Biochar as an effective carrier for poultry-feather hydrolysate with positive effects on nutrient retention across soil textures

Abstract

Rapid nutrient loss from soluble organic amendments remains a key challenge in sustainable soil management, limiting their long-term effectiveness in maintaining crop productivity, soil fertility, and ecosystem functioning. While organic amendments like poultry-feather hydrolysate and plant-derived biochar are being explored for improving soil health, their combined effects particularly across different soil textures remain underexplored. A four-month laboratory microcosm experiment was conducted to determine the effects of hydrolysate applied alone or absorbed into biochar on nutrient availability and microbial properties in two soils differing in texture (loamy sand and sandy loam). Hydrolysate absorbed into biochar reduced early nutrient losses especially for phosphorus, calcium (Ca) and magnesium (Mg) by 24%, 72% and 14%, respectively, and enhanced later soil nutrient retention, increasing soil content of dissolved organic carbon (by 50%), Ca (by 47%) and Mg (by 523%) than the hydrolysate alone. Hydrolysate absorbed into biochar also maintained higher and more consistent soil pH, indicating improved buffering capacity. The effects were more pronounced in sandy loam soil, where microbial activity increased by 25% and dissolved Ca and Mg content increased by 30% and 14%, respectively, compared to the loamy sand soil, reflecting the higher sorption capacity of the finer-textured soil. These findings position hydrolysate absorbed into biochar as an effective slow-release amendment, with maximum potential for optimizing nutrient retention, particularly in finer-textured soils. Future field- and plant-based studies should test whether these benefits extend to nutrient uptake and crop productivity.

1 Introduction

Organic soil amendments derived from agricultural by-products have increasingly been explored as sustainable tools to improve soil quality and fertility. These amendments improve soil structure, supply essential nutrients, and stimulate microbial activity, thereby promoting soil health and crop production (Bamdad et al., 2022). Among the diverse organic amendments, poultry-feather hydrolysate has been increasingly used in agriculture due to its high bioavailability and nutrient-rich composition (Kaur et al., 2021). Rich in organic carbon (C), nitrogen (N), and trace elements, hydrolysate stimulates microbial activity, thus improving the uptake and utilization of soil nutrients (Roy and Jílková, 2025a), while also enhancing moisture retention and structural stability against erosion (Jagadeesan et al., 2023). Due to its inherent stickiness, hydrolysate further stabilizes soil organic matter (SOM) through aggregate formation or mineral particle adsorption, thereby supporting C sequestration (Kellerová and Jílková, 2025). However, despite these benefits, the effects are often short-lived, largely due to high solubility and susceptibility to rapid microbial consumption, leaching, and losses from the soil system and approaches to prolong hydrolysate effectivity remain largely unexplored.

Alongside hydrolysate, plant-derived biochar represents a complementary soil amendment with well-established benefits for soil functioning. Produced via pyrolysis of plant biomass under limited oxygen conditions, biochar is characterized by a highly porous structure, large specific surface area, and diverse surface functional groups, offering a potential solution to nutrient retention challenges (Shyam et al., 2025). These properties enable biochar to improve soil structure, increase water-holding capacity, enhance nutrient retention, and provide protected microhabitats for soil microorganisms (Kang et al., 2022). Numerous studies have demonstrated its capacity to enhance microbial activity and biomass, promote soil aggregation, and strengthen SOM stability (Kellerová and Jílková, 2025; Sheng et al., 2023; Wu et al., 2025). In addition to its direct effects, biochar can interact with other organic and inorganic soil constituents, influencing soil nutrients availability and microbial processes (Hu et al., 2024). Crucially, from nutrient retention perspective, the high surface area and sorptive capacity of biochar may also allow sorption of hydrolysate onto biochar surface or within its pores, thereby reducing rapid microbial consumption and leaching of hydrolysate, while still maintaining its biological availability. As a result, biochar-hydrolysate interactions are expected to enable a more gradual and sustained release of nutrients into the soil. However, the extent to which biochar surface properties drive hydrolysate absorption and whether this specific synergistic interaction regulate the slow release of nutrients rather than rapid loss, remains a critical and unaddressed knowledge gap.

