Phosphorus fraction transformation and soil acidity mitigation through integration of water hyacinth biochar with organic and synthetic phosphorus sources in acidic soil

Phosphorus (P) deficiency is a major problem in acidic and tropical soils, including in Ethiopia, because more than 80% of P derived from synthetic fertilizers becomes fixed and unavailable. The short-lived effect of lime and the limited application of fertilizers due to increasing costs cannot solve the problem, leading to significant declines in crop productivity. This study examined how integrating water hyacinth biochar with organic and synthetic P sources affects the transformation of soil P fractions and soil acidity amelioration in Ethiopian soil. We set up a 90-day incubation experiment using P fertilizer, poultry manure (PM), and their pairwise combinations with 1% and 2% biochar and lime, along with a negative control. Soil samples were taken on days 0, 3, 7, 15, 30, 45, 60, and 90 for analyzing pH, exchangeable acidity, exchangeable Al³⁺, and available P, while samples taken on days 3, 30, and 90 were used to analyze the transformations of P fractions. The results indicated that the efficacy of biochar–fertilizer and biochar–PM integrations outperformed that of fertilizer and PM alone and their combinations with lime, significantly increasing the soil pH by 0.99–1.79 units, available P by 611%–839%, labile P fractions by 1,180%–1,795%, and calcium-associated P by 372%–611%. In contrast, they decreased exchangeable acidity by 80.0%–95.7%, exchangeable Al³⁺ by 92.5%–100%, Al/Fe-bound P fractions by 35%–60%, and recalcitrant P by 18%–20% compared with the control after incubation. More importantly, co-application of 2% biochar with PM and fertilizer eliminated exchangeable Al³⁺. Overall, biochar–PM combinations exhibited better efficacy compared to biochar–fertilizer counterparts. Therefore, it can be concluded that valorizing invasive water hyacinth into biochar and co-applying it with organic and synthetic P fertilizers can be an integrated strategy for repurposing waste to address soil acidity and P deficiency, enhance soil fertility, and reduce reliance on fertilizers, thereby promoting sustainable agriculture and effective waste management.

1 Introduction

Phosphorus (P) is the second-most crop yield-limiting nutrient after nitrogen (N), playing vital roles in key plant activities such as photosynthesis and respiration (Kamran et al., 2018). The average P content in soil is 0.05%, w/w, out of which approximately only <0.1% is present in soil solution and <1% of the total P is accessible for plant uptake (Bünemann, 2015). Much of the soil P remains unavailable primarily due to fixation and precipitation by soil components, such as aluminum (Al³⁺) and iron (Fe³⁺) in acidic soils and calcium (Ca²⁺) in alkaline soils, along with immobilization by soil microorganisms (Zhu et al., 2018). Additionally, leaching and erosion lead to considerable P loss. Due to this, over 40% of global agricultural soils have deficient plant-available P (Balemi and Negisho, 2012).

To address the shortage of plant-available P, more than 15 million tons of P fertilizers are applied to agricultural soils worldwide each year, with a projected annual increase of 2%. However, from this, only <20% is absorbed by plants during a growing season. This substantial reliance on synthetic fertilizers conflicts with the few phosphate rocks used for its production (Zhang et al., 2016). As global P demand is projected to double by 2050, without alternative sources to meet this demand, worldwide reserves will last only approximately 50–125 years (Chowdhury et al., 2018). This will likely lead to food insecurity worldwide, particularly in developing countries such as Ethiopia (Cordell and White, 2011). Additionally, excess P in cultivated lands can be exposed to surface runoff, leading to environmental pollution.

In Ethiopia, where agriculture is the main pillar of the economy, over 43% of the agricultural soils are affected by soil acidity (pH < 5.5), significantly reducing nutrient availability, particularly plant-available P (Warke, 2024). Although the total P content of Ethiopian agricultural soils ranges from 185 to 1,981 mg kg⁻¹, plant-available P is only <5 mg kg⁻¹ (Asmare Melese et al., 2015), mainly due to P fixation, coupled with losses of approximately 13 kg ha^–1 yr^–1 through erosion and leaching (Abebe et al., 2022). Although liming is a common method used to reduce soil acidity, it is ineffective due to its short-lived effects, limited availability, high costs, and transportation challenges from production sites (Mosissa and Taye, 2019). Consequently, in Ethiopia, agricultural soils, including many tropical soils, are highly acidic and deficient in plant-available P, making P one of the main limiting factors for crop production in the highlands (Asmare Melese et al., 2015). Therefore, ensuring an adequate P supply is vital for effective soil management and sustainable crop production in Ethiopia’s tropical regions.

