Recycled Phosphorus from Biomass Ash: Fertilizer Performance Across Crops

Abstract

Biomass ashes represent a promising secondary phosphorus (P) source, yet their agronomic performance depends on feedstock origin, processing, and crop-specific interactions. This study evaluated the P fertilizer efficacy of raw and processed biomass ashes derived from cereal straw and paludiculture biomass, compared with triple superphosphate (TSP), using two sequential greenhouse pot experiments with maize, amaranth, and blue lupine. Processed ash products, particularly compacted ashes and ash–straw mixtures, increased plant biomass and P uptake to levels comparable to or exceeding those achieved with TSP. The cumulative P uptake of the three crops reached up to 250–300 mg pot⁻¹ under processed ash treatments, exceeding the uptake under TSP (≈150–180 mg pot⁻¹) and the unfertilized control (≤80 mg pot⁻¹). However, crop-specific differences were observed: amaranth benefited most from the ash products, whereas combinations of ashes with lupine were less favorable. Beside acting as a P source, processed biomass ashes also increased soil pH by about 0.5 units, improved soil aggregation by increasing macroaggregates (>2 mm) to up to 20% compared with only about 7% in TSP and the control, and promoted favorable shifts in Hedley P fractions. Soil enzyme activities were governed primarily by crop species, with amaranth stimulating phosphatase activity the most. Further research should aim to refine crop-specific application strategies for processed biomass ashes and to elucidate their impacts on soil structure and P dynamics.

1. Introduction

In the European Union, the use of biomass energy is largely driven by political efforts to mitigate climate change, reduce greenhouse gas emissions, and decrease dependence on fossil fuels [1]. Biomass combustion plays a role in heat and power generation. However, it can only be considered environmentally sustainable if biomass production and supply chains are responsibly regulated and managed, emissions from combustion are minimized, and valuable or scarce elements in combustion residues are efficiently recovered and recycled [2]. Biomass ashes pose both a disposal challenge and an opportunity for resource recovery. Their agricultural application offers a promising pathway for nutrient recycling and a reduced reliance on mineral fertilizers [3,4].

Phosphorus (P) recovery is of particular concern. Global P reserves are finite and are considered a limiting factor for long-term food security. Given the intertwined challenges of mineral P dependency, unequal global distribution, and problems with eutrophication, it is essential to investigate sustainable, circular P use strategies that can overcome the existing barriers [5,6,7]. Recycling P from agricultural and bioenergy residues is therefore critical to close nutrient cycles and safeguard future food systems.

Biomass ashes—originating from the combustion of plant materials such as wood, straw, or crop residues—are among the oldest mineral fertilizers and contain essential nutrients such as P, potassium (K), calcium (Ca), and magnesium (Mg), while nitrogen (N) is largely absent due to volatilization during combustion [8,9]. Previous studies indicate that biomass ashes can sustainably substitute mineral P fertilizers by increasing soil P availability to an extent comparable to highly soluble P fertilizer [4,10,11]. In addition to nutrient supply, ashes may act as liming agents, improving soil pH and soil structure, as well as stimulate microbial activity and organic matter mineralization [12,13,14,15]. On the other hand, biomass ashes may be loaded with pollutants, of which especially heavy metals are of concern [10,11].

The combustion conditions and feedstock characteristics strongly influence the nutrient content and release dynamics of biomass ashes, also impacting the P availability in soil [4,9,16,17,18]. A frequently used feedstock for biomass combustion is agricultural straw from cereals, sugarcane, and rice, as it is widely available and generally associated with a low risk of contamination [11,19]. Other feedstocks include animal manure and sewage sludge. While these materials contain relatively high nutrient concentrations and represent important P sources, they may also pose a considerable risk of contamination with heavy metals [20,21]. Energy grasses may also be used for combustion [22]. However, the cultivation of dedicated energy crops on agricultural land has been increasingly debated in recent years due to concerns related to land use competition, food security, and environmental impacts. In contrast, paludiculture systems based on rewetted peatlands are gaining increasing interest, serving as biomass feedstocks and offering the potential for nutrient removal in wetlands. While enabling biomass production, they deliver ecological and climate benefits, including reduced greenhouse gas emissions and the conservation of peat soils [23,24]. However, research on the composition and secondary use of ashes from these wetland crops is nearly absent.

Crop species differ substantially in their capacity to mobilize P, and their responses to fertilizer application are influenced by root traits, nutrient uptake efficiency, and interactions with soil microbial communities [25,26,27,28]. Consequently, the choice of crop species, in combination with biomass ash application, can play a decisive role in enhancing the agronomic efficacy of these amendments. The effects of biomass ashes on P utilization have been studied for various crops, including wheat [29], barley [10], canary grass [30], wavy hair-grass [31], Brassica species [32,33], and a broad selection of catch crops [4].

Legumes have a high P demand to meet the energy requirements of nitrogen fixation, and biomass ash application can be beneficial, as ashes supply P but no nitrogen. Positive effects have been reported, for example, for peas [33]. Conversely, soil-applied ashes are alkaline and may limit the P-mobilizing potential of legumes, which is often associated with an acidification of the rhizosphere through the exudation of organic acids or cations. In this context, Dombinov et al. [34] observed a positive effect of biomass ashes on soybean growth only when combined with farmyard manure.

Biomass ashes may be applied as raw material, but, to enable efficient agronomic handling and environmentally sound application, they are often treated to modify their physical and chemical properties prior to soil use [2,35]. That can be, for example, mixtures with straw [36] or compost [37], or compacted products such as pellets or briquettes [2,38]. Although the agronomic efficacy of biomass ashes has been widely studied, comparatively little research has addressed processed ashes and their application in combination with different crop species. Wood ash pellets were used by Greinert et al. [39] in combination with begonia, while a study by Ochecova et al. [40] used those pellets in ryegrass.

Against this background, our study aims to evaluate the complex interactions between ash feedstock, treatment methods, and the cultivated crop species, thereby improving our understanding of the effects of ash-based products on plant growth. We investigated cereal straw and paludiculture biomass as combustion feedstocks. Although the availability of paludiculture biomass is increasing with peatland rewetting, its ash has not yet been assessed for its P fertilizing effects. These ashes were tested both untreated and processed, in combination with maize, amaranth, and blue lupine. These three crops were selected because of their different P requirements and their ability to mobilize P [4,41]. To assess not only the short-term effects but also the residual effects of the biobased fertilizers, two greenhouse experiments were established. In the first experiment, the biobased fertilizers were applied to each crop, whereas in the second experiment the products were applied only once to lupine, and the residual effects on maize followed by amaranth were tested.

The specific objectives were to (i) compare the P fertilizer efficacy of biobased products with that of a highly soluble mineral P fertilizer (triple superphosphate, TSP); (ii) assess the effects of biomass ash treatments; and (iii) investigate the role of crop species in modulating the effects of biomass ashes.

2. Materials and Methods

2.1. Soil Characteristics

To elucidate the impact of products based on biomass ash on P utilization, two pot experiments (PE I and PE II) were established, employing a randomized complete block design within the controlled environment of a greenhouse at the University of Rostock. Non-calcareous topsoil was sourced from a long-term field experiment located in Northern Germany (Experimental Station Biestow, Rostock). The soil texture is characterized as loamy sand and classified as a Stagnic Cambisol according to the World Reference Base for Soil Resources. Following an absence of P fertilizer for over two decades, the initial concentration of plant-available soil P, determined as double lactate-extractable P (Pdl; see

Table 1. Initial soil properties at the onset of the pot experiments.

Experiment	Pdl	Kdl	Mgdl	pH	N	C	SOM
	mg kg−1				%
PE I	30.1	83.6	103	5.39	0.09	0.84	2.52
PE II	37.8	82.7	106	5.46	0.09	0.86	2.60

PE I and II = pot experiment I and II, dl = double lactate-soluble of P, K, and Mg; SOM = soil organic matter.

), was markedly low, reflecting a suboptimal P status [42]. Conversely, the concentrations of double lactate-extractable potassium (Kdl) and magnesium (Mgdl) were within optimal ranges.

2.2. Design of the Experiment and Treatment Applications

A diverse set of biomass ash materials was employed to capture a broad range of biobased feedstocks, as well as further processed or blended products (

Table 2. Treatments, pH values, P concentrations of the ashes, and P supply of the two pot experiments (PE I and PE II).