The variability in nutrient retention capacity is largely governed by soil texture, which plays a critical role in determining the effectiveness of organic amendments on soil properties. Coarser-textured soils are particularly prone to nutrient leaching and organic matter loss, whereas finer-textured soils with smaller particle size and greater surface area can adsorb soil amendments more effectively, enhancing microbial activity, nutrient availability, and water retention (Kellerová and Jílková, 2025). Poultry-feather hydrolysate generally exerts stronger and longer-lasting effects in finer-textured sandy loam soils, where greater adsorption capacity supports prolonged nutrient retention and sustained microbial activity, whereas responses in coarser-textured soils are often weaker and shorter-lived due to rapid leaching (Roy and Jílková, 2025a, b). The use of biochar as a carrier for hydrolysate may therefore be particularly beneficial in coarse-textured soils, as it immobilizes hydrolysate, balances limited retention on mineral particles, and reduces rapid leaching, thereby enhancing efficiency and persistence of hydrolysate in the soil. Previous studies indicate that biochar application in coarse-textured soils improves water-holding capacity, reduces nutrient leaching, and enhances microbial activity and biomass by increasing surface area and providing protected microhabitats (Azeez et al., 2024; Jílková, 2023; Thao et al., 2024). Although the effects of hydrolysate and biochar have been studied individually across different soil textures, their combined effects have not yet been investigated.

Prior work has addressed hydrolysate and biochar only separately: hydrolysate studies have focused on its short-term biostimulant and fertility effects (Kaur et al., 2021; Gupta et al., 2023; Adelere and Lateef, 2023; Roy and Jílková, 2025a, b), while biochar studies have emphasized its standalone capacity to improve nutrient retention, water-holding capacity, and microbial habitat (Jílková, 2023; Azeez et al., 2024; Thao et al., 2024; Bekchanova et al., 2024). No study has so far examined biochar as a physical carrier that modifies the release dynamics of liquid hydrolysate, nor tracked how such a combination behaves over time. To address this gap, we conducted a 4-month laboratory microcosm incubation to determine the effects of hydrolysate and biochar, applied individually and in combination, on the chemical and microbial properties of agricultural soils. Temperate coarser-textured (loamy sand) and finer-textured (sandy loam) soils were incubated with either no addition, or with hydrolysate, biochar or hydrolysate absorbed into biochar addition. Soil microbial activity and available nutrient content in the soil leachates were followed regularly during the incubation and soil chemical and microbial properties were analyzed in soils from two harvests after one and four months. The present study aimed to test two hypotheses: (1) the effect of hydrolysate absorbed into biochar on soil properties will be stronger and last longer than that of the hydrolysate alone as the hydrolysate will be less prone to leaching and (2) the effect will be more evident in loamy sand than sandy loam soil as the hydrolysate absorbed into biochar will prolong its effect in the coarser-textured soil where otherwise hydrolysate is prone to fast leaching and loss compared to the finer-textured soil. By addressing these hypotheses, the study evaluates the potential of hydrolysate-biochar combination to provide sustained-release benefits and enhance the resilience of coarse-textured agricultural soils.

2 Materials and methods

2.1 Collection and preparation of soils and additives

Soils were collected from two fields under conventional farming near České Budějovice (Czech Republic; 49°01′38′′N, 14°27′51′′E and 49°02′13′′N, 14°27′46′′E, respectively) in March 2024. Soils (Fluvisols, WRB) were collected from 0–10 cm depth at 5 locations in a 0.25-ha area. The collected soils were thoroughly mixed, passed through a 2-mm sieve, and stored at 4°C before being used in the experiment. Soil texture was determined based on wet-sieving and sedimentation according to Gee and Bauder (1986). The soils were characterized as loamy sand with 67 ± 1.2% sand (2000-63 µm), 21 ± 1.0% silt (63-2 µm), and 12 ± 0.3% clay (< 2 µm) and sandy loam with 60 ± 1.1% sand (2000-63 µm), 23 ± 1.1% silt (63-2 µm), and 17 ± 0.6% clay (< 2 µm) (USDA).