The use of chemical fertilizers, including P, has been a common strategy to combat decreasing crop yields in Ethiopia. However, low application rates caused by high costs and limited availability, coupled with P fixation and nutrient leaching, have hindered this approach from effectively addressing the issue. Instead, continued fertilizer use has further worsened soil acidity and reduced P availability (Agegnehu et al., 2017). Therefore, sustainable and efficient alternative soil nutrient management strategies, including for P, are urgently needed.

The recovery and reuse of P from organic waste materials can be sustainable alternative to reduce reliance on synthetic P fertilizers and balance future P demand (Chowdhury et al., 2018). This is particularly advantageous for developing countries such as Ethiopia, consisting of abundant organic waste rich in phosphate. Organic materials such as poultry manure (PM), farmyard manure, and compost serve as natural fertilizers, offering a viable substitute of chemical fertilizers (Du-Preez et al., 2011). Research findings demonstrated that P from organic materials is as or even more available than P from inorganic P fertilizers (Kamran et al., 2018). Among these, PM stands out due to its higher nutrient content, affordability, and the fact that, unlike other manures, it is not used for fuel. Ethiopia has a large poultry population, exceeding 60 million (CSA, 2021), offering a valuable opportunity to use PM as an agricultural input, thereby reducing the reliance on costly fertilizer and promoting sustainable agriculture and effective waste management. However, repeated application of only PM may pose risk of pathogenic exposure, heavy-metal accumulation, and nutrient leaching and volatilization, leading to environmental pollution (Vakal et al., 2021). Consequently, alternative strategies are needed to provide adequate nutrients while minimizing environmental risks.

Biochar has emerged as a promising soil conditioner due to its alkaline pH, high carbon content, and cation exchange capacity (CEC) and porous, functionalized surface, which improve soil acidity, enhance nutrient retention and plant uptake, and boost crop productivity, while also reducing environmental hazards, making it a sustainable alternative to conventional fertilizers (Agegnehu et al., 2017). Studies have shown that application of biochar can increase P availability in acidic soil through: 1) increasing the desorption of fixed phosphate due to stronger anion repulsion at high pH and CEC (Jiang et al., 2015), 2) complexing acidic Al³⁺ and Fe³⁺ with its functional groups such as –O^– and –COO^– and freeing up fixed P (Wang et al., 2012), 3) direct supply of P with values depending on the nature of feedstocks and pyrolysis conditions (Glaser and Lehr, 2019), and 4) increasing the microbial and enzymatic activities that promote P cycling (Ghodszad et al., 2021). According to Sun et al. (2018), biochar application represents an effective means of recovering and reusing P in soils. A comprehensive review by Ghodszad et al. (2021) concluded that biochar addition decreases P fixation and enhances labile P, often outperforming the approaches of traditional liming and sole application of fertilizers.

Previous studies demonstrated that biochar application markedly alters the transformation of P fractions in soils (Lutfunnahar et al., 2021; Xu et al., 2024; Khan et al., 2025). When biochar is applied to acidic soils, the long-term available P (Al/Fe-bound P) can be transformed into short-term available forms (Kamran et al., 2019; Wu et al., 2022). Studies on the chemical fractionation of soil P with biochar application help identify the dominant inorganic and organic P forms in soils after biochar amendment and clarify how biochar transforms non-labile P into plant-available labile fractions (Wu et al., 2022).

