulvic acid (SRFA) with higher MW, aromaticity, carbon unsaturation degree, nominal oxidation state of carbon (NOSC), and oxygen (O) and nitrogen (N) contents but lower hydrogen (H) content exhibit greater reactivity in reducing Mn oxides [21]. Interestingly, soil DOM molecules with lower O content are preferentially oxidized by birnessite to O-rich compounds, owing to their typically higher phenolic content [22]. The reaction pathways of phenolic compounds contained the opening of the aromatic ring to form dicarboxylic and/or monocarboxylic acids [23], oxidation to quinones [20], and polymerization of oxidation products [16]. The conversion of phenols to quinones represents a common Mn oxide-induced DOM oxidation pathway, resulting in quinone products being consistently identified across all DOM samples regardless of their initial composition [20]. Furthermore, Mn oxides catalyze the polyphenol Maillard reaction and generate humic polymers, which is an important pathway in humification processes [24]. Recent research has demonstrated that Mn ions and minerals can enhance abiotic Maillard reaction rates by up to two orders of magnitude under continental margin temperature conditions, potentially generating approximately 4.1 Tg C yr−1 for preservation in marine sediments [25]. Although the oxidation of aquatic DOM and model organic compounds by Mn oxides has been extensively studied, the mechanisms governing soil DOM-Mn oxide interactions remain poorly understood. In addition, the specific conditions favoring Mn oxide-induced DOM oxidation versus polymerization remain unclear.
Humic acid (HA) and fulvic acid (FA) are natural bio-stimulants and also environmentally friendly and efficient soil amendments [26]. In recent years, HA and FA have been widely adopted as functional fertilizers in agricultural systems due to their multifaceted benefits, including nutrient provision, heavy metal immobilization, and enhanced soil carbon sequestration [27,28,29,30]. Previous studies have demonstrated that long-term organic amendments significantly enhance the recalcitrant DOM fraction while reducing labile components [31,32]. The application of HA and FA may substantially alter soil DOM composition. However, the impacts of HA/FA amendments on soil DOM-Mn oxide reactions remain poorly characterized, particularly regarding molecular-scale transformation pathways.
The present study is aimed at investigating the molecular transformation of soil DOM upon oxidation and polymerization by birnessite with HA and FA amendments using electrospray ionization (ESI) coupled with ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Our study offered a comprehensive understanding on the underlying mechanisms for the oxidation and polymerization of soil DOM by birnessite, which were governed by the chemical composition of DOM exogenously added.

2. Methods and Materials

2.1. Sampling and Characterization

Surface soil samples (0–20 cm depth) were collected from a paddy field in Shangyu, Zhejiang Province, China (29°59′43″ N, 120°47′13″ E) (Figure S1). The soil was classified as loam type with a pH of 5.80, containing total organic C, Fe, and Mn with concentrations of 25.19 g kg−1, 27.19 g kg−1, and 0.23g kg−1, respectively [33]. Soil samples were air-dried, homogenized through a 2mm mesh sieve, and subsequently used for characterization and incubation experiments.

2.2. Preparation of Birnessite, HA, and FA

Acid birnessite was synthesized as described previously [34]. Briefly, the synthesis of acid birnessite involves the dropwise addition (<1 mL min−1) of a 12 M hydrochloric acid (HCl) solution into a boiling 0.67 M KMnO4 solution. The solution remained boiling for 30 min and aged in an oven at 60 °C for 12 h, all while continuously stirring and then cooled to room temperature. The manganese oxide particles were filtered by vacuum filtration through a 0.22 μm nylon filter, rinsed thoroughly with Milli-Q water to remove any remaining reactants until the suspension conductivity was <20 s∙cm−1 and freeze dried. The dried solids were stored dark at room temperature until reaction. The X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) patterns of birnessite are shown in our previous study [33]. HA and FA were extracted and purified from pig manure (PM), which was obtained from Taobao (Alibaba, Hangzhou, China) following established protocols [35,36]. Briefly, freeze-dried compost samples were extracted with a mixture of 0.1 M Na2P2O7 and 0.1 M NaOH at a solid-to-liquid ratio (w/v) of 1:10. The suspension was constantly shaken at 200  rpm for 24  h at 25 °C and then centrifuged at 8000× g for 10 min to obtain the supernatant humic substance. The supernatant was collected and acidified to pH 1.5 using 6 M HCl and maintained at 4 °C in darkness for 12 h. Then, the HA was separated from FA by centrifugation at 11,000× g for 15 min, the residue was collected as HA, and the suspension was FA. The HA fraction was repeatedly washed with 0.1 M HCl to ensure purity. The FA was purified using XAD-8 macro-porous adsorption resin and hydrogen-type cation exchange resin (Amberlite IR-120, Fluka, Buchs, Switzerland). Finally, both purified HA and FA fractions were freeze-dried and stored for subsequent use. The physicochemical properties of HA and FA were characterized by elemental composition (Vario EL III, Elementar, Mt. Laurel, NJ, USA) and a Fourier Transform Infrared Spectrometer (FTIR, Nicolet iS20, Thermo Fisher Scientific, Waltham, MA, USA).