The hydrolysate was prepared from the waste chicken feather (directly from production without any treatment) with water content of approximately 35 wt. % (Rabbit Trhový Štěpánov, corp. Czech Republic) in the batch stirred reactor. Exactly 2 kg of feather was put into the reactor of 25 L volume together with 100 g of malic acid and 15 L of water, and the batch was heated to the temperature of 115-125°C. After 5 hours, the reactor was cooled down and the reaction product separated by filtration to liquid hydrolysate and solid residue (below 3% wt). Approximately 15–17 L of liquid hydrolysate was prepared from one batch and stored at 4°C before use.

The biochar was produced from spruce wood chips under the exact pyrolysis conditions previously described in Dlasková et al. (2026), where its detailed morphological and pore structure characterization has been done. Pyrolysis of a dry sample in a sealed tube reactor under an N₂ atmosphere with a flow rate of 0.1 L min^-1. The reactor was heated in a muffle furnace 700°C for 90 min. The heating rate of the furnace was 10°C min^–1. Off-gas of the primary pyrolysis gas were ensured by a continuous supply of N₂ through a condensation vessel filled with water. Subsequently, the pyrolyzed wood chips were left in the furnace to cool down under the N₂ atmosphere. The prepared biochar was characterized by nitrogen adsorption at 77 K (ASAP 2050, Micromeritics). The specific surface of biochar S_BET was determined using the t-plot method with a standard isotherm for carbonaceous materials according to the standard defined by ASTM D6556. The biochar had specific surface of 450 m² g^–1. The morphological and pore structure properties of the biochar, including SEM imaging, have been characterized previously (Dlasková et al., 2026).

Hydrolysate was absorbed into biochar by submerging the biochar into the hydrolysate in a ratio of 10 g of biochar per 24 mL of hydrolysate. The biochar was submerged in an excess of hydrolysate for 30 minutes; after this time, it was removed and placed on a sieve to allow the excess hydrolysate to drain off. Gravimetric analysis determined the amount of hydrolysate absorbed, which was 2.4 g per 1 g of biochar.

The chemical and microbial properties of the soils, hydrolysate, and biochar are listed in Table 1. Organic matter (OM) content was determined based on loss on ignition at 450°C for 5 h. For determination of the contents of total organic C (TOC), total N (TN), total P (TP), and total basic cations (TCa, TMg, TK), air-dried samples were ball-milled and analyzed using a Flash Elemental Analyzer (Thermo Scientific) (TOC and TN), an inductively coupled plasma optical emission spectroscopy (ICP-OES) (TP), or a flame atomic absorption spectrometer (SensAA, GBC) (TCa, TMg, TK). Dissolved organic C (DOC), dissolved N (DN), dissolved P (DP), and dissolved basic cations (DCa, DMg, DK) were extracted in deionized water (dH₂O) (1:10 sample:dH₂O ratio) and analyzed in leachates using a TOC-L_CPH/CPN analyzer (Shimadzu) (DOC and DN), spectrophotometry according to Murphy and Riley (1962) (DP), and a flame atomic absorption spectrometer (SensAA, GBC) (DCa, DMg, DK). pH was assessed in a 1:10 sample:dH₂O suspension using a glass electrode. Microbial biomass C and N (C_mic and N_mic) were extracted using the fumigation-extraction method (Vance et al., 1987) and measured using a TOC analyzer (model TOC-LCPH/CPN, Shimadzu).

2.2 Microcosm experiment

An experiment was conducted using microcosms that enabled respiration measurements and leachate collection as described in Jílková et al. (2019). A 100-g quantity of fresh soil was packed into each microcosm chamber to 1.0 g cm^-3 dry bulk density, resulting in a soil profile height of 4 cm. The microcosm chambers were treated with no additive (NA), 1.2 mL of hydrolysate (HYDRO), 0.5 g of biochar (BIO), or 1.2 mL of hydrolysate absorbed into 0.5 g of biochar (HYDRO+BIO) applied onto the soil surface. The amount of hydrolysate resulted in an addition of 0.14 t dw ha^-1 (Roy and Jílkova, 2025a) and the amount of biochar resulted in an addition of 2 t dw ha^-1 (Major et al., 2010). Each treatment had six replicates, giving a total of 48 microcosms.