Research findings suggest that valorization of invasive plants into biochar is an effective approach to mitigate ecological harm and enhance soil nutrient cycling (Lewoyehu et al., 2024). Since 2011, water hyacinth, a global invasive weed, has invaded Ethiopia’s largest lake, Lake Tana, and has been negatively affecting the lake ecosystem and the quality of life of the surrounding communities (Dersseh et al., 2019). Thus, converting this invasive weed into biochar transforms an environmental problem into a valuable agricultural input, making it beneficial. Since water hyacinth has very high moisture content, converting it into biochar is preferred over composting or using it as green manure. Because pyrolysis reduces the biomass volume and increases the alkalinity (Dai et al., 2017), while also destroying contaminants and pathogens and reducing the mobility and bioavailability of heavy metals (Jin et al., 2016). It also prevents the risk of spreading weed seeds. The resulting stable, alkaline biochar with a high surface area more effectively and durably neutralizes soil acidity and enhances nutrient retention, long-term availability, and plant use efficiency, thereby boosting crop yields, which are critical constraints in Ethiopian acidic soils (Agegnehu et al., 2017).

Since PM, like water hyacinth biochar, is an underutilized organic waste, integrating these organic wastes transforms two problematic materials into valuable soil amendments with complementary benefits—biochar’s liming and sorption capacity coupled with PM’s nutrient richness—thereby simultaneously achieving soil waste valorization and soil acidity amelioration. Enriching acidic and degraded soils through such integrated use of organic wastes promotes sustainable agriculture, supports the European Union’s 2030 objectives of reducing dependence on chemical fertilizers and expanding organic farming, and mitigates environmental impacts (Rivelli and Libutti, 2022). However, no research has been reported in the Ethiopian highlands on how organic and inorganic soil amendments, such as water hyacinth biochar and lime, affect P fractions derived from organic and inorganic sources in acidic soils, although the content and forms of P fractions largely depend on soil properties, nutrient sources, and soil amendment types.

Therefore, this study examined how integrating water hyacinth biochar with organic and inorganic inputs influences the transformation of P fractions and amelioration of soil acidity in Ethiopia. We hypothesized that integrating water hyacinth biochar with organic and synthetic P fertilizers would significantly reduce soil acidity and exchangeable Al³⁺, while increasing the transformation of Al/Fe-fixed P and recalcitrant P into plant-available labile P fractions. Biochar would perform better than traditional lime in reducing soil acidity and increasing labile P forms. The results of this study could have significant relevance for crop production and environmental mitigation.

2 Materials and methods

2.1 Soil sample, poultry manure, and biochar preparation

The soil samples with an acidic surface (0 cm–20 cm) used in this experiment were collected from the Koga research site in northwestern Ethiopia (11°10′ and 11°25′ N, 37°02′ and 37°17′ E). Debris and stones were removed, and the samples were air-dried, crushed, screened through a 2-mm sieve, and analyzed for basic physicochemical properties (Table 1).

Table 1

Table 1. Basic physicochemical properties and phosphorus fractions of the soil, poultry manure, and water hyacinth biochar used in the experiment.

Soil pH was measured using a digital pH meter (LAQUA F-71, Horiba Scientific, Kyoto, Japan) in a 1:2.5 soil-to-water ratio (Robinson, 1994). Exchangeable acidity and exchangeable Al³⁺ were determined by extracting the soil with 1 mol L^–1 KCl, followed by titration with 0.02 mol L^–1 sodium hydroxide (NaOH) and hydrochloric acid (HCl), respectively (Anderson and Ingram, 1994). Available P was measured using a flow injection autoanalyzer (FIAlyzer-1000, FIAlab Instruments, Inc., Seattle, WA, USA) after extraction with Mehlich 3 solution (Mehlich, 1984), while total P was determined after digesting 2 g of the soil sample using a mixture of concentrated nitric (HNO₃), sulfuric (H₂SO₄), and perchloric (HClO₄) acids (Chapman and Pratt, 1962). Organic carbon (OC) was analyzed using the wet digestion method, with potassium dichromate (K₂Cr₂O₇) as an oxidizing agent (Walkley and Black, 1934), while CEC was determined after extraction with ammonium acetate (NH₄OAc, pH 7; Singh et al., 2017).

The fresh PM, collected from a layer poultry farm in Tokyo, Japan, was spread on a plastic sheet and air-dried in a ventilated shade for 1 week. It was then oven-dried at 45 °C for 24 h, ground, and sieved using a 2-mm sieve. The characterizations of basic properties (Table 1) were conducted using the same procedures used for soil analysis.