2.3. Anaerobic Incubation Experiment

Anaerobic incubation systems were set up to simulate the flooded conditions in paddy soils. All glassware and rubber plugs were autoclaved before use. Six treatments were set up, including (i) control (CK); (ii) birnessite (MN); (iii) HA; (iv) HA + birnessite (HM); (v) FA; (vi) FA + birnessite (FM). For each treatment, 20 g of soil was homogenized with 40 mL deionized water. HA and FA were applied at 1‰ (w/w) [37], while birnessite was added at 0.5% (w/w) of soil mass [33]. Soil without HA, FA, or Mn amendment served as the control (CK). All bottles were purged with N2 for 20 min and immediately sealed with butyl rubber stoppers. The mixtures were incubated at 25 °C in the dark. Soil samples were collected at designated time intervals (3, 7, 14, 21, and 42 days) during incubation. The pH of the sample was detected, and then, soil samples were homogenized and centrifuged at 8000× g for 10 min to extract porewater. The supernatant was filtered through 0.22 μm PTFE membrane filters and stored at 4 °C for subsequent analysis. Dissolved organic carbon (DOC) concentrations in porewater were quantified using a TOC analysis (Multi N/C, Jena 3100, Analytik Jena, Jena, Germany). Mn(II) concentrations were determined colorimetrically at 540 nm using the potassium permanganate method [38].

2.4. FT-ICR-MS Analysis

The molecular composition of DOM in soil porewater was characterized using FT-ICR-MS (Bruker SolariX XR 12T, Bruker Daltonics, Billerica, MA, USA). Solid phase extraction (SPE) was performed on all the samples to desalinate and concentrate the DOC solution prior to FT-ICR-MS measurement (Text S1, Text S2). Key molecular indices including modified aromaticity index (AImod), double bond equivalence (DBE), and NOSC, O/C ratio, H/C ratio were calculated, referring to the previous study [20]. The measured DOM compounds were categorized into the following five compound classes based on neutral elemental compositions: (i) condensed aromatic compounds (AImod > 0.66), (ii) polyphenol compounds (0.5 < AImod ≤ 0.66), (iii) highly unsaturated and phenolic compounds (AImod ≤ 0.5 and H/C < 1.5), (iv) aliphatic compounds (1.5 ≤ H/C ≤ 2.0), and (v) saturated fatty acids and carbohydrates (H/C ≥ 2.0) [39]. To determine the changes in DOM composition after reacting with birnessite, a comparative analysis of DOM transformation was conducted through paired treatment comparisons, as follows: CK vs. MN, HA vs. HM, and FA vs. FM, respectively. The organic compounds were grouped into the following three parts: persistent, removed, and formed compounds. Persistent compounds were defined as molecular formulae detected in both control and corresponding treated groups (CK-MN, HA-HM, FA-FM pairs). Removed compounds were exclusively identified in control groups (CK, HA, FA), while formed compounds were unique to treated groups (MN, HM, FM). In addition, formulas of compounds (i.e., CHO, CHON, CHOS, CHONS) were grouped into removed, persistent, and formed compounds as well; compounds were categorized into the following seven compound classes based on neutral elemental compositions: (i) lipids (O/C = 0–0.3, H/C = 1.5–2.0), (ii) proteins (O/C = 0.3–0.67, H/C = 1.5–2.2), (iii) lignins (O/C = 0.1–0.67, H/C = 0.7–1.5), (iv) carbohydrates (O/C = 0.67–1.2, H/C = 1.5–2.2), (v) unsaturated hydrocarbons (O/C = 0–0.1, H/C = 0.7–1.5), (vi) condensed aromatics (O/C = 0–0.67, H/C = 0.2–0.7), and (vii) tannins (O/C = 0.67–1.2, H/C = 0.5–1.5) [40]. In order to test the reproducibility of the FT-ICR-MS analysis, we selected three original soil DOM samples for testing and analysis. The results showed excellent reproducibility (Figure S2).