Microcosms were incubated at ~20°C for 4 months (April–July 2024) and during the incubation, microcosms were watered with 65 mL of dH₂O every 2 weeks (corresponding to a mean annual precipitation of 676 mm, which is typical for the area where the soil was collected), and soil leachates were collected at the bottom of each microcosm. Soil leachates on day 14, 28, 42, 70, 97, and 125 were stored at –20°C until they were analyzed for DOC, DN, DP, DCa, DMg, and DK as described earlier. Soil respiration was determined on day 15, 29, 43, 57, 71, 85, 98, and 126, which in each case was 1 day after the microcosms were watered. Gas samples were analyzed within 24 h with an HP 5890 gas chromatograph. Three replicates of microcosms were destructively harvested after one month (day 30; first harvest) and at the end of the experiment (day 127; second harvest). The soil was homogenized and used for the chemical and microbial analyses as described earlier.

2.3 Statistical analyses

The effects of the soil type and additive on the cumulative respiration and cumulative nutrient contents in the leachates were tested using two-way ANOVAs. The effects of the harvest, soil type and additive on the soil properties were tested using three-way ANOVAs. When tests indicated significant differences, Tukey LSD post-hoc tests were used to compare means. Dependent variables were log-transformed to satisfy the assumptions of normality and homoscedasticity if needed. Statistica 13 (StatSoft Inc., USA) was used for statistical analyses.

3 Results

3.1 Cumulative respiration and nutrient content in the leachates

Cumulative respiration and nutrient content in the leachates were generally affected by the harvest, soil type, additive and their interactions (Table 2). Cumulative respiration was on average 265% higher at the second harvest than at the first harvest and was on average 156% higher in loamy sand soil than in sandy loam soil (Figure 1A). Cumulative respiration was lowest in the NA treatment, followed by 4% higher respiration in the BIO and HYDRO+BIO treatments and 8% higher respiration in the HYDRO treatment. The effect of the additive was stronger at the second harvest, with on average 1% higher respiration in the BIO treatment and 8% higher respiration in both the HYDRO and HYDRO+BIO treatments than in the NA treatment, while no significant differences occurred at the first harvest. The effect of the additive was stronger in sandy loam soil than in loamy sand soil, with on average 10%, 25% and 30% higher respiration in the BIO, HYDRO+BIO and HYDRO treatments, respectively, than in the NA treatment in sandy loam soil, while no significant differences occurred in loamy sand soil.

Figure 1

Cumulative DOC content was on average 270% higher at the second harvest than at the first harvest and was on average 46% higher in loamy sand soil than in sandy loam soil (Figure 1B). Cumulative DN content was on average 130% higher at the first harvest than at the second harvest, was on average 62% higher in loamy sand soil than in sandy loam soil and was on average 12%, 28% and 43% higher in the HYDRO+BIO, NA and BIO treatments than in the HYDRO treatment, respectively (Figure 1C). The effect of the additive was stronger at the first harvest, with on average 17%, 37%, and 64% higher DN content in the HYDRO+BIO, NA and BIO treatments, respectively, than in the HYDRO treatment, while no significant differences occurred at the second harvest. Cumulative DP content was on average 21% higher at the second harvest than at the first harvest and was on average 180% higher in loamy sand soil than in sandy loam soil (Figure 1D).