For biochar preparation, water hyacinth biomass was collected from Lake Tana, Ethiopia. The collected biomass was cleaned, cut into small pieces, and dried at 75 °C for 48 h. The dried biomass was then added to a cylindrical Al tin with a closer containing four holes to allow minimum gas release and then was pyrolyzed in an oxygen-limited environment using an electric furnace (LT 40/12, Nabertherm GmbH, Lilienthal, Germany) at 450 °C for 2 h residence time with a heating rate of 10 °C min^–1. The biochar was pulverized and screened with 0.5-mm and 2-mm sieves for characterization (Table 1) and incubation, respectively. Fixed carbon, volatile matter, and ash content were measured using simultaneous differential thermogravimetry (SDT Q600, TA Instruments, Lukens Drive, New Castle, DE, USA). The surface area and porosity were measured using the Brunauer–Emmett–Teller (BET) method via an isothermal N₂ adsorption at −196 °C using an automatic surface area and porosimeter (ASAP 2010; Micrometric, Lincoln, UK) after degassing the sample at 273 °C for 10 h (Brunauer et al., 1938). Surface functional groups (phenolic, lactonic, and carboxylic groups) were analyzed using the Boehm titration technique (Pereira et al., 2015). The pH and CEC were analyzed using a 1:10 biochar to water ratio and the NH₄OAc method, respectively (Singh et al., 2017). Total carbon (C) was determined using a dry combustion method with an elemental analyzer (Perkin Elmer, 2400 series II, Waltham, MA, USA; Yeomans and Bremner, 1991). Total P was determined by digesting 0.5 g of air-dried biochar using concentrated HNO₃, H₂SO₄, and HClO₄. Available P and OC were analyzed using a flow injection autoanalyzer and the wet digestion method, respectively. Additionally, the modified sequential extraction method of Hedley et al. (1982) was used to analyze P fractions in the soil, PM, and biochar using 2, 1, and 1 g of the samples, respectively.

2.2 Experimental treatments

A negative control (CK; soil only), P fertilizer (F), PM, and factorial combinations of lime (L) and 1% and 2% water hyacinth biochar (B) with F and PM (LF, LPM, 1BF, 1BPM, 2BF, and 2BPM, respectively) were set up in a completely randomized design for the incubation experiment. The biochar rates were selected based on their demonstrated effectiveness for ameliorating acidic soil (Lewoyehu et al., 2024; Lewoyehu et al., 2025). The fertilizer triple superphosphate (TSP) was applied at 138 kg P₂O₅ ha^–1, which is the recommended rate for maize cultivation (common crop in the soil sampling area; Alemu et al., 2022). The PM application rate was calculated based on the total P content, assuming 80% availability. An availability coefficient of 80% was used for PM P as it represents the conservative lower end of the commonly cited availability range of 80%–100% and is widely used in nutrient-management guidelines and extension recommendations (Zhang, 2017; Lory, 2018). Because soil acidity promotes P fixation by Al and Fe oxides, using 80% prevents overestimation of manure-derived available P and aligns with standard agronomic practices. The lime rate (LR) was calculated using Equation 1 (Agegnehu et al., 2019). Each treatment was done in triplicate.

$\text{LR}\left({\text{CaCO}}_3\text{\hspace{0.17em}kg\hspace{0.17em}}{\text{ha}}^{–1}\right)=\frac{\text{Ex}.\text{Ac}*\text{BD}*\text{depth}*{10}^4*1000}{2000},(1)$

where Ex. Ac and BD are exchangeable acidity and bulk density of the experimental soil, respectively (Table 1), with a depth of 0.20 m.

2.3 Incubation experiment

Soil samples (2 mm, 500 g each) in 1-L plastic bottles were preincubated at 30 °C for 2 weeks at 55% field capacity. Bottles were covered with a perforated parafilm to reduce water loss while maintaining aerobic conditions and were weighed every 3 days to adjust moisture with deionized water. After preincubation, soils were sampled (day 0), amended with the calculated treatment rates, thoroughly mixed, and incubated for 90 days. During incubation, soil samples were taken on days 3, 7, 15, 30, 45, 60, and 90. The collected soil samples were air-dried, crushed, and the pH, available P, Ex. Ac, and Ex. Al³⁺ were analyzed using the methods stated earlier.