Treatments	Abbrev.	Part of	pH	Ptot	Application	P Amount
				%	g pot−1	g pot−1
Control	CON	PE I and PE II	-	-	-	-
TSP	TSP	PE I and PE II	2.97	20.3	1.04	0.21
Raw Straw	S	PE I	7.22	0.15	70.4	0.11
Straw Ash 1	SA1	PE I and PE II	12.2	1.31	16.1	0.21
Straw Compactate 1	SC1	PE I and PE II	12.2	1.31	16.1	0.21
Straw Ash Mix 1	SM1	PE II	11.7	1.19	17.7	0.21
Straw Ash 2	SA2	PE I and PE II	10.8	0.44	48.0	0.21
Straw Compactate 2	SC2	PE I and PE II	10.7	0.44	48.0	0.21
Straw Ash Mix 2	SM2	PE I and PE II	10.3	0.89	25.4	0.21
Paludic. Ash	PA	PE I and PE II	13.1	2.09	10.1	0.21
Paludic. Compactate	PC	PE II	13.2	2.09	10.1	0.21
Paludic. Ash Mix	PM	PE II	12.5	1.92	11.0	0.21

Paludic. = paludiculture, Ash mix = ash-based mixtures comprising 10% straw/paludiculture and 90% ash.

). These materials included ashes derived from agricultural crop residues (predominantly Triticum spp.) and biomass from paludiculture systems (mainly Phalaris arundinacea L.). Additionally, straw ash compactates (granulated to a particle size distribution of 0.5–6.3 mm) and organic–mineral mixtures based on biomass and the corresponding ash (comprising 10% biomass and 90% ash, in the following referred to as ash-based mixtures) were included in the study.

The experimental setup comprised a non-fertilized control, a treatment with conventional mineral P fertilization (triple superphosphate; TSP), and a treatment using untreated straw material for comparative purposes. The biomass ashes were sourced directly from biomass combustion facilities. The compacted straw ash granules (compactates) were provided by the Thuringian State Office for Agriculture and Rural Areas (TLLLR). Due to the heterogeneity of the input materials and variation in combustion processes, the nutrient compositions of the ash materials and compactates differed (Table 2). The corresponding concentrations of heavy metals are presented in the Supplementary Materials (Table S1). All ash-based fertilizers were applied at a standardized P application rate equivalent to 210 mg P per pot (Table 2).

Mitscherlich pots (height: 21 cm; diameter: 19.5 cm) were each filled with 6 kg of air-dried, sieved soil (mesh size > 2 mm). Prior to fertilization, composite soil samples were collected to determine the initial physicochemical soil properties. The biomass ashes, ash-based mixtures, compactates, and the highly soluble mineral P fertilizer (triple superphosphate; TSP) were applied to the soil surface and subsequently incorporated into the upper 5 cm of the soil. Due to the inherently low P content of wheat straw, a compromise had to be made between achieving the target P supply and maintaining a realistic straw application rate. Consequently, in the straw treatment, a relatively large amount of straw was incorporated, but the resulting P input was comparatively low. In this case, the applied P quantity was reduced by half relative to the standard application rate. To facilitate uniform incorporation into the soil matrix, the straw was pre-shredded to a particle size of <5 mm. A control treatment (CON) without the application of P was also established. All treatments were arranged in a randomized design with four replicates per treatment.

In PE I, additional treatment variants were included to evaluate the potential fertilization effects of potassium (K), despite the initially sufficient plant-available K content in the soil, potassium chloride (KCl), and a combination of TSP and KCl. As no statistically significant K fertilization effect was observed (Supplementary Materials, Tables S2 and S3), these treatments were not included in the Section 3.

The experiments were established under controlled greenhouse conditions. Natural sunlight was supplemented with adjustable artificial lighting to maintain a 16 h light/8 h dark photoperiod. Shading was applied as needed to prevent excessive light stress. Air temperature was maintained between 22 and 25 °C during the day and 18 and 20 °C at night.

2.2.1. Pot Experiment I

Pot experiment I was designed to assess the agronomic effects of the different ash-based fertilizers on plant growth and P availability. Maize (Zea mays L. cv. Ronaldinho), blue lupine (Lupinus angustifolius L. cv. Probor), and amaranth (Amaranthus cruentus L. cv. Bärnkraft) were cultivated under controlled conditions for a period of eight weeks. Maize was selected due to its widespread cultivation and relevance in biogas production systems. Amaranth was included as a test crop owing to its high P demand, and lupine was included as a legume crop with a high capacity for P mobilization. Prior to sowing, lupine seeds were inoculated with RADICIN-Lupin (Jost GmbH, Iserlohn, Germany) to promote symbiotic N fixation. To ensure an adequate supply of N, magnesium (Mg), and sulfur (S), each pot received 50 mL of a nutrient solution containing 1.46 g of NH₄NO₃ (equivalent to 0.52 g N) and 1.52 g of MgSO₄ (providing 0.15 g Mg and 0.20 g S). An exception was made for the lupine treatments, which received only 50% of the standard N application rate, to account for their ability to fix atmospheric N. All fertilization treatments were administered two days prior to sowing. Irrigation was performed using distilled water as needed to maintain optimal moisture conditions. The pots were designed to allow for free drainage to field capacity. Leachate was collected in bowls placed beneath each pot and was replenished to prevent nutrient loss through leaching. Plants were grown until the onset of flowering and harvested after a cultivation period of 66 days.

2.2.2. Pot Experiment II

Pot experiment II was conducted to verify the results obtained in PE I and to assess the residual P fertilization effects of applied ash products and substrates. With the same experimental setup, but in contrast to the parallel cultivation approach used in PE I, crops in PE II were cultivated sequentially in the same pots to simulate longer-term nutrient dynamics. The sequence began with blue lupine, followed by maize and then amaranth. Each crop was grown for 61 days from sowing to harvest. After each harvest, the root residues were chopped and reincorporated into the soil to maintain a consistent organic matter input and mimic field-like conditions. Macronutrients (N, K, Mg, and S) were reapplied before each sowing to ensure adequate nutrient supply for each crop. In contrast, P was applied only once at the beginning of the experiment, either as TSP or via ash-based products. This design enabled the evaluation of residual P availability throughout successive cropping cycles. Furthermore, the range of substrates was expanded to complementary treatments (compactates and ash-based mixtures) to further explore the agronomic value of blended organic–mineral fertilizer formulations.

2.3. Analysis of Ashes and Applied Substrates

The pH values (measured in CaCl₂ solution) and the total concentrations of P, K, and Mg in the substrates were determined following aqua regia digestion using a microwave-assisted procedure. Elemental concentrations in the digests were quantified using inductively coupled plasma optical emission spectrometry (ICP-OES; Optima 8300, Perkin Elmer, Waltham, MA, USA). Additionally, the citrate-soluble P fraction (Pcit), which is considered an indicator of fertilizer effectiveness, was extracted from the biomass ashes using a 2% citric acid solution. The analysis of heavy metals in the ashes was conducted by LUFA Rostock in accordance with DIN standards: DIN EN ISO 11885-E22 for Pb, Cd, Cr, Cu, Ni, and Zn and DIN EN 1483-E12 for Hg. The measured concentrations were compared with relevant precautionary and threshold values defined by applicable regulations; the measured concentrations remained within permissible limits.

2.4. Plant Analysis

The harvested biomass was initially dried at 60 °C and subsequently ground. Final biomass yield was determined after additional drying at 105 °C. P content in the plant dry matter was measured using the vanadate–molybdate method [4]. Phosphorus concentration was quantified spectrophotometrically (Specord 40, Analytik Jena, Jena, Germany). Total C, N, and S in air-dried, milled plant material were determined with a CNS analyzer (Vario EL, Foss Heraeus, Hanau, Germany; DIN ISO 10694:1996-08). Mg, K, and calcium (Ca) were quantified with ICP-OES (Optima 8300, PerkinElmer, Waltham, MA, USA) after acid digestion (results provided in Supplementary Materials). Uptake of nutrients was calculated by multiplying the dry weight of the harvested biomass by the corresponding nutrient concentrations. In addition to aboveground biomass, belowground biomass was also analyzed for nutrient contents following the same analytical procedures.

2.5. Soil Analysis

At the end of the experimental period, composite soil samples were collected from each pot. Prior to analysis, samples were air-dried and sieved through a 2 mm mesh for general analyses. For P fractionation, the soil material was further ground to pass a 0.1 mm sieve. Fresh soil aliquots destined for enzymatic assays were immediately frozen at −18 °C to preserve activity.

Total concentrations of C, S, and N in soil were determined with a CNS-Analyzer (Vario EL, Fa. Foss Heraeus, Hanau, Germany) using air-dried and milled soil material (DIN ISO 13878: 1998-11) (results provided in Supplementary Materials). Soil pH was measured in 0.01 M CaCl₂ at a soil/solution ratio of 1:2.5 using a pH electrode (pH 1100 L, VWR International, Darmstadt, Germany). The SOM content was determined as the difference in soil weight (air-dried and sieved through 2 mm) before and after incineration (550 °C in muffle furnace). P availability was assessed by employing multiple extraction procedures: water-soluble P (Pw) was extracted according to van der Paauw [5] and quantified using the molybdenum blue colorimetric method. Plant-available P (Pdl), K (Kdl), and Mg (Mgdl) were extracted using the double lactate (dl) method as described by Hoffmann [43]. The concentration of P in DL extracts was determined spectrophotometrically using the vanadate–molybdate assay.