Cumulative DCa content was on average 123% higher at the second harvest than at the first harvest and was on average 68% higher in loamy sand soil than in sandy loam soil (Figure 2A). Cumulative DCa content was lowest in both the NA and BIO treatments, followed by 15% higher content in the HYDRO+BIO treatment and 72% higher content in the HYDRO treatment. The effect of the additive was stronger at the first harvest than at the second harvest, with on average 21% and 96% (first harvest) and 11% and 53% (second harvest) higher DCa content in the HYDRO+BIO and HYDRO treatments, respectively, than in both the NA and BIO treatments. The effect of the additive was stronger in sandy loam soil than in loamy sand soil, with on average 74% (sandy loam) and 58% (loamy sand) higher DCa content in the HYDRO treatment than in the other treatments. Cumulative DMg was on average 152% higher at the second harvest than at the first harvest and was on average 50% higher in loamy sand soil than in sandy loam soil (Figure 2B). Cumulative DMg content was lowest in the BIO treatment, followed by 14% higher content in both the NA and HYDRO+BIO treatments and 78% higher content in the HYDRO treatment. The effect of the additive was stronger at the first harvest than at the second harvest, with on average 98% (first harvest) and 38% (second harvest) higher DMg content in the HYDRO treatment than in the other treatments. The effect of the additive was stronger in sandy loam soil than in loamy loam soil, with on average 14% and 72% (sandy loam) and 6% and 65% (loamy sand) higher DMg content in the HYDRO+BIO and HYDRO treatments, respectively, than in both the NA and BIO treatments. Cumulative DK was on average 172% higher at the second harvest than at the first harvest and was on average 225% higher in loamy sand soil than in sandy loam soil (Figure 2C). Cumulative DK content was lowest in both the NA and BIO treatments, followed by 3% higher content in the HYDRO+BIO treatment and 10% higher content in the HYDRO treatment.

Figure 2

3.2 Final soil properties

Soil properties were generally affected by the harvest, soil type, additive and their interactions (Table 3). DOC content was on average 97% higher at the second harvest than at the first harvest, was on average 35% higher in sandy loam soil than in loamy sand soil and was on average 17%, 24% and 40% higher in the HYDRO, NA and HYDRO+BIO treatments, respectively, than in the BIO treatment (Figure 3A). The effect of the additive was stronger at the second harvest, with on average 33% higher content in the HYDRO treatment and 50% higher content in both the NA and HYDRO+BIO treatments than in the BIO treatment, while no significant differences occurred at the first harvest. DN content was on average 57% higher at the second harvest than at the first harvest (Figure 3B). DP content was on average 121% higher at the second harvest than at the first harvest and was on average 20% higher in loamy sand soil than in sandy loam soil (Figure 3C). DP content was lowest in the BIO treatment, followed by 14% higher content in the NA and HYDRO treatments and 24% higher content in the HYDRO+BIO treatment.

Figure 3

DCa content was on average 97% higher at the first harvest than at the second harvest and was on average 21% higher in loamy sand soil than in sandy loam soil (Figure 3D). DCa content was lowest in the NA and HYDRO treatments, followed by 27% higher content in the HYDRO+BIO treatment and 69% higher content in the BIO treatment. The effect of the additive was stronger at the first harvest, with on average 12% higher content in the HYDRO+BIO treatment and 137% higher content in the HYDRO and BIO treatments than in the NA treatment. In contrast, at the second harvest, DCa content in the HYDRO+BIO treatment was on average 47% higher than in the NA and HYDRO treatments and 93% higher than in the BIO treatment. The effect of the additive was stronger in loamy sand soil, with DCa content on average 30% and 164% higher in the HYDRO+BIO and BIO treatments, respectively, than in both the NA and HYDRO treatments, whereas in sandy loam soil, DCa content was on average 30% higher in the HYDRO+BIO treatment than in the other treatments. DMg content was on average 81% higher at the first harvest than at the second harvest and was on average 36%, 53% and 78% higher in the NA, HYDRO+BIO and BIO treatments, respectively, than in the HYDRO treatment (Figure 3E). The effect of the additive was stronger at the second harvest, with on average 523% higher content in the BIO and HYDRO+BIO treatments and 927% higher content in the NA treatment than in the HYDRO treatment, while no significant differences occured at the first harvest. The effect of the additive was stronger in loamy sand soil, with on average 16%, 103% and 112% higher content in the HYDRO+BIO treatment than in the NA, HYDRO and BIO treatments, respectively, while no significant differences occured in sandy loam soil. DK content was on average 923% higher at the second harvest than at the first harvest, was on average 76% higher in loamy sand soil than in sandy loam soil and was on average 75%, 122% and 200% higher in the HYDRO, HYDRO+BIO and NA treatments, respectively, than in the BIO treatment (Figure 3F). The effect of the additive was stronger at the second harvest, with on average 120% higher content in the HYDRO and HYDRO+BIO treatments and 242% higher content in the NA treatment than in the BIO treatment, while no significant differences occured at the first harvest.