Transformation of P fractions in the soil samples taken on days 3, 30, and 90 was examined using the modified method of Hedley et al. (1982). Soil P fractionation was conducted on day 3 to capture the immediate reactions following application of amendments such as rapid dissolution of soluble P from fertilizer and PM, initial adsorption of P onto biochar surfaces, and short-term pH changes that influence labile P availability. It captures the initial flush of easily mineralizable and water-soluble P fractions. P fractionation on day 30 was conducted to represent a transitional phase where microbial activity, organic matter mineralization from PM, and biochar–soil interactions begin to stabilize. By this time, redistribution among labile, moderately labile, and moderately stable P pools occurs, allowing assessment of how amendments regulate P retention and transformation beyond the initial reaction period. Finally, P fractionation on day 90 reflects the longer-term stabilization of P fractions as the system approaches equilibrium, capturing slow mineralization, gradual release of organically bound P, formation of more recalcitrant P fractions, and the sustained liming effects of biochar. This helps determine whether the amendments can maintain improved P availability and reduce fixation over time.

In brief, 2 g of air-dried soil was added to centrifuge tubes (50 mL) and sequentially extracted with 30 mL of 1 mol L^–1 ammonium chloride (NH₄Cl), 0.5 mol L^–1 sodium bicarbonate (NaHCO₃, pH 8.5), 0.1 mol L^–1 NaOH, and 1 mol L^–1 HCl. For each extractant, the soil was shaken for 16 h and centrifuged at 5,000 g for 15 min. The extract was sifted through a 0.45-μm membrane filter and analyzed using an autoanalyzer to determine the inorganic P fraction (Pi). For NaHCO₃ and NaOH extractants, 5 mL of the extract (before filtration) was subjected to ammonium persulfate [(NH₄)₂S₂O₈] oxidation in an autoclave at 121 °C for 1 and 1.5 h, respectively, to determine the total P. Pi was subtracted from total P to obtain the organic P fraction (Po) in these extractants. After extraction with HCl, the solid residue was oven-dried at 45 °C for 24 h and digested with a mixture of concentrated HNO₃, H₂SO₄, and HClO₄ until it turned colorless. The digested sample was filtered, pH-adjusted, and analyzed using an autoanalyzer to determine residual P. Additionally, 2 g of the air-dried soil was directly digested with the aforementioned concentrated acids to determine total P and compare it with the sum of the P fractions obtained via sequential extraction (Supplementary Table S2).

2.4 Statistical analysis

Data were analyzed using the Statistical Package for Social Science (SPSS 22, IBM Inc., Armonk, NY, USA). One-way ANOVA followed by Tukey’s HSD test (P < 0.05) was used to determine significant differences in soil pH, Ex. Ac, Ex. Al³⁺, available P, and P fractions among the treatments at each specific sampling date. Linear regression and Pearson correlation analyses were conducted to examine the relationships among these soil parameters.

3 Results

3.1 Effects of the amendments on soil pH and exchangeable acidity

Integrated application of water hyacinth biochar with organic or synthetic P fertilizers compared to PM and fertilizer alone and their combinations with lime markedly ameliorated soil acidity, resulting in significantly (P < 0.05) higher soil pH (Figures 1a,b; Supplementary Table S1). The efficacy of the B–PM integrations outperformed that of the B–F counterparts. The 2BPM treatment consistently produced the highest soil pH, recording 1.79 units higher than CK after incubation. Although less effective than biochar, lime application also increased soil pH, showing a modest increase of 0.49–0.59 units above the control after incubation (Figure 1b; Supplementary Table S1). The LPM treatment exhibited significantly higher pH than the LF treatment throughout the incubation period. The sole application of PM increased soil pH by only 0.32 units compared with the control. In contrast, the F-only treatment slightly acidified the soil, although it did not differ significantly from the control.

Figure 1

Figure 1. Soil pH dynamics during the incubation period (a) and overall pH changes after incubation (b). Different letters on the bars indicate significant differences (Tukey’s HSD) among the treatments at P < 0.05. Error bars represent standard deviations (n = 3). CK, control (soil only); F, recommended rate of phosphorus fertilizers for maize cultivation (138 kg P₂O₅ ha^–1) as triple super phosphate; PM, poultry manure at a recommended rate of phosphorus fertilizers, assuming 80% of total phosphorus in PM is available; L, lime with a rate determined using the exchangeable acidity method; B, water hyacinth biochar at 1% (1B) and 2% (2B) application rates.