3. Results and Discussion

3.1. Oxidation and Adsorption of DOM by Birnessite

The DOC concentrations were monitored throughout the incubation period. HA and FA amendments significantly enhanced soil DOC concentrations to 1.6–2.5-fold and 5.1–12.8-fold of CK, respectively (Figure 1a). It was consistent with the result of previous studies that the applications of organic fertilizer increase the SOC [32,41]. The HA and FA amendment substantially altered both the molecular diversity and compositional profile of detected compounds. The original soil contained 4868 molecular formulae, which decreased to 3825 in CK samples following incubation. The HA amendment reduced molecular diversity to 3505 formulae, whereas FA amendment increased it to 4828 relative to CK (Figure 2a). Comparative analysis revealed 1001, 623, and 2075 unique molecular formulae in CK, HA, and FA treatments, respectively (Figure 2b). The HA amendment significantly increased CHO (4.5%) and CHOS (3.0%) compounds, while decreasing CHON (3.5%) and CHONS (4.0%) compounds compared with that of CK (Table 1). In contrast, FA amendment enhanced CHON (4.2%) and CHOS (1.4%) abundances while reducing CHO (1.1%) and CHONS (4.5%) components compared to CK. Notably, FA amendment exhibited 5.6% lower CHO and 7.7% higher CHON compared to that in HA (Table 1). These findings demonstrate distinct molecular signatures between HA and FA amendment, with FA particularly enriching N-containing compounds that substantially modified DOM composition [35]. The amendment-induced DOM compositional changes significantly modulated subsequent birnessite interactions. The concentration of soil DOC in the HM treatment showed a trend of increasing in the first 14 days and then decreasing until day 42, with increases in the first 7 days and then decreases until day 42 in FM. The removal rates of DOC in HM and FM were 3.6 and 23.6 mg L−1day−1, respectively. By day 42, birnessite amendment reduced DOC concentrations by 18.5% (MN), 17.9% (HM), and 13.1% (FM) compared to corresponding non-birnessite treatments (Figure 1a). These DOC reductions indicate concurrent sorption and oxidative processes, both contributing to DOM removal from solution. Simultaneously, increases in aqueous Mn(II) with the addition of birnessite in MN, HM, and FM treatments were indirect indicators of DOM oxidation (Figure 1b). This redox coupling aligns with established mechanisms where DOM oxidation drives Mn(IV) reduction to Mn(III/II) species [42]. Oxidized organic C species and reduced Mn may either be removed from the DOM pool or desorbed into solution [43]. The faster removal rates of DOC and production of much more dissolved Mn(II) in FM than HM (Figure 1b) may be due to the lower pH of FM than that in HM (Figure 1c), which is consistent with that lower pH favoring OM oxidation by Mn oxides [43]. However, a low pH value is not conducive to the adsorption of DOM by Mn oxides [44]. DOM adsorption is positively correlated with the crystallinity of Mn oxides; the Mn oxides with lower crystallinity have the largest specific surface area and adsorb the highest amount of DOM, while the Mn oxides with higher crystallinity could induce the Maillard reaction and thereby promote the formation of geopolymerized organic matter, leading to reduced bioavailability of DOM [45]. In addition, an increase in the concentration of DOM will promote its adsorption on the surface of Mn oxides [44]. Moreover, birnessite amendment significantly altered both molecular diversity and key molecular indices of DOM. The addition of birnessite decreased the DOM compound numbers from 3825 in CK to 3121 in MN and from 4828 in FA to 4342 in FM, respectively (Figure 2a). Unique molecular formulae decreased from 1683 (CK) to 979 (MN) and from 1619 (FA) to 1133 (FM) following birnessite amendment (Figure 2c,e). These results indicated that more molecule removed than formed by birnessite in CK and with FA amendment through oxidation and/or selective sorption. However, birnessite amendment slightly increased the DOM compounds from 3505 to 3666 in HM, and 1384 compounds were only present in HA compared to 1545 compounds only present in HM, indicating that soil DOM reacted with birnessite to form more molecules with HA amendment.
Compared to CK, the MN treatment decreased the distribution of aliphatic compounds from 27.3% to 24.8%, while increasing condensed aromatics from 10.2% to 12.9% (Table 1). HA amendment showed more pronounced compositional shifts by decreasing highly unsaturated and phenolic compounds from 42.0% to 37.5% and increasing condensed aromatic compounds from 9.4% to 15.5% compared to HA with HM. In contrast, HM increased saturated fatty acids and carbohydrates and aliphatic compounds by 1.3% and 2.1% and decreased highly unsaturated and phenolic compounds by 2.5%. As to abundance-weighted molecular indices, the oxidation decreased the C/H ratio, O/C ratio, and NOSC but increased AImod and DBE with HA amendment, while it increased the H/C ratio but decreased O/C ratio, NOSC, AImod, and DBE with FA amendment (Table 1). Thus, the difference between with HA and FA amendment suggested that HA and FA induced a contrasting reaction of DOM with birnessite in soil, with detailed molecular-level evidence presented in subsequent sections.