Pedogenic P fractions associated with amorphous and poorly crystalline aluminum and iron oxides were quantified as oxalate-extractable P (Pox), iron (Feox), and aluminum (Alox) following the method of Schwertmann [6]. These values were subsequently used to calculate the P sorption capacity (PSC) and degree of P saturation (DPS) according to Schoumans [7], where

PSC = (Alox + Feox)/2 [mmol kg⁻¹]

and

DPS = Pox/PSC × 100 [%].

Sequential P fractionation was performed on selected samples following the methodologies of Tiessen et al. [44] and Dou et al. [45]. The operationally defined P pools extracted included water-extractable P (H₂O-P, pH 7), bicarbonate-extractable P (NaHCO₃-P, pH 8.5), hydroxide-extractable P (NaOH-P, pH 13), and sulfuric acid-extractable P (H₂SO₄-P, pH ~ 0). Residual P was calculated as the difference between total P (Pt) and the sum of the sequentially extracted fractions. P concentrations in all fractions were determined via ICP-OES.

The activity of acid and alkaline phosphatases was measured according to Tabatabai & Bremner [46] and was expressed as mg p-nitrophenol released from a pre-given p-nitrophenyl phosphate solution in 1 g soil after incubation at 37 °C for 24 h (mg p-nitrophenol g⁻¹ 24 h⁻¹). The activity of dehydrogenase was determined according to Thalmann [47] by suspending 1 g soil in 0.8% triphenyltetrazolium chloride solution and incubating it at 37 °C for 24 h. Triphenyltetrazolium chloride in suspension was reduced by soil microorganisms to triphenylformazan (TPF), which was extracted with acetone and measured photometrically at 546 nm (Spekol 11, Carl Zeiss Jena, Jena, Germany). The activity was expressed as mg TPF per g soil released within 24 h (mg TPF g⁻¹ 24 h⁻¹).

Aggregate stability was evaluated using a modified wet-sieving method based on Kemper and Rosenau [48]. The results were expressed as mean weight diameter (MWD), which reflects the average size of water-stable aggregates across the measured size fractions. In pot experiment I, this analysis was performed on a subset of treatments (amaranth: CON, TSP, SA2, SC2, SM2) selected to represent the main fertilizer types (control, mineral P, raw ash, compacted ash, and ash–straw mixture) as a preliminary assessment. Following confirmation of treatment effects, aggregate stability was subsequently measured for all treatments in pot experiment II to provide a comprehensive dataset.

2.6. Data Processing, Standardization and Statistics

Prior to statistical analysis, all data were screened for consistency and checked for outliers. To enable cross-species comparisons among crops with inherently different P uptake capacities, values were standardized within each crop species using Z-transformation:

Z = (X − μ)/σ

where X is the individual observation, μ is the mean, and σ is the standard deviation of the respective crop species. This transformation converts the original data into dimensionless units describing how strongly a given observation deviates from the species-specific mean, thereby allowing fertilizer effects to be compared across crop species on a common scale.

To detect potential treatment differences in soil and plant data, an analysis of variance (Anova) followed by Tukey’s test (p < 0.05) was performed, and significantly different means were indicated by using different letters. To assess differences between two treatment groups, an unpaired Student’s test was conducted. Residuals were normally distributed (Shapiro–Wilk test, p < 0.05) and variances were homogenous (Levene, p < 0.05). All statistical analysis was conducted using the programming language R (version 4.5.1).

3. Results

3.1. Pot Experiment I

3.1.1. Effects of Biomass Ashes on Soil Properties on Plant P Uptake

The biobased products had a significant effect on plant growth and P uptake (

Table 3. Plant biomass (shoot and root, dry matter) and P uptake as affected by the fertilizer treatment in pot experiment I.

Ash/ Substrate	Maize	Lupine	Amaranth	Maize	Lupine	Amaranth
	Plant Biomass [g pot−1]			Plant P Uptake [mg pot−1]
SA1	117 (±9.53) abc	18.7 (±2.08) cd	49.5 (±7.15) bc	163 (±15.0) b	22.8 (±4.12) cd	216 (±53.3) bc
SC1	126 (±9.01) ab	21.1 (±1.02) abc	71.6 (±6.77) a	195 (±13.5) ab	29.8 (±3.33) bc	376 (±75.3) a
S	78.7 (±9.61) d	14.8 (±1.32) ef	30.5 (±9.53) de	160 (±10.7) b	16.9 (±1.59) de	123 (±34.5) cd
SA2	106 (±7.31) bc	16.3 (±1.74) def	45.8 (±10.1) cd	160 (±23.9) b	18.6 (±4.95) de	206 (±37.0) bc
SC2	119 (±6.32) abc	20.0 (±2.30) bcd	62.8 (±4.50) abc	187 (±18.5) ab	25.0 (±3.97) bcd	295 (±54.3) ab
SM2	129 (±8.34) a	23.9 (±1.85) a	66.6 (±8.63) ab	207 (±19.7) a	34.1 (±6.46) ab	285 (±25.9) ab
PA	104 (±9.81) c	17.0 (±1.21) de	44.8 (±4.81) cde	159 (±23.6) b	20.8 (±3.66) cde	209 (±28.6) bc
TSP	124 (±8.15) abc	23.4 (±0.97) ab	67.3 (±8.41) ab	214 (±12.3) a	39.7 (±4.45) a	362 (±33.2) a
CON	77.4 (±4.02) d	12.7 (±1.30) f	26.6 (±5.61) e	98.4 (±12.5) c	12.7 (±2.25) e	99.1 (±28.2) d
MEAN	109 (±20.1) A	18.6 (±3.90) C	50.8 (±17.0) B	171 (±36.7) B	24.5 (±9.04) C	236 (±98.6) A

CON = control, TSP = triple superphosphate, SA = straw ash, SC = straw compactate, S = raw straw, SM = straw ash-based mixture, PA = paludiculture ash; different small letters indicate significant differences between the fertilization treatments within a column; different capital letters indicate significant differences between crops within a line; p < 0.05 (Tukey).

). Relative to the control without P supply, most treatments enhanced these parameters. Raw straw was the sole exception, as it proved unsuitable and increased plant P uptake only in maize. By contrast, the ash-based compactates and the straw ash-based mixture produced biomass increases comparable to, or greater than, those achieved with TSP. Plant biomass was highest in maize, followed by amaranth and lupine. In contrast, owing to the higher P concentration in amaranth tissue (Supplementary Materials, Table S4), the total plant P uptake followed a different ranking, with amaranth showing the highest uptake, followed by maize and lupine.

When standardized across all crop species, plant P uptake revealed a clear ranking of fertilizer effectiveness, with TSP, compacted ash, and ash-based mixtures showing the highest positive Z-scores, followed by untreated biomass ashes, whereas straw and the unfertilized control exhibited predominantly negative Z-scores (Figure 1).

From the total plant biomass given in Table 3, the root biomass had a relatively small part of about 18% for lupine (3 g), 22% for maize (20 g), and 37% for amaranth (13 g) (Supplementary Materials, Table S5). Following the results of total plant biomass (Table 3), the strongest shoot biomass responses were observed following the application of compacted ash and the straw ash-based formulation, resulting in shoot biomass increases of 48–187% compared with the control (Figure 2). Root biomass responses generally followed similar patterns. However, species-specific differences were evident: amaranth benefited primarily through enhanced shoot growth, whereas maize exhibited a relatively greater increase in root biomass than in shoot biomass. Overall, amaranth showed the most pronounced responsiveness to P supply.

3.1.2. Effects of Biomass Ashes on Soil Properties

The biobased products had significant effects on soil pH values, soil P concentration, and the activity of enzymes.

Usually, the ash products resulted in an increase in soil pH compared to treatments without ash application (

Table 4. Soil pH and bioavailable P contents as affected by fertilization treatments and crop species of pot experiment I.