Soil pH was on average 4% higher at the first harvest than at the second harvest and was on average 2% higher in the sandy loam soil than in loamy sand soil (Figure 4A). Soil pH was lowest in the HYDRO treatment, followed by 0.4% higher value in the NA treatment and 1% higher value in both the BIO and HYDRO+BIO treatments. Soil pH was marginally significantly affected by the interaction of the harvest and the additive (p = 0.06), with on average 1% higher value in the HYDRO and BIO treatments and 2% higher value in the HYDRO+BIO treatment than in the NA treatment at the first harvest. In contrast, at the second harvest, soil pH in both the HYDRO+BIO and BIO treatment was on average 1% higher than in the NA and HYDRO treatments. The effect of the additive was stronger in loamy sand soil than in sandy loam soil, with on average 1%, 2% and 3% higher value in the NA, BIO and HYDRO+BIO treatments, respectively, than in the HYDRO treatment, while no significant differences occured in the sandy loam soil. C_mic content was on average 13% higher in loamy sand soil than in sandy loam soil (Figure 4B). C_mic content was marginally significantly affected by the additive (p = 0.06), with lowest content in the HYDRO+BIO treatment, followed by 14% higher content in the NA and HYDRO treatments and 22% higher content in the BIO treatment. N_mic content was on average 18% higher at the first harvest than at the second harvest and was on average 8% higher in loamy sand soil than in sandy loam soil (Figure 4C).

Figure 4

4 Discussion

Poultry-feather hydrolysate and plant-derived biochar hold potential for improving soil nutrient availability and enhancing soil health (Bamdad et al., 2022; Kellerová and Jílková, 2025; Khaledi et al., 2023). However, previous studies have largely focused on the individual effects of hydrolysate (Adelere and Lateef, 2023; Gupta et al., 2023; Roy and Jílková, 2025b) or biochar (Bekchanova et al., 2024; Sheng et al., 2023; Sun et al., 2022), and there is limited information on their combined effects and on their effects in different soil textures. To address these knowledge gaps, the present study investigates the impact of hydrolysate applied alone or absorbed into biochar on nutrient availability and microbial properties in loamy sand and sandy loam soils. Uniquely, this is the first study to examine the temporal dynamics of nutrient retention and leaching, providing new insights into the role of biochar as a carrier for hydrolysate and its potential for sustaining soil fertility over time.

The absorption of hydrolysate into biochar reduced nutrient losses and prolonged the influence of hydrolysate on soil nutrients and microbial activity relative to hydrolysate applied alone, although this effect varied among respective nutrients and with time of the experiment, thus supporting the first hypothesis only partially. Notably, the hydrolysate-biochar mixture itself contained higher OM, TOC, TN, TP, and DOC content than biochar alone (Table 1). Given the 2:0.14 biochar-to-hydrolysate mass ratio, this largely reflects an additive contribution of hydrolysate-derived organic matter and nutrient content to the biochar matrix, rather than a synergistic transformation. The proportionally larger increases in TN, TP, and DOC content relative to OM content are consistent with hydrolysate contributing N, P and readily soluble C-rich compounds that are less represented in the pyrolyzed biochar. Hydrolysate applied alone generally resulted in higher cumulative respiration (Figure 1A) and higher release of dissolved nutrients in the leachates, particularly DCa (Figure 2A) and DMg (Figure 2B), indicating rapid microbial utilization followed by pronounced leaching. This suggests that the hydrolysate acted primarily as a readily available substrate with limited persistence in the soil. These findings align with our previous study (Roy and Jílková, 2025a), which demonstrated rapid microbial activity followed by limited stabilization of hydrolysate-derived C. The similar respiration rates with hydrolysate absorbed into biochar and biochar treatments suggest that absorption of hydrolysate into biochar surfaces physically reduced substrate accessibility to microbial enzymes, likely through occlusion within biochar micropores or binding to surface functional groups, thereby slowing the rate of microbial decomposition and C mineralization (Dai et al., 2021). This is supported by the porous morphology of this biochar, as revealed by SEM imaging and pore structure analysis (Dlasková et al., 2026).