3.2. Molecular Reactivity of DOM

In CK, birnessite oxidation preferentially removed aliphatics (27.3%), highly unsaturated/phenolics (38.3%), and polyphenols (15.3%), representing the dominant removal categories (Figure 3a and Figure 4c). The higher abundance of highly unsaturated and phenolic compounds in initial soil (58.2%) would contribute to the higher proportion removed than others (Figure 4a). The removed compound with HA amendment (Figure 3b) was similar to that in CK, whereas FA amendment exhibited a fundamentally different removal profile. The group 1–5 compounds removed with HA amendment contributed to 14.4%, 15.3%, 39.1%, 26.4%, and 4.8%, respectively (Figure 4f). While group 1–5 compounds of FA amendment displayed distinct removal proportions of 11.2%, 14.7%, 49.7%, 21.1%, and 3.3%, respectively (Figure 4i). The most compounds removed with FA amendment were highly unsaturated and phenolic compounds and polyphenol compounds (Figure 3c). These differential removal patterns between HA and FA amendment originated from their distinct initial DOM compositions. The higher reduction in highly unsaturated and phenolic compounds in FA than that in HA was due to its higher abundance in FA (Table 1). Furthermore, relative compound reactivity was quantified using class-specific removal ratios (removed/total), revealing distinct redox susceptibilities [21]. The highly unsaturated and phenolic compounds, polyphenol compounds, and condensed aromatic compounds has higher relative reactivity than others (Figure 4k), which because aromatic, DOM is reactive and preferentially oxidized relative to others [20]. A comparison between the removed and persistent compounds provided further insights into the preferential removed. Van Krevelen analysis revealed removed compounds were enriched in O, N, and S, predominantly comprising CHON and CHONS formulae (Figure 5). In CK and HA, the removed compounds contained variable amounts of CHO formulas (Figure 5a,b), whereas removed compounds with FA amendment contained more N-containing formulas (Figure 5f). The FTIR results showed that FA contained more -NH2 (1560 cm−1) and -COOH compounds (1410 cm−1) (Figure S3), and -COOH and -NH2 are common redox-active moieties. The DBE could indicate the molecular unsaturation, and the total number of rings [46] and greater AImod indicated stronger aromaticity [47,48]. Regardless of HA or FA amendment, the numbers of removed compounds that had substantially higher DBE, AImod, NOSC, and mass (Figure 6) were more than that of persistent compounds. Removed compounds with FA amendment had higher AImod and MW than those with HA amendment (Figure 6e,k). These results were consistent with others that showed reducing the activity of functional groups attached to unsaturated C is higher than that of saturated C [47].