Ash/ Substrate	Maize	Lupine	Amaranth	Maize	Lupine	Amaranth	Maize	Lupine	Amaranth
	pH			Pw [mg kg−1]			Pdl [mg kg−1]
SA1	5.24 (±0.16) a	5.47 (±0.03) a	5.51 (±0.35) ns	8.48 (±0.87) ab	12.9 (±2.14) ab	9.79 (±1.46) ns	42.9 (±2.74) a	42.8 (±3.96) a	33.5 (±5.79) ns
SC1	5.19 (±0.20) ab	5.45 (±0.08) ab	5.37 (±0.05) ns	8.89 (±1.14) a	15.3 (±2.91) a	10.2 (±1.65) ns	41.7 (±2.33) a	45.6 (±4.31) a	41.5 (±4.69) ns
S	4.96 (±0.18) ab	5.21 (±0.13) c	5.09 (±0.12) ns	8.40 (±1.05) ab	8.81 (±6.08) ab	8.40 (±3.83) ns	25.2 (±4.46) b	29.6 (±2.98) bc	26.6 (±10.7) ns
SA2	5.15 (±0.07) ab	5.52 (±0.14) a	5.37 (±0.26) ns	9.15 (±0.51) ab	13.5 (±3.54) ab	9.90 (±1.41) ns	42.2 (±2.37) a	47.0 (±8.64) a	28.0 (±8.60) ns
SC2	5.14 (±0.05) ab	5.39 (±0.09) abc	5.45 (±0.32) ns	9.60 (±0.53) ab	12.7 (±2.38) a	9.71 (±2.17) ns	38.2 (±2.10) a	41.5 (±1.77) a	35.2 (±9.32) ns
SM2	5.04 (±0.03) ab	5.39 (±0.07) abc	5.38 (±0.22) ns	9.23 (±0.62) ab	11.7 (±1.49) a	10.9 (±2.91) ns	36.6 (±3.67) a	40.3 (±2.49) ab	36.9 (±10.0) ns
PA	5.17 (±0.22) ab	5.39 (±0.08) abc	5.46 (±0.29) ns	9.15 (±0.89) ab	11.5 (±0.46) ab	9.79 (±1.46) ns	40.3 (±2.29) a	42.6 (±5.74) a	27.7 (±6.77) ns
TSP	4.92 (±0.03) ab	5.21 (±0.07) c	5.10 (±0.18) ns	9.64 (±0.94) a	11.8 (±0.33) ab	11.0 (±3.01) ns	38.2 (±3.27) a	47.3 (±6.19) a	40.7 (±8.75) ns
CON	4.88 (±0.11) b	5.25 (±0.02) bc	5.02 (±0.07) ns	7.43 (±0.56) b	4.90 (±0.31) b	6.04 (±1.20) ns	25.8 (±1.32) b	28.2 (±3.82) c	26.0 (±5.45) ns
Mean	5.08 (±0.17) B	5.37 (±0.13) A	5.30 (±0.27) A	8.88 (±0.98) B	11.6 (±3.73) A	9.51 (±2.54) B	36.8 (±6.90) AB	40.6 (±7.97) A	32.9 (±9.14) B

Pw = water soluble P, Pdl = double-lactate soluble P, CON = control, TSP = triple superphosphate, SA = straw ash, SC = straw compactate, S = raw straw, SM = straw ash-based mixture, PA = paludiculture ash; different small letters indicate significant differences between the fertilization treatments within a column; different capital letters indicate significant differences between crops within a line; ns = not significant; p < 0.05 (Tukey).

). Although differences between raw ashes and their compacted forms were marginal, treatments without ash application exhibited a decrease in soil pH relative to the initial value (pH 5.39), likely due to crop growth and root exudation. Maize, which developed the most extensive root system, showed lower soil pH values than the other two crops.

The contents of readily available P in the soil—measured as water-soluble P (Pw) and double-lactate-soluble P (Pdl)—were significantly influenced by both fertilization treatments and crop species (Table 4). Phosphorus application, whether through TSP or ash-based products, generally increased the levels of plant-available P in the soil compared to the unfertilized control. This effect was significant for maize and lupine, while amaranth showed a similar trend without reaching significance. Notably, no consistent differences were observed between compacted and non-compacted ash treatments, indicating that the physical form of the ash products had no measurable impact on short-term P availability in the soil. Exchangeable K (Kdl) and Mg (Mgdl) remained at optimal levels across all treatments at the end of the experiment. Likewise, total sulphur, total N, and SOM did not differ significantly among treatments (Supplementary Materials, Table S6). The phosphorus extracted with ammonium oxalate (Pox), which represents sorbed P associated with Fe and Al oxides and hydroxides, was not markedly affected by either crop species or fertilizer treatments (Supplementary Materials, Table S7). Likewise, the total P contents in the soil (Pt) were only slightly affected by the treatments (Supplementary Materials, Table S8), indicating a limited overall P accumulation under the applied conditions.

Soil P fractions were determined for selected treatments using the Hedley sequential extraction to assess P solubility and distribution (Figure 3). Consistent with Pw and Pdl results, highly labile P fractions (H₂O-P and NaHCO₃-P) generally increased following ash fertilization compared with the unfertilized control, and in some cases exceeded the levels observed with TSP. Interestingly, the NaHCO₃-P fraction was, besides TSP, especially increased by the paludiculture ash. No systematic differences were observed between compacted and raw ash treatments. The moderately stable NaOH-P did not differ among treatments, whereas the stable H₂SO₄-P fraction was highest in the straw ash-based mixture and compacted ash treatments. Averaged across crops, the sum of these four Hedley P fractions was lowest in the control (236 mg kg⁻¹) and clearly higher in TSP (300 mg kg⁻¹), paludiculture ash (279 mg kg⁻¹), and compacted straw ash treatments (289 mg kg⁻¹). Crop P uptake had a small impact on soil P fractions, except in the TSP treatment, where the high P removal by amaranth corresponded to slightly lower P concentrations (sum of all fractions) compared with maize and lupine.

Soil enzyme activities were influenced by both crop species and fertilizer treatments, with the effects varying among the three enzymes (

Table 5. Activities of soil enzymes (acid and alkaline phosphatase, dehydrogenase) as affected by fertilization treatments and crop species of pot experiment I.

Ash/ Substrate	Maize	Lupine	Amaranth	Maize	Lupine	Amaranth	Maize	Lupine	Amaranth
	Acid Phosphatase Activity [μg p-Nitrophenol g−1 TM 24 h−1]			Alkaline Phosphatase Activity [μg p-Nitrophenol g−1 TM 24 h−1]			Dehydrogenase Activity mg TPF g−1 24 h−1
SA1	105 (±25.7) ns	94.1 (±38.5) b	174 (±53.2) ns	35.1 (±7.59) ns	59.6 (±15.8) ns	61.7 (±10.8) ab	36.5 (±2.62) ns	53.1 (±5.29) b	41.0 (±21.4) ns
SC1	117 (±50.4) ns	116 (±18.1) ab	162 (±62.0) ns	31.2 (±9.78) ns	49.8 (±9.25) ns	56.2 (±25.7) ab	43.8 (±12.1) ns	41.5 (±8.28) b	30.2 (±16.5) ns
S	124 (±41.6) ns	145 (±15.6) ab	181 (±51.2) ns	47.7 (±9.06) ns	65.2 (±6.52) ns	86.0 (±17.9) a	62.3 (±15.2) ns	128 (±21.0) a	64.8 (±25.6) ns
SA2	93.2 (±24.3) ns	105 (±8.34) ab	172 (±11.7) ns	40.7 (±4.15) ns	68.0 (±20.0) ns	54.1 (±14.1) ab	44.4 (±4.81) ns	58.0 (±15.6) b	31.7 (±18.9) ns
SC2	101 (±20.9) ns	124 (±8.12) ab	152 (±20.7) ns	40.3 (±11.6) ns	75.8 (±21.0) ns	48.6 (±14.2) b	50.0 (±7.02) ns	55.9 (±12.2) b	40.3 (±10.3) ns
SM2	107 (±38.1) ns	113 (±11.2) ab	169 (±33.4) ns	33.4 (±8.46) ns	65.7 (±11.7) ns	71.5 (±10.1) ab	51.7 (±12.0) ns	48.5 (±20.8) b	43.8 (±15.2) ns
PA	96.2 (±10.6) ns	122 (±30.4) ab	169 (±40.5) ns	32.8 (±17.4) ns	60.5 (±18.6) ns	55.0 (±9.22) ab	47.8 (±12.2) ns	49.3 (±14.0) b	28.4 (±4.79) ns
TSP	121 (±28.7) ns	105 (±12.9) ab	180 (±29.5) ns	30.8 (±8.64) ns	56.6 (±5.72) ns	49.1 (±8.45) b	41.9 (±15.3) ns	46.8 (±11.8) b	27.2 (±12.0) ns
CON	123 (±31.8) ns	157 (±35.1) a	211 (±52.4) ns	31.6 (±8.09) ns	65.9 (±21.3) ns	66.8 (±10.9) ab	42.5 (±9.09) ns	53.4 (±7.48) b	33.4 (±16.2) ns
Mean	110 (±30.5) B	120 (±27.6) B	175 (±39.5) A	36.0 (±10.4) B	63.0 (±15.4) A	61.1 (±16.8) A	46.8 (±11.9) B	59.4 (±27.9) A	38.1 (±18.5) B

CON = control, TSP = triple superphosphate, SA = straw ash, SC = straw compactate, S = raw straw, SM = straw ash-based mixture, PA = paludiculture ash; different small letters indicate significant differences between the fertilization treatments within a column; different capital letters indicate significant differences between crops within a line; ns = not significant; p < 0.05 (Tukey).