In contrast, hydrolysate absorbed into biochar tended to moderate nutrient leaching, particularly for DP (Figure 1D), DCa (Figure 2A) and DMg (Figure 2B). This indicates that absorption of hydrolysate into biochar reduced its immediate availability and leaching, thereby prolonging its retention in the soil. Specifically, biochar surface functional groups including carboxyl and hydroxyl groups and its inherent cation exchange capacity (CEC), likely facilitated electrostatic retention of Ca²⁺ and Mg²⁺ ions, reducing their mobility in the soil and limiting nutrient leaching losses (Ippolito et al., 2015). The application of hydrolysate absorbed into biochar thus resulted in higher DOC (Figure 3A), DP (Figure 3C), DCa (Figure 3D) and DMg (Figure 3E) content in the soil compared to hydrolysate alone at later stages of the experiment, indicating that biochar slowed down the release and leaching of nutrients. The delayed availability likely reflects absorption of soluble organic molecules and cations into biochar, which reduced their immediate mobility and protected them from rapid mineralization, consistent with a paddy soil study reporting that biochar increased complexed aluminum oxide content by up to 193 mg kg^-1, reflecting enhanced organo-mineral complexation capacity for cations (Wu et al., 2025). This pattern also aligns with findings that biochar reduces nitrate leaching progressively as application rate increases, consistent with strong retention of ammonium and nitrate ions (Das, 2024).

The substantially higher soil DMg content with hydrolysate absorbed into biochar reflects the very low DMg content retained under hydrolysate alone, where rapid leaching depleted Mg from the soil profile, amplifying the relative difference. Biochar selectively retains divalent cations such as Mg²⁺ through electrostatic attraction at negatively charged surface sites, inner-sphere complexation, and precipitation within micropores (Li et al., 2017). The high surface area of the biochar likely maximized available sorption sites, while gradual desorption over time explains the pronounced difference at the later harvest, consistent with reported reductions in nitrate leaching of 11-72% following biochar amendment (Hossain et al., 2020). Moreover, hydrolysate absorbed into biochar maintained higher and more consistent soil pH during the experiment compared to hydrolysate alone (Figure 4A), suggesting that biochar contributed to buffering the acidifying effect of hydrolysate likely attributed to the alkaline nature of biochar, thereby creating more favorable conditions for sustained nutrient retention (Sun et al., 2022). Overall, these results suggest that biochar can mitigate the rapid loss of hydrolysate and thus nutrients, and shift its effects from short to more persistent.

The effect of hydrolysate absorbed into biochar on nutrient availability and microbial properties was stronger in sandy loam than in loamy sand soil, contradicting the second hypothesis that assumed biochar’s retention capacity would compensate for limited mineral sorption by loamy sand soil. Instead, these results suggest that biochar-mediated retention is not simply additive to soil mineral sorption but depends on it. Biochar appears to amplify a soil’s existing retention capacity rather than substitute for its absence in coarse-textured soils. Soil texture can strongly mediate the persistence of organic amendments by affecting nutrient retention and soil properties (Wei et al., 2023). Finer-textured soils generally exhibit higher retention and slower leaching of dissolved compounds due to greater surface area and CEC (Matichenkov et al., 2020). In this study, hydrolysate absorbed into biochar moderated nutrient leaching (Figure 2) and enhanced soil content of DCa (Figure 3D) and DMg (Figure 3E), particularly in sandy loam soil, indicating delayed release and prolonged nutrient retention. This pattern suggests that the higher clay content and associated CEC of sandy loam soil provided abundant negatively charged mineral surfaces for electrostatic interactions with biochar particles and hydrolysate-derived cations, facilitating stronger biochar-hydrolysate-soil particle associations and allowing nutrients to be retained more effectively within the soil profile (Blanco-Canqui, 2017; Singh et al., 2022). Three mechanisms likely contributed jointly to this texture-dependent retention. First, direct electrostatic and cation-bridging interactions between clay mineral surfaces and biochar-hydrolysate complexes formed stable organo-mineral associations, consistent with reports that biochar increases complexed aluminum oxide content in soil (Wu et al., 2025). Second, the finer-textured sandy loam likely had a shorter, less connected leaching pathway, physically slowing nutrient transport independent of biochar-specific sorption. Third, biochar particles may have anchored to clay surfaces, extending the mineral’s sorption capacity rather than acting as an independent sorbent. The first mechanism was likely the primary driver, since the two soils differed in clay content while treatment rates remained fixed. In contrast, loamy sand soil, dominated by sand particles with minimal clay content, low specific surface area, and reduced CEC, offered fewer mineral absorption sites for biochar-hydrolysate interactions, thereby shortening nutrient residence time and limiting the capacity of biochar to mitigate leaching losses, even though some soil nutrient retention and pH buffering were observed (Figure 4A).