3.3. Oxidation Product Formed

The numbers of compounds showed that 979, 1545, and 1133 specific molecules were determined only in MN, HM, and FM, compared to 1683, 1384, and 1619 specific molecules that were determined in CK, HA, and FA, respectively (Figure 2c–e). The decreased numbers with birnessite amendment indicated that more compounds were removed than formed with birnessite amendment in MN and FM treatment, but the result for HM was opposite. Newly formed compounds exhibited distinct compositional profiles compared to removed and persistent species. The oxidation of DOM by birnessite resulted in an increase in condensed aromatic and polyphenol compounds in MN (Figure 3a) and condensed aromatic, polyphenol, and aliphatic compounds in HM treatments (Figure 3b). While only aliphatic compounds increased in FM (Figure 3c). The relative abundance-weighted contributions of the five compounds of formed compounds showed that condensed aromatic, polyphenol, highly unsaturated and phenolic, aliphatic compounds, and saturated fatty acids and carbohydrates contributed to 25.3%, 15.9%, 34.2%, 19.3%, and 5.3%, respectively, in CK (Figure 4d) and contributed to 28.3%, 14.7%, 28.8%, 25.0%, and 3.4%, respectively, in HA (Figure 4g). However, the newly formed compounds with FA amendment mainly increased the highly unsaturated and phenolic compounds (39.1%), aliphatic compounds (28.0%), and saturated fatty acids and carbohydrates (8.6%) (Figure 4j). The previous study reported that soil DOM has a higher proportion of phenolic compounds [22]. Three oxidation pathways of phenolic compounds are likely to occur in this system with MnO2 [44]; phenolic compounds open the aromatic ring and transfer to dicarboxylic acids and/or monocarboxylic acids; phenolic compounds react with Mn oxides to form phenoxy radicals and then produce polymeric products and phenolic compounds oxidize to quinones (Table 2).
Relative to FA amendment, HA amendment showed significantly higher proportions of newly formed condensed aromatic compounds (Figure 4l). The molecular transformation of DOM was further analyzed with the changes of MW before and after reactions with birnessite, which showed that it produced more compounds in high-MW regions with HA amendment (Figure 6j–l). These findings demonstrate HA amendment enhanced birnessite-induced DOM polymerization, yielding high-MW organic products. The polymerization of DOM is an important geochemical reaction catalyzed by Mn oxides [49]. The polymerization of DOM could increase DBE, AImod, and NOSC values. Therefore, the distributions of DBE, AImod, and NOSC of formed compounds shifted to higher values with HA amendment after oxidation by birnessite compared to that of removed compounds (Figure 6a,b,d,e,g,h), which also suggests polymerization reactions occurred. Molecular formula analysis provided additional evidence supporting polymerization pathways. As shown in Equation (2), polymerization products followed the stoichiometric pattern [+(6 + 6n)C + (4 + 4n)H + (1 + n)O] (n ≥ 0). There were 47, 77, and 54 polymeric products that were matched from the newly formed molecules based on this rule in CK, with HA or FA amendment, respectively (Figure 7). Therefore, the FT-ICR-MS results supported that the HA amendment enhanced the polymerization of soil DOM during soil DOM reaction with birnessite. Previous studies have also reported that polymerization reactions of phenolic compounds or aromatic amines can be catalyzed by Mn oxides [16,49,50]. Mn(IV) oxides could accelerate the oxidative polymerization of polyphenols under common soil pH conditions [47]. In contrast, the formed compounds with FA amendment had substantially lower NOSC and AImod (Figure 6f,i). The lower abundance of high-MW compounds in FA compared with that in HA indicated preferential oxidative pathways over polymerization. Following the rules of +3O and +O-5C-4H based on the opening of the aromatic ring (Equation (1)), we identified 19, 18, and 29 dicarboxylic acid products (DA) and 13, 20, and 30 monocarboxylic acid (MA) products (Figure 7b,c). Moreover, the oxidation of phenols to quinones appeared to be a common pathway as 51, 56, and 69 quinone products (QP) in CK, with HA and FA amendment, respectively, following the rules of -2H (Equation (3)) (Figure 7d). These findings demonstrate that FA amendment enhanced oxidation pathways of DOM by birnessite, favoring carboxylic acid and quinone formation. The different chemical compositions of HA and FA may be responsible for the difference (Table S1, Figure S3). Humic acid is a complex mixture of organic molecules, which has a strong absorption peak at 3270, 2930, 1620, and 1510 cm−1 (Figure S3). According to the previous study, the absorption peak at 3270 cm−1 mainly originates from the O-H stretching vibration in carboxylic acids and phenols; the absorption peak at 2930 cm−1 characterizes the C-H3 and C-H2 of aliphatic groups; the absorption peaks at 1620 cm−1 belong to the C=C or C=O vibrations in aromatic or carboxylic acids; the absorption peak at 1510 cm−1 is related to the vibration of the aromatic ring framework, indicating that the HA mainly composed of phenolic, carboxylic acid, enolic, and quinone functional groups and has higher aromaticity [51]. Humic acid can strongly adsorb onto the surface of birnessite through functional groups such as carboxyl and phenolic hydroxyl groups, partially occupying its oxidation sites. The adsorption may reduce the oxidation ability of birnessite. The birnessite can activate the inert components (such as aromatic rings) in HA, generating phenoxide radicals or semi-quinone radicals. This, in turn, promotes the polymerization reactions of other small molecules (such as phenols, amino acids) in soil DOM, forming larger molecular weight organic substances. In contrast, FA is the smallest component of humic substances, and the absorption peaks located around 1720, 1410, and 1050 cm−1 were higher than that of HA (Figure S3), indicating that the HA contained more carboxylic acid and polysaccharide structure [51]. Compared to HA, the benzene ring connection of FA is loose, with lower C and H and higher O content [52]. FA contains more oxidizable functional groups, which can directly act as electron donors and be oxidized by birnessite to produce CO2 and small molecular organic acids, while birnessite is reduced. The free radicals generated by the oxidation of FA may further attack other soil DOM molecules, leading to the overall degradation of soil DOM. The higher amounts of polymeric products formed with HA amendment than FA probably reflect the preferential reaction of phenolic compounds with aromatic structures. A previous study reported that in the presence of HA, residues of catechol and p-coumaric acid are homogenously bound to humic molecules of various sizes via ester and ether bonds [53]. The amendment of HA increased the binding of the phenolic compounds with HA in soil catalyzed by birnessite, inevitably enhancing the resident time of the phenolic C in soil and, therefore, contribute to C storage in soil. Conversely, the higher N compounds and lower pH of FA favors birnessite-induced DOM oxidation pathways, potentially enhancing C destabilization in soil.