). Acid phosphatase activity was more strongly affected by crop type than by fertilizer application, with the highest activities observed under amaranth cultivation, reflecting the species’ high P demand. Notably, in this context, treatments without P fertilization tended to have a higher acid phosphatase activity than P-amended treatments, highlighting the enzyme’s role in mobilizing P under nutrient-limited conditions. Amaranth cultivation was also associated with elevated activities of alkaline phosphatase, while the highest activity of the dehydrogenase was found for lupine. Conversely, maize cultivation corresponded to the lowest activities across all enzymes. Like acid phosphatase, the alkaline phosphatase was barely affected by the ash application, although the pH had increased after the application of the ash products (see above). The activity of dehydrogenase in the soil, which can be considered a measure of microbial activity, was more activated by the straw than by ashes, which is related to the supply of organic matter in this treatment.

The application of different ash-based substrates resulted in distinct effects on the soil aggregate size distribution and structural stability, as measured by the mean weight diameter (MWD) (

Table 6. Soil aggregate size distribution after dry-sieving, and mean weight diameter (MWD) of water-stable aggregates at the end of PE I (crop amaranth, selected treatments).

Ash/ Substrate	Aggregates Size Composition [%]			MWD
	>2 mm	2–0.25 mm	<0.25 mm	mm
SA2	7.8 (±1.3) b	50.3 (±3.7) ns	41.9 (±3.1) ab	0.82 (±0.04) b
SC2	14.1 (±2.3) a	56.0 (±4.3) ns	29.9 (±6.5) b	0.91 (±0.03) a
SM2	13.3 (±3.4) a	54.5 (±2.9) ns	32.2 (±3.3) b	0.88 (±0.05) ab
TSP	6.2 (±1.5) b	48.5 (±4.7) ns	45.3 (±4.1) a	0.77 (±0.04) b
CON	5.9 (±2.0) b	48.7 (±4.0) ns	45.4 (±5.1) a	0.79 (±0.03) b

CON = control, TSP = triple superphosphate, SA = straw ash, SC = straw compactate, SM = straw ash-based mixture; different small letters indicate significant differences between the fertilization treatments within a column; ns = not significant; p < 0.05 (Tukey).

). Both the compacted ash and the straw ash-based mixture increased the proportion of macroaggregates (>2 mm), reaching 14.1% and 13.3%, respectively, compared to straw ash 2 (7.8%), TSP (6.2%), and the control treatment (5.9%) (p < 0.05), while resulting in lower proportions of microaggregates (<0.25 mm). Correspondingly, the highest MWD values were observed in both these treatments, reflecting an enhanced aggregate stability. Differences in the intermediate fraction (2–0.25 mm) were not statistically significant across treatments.

3.2. Pot Experiment II

3.2.1. Residual Effects of Biomass Ashes on Crop P Uptake

In the second pot experiment, crops were grown sequentially and received only a P application of 200 g per pot at the beginning of the experiment. Based on the results of experiment I, lupine—characterized by the lowest P uptake—was cultivated first, followed by maize and amaranth. This sequential cropping approach was used to assess the residual fertilizer effect of P.

The crop yields were strongly influenced by the type of fertilizer applied (

Table 7. Shoot biomass (dry matter) and P uptake as affected by the fertilizer treatment in pot experiment II.

Ash/ Substrate	Lupine→	Maize→	Amaranth	Lupine→	Maize→	Amaranth
	Shoot Biomass [g pot−1]			Shoot P Uptake [g pot−1]
SA1	7.71 (±0.51) c	70.3 (±19.8) c	20.6 (±2.60) cd	5.36 (±1.88) ab	58.0 (±9.19) def	113 (±14.6) c
SC1	9.63 (±0.80) ab	88.0 (±19.9) a	29.1 (±1.57) ab	5.28 (±0.12) ab	97.3 (±12.9) ab	175 (±5.17) ab
SM1	10.7 (±0.28) a	82.6 (±12.9) ab	25.5 (±3.73) bc	6.71 (±2.21) ab	86.9 (±12.5) abc	157 (±25.9) b
SA2	7.26 (±0.71) cd	69.0 (±33.5) c	18.2 (±2.94) d	5.66 (±2.15) ab	49.2 (±9.97) ef	104 (±15.1) c
SC2	9.16 (±0.32) b	83.0 (±4.57) ab	31.6 (±1.56) ab	5.84 (±1.24) ab	70.7 (±15.3) bcde	196 (±32.2) ab
SM2	10.0 (±0.34) ab	83.1 (±0.59) ab	27.9 (±2.54) ab	8.54 (±0.98) a	87.3 (±15.3) abc	176 (±21.8) ab
PA	6.47 (±0.34) d	76.6 (±2.42) bc	27.3 (±2.68) ab	5.10 (±1.34) ab	60.1 (±7.52) cdef	168 (±6.95) b
PC	7.76 (±0.38) cd	89.3 (±2.34) a	32.8 (±1.07) a	5.15 (±1.64) ab	101 (±8.99) a	213 (±6.64) a
PM	9.08 (±0.30) b	83.6 (±2.25) ab	28.1 (±2.46) ab	7.80 (±2.96) a	79.2 (±16.3) abcd	167 (±16.6) b
TSP	10.8 (±0.38) a	72.1 (±7.31) c	17.8 (±1.30) d	9.68 (±3.29) a	67.6 (±5.21) bcde	88.1 (±10.5) cd
CON	4.43 (±0.73) e	57.3 (±1.90) d	11.3 (±1.09) e	1.96 (±0.37) b	33.5 (±3.63) f	52.5 (±3.27) d
MEAN	8.44 (±1.94) C	77.7 (±9.68) A	24.6 (±6.69) B	6.10 (±2.06) C	71.9 (±20.9) B	146 (±49.7) A

CON = control, TSP = triple superphosphate, SA = straw ash, SC/PC = straw/paludiculture compactate, SM/PM = straw/paludiculture ash-based mix, PA = paludiculture ash; different small letters indicate significant differences between the fertilization treatments within a column; different capital letters indicate significant differences between crops within a line; p < 0.05 (Tukey).

, Figure 4). Consistently higher biomass production was observed when ashes were applied in compacted form, confirming the findings from pot experiment I. It is noteworthy that the fertilizing effect of the ash-based products did not diminish over time. Even for amaranth—the final crop grown in the rotation—substantial yield differences were observed between treatments. For example, dry matter yields reached 32.8 g per pot with compacted paludiculture ash, compared to only 11.3 g per pot in the unfertilized control. Over time, the compacted ash products and ash-based mixtures outperformed the TSP treatment in terms of biomass production. While the raw ashes showed comparable effects to TSP, the treatment with pure paludiculture ash resulted in a notably higher shoot biomass, highlighting its potential as an effective and plant-available P source.

Biomass production differed markedly among crop species, with maize again producing the highest yields, as already observed in the first experiment. In general, the overall yield levels in experiment II were lower than those recorded in PE I, potentially reflecting the shorter growing time, residual nutrient availability, and cumulative system effects.

The differences between treatments were even more pronounced with respect to plant P uptake (Table 7, Figure 4), as the fertilizer treatments also affected the shoot P concentration. The application of compacted ash products led to a substantial increase in P uptake compared to the control. Compacted paludiculture ash resulted in a threefold increase in total P uptake across all crop species, with average values rising from 29 mg P per pot (CON) to 107 mg P per pot (PC). Among the crop species tested, amaranth exhibited the highest P uptake, which is consistent with the observations from PE I. When averaged across all crops, a clear ranking of fertilizer effectiveness emerged: PC > SC > SM/PM > PA > SA/TSP > CON. However, the P utilization from the ash-based products varied markedly among crops. When the ash-based products were combined with lupine, the P utilization was lowest for all crops; while with TSP the P uptake was of about 10 g per pot, it was reduced by nearly half in the ash-based treatments. Only in the ash-based mixtures with straw was this decline less pronounced. In maize, the P utilization from the ash-based products was comparable to that obtained with TSP (about 70 mg per pot). For amaranth, however, the P utilization from the ash-based products (e.g., 213 mg per pot in the compacted paludiculture ash) was substantially even higher than with TSP (88 mg per pot). The concentrations of K, Mg, Ca, C, N, and S in plant material remained consistent across all treatments (Supplementary Materials, Table S9).