In sandy loam soil, inherent soil properties that promote greater nutrient retention likely created a more favorable environment for microbial activity (Figure 1A), resulting in sustained nutrient availability over time. These findings are broadly consistent with a meta-analysis of 92 studies reporting that biochar amendment reduces mineral nutrient leaching by an average of 38% and increases soil N, P, and K by 36%, 34%, and 15%, respectively (Bekchanova et al., 2024), with greater benefits typically observed in finer-textured soils where higher surface area and CEC promote stronger biochar-soil interactions (Liu et al., 2025). The observed increases in DCa and DMg in sandy loam soil (up to 74% and 72%, respectively, at the second harvest) fall within a comparable order of magnitude to these broader trends, despite differences in the specific nutrients and amendment types assessed. Overall, these findings indicate that soil texture strongly governs the magnitude and persistence of nutrient retention, with finer-textured soils showing more prolonged interactions between hydrolysate and biochar.

As a short-term laboratory microcosm study conducted without plants, the present study does not capture field conditions, where processes such as plant uptake, root exudation, temperature and moisture fluctuation, and microbial interactions with vegetation operate simultaneously and may substantially alter nutrient dynamics. Moreover, the study only included loamy sand and sandy loam soil, limiting generalizability to other soil types. Finally, microbial responses were assessed only through respiration and biomass, reflecting activity rather than community composition. Future studies incorporating diverse soil types, plant-growth or field trials, and direct biochar-hydrolysate characterization would help address these limitations.

5 Conclusion

The present study demonstrates that poultry-feather hydrolysate and plant-derived biochar can interact synergistically to enhance soil nutrient retention and support sustained soil fertility. Absorption of hydrolysate into biochar reduced immediate nutrient losses, moderated leaching, and prolonged nutrient availability, particularly for DP, DCa and DMg, compared with hydrolysate applied alone. These differential retention patterns across nutrients reflect the distinct absorption affinities and release kinetics of phosphate and divalent cations on biochar surfaces, highlighting that the slow-release benefit of biochar as a carrier is nutrient-specific and temporally dynamic. The treatment also maintained consistent soil pH and promoted more persistent microbial activity, indicating that biochar shifts the effects of hydrolysate from short to longer lasting. Soil texture strongly influenced these outcomes, with sandy loam soil exhibiting greater nutrient retention and microbial activity than loamy sand soil, contrary to the initial expectation that the effect would be more pronounced in the coarser-textured soil. This indicates that biochar’s carrier benefit does not compensate for a coarse soil’s limited mineral sorption capacity, but instead is amplified where the soil’s own retention capacity, mediated by clay content and CEC, is already greater. Practically, this suggests that biochar-hydrolysate combinations may deliver more nutrient-retention benefit when applied to finer-textured soils such as sandy loam, rather than to coarse-textured soils where biochar was originally expected to offer the most compensatory value. Overall, these findings highlight the potential of biochar as a carrier for hydrolysate to improve nutrient retention and soil health, while emphasizing that soil texture critically modulates the magnitude and persistence of these benefits. Future work should test whether these soil-level benefits translate into improved plant nutrient uptake, crop productivity, and field-scale nutrient-use efficiency, particularly in finer-textured soils where the strongest effects were observed.

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