4. Conclusions

Soil functionality is critically dependent on the balance between SOC stabilization and nutrient cycling, both of which are influenced by the interactions between dissolved organic matter (DOM) and reactive mineral phases such as manganese oxides. This study systematically investigated the differential transformation pathways of DOM from HA and FA amendment paddy soil during birnessite-induced reactions. Our findings demonstrate that HA amendment significantly enhanced the formation of condensed aromatic compounds and PP, increasing the MW and aromaticity of products. These results indicate HA-enhanced birnessite-induced DOM polymerization, representing a potential mechanism for long-term C stabilization in terrestrial ecosystems. In contrast, FA amendment preferentially directed DOM transformation toward low MW oxidation products (DA, MA, and QP), indicating enhanced oxidative pathways of DOM with birnessite, which may accelerate SOC turnover and release bioavailable nutrients. This study emphasizes the contrasting roles of HA and FA amendment in regulating C cycling in soil by Mn oxides. Future efforts should focus on developing tailored HA and FA amendment strategies and investigating the long-term effects of HA and FA on SOC stabilization. By elucidating these processes, we can advance strategies to enhance soil fertility, mitigate climate change through carbon sequestration, and reduce reliance on synthetic fertilizers to promote the sustainability of soil.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su17167203/s1, Text S1. Solid phase extraction of DOM. Text S2. FT-ICR-MS data collection and processing. Table S1. Elemental compositions and ratios of compost-derived HA and FA. Figure S1. Location map of the soil area. Figure S2. Comparison of percentages of compound classes between the three replicates of the original soil DOM sample. Figure S3. FTIR spectra of HA and FA derived from manure compost.