3.2.2. Residual Effects of Biomass Ashes on Soil Properties

Soil pH was affected by the crop species, confirming the trends observed in pot experiment I (

Table 8. Soil pH and bioavailable P contents as affected by fertilization treatments and crop species of pot experiment II.

Ash/ Substrate	Lupine→	Maize→	Amaranth	Lupine→	Maize→	Amaranth	Lupine→	Maize→	Amaranth
	pH			Pw [mg kg−1]			Pdl [mg kg−1]
SA1	5.79 (±0.54) ns	5.37 (±0.48) ns	5.42 (±0.47) ab	5.65 (±1.04) ab	4.04 (±1.55) ns	13.1 (±1.38) a	89.7 (±18.2) a	47.7 (±8.85) ab	51.5 (±2.88) ab
SC1	5.85 (±0.29) ns	5.51 (±0.42) ns	5.62 (±0.51) ab	5.49 (±0.89) ab	2.81 (±0.78) ns	15.0 (±2.88) a	56.3 (±9.53) bcd	66.6 (±12.1) a	70.3 (±20.5) a
SM1	5.67 (±0.53) ns	5.36 (±0.17) ns	6.02 (±0.70) b	5.18 (±2.31) ab	3.03 (±0.93) ns	13.8 (±5.25) a	47.8 (±5.36) cd	54.3 (±8.67) ab	65.2 (±3.66) ab
SA2	5.85 (±0.50) ns	5.40 (±0.39) ns	5.36 (±0.40) ab	6.88 (±1.85) ab	4.12 (±1.36) ns	9.67 (±2.53) ab	73.7 (±15.7) abc	48.1 (±4.32) ab	51.1 (±7.23) ab
SC2	5.86 (±0.68) ns	5.51 (±0.34) ns	6.41 (±0.05) b	5.17 (±0.34) ab	2.80 (±0.71) ns	18.7 (±6.86) a	54.0 (±12.7) bcd	64.5 (±2.84) a	74.9 (±6.70) a
SM2	5.63 (±0.33) ns	5.38 (±0.44) ns	5.36 (±0.68) ab	4.78 (±0.37) a	3.26 (±0.48) ns	15.4 (±4.78) a	57.6 (±12.4) bcd	67.2 (±21.2) a	62.2 (±4.03) ab
PA	5.78 (±0.50) ns	5.39 (±0.59) ns	6.13 (±0.20) b	5.91 (±1.57) ab	3.57 (±0.97) ns	13.9 (±3.86) a	64.9 (±10.0) abc	42.4 (±3.55) ab	55.1 (±4.71) ab
PC	5.81 (±0.49) ns	5.52 (±0.27) ns	5.46 (±0.58) ab	5.32 (±1.28) ab	3.03 (±0.58) ns	16.5 (±4.18) a	54.7 (±6.26) bcd	56.9 (±2.59) ab	69.2 (±11.2) ab
PM	5.56 (±0.55) ns	5.41 (±0.50) ns	5.49 (±0.59) ab	5.70 (±1.00) ab	4.34 (±1.56) ns	16.1 (±3.80)a	59.9 (±9.71) bcd	68.0 (±24.8) a	65.9 (±14.4) ab
TSP	5.11 (±0.12) ns	5.13 (±0.18) ns	5.15 (±0.12) a	7.81 (±0.72) a	3.92 (±1.51) ns	9.44 (±2.05) b	79.9 (±13.7) ab	59.8 (±8.24) ab	49.1 (±6.74) ab
CON	5.22 (±0.08) ns	5.04 (±0.21) ns	5.06 (±0.23) a	4.27 (±0.82) b	3.02 (±0.68) ns	9.17 (±1.01) b	34.7 (±2.77) d	33.6 (±3.68) b	42.5 (±4.92) b
Mean	5.65 (±0.47) A	5.37 (±0.37) B	5.58 (±0.57) AB	5.65 (±1.44) B	3.45 (±1.09) C	13.7 (±4.41) A	61.3 (±18.0) NS	55.3 (±15.1) NS	59.8 (±13.0) NS

Pw = water soluble P, Pdl = double-lactate soluble P, CON = control, TSP = triple superphosphate, SA = straw ash, SC/PC = straw/paludiculture compactate, SM/PM = straw/paludiculture ash-based mixture, PA = paludiculture ash; different small letters indicate significant differences between the fertilization treatments within a column; different capital letters indicate significant differences between crops within a line; ns/NS = not significant; p < 0.05 (Tukey).

). Fertilizer treatments also had effects; however, these were not statistically significant in combination with lupine and maize, likely due to relatively high standard variation in the values. On average, for all crops, the application of ash-based products led to an increase in soil pH of approximately 0.5 units. In contrast, the cultivation without fertilization (CON) resulted in a decrease in soil pH. This acidification can be attributed to the release of root exudates during plant growth.

The contents of readily available P in the soil—measured as Pw and Pdl—were influenced by both the fertilizer treatments and the cultivated crop species (Table 8). P application, whether as TSP or ash-based fertilizers, typically led to increased levels of plant-available P in the soil compared to treatments without P supply. Initially, compacted ash products tended to result in a lower soil P availability compared to their unprocessed counterparts. However, as the experiment progressed, typically higher available P concentrations were observed in soils treated with compactates than with raw ashes, despite the greater P uptake and removal by the crops that received a P supply with ash compactates. A similar trend was observed for the ash-based mixtures, although less pronounced. Surprisingly, the highest soil Pw concentrations were observed in combination with amaranth, even though it was the final crop in the sequence and exhibited a particularly high P uptake.

The results for the other plant nutrients in soil were generally consistent with the findings from pot experiment I. By the end of the experiment, Kdl and Mgdl remained within optimal ranges across all treatments (Kdl: 87.1–146 mg kg⁻¹; Mgdl: 92.0–152 mg kg⁻¹). Furthermore, similarly to pot experiment I, no significant differences were observed in St, Nt, or SOC between treatments (Supplementary Materials, Table S6). The soil Pox contents and the DPS values were only marginally affected by the fertilization treatments, and compacted ash products did not result in higher Pox concentrations than their corresponding raw ashes (Supplementary Materials, Table S10). The influence of crop species on these parameters was also minimal and agronomically negligible, confirming the findings from pot experiment I.

Soil acid phosphatase activity was significantly affected by the cultivated crop species (

Table 9. Activities of soil enzymes (acid and alkaline phosphatase, dehydrogenase) as affected by fertilization treatments and crop species of pot experiment II.

Ash/ Substrate	Lupine→	Maize→	Amaranth	Lupine→	Maize→	Amaranth	Lupine→	Maize→	Amaranth
	Acid Phosphatase Activity [μg p-Nitrophenol g−1 TM h−1]			Alkaline Phosphatase Activity [μg p-Nitrophenol g−1 TM h−1]			Dehydrogenase Activity [lg (g DW)−1 h−1]
SA1	85.1 (±10.5) ns	72.4 (±19.8) ab	155 (±36.8) abc	15.0 (±7.53) ns	24.0 (±3.47) ns	25.9 (±3.36) c	9.37 (±2.53) b	62.2 (±6.43) ab	29.4 (±6.53) ab
SC1	113 (±40.2) ns	52.5 (±19.9) a	69.3 (±31.2) c	25.7 (±5.55) ns	17.7 (±7.80) ns	41.6 (±11.4) abc	12.6 (±5.20) b	33.4 (±18.2) ab	32.6 (±9.25) ab
SM1	90.8 (±13.2) ns	82.6 (±12.9) ab	127 (±33.8)abc	17.4 (±18.6) ns	10.8 (±6.85) ns	39.0 (±1.30) abc	47.8 (±14.1) ab	109 (±29.6) a	47.8 (±14.1) ab
SA2	93.4 (±7.54) ns	83.1 (±33.5) ab	151 (±22.9) abc	23.0 (±3.64) ns	15.5 (±8.25) ns	26.0 (±6.98) bc	15.2 (±7.30) b	64.0 (±4.76) ab	28.2 (±7.30) ab
SC2	96.3 (±4.83) ns	59.4 (±26.4) ab	86.1 (±51.3) bc	11.7 (±3.57) ns	13.3 (±9.57) ns	49.1 (±3.47) ab	44.7 (±6.59) ab	43.8 (±33.1) ab	44.7 (±6.59) ab
SM2	92.2 (±42.6) ns	66.1 (±15.3) ab	111 (±37.1) abc	21.6 (±10.8) ns	18.3 (±2.98) ns	44.4 (±14.4) abc	59.2 (±16.5) a	87.5 (±51.7) ab	59.2 (±16.5) a
PA	78.8 (±42.3) ns	77.6 (±34.4) ab	138 (±12.2) abc	25.5 (±9.88) ns	14.2 (±10.5) ns	32.9 (±9.42) abc	21.0 (±20.2) ab	49.1 (±37.7) ab	41.0 (±20.2) ab
PC	93.6 (±65.2) ns	45.7 (±25.8) a	79.8 (±48.0) bc	25.1 (±6.92) ns	11.5 (±6.56) ns	50.9 (±17.4) a	36.5 (±18.1) ab	57.3 (±10.8) ab	36.5 (±18.1) ab
PM	93.4 (±43.8) ns	45.9 (±17.5) a	102 (±7.14) abc	29.6 (±13.9) ns	10.5 (±6.97) ns	32.8 (±2.49) ab	56.9 (±20.7) a	61.8 (±35.5) ab	56.9 (±20.6) ab
TSP	54.3 (±24.1) ns	84.5 (±11.7) ab	179 (±10.7) a	29.3 (±14.3) ns	18.2 (±4.59) ns	30.5 (±8.60) abc	16.8 (±14.8) b	26.9 (±22.1) b	16.8 (±4.81) c
CON	127 (±17.0) ns	114 (±35.1) a	166 (±31.8) ab	9.00 (±3.17) ns	21.4 (±14.1) ns	23.4 (±3.67) c	12.6 (±5.83) b	21.7 (±16.0) b	12.6 (±5.83) c
Mean	92.6 (±34.6) B	71.2 (±27.8) C	123 (±44.8) A	21.1 (±11.2) B	15.8 (±7.99) B	37.4 (±12.1) A	31.5 (±21.8) B	56.3 (±34.2) A	27.7 (±15.6) B