Author Contributions

X.Z. (Xiangbiao Zhang): data curation, investigation, methodology. X.Z. (Xin Zhou): data curation, investigation, methodology. Y.M.: data curation, investigation, methodology. W.Z. (Wenjin Zhang): investigation, methodology. R.Z.: investigation, writing—review and editing. W.Z. (Weiwei Zhai): conceptualization, funding acquisition, writing—original draft, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (41907101) and Fundamental Research Funds for the Provincial Universities of Zhejiang (RF-A2024012).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Variations of DOC (a), Mn(II) concentrations (b), and pH (c) in different treatments during the incubation. CK: control; MN: birnessite; HA: humic acid; HM: humic acid + birnessite; FA: fulvic acid; FM: fulvic acid + birnessite. Data are average ± standard deviation (n = 3).
Figure 1. Variations of DOC (a), Mn(II) concentrations (b), and pH (c) in different treatments during the incubation. CK: control; MN: birnessite; HA: humic acid; HM: humic acid + birnessite; FA: fulvic acid; FM: fulvic acid + birnessite. Data are average ± standard deviation (n = 3).
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Figure 2. Venn diagrams showing the number of unique and shared DOM molecules among the six treatments CK, MN, HA, HM, FA and FM (a), between CK, HA and FA (b), CK and MN (c), HA and HM (d), FA and FM (e) treatments. CK: control; MN: birnessite; HA: humic acid; HM: humic acid + birnessite; FA: fulvic acid; FM: fulvic acid + birnessite.
Figure 2. Venn diagrams showing the number of unique and shared DOM molecules among the six treatments CK, MN, HA, HM, FA and FM (a), between CK, HA and FA (b), CK and MN (c), HA and HM (d), FA and FM (e) treatments. CK: control; MN: birnessite; HA: humic acid; HM: humic acid + birnessite; FA: fulvic acid; FM: fulvic acid + birnessite.
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Figure 3. Van Krevelen diagrams of assigned molecular formulas between CK and MN (a), HA and HM (b), FA and FM (c) treatments. Formulas were assessed to persistent, removed, formed compounds using presence/absence analysis. The molecular components were assigned to five groups according to the modified aromaticity index (AImod) and H/C ratio, where group 1 represents condensed aromatic compounds (AImod > 0.66), group 2 represents polyphenol compounds (0.5 < AImod ≤ 0.66), group 3 represents highly unsaturated and phenolic compounds (AImod ≤ 0.5 and H/C < 1.5), group 4 represents aliphatic compounds (1.5 ≤ H/C ≤ 2.0), and group 5 represents saturated fatty acids and carbohydrates (H/C ≥ 2.0).
Figure 3. Van Krevelen diagrams of assigned molecular formulas between CK and MN (a), HA and HM (b), FA and FM (c) treatments. Formulas were assessed to persistent, removed, formed compounds using presence/absence analysis. The molecular components were assigned to five groups according to the modified aromaticity index (AImod) and H/C ratio, where group 1 represents condensed aromatic compounds (AImod > 0.66), group 2 represents polyphenol compounds (0.5 < AImod ≤ 0.66), group 3 represents highly unsaturated and phenolic compounds (AImod ≤ 0.5 and H/C < 1.5), group 4 represents aliphatic compounds (1.5 ≤ H/C ≤ 2.0), and group 5 represents saturated fatty acids and carbohydrates (H/C ≥ 2.0).
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Figure 4. Abundance-weighted contributions of compounds of initial soil (a), and persistent (b,e,h), removed (c,f,i), formed (d,g,j) compounds and percentage of the number of removed (k) and formed compounds (l) over the total number in each compound class. CK: control; HA: humic acid; FA: fulvic acid.
Figure 4. Abundance-weighted contributions of compounds of initial soil (a), and persistent (b,e,h), removed (c,f,i), formed (d,g,j) compounds and percentage of the number of removed (k) and formed compounds (l) over the total number in each compound class. CK: control; HA: humic acid; FA: fulvic acid.
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Figure 5. Van Krevelen diagrams (H/C versus O/C molar ratios) of assigned molecular formulas (CHO, (a–c); CHON, (d–f); CHONS, (g–i); CHOS, (j–l)). Formulas were assessed to persistent, removed, formed compounds using presence/absence analysis.
Figure 5. Van Krevelen diagrams (H/C versus O/C molar ratios) of assigned molecular formulas (CHO, (a–c); CHON, (d–f); CHONS, (g–i); CHOS, (j–l)). Formulas were assessed to persistent, removed, formed compounds using presence/absence analysis.