CON = control, TSP = triple superphosphate, SA = straw ash, SC/PC = straw/paludiculture compactate, SM/PM = straw/paludiculture ash-based mix, PA = paludiculture ash; different small letters indicate significant differences between the fertilization treatments within a column; different capital letters indicate significant differences between crops within a line; ns = not significant; p < 0.05 (Tukey).

). Consistent with pot experiment I, the highest activities were observed in association with amaranth, thereby likely reflecting the high P demand of this crop. Across treatments, the greatest acid phosphatase activities were found in the control without P input, also in agreement with the first experiment. The differences in acid phosphatase activity among fertilizer products applied were generally minor and showed inconsistent patterns across crop species.

The highest alkaline phosphatase activities were again associated with amaranth, indicating a strong crop-specific influence on soil enzymatic activity across both types of phosphatases. Especially high activities were measured in soil amended with compacted ash-based fertilizers (Table 9). This effect may be partially explained by the increase in soil pH following ash application in amaranth, which was more pronounced in pot experiment II than in experiment I. A similar trend was observed for alkaline phosphatase activity in the lupine pots, whereas, in the maize pots, enzyme activity was generally low and largely unaffected by the fertilizer treatments.

The influence of organic matter addition on the activity of the dehydrogenase was also confirmed in this experiment. Treatments combining ash with straw (SM1, SM2, and PM) resulted in elevated activity levels (Table 9). In contrast to the fertilizer treatments, the crops cultivation did not affect the activity of the dehydrogenase.

The application of various ash-based treatments resulted in significant differences in soil aggregate size distribution and structural stability (

Table 10. Soil aggregate size distribution after dry-sieving and mean weight diameter (MWD) of water-stable aggregates at the end of pot experiment II.

Ash/ Substrate	Aggregates Size Composition [%]		MWD
	>2 mm	2–0.25 mm	<0.25 mm	mm
SA1	9.2 (±0.9) bc	50.3 (±5.1) ns	40.5 (±4.1) ab	0.80 (±0.04) bc
SC1	19.5 (±2.2) a	55.0 (±4.4) ns	25.5 (±5.5) c	0.92 (±0.02) a
SM1	16.7 (±3.2) a	49.4 (±5.7) ns	33.9 (±4.7) abc	0.84 (±0.04) ab
SA2	8.4 (±1.2) bc	48.5 (±6.0) ns	43.1 (±5.5) a	0.79 (±0.04) bc
SC2	18.9 (±2.0) a	52.8 (±5.4) ns	28.3 (±5.6) bc	0.89 (±0.02) ab
SM2	16.0 (±3.6) ab	46.9 (±6.3) ns	37.1 (±7.0) ab	0.86 (±0.03) ab
PA	10.6 (±4.3) abc	48.3 (±4.9) ns	41.1 (±6.4) ab	0.78 (±0.06) bc
PC	19.9 (±5.1) a	53.0 (±5.5) ns	27.1 (±5.1) c	0.94 (±0.03) a
PM	18.2 (±4.8) a	49.9 (±7.2) ns	31.9 (±5.4) bc	0.86 (±0.07) ab
TSP	7.2 (±3.8) c	47.0 (±5.9) ns	45.8 (±7.8) a	0.73 (±0.03) c
CON	6.7 (±3.0) c	46.9 (±7.5) ns	46.4 (±7.0) a	0.69 (±0.05) c

CON = control, TSP = triple superphosphate, SA = straw ash, SC/PC = straw/paludiculture compactate, SM/PM = straw/paludiculture ash-based mixtures, PA = paludiculture ash; different small letters indicate significant differences between the fertilization treatments within a column; ns = not significant; p < 0.05 (Tukey).

). The compacted ash treatments (SC1, SC2, PC), as well as the straw ash-based mixtures (PM, SM1), increased the proportion of macroaggregates (>2 mm), ranging from 16.0% to 19.9%, compared to the lower values observed in TSP (7.2%) and the control (6.7%) (p < 0.05). No statistically significant differences were detected in the intermediate fraction (2–0.25 mm) among treatments. In terms of microaggregates (<0.25 mm), the compacted ashes exhibited the lowest proportions (between 25.5 and 28.3%), indicating an improved soil structural stability. In contrast, TSP (45.8%) and the control (46.4%) showed the highest percentages in this fraction, suggesting a poorer aggregation. Correspondingly, mean weight diameter (MWD) values were higher in the compacted ashes, followed by the straw ash-based mixtures. The lowest MWDs were recorded for TSP (0.73 mm) and the control (0.69 mm). These results underscore the beneficial effects of selected ash-based amendments in enhancing soil structural stability and aggregate formation.

4. Discussion

4.1. Impact on Untreated and Treated Biomass Ashes on Plant P Uptake and Soil P Availability

The results showed that the P supply with the fertilizer treatments was able to increase both the growth and P uptake of the different crop species. The untreated biomass ashes performed quite similar, so no strong influence of the combustion process itself could be expected, even though other studies have reported pronounced effects of combustion conditions on ash properties [16,17]. Remarkable differences were observed only in the readily available NaHCO₃-extractable P fraction (Hedley) of the soil, which was higher following the application of paludiculture ash. However, this was not translated into an enhanced plant P nutrition. While a number of studies already exist on biomass ashes, often based on cereal straw and wood [4,49,50], ashes based on paludiculture were less commonly used, and studies on their P effects have so far not been available. The results of the study show that ashes based on this material behave similarly to other biomass ashes and exhibit a relatively high P availability and relatively low contents of heavy metals. This distinguishes them, for example, from ashes based on sewage sludge, which may have a higher total P content but often have a comparatively low P availability [21]. The P concentration in the paludiculture ash, at approximately 2%, was slightly higher than that in the straw ashes. While this difference was not relevant for the pot trials, as ash application rates were adjusted to provide a uniform P input of 210 g P per pot, it may become important under field conditions by allowing lower application rates for P supply.

The superior performance of the processed ashes likely results from a combination of (i) increased P solubility, together with modified P release kinetics, (ii) improvements in soil physical conditions, and (iii) enhanced rhizosphere processes. Regarding the P availability of the amendments, previous studies have shown that pre-treatment and processing methods (e.g., pelletization, acidification) can modify ash mineralogy and increase the proportion of readily bioavailable P forms [15,51]. Similarly, mixing biomass ash with organic residues or subjecting ash to biological processing, such as vermicomposting, may transform otherwise recalcitrant P forms into more available fractions [52,53]. The processing of ashes can also reduce the rapid surface precipitation of Ca-P, resulting in a slower but more sustained release of P compared with finely dispersed raw ashes [54]. However, this would rather be relevant in soils with a higher pH than in our study. The role of the pH values is often discussed in relation to P availability in soil, and the liming effects of alkaline biomass ashes were highlighted in several studies [31,55]. In our experiments with a loamy sand, soil pH in the control treatment was low after crop cultivation, ranging from approximately 4.9 to 5.2. At this acidic soil reaction, P sorption onto Fe and Al oxides and hydroxides is expected to be significant [56]. On average, ash application increased soil pH by approximately 0.5 units, potentially enhancing P availability.

It can be assumed that the improved growing conditions resulting from modifications to the soil physical properties were one main driver for the enhanced P uptake observed following the application of processed ashes. In particular, treatments with compacted ashes and ash–straw mixtures showed significant shifts in aggregate size distribution, accompanied by an increase in mean weight diameter (MWD), which may have contributed to more favorable soil conditions. Carbonaceous amendments, especially when combined with organic residues, have been shown to enhance soil structural stability and pore connectivity, thereby promoting root penetration and nutrient diffusion in the rhizosphere and increasing the proportion of macroaggregates [13,57,58,59]. In contrast, mineral fertilizers such as TSP lack these structural benefits, resulting in higher microaggregate proportions and lower MWD, as also observed in a study comparing organic versus inorganic amendments [60].

Enzymes play a key role in the transformation of organic compounds, and phosphatases support plant P nutrition from the organic P pool in soil. Although ashes do not contribute to the organic P pool, they might influence the activity of alkaline and acidic phosphatases through their effects on pH values. However, the influence of ashes in the present study was relatively minor. To achieve a pronounced shift in phosphatase activities driven by the pH effect, soil pH values would have needed to increase above 7, a range in which the activity of acid phosphatases declines, while that of alkaline phosphatases increases markedly [61]. Dehydrogenase activity is commonly used as an indicator of soil microbial activity. It depends on organic matter supply and reflects increases in microbial biomass and metabolic activity following organic carbon inputs [37,62]. Consequently, the highest dehydrogenase activities were observed in treatments with straw incorporation. Stimulating microbial activity is also relevant regarding the production of extracellular polysaccharides and organic acids, which further stabilize aggregates and mobilize P from sparingly soluble Ca-, Fe-, and Al-phosphates [25,63].

Although crop P uptake increased following the application of the compacted ashes and ash-based mixtures, this effect was not always reflected in higher concentrations of available P or in the degree of P saturation in the soil in experiment I, even though trends indicated increases in these parameters. This discrepancy may be partly explained by the depletion of the soil P pools in the treatments with higher P uptakes. Additionally, ash-derived P might not be detected by soil P tests but can still supply P to plants through gradual dissolution and rhizosphere-driven mobilization. Consequently, plant species with strong P acquisition strategies are able to exploit these pools efficiently, even when conventional soil P indicators remain unchanged [64].

In the second experiment, involving the successive cultivation of lupine, maize, and amaranth, differences between fertilization treatments regarding plant P nutrition became even more pronounced. Remarkably, compacted paludiculture ash outperformed TSP, driving plant P uptake to levels nearly twice as high. Across crop species, fertilizer effectiveness followed the order compacted ashes > ash-based mixtures > paludiculture ash > straw ash/TSP > CON, reflecting differences in P solubility and release dynamics between processed and raw ash materials and their effects on plant growing conditions (see above). Interestingly, as the experiment progressed, soils treated with ash compactates exhibited higher concentrations of available P, despite the accumulated P removal by the crops. After amaranth, the final crop in the sequence, soil P concentrations in the treatments with compactates reached approximately 70 mg kg⁻¹ Pdl, about 15–20 mg kg⁻¹ higher than after lupine, the first crop. These results suggest that compacted ashes may be particularly effective in providing a sustained P supply over time.

Although the concentrations of Cd, Pb, and Zn in all ash materials used in this study were below current regulatory thresholds, the potential for long-term accumulation under repeated ash application needs to be considered. Field studies indicate that soil metal pools can gradually increase depending on ash composition, soil texture, and organic matter content [65], underscoring the need for monitoring during the extended use of biomass ashes as fertilizer. Extrapolating pot experiment results to field conditions requires caution, as nutrient release, metal mobility, and biological responses may differ in dependence of environmental conditions and are influenced by spatial heterogeneity, factors only partly captured in pots. Ash properties also vary with biomass type and combustion conditions, affecting mineralogy, alkalinity, and trace metal content [17,54]. While our pot experiments demonstrate the short-term agronomic potential and environmental safety of processed ashes, long-term field trials are essential to assess cumulative trace metal behavior and nutrient dynamics under realistic management.

4.2. Crop Effects on Soil Characteristics in Combination with Fertilizer Treatments

The crops investigated in this study differed markedly in P uptake, the utilization of supplied P, and the effects on soil characteristics, particularly soil enzyme activities. In both experiments, distinct activity levels were observed, with amaranth exhibiting the highest acid phosphatase activity. This could have helped to cover the P demand by mobilizing P from soil reserves in the second experiment, as the cumulative P uptake exceeded the 210 g P supplied per pot. Although acid phosphatase is predominantly released by plant roots [66], amaranth exhibited a higher soil acid phosphatase activity than maize, despite having a smaller root system. The high rhizosphere phosphatase activity in amaranth, which enhances the mineralization of organic P pools and thereby increases P availability, was also reported by Mndzebele et al. [67]. The cultivation of lupine, which had the least extensive root system in this study, resulted in low activities of acid phosphatase, but was associated with relatively high activities of alkaline phosphatases, whose excretion is, other than acid phosphatase, closely linked to microbial activity and population size [66,68]. Since the soil microbial community structure varies with plant species, crops may also influence the microbial contribution to enzyme activity. This was also underlined by the highest dehydrogenase activity after lupine cultivation in the first experiment and confirmed the ability of legumes to stimulate microbial processes in the soil. Higher activities of the dehydrogenase and alkaline phosphatase were also found, e.g., for different bean species [69] and small grain legumes such as alfalfa [70]. In the second experiment, this effect was not observed, and the overall enzyme activities were lower, likely due to the reduced plant biomass in pot experiment II. Beyond phosphatase activity, plants may employ additional P mobilization mechanisms that enable a high P uptake under deficiency. These likely include the enhanced utilization of Fe and Al phosphates, mediated by the root exudation of organic acids that form stable complexes with Fe³⁺ and Al³⁺ [71]. It is conceivable that this has particularly helped amaranth meet its P demand.

The crop effect on soil pH was probably also related to root system size, showing lower values for maize. While soil acidification through the excretion of, e.g., organic acid and cations was often reported in relation to legume cultivation [72,73,74], lupine did not result in lower pH values than the other crops. As described above, this is likely related to the low root biomass.

The crop effects on soil P content likely reflected the interplay between P removal and P mobilization. In pot experiment I, the high P uptake by amaranth resulted in low soil Pdl levels, whereas lupine exhibited considerably higher values, which may be attributed either to active P mobilization or, more likely, to the lower P uptake. Surprisingly, in pot experiment II, the Pw contents in soil were found to be highest for amaranth, despite the high P uptake and the last crop in the sequence. This was also the case for Pdl and was found in the control without P supply, as well as in the treatments with compacted ashes. While a long-lasting P effect can be assumed for the compactates (see Section 4.1), higher Pdl values even in the control are likely caused by P mobilization and can also be linked to the high acid phosphatase activity associated with the cultivation of amaranth (see above). In experiment II, the observed effects on soil characteristics could be explained by the ongoing experiment duration, as well as the influence of the crop species, making it difficult to separate their individual contributions.

A notable outcome of this study is the differential response of crop species to ash treatments. When the ash-based products were combined with lupine, the P utilization was relatively low. In maize, the P utilization from the ash-based products was comparable to that obtained with TSP, whereas in amaranth it was substantially higher than with TSP. This gradient reflects the inherent differences in P solubility among the applied materials and release dynamics, as well as plant P mobilization. The apparently limited effectiveness of ash-based products in combination with lupine cultivation may be due to interference with its main P mobilization mechanism. Lupine is a typical cluster-rooted legume that mobilizes sparingly soluble soil P mainly by releasing large amounts of organic acids (particularly citrate and malate), thereby acidifying the rhizosphere and dissolving Ca-, Fe-, and Al-phosphates [25,72]. Although the biomass ashes increased soil pH by only about 0.5 units in this study, their liming effect likely counteracted the acidification-driven P mobilization mechanism of lupine. Crops such as maize and amaranth rely less on rhizosphere acidification and more on the uptake of soluble or weakly sorbed phosphate, making them better suited to exploit the gradual P release after the application of ash-based products.

5. Conclusions

The results support the potential of biomass ash as a P fertilizer, which aligns well with the principles of the circular economy. Provided that heavy metal safety criteria are met, biomass ash can supply plant-available P, enhance soil fertility, and reduce the dependence on finite phosphate rock resources. Ashes derived from paludiculture feedstocks—being a relatively new biomass source—exhibited fertilizer effects comparable to those of conventional straw ashes. Compacted biomass ashes provided even more sustained agronomic benefits, possibly due to the improved soil physical properties. However, further studies are needed to better understand their effects on soil structure and P dynamics. Crop-specific responses indicate that ash application is especially suitable for crops with a high P demand, such as amaranth, whereas combinations with legumes should be approached with caution, possibly due to an interference with their primary P mobilization mechanisms. Further research is needed to refine crop-specific recommendations and application strategies for ash-based fertilizers under practical farming conditions.