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Figure 6. Distributions of double bond equivalency (DBE, (a–c)), modified aromaticity index (AImod, (d–f)), nominal oxidation state of C (NOSC, (g–i)), mass (j–l), H/C (m–o), and O/C ratios (p–r) of removed, formed, and persistent compounds.
Figure 6. Distributions of double bond equivalency (DBE, (a–c)), modified aromaticity index (AImod, (d–f)), nominal oxidation state of C (NOSC, (g–i)), mass (j–l), H/C (m–o), and O/C ratios (p–r) of removed, formed, and persistent compounds.
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Figure 7. Numbers of the DOM molecules formed from the potential transformation pathways after reactions with birnessite. (a) PP: polymeric products; (b) DA: dicarboxylic acid products; (c) MA: monocarboxylic acid products; (d) QP: quinone products.
Figure 7. Numbers of the DOM molecules formed from the potential transformation pathways after reactions with birnessite. (a) PP: polymeric products; (b) DA: dicarboxylic acid products; (c) MA: monocarboxylic acid products; (d) QP: quinone products.
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Table 1. DOM molecular characterization of the initial soil and initial soil DOM after flooded incubation with HA, FA, and birnessite amendment a.
Table 1. DOM molecular characterization of the initial soil and initial soil DOM after flooded incubation with HA, FA, and birnessite amendment a.
Initial
DOM		CKMNHAHMFAFM
Total DOM4868382531213505366648284342
CHO (%)44.9
(2186)		37.5
(1568)		43.1
(1455)		42.0
(1472)		46.9
(1721)		36.4
(1757)		37.5
(1626)
CHON (%)45.3
(2204)		32.0
(1225)		27.3
(854)		28.5
(1000)		24.8
(908)		36.2
(1747)		34.7
(1506)
CHOS (%)7.8
(378)		20.2
(740)		19.4
(593)		23.2
(814)		22.4
(822)		21.6
(1043)		21.8
(946)
CHONS (%)2.1
(101)		10.3
(292)		6.8
(219)		6.3
(219)		5.9
(215)		5.8
(281)		6.1
(264)
Saturated fatty acids and carbohydrates %
(Numbers)		3.0
(146)		5.0
(192)		5.1
(160)		4.7
(166)		4.2
(152)		3.0
(144)		4.3
(187)
Aliphatic compounds %
(Numbers)		28.0
(1362)		27.3
(1043)		24.8
(773)		28.7
(1006)		28.0
(1026)		24.0
(1160)		26.2
(1136)
Highly unsaturated and phenolic compounds %
(Numbers)		58.1
(2831)		42.9
(1639)		42.5
(1326)		42.0
(1471)		37.5
(1373)		52.8
(2547)		50.3
(2184)
Polyphenol compounds %
(Numbers)		8.1
(395)		14.6
(560)		14.7
(458)		15.2
(533)		14.9
(547)		13.9
(670)		13.1
(570)
Condensed aromatic compounds %
(Numbers)		2.8
(134)		10.2
(391)		12.9
(404)		9.4
(329)		15.5
(568)		6.4
(307)		6.10
(265)
Average molecular (Da)395.55431.23411.42427.42413.37428.50428.39
H/C1.381.231.181.241.181.231.26
O/C0.460.400.400.390.360.390.37
AImod0.130.230.260.240.280.250.24
DBE7.1010.5310.9610.5611.8110.3310.18
NOSC−0.30−0.29−0.24−0.33−0.35−0.29−0.39
a The percentage distributions of compound classes were abundance-weighted. In parentheses are the number of compounds in each compound class. Almod, modified aromaticity index; DBE, double bond equivalent; NOSC, nominal oxidation state of carbon; O/C, H/C, average elemental ratios; molecular formulas in elemental groups (CHO, CHON, CHON, and CHONS). CK: control; MN: MnO2; HA: humic acid; HM: humic acid + birnessite; FA: fulvic acid; FM: fulvic acid + birnessite.
Table 2. The potential transformation pathways and expected molecular changes for each reaction pathway.
Table 2. The potential transformation pathways and expected molecular changes for each reaction pathway.
PathwayReaction
Open the aromatic ringSustainability 17 07203 i001(1)
PolymerizationSustainability 17 07203 i002(2)
Phenols to quinonesSustainability 17 07203 i003(3)
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Zhang, X.; Zhou, X.; Ma, Y.; Zhang, W.; Zhang, R.; Zhai, W. Contrasting Reaction of Dissolved Organic Matter with Birnessite Induced by Humic and Fulvic Acids in Flooded Paddy Soil. Sustainability 2025, 17, 7203. https://doi.org/10.3390/su17167203

AMA Style

Zhang X, Zhou X, Ma Y, Zhang W, Zhang R, Zhai W. Contrasting Reaction of Dissolved Organic Matter with Birnessite Induced by Humic and Fulvic Acids in Flooded Paddy Soil. Sustainability. 2025; 17(16):7203. https://doi.org/10.3390/su17167203

Chicago/Turabian Style

Zhang, Xiangbiao, Xin Zhou, Yanyue Ma, Wenjin Zhang, Ruihua Zhang, and Weiwei Zhai. 2025. "Contrasting Reaction of Dissolved Organic Matter with Birnessite Induced by Humic and Fulvic Acids in Flooded Paddy Soil" Sustainability 17, no. 16: 7203. https://doi.org/10.3390/su17167203

APA Style

Zhang, X., Zhou, X., Ma, Y., Zhang, W., Zhang, R., & Zhai, W. (2025). Contrasting Reaction of Dissolved Organic Matter with Birnessite Induced by Humic and Fulvic Acids in Flooded Paddy Soil. Sustainability, 17(16), 7203. https://doi.org/10.3390/su17167203

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