Mirroring the pH response, co-application of biochar with PM or fertilizer achieved greater reductions in Ex. Ac and Ex. Al³⁺ than the sole application of these nutrient sources, as well as their combinations with lime (Figures 2a,b; Supplementary Table S1). After incubation, Ex. Ac and Ex. Al³⁺ levels in biochar-treated soils were 64.6%–92.4% and 87.6%–100% lower, respectively, compared to the lime-treated soils. More importantly, the 2BF and 2BPM treatments eliminated Ex. Al³⁺ throughout the incubation period. Additionally, no Ex. Al³⁺ was observed until day 45 in the 1BF and 1BPM treatments, showing only 0.23 and 0.19 cmol₍₊₎ kg^–1, respectively, after incubation (Figure 2b; Supplementary Table S1). Neither 1BF and 1BPM nor 2BF and 2BPM had significant variations in Ex. Ac and Ex. Al³⁺ (Supplementary Table S1). However, significant differences were observed between LPM and LF. Although the reductions were considerably smaller than those achieved with biochar and lime, addition of PM alone still lowered Ex. Ac and Ex. Al³⁺ by 25% and 18%, respectively, after incubation. In contrast, the sole application of F increased Ex. Ac and Ex. Al³⁺ throughout the incubation period, peaking to 4.75 and 3.13 cmol₍₊₎ kg^–1, respectively, on day 90.

Figure 2

Figure 2. Dynamics of soil exchangeable acidity (a) and exchangeable Al³⁺ (b) during the incubation period. Error bars represent standard deviations (n = 3). Treatment abbreviations are the same as in Figure 1.

3.2 Effects of the amendments on soil-available phosphorus

Soil-available P dynamics significantly (P < 0.05) varied among the applied treatments during incubation (Figure 3; Supplementary Table S1). Initially, biochar–fertilizer integrations enhanced available P more than the corresponding biochar–PM combinations. However, in later stages, the trend reversed, with biochar–PM sustaining higher available P, recording 611%–782% above the control by the end of incubation. Biochar treatments consistently outperformed lime and sole nutrient applications in efficacy, which produced moderate or minimal increases. The sole application of PM showed a delayed increase in available P compared to its combined application with biochar. In contrast, the F-only treatment increased available P early on but decreased rapidly, showing the lowest levels after the control from the second week onward and reaching 13.0 mg kg^-1 by day 90. Across all treatments, available P decreased toward the end of incubation, yet lower reductions were observed in biochar-amended soils compared to other treatments. Overall, the temporal trends of available P mirrored soil pH patterns and were inversely associated with Ex. Ac and Ex. Al³⁺ (Figures 1a, 2a,b, 3).

Figure 3

Figure 3. Soil-available phosphorus dynamics during the incubation period. Error bars represent standard deviations (n = 3). Treatment abbreviations are the same as in Figure 1.

3.3 Effects of the amendments on soil total and fractional phosphorus

Soil total P exhibited significant variations (P < 0.05) among the treatments (Supplementary Table S2). The B–PM combinations produced greater increases, with 2BPM achieving the highest total P, while the control had the lowest. PM alone produced markedly higher total P than F alone. However, lime application did not affect total P, and neither F and LF nor PM and LPM exhibited significant variability (Supplementary Table S2).

3.3.1 Phosphorus fractions

Co-application of water hyacinth biochar with either PM or fertilizer significantly (P < 0.05) altered the transformation of P fractions relative to individual applications of PM and fertilizer, and their combinations with lime. Initially, fertilizer-based treatments (B–F, LF, and F) produced higher labile P fractions (NH₄Cl–P and NaHCO₃–Pi) than the corresponding PM-based treatments (B–PM, LPM, and PM), with B–F treatments increasing them by 307%–531% and 616%–1184%, respectively, relative to CK (Table 2). However, PM–based amendments ultimately resulted in higher labile P fractions, with the 2BPM treatment showing the greatest enhancements of 646% of NH₄Cl–P and 1293% of NaHCO₃–Pi. The B–PM, LPM, and PM also entirely provided a higher labile organic P fraction (NaHCO₃–Po) than the fertilizer-based treatments, with 2BPM recording the highest values across all sampling days. Although less effective than biochar, lime amendment still produced substantially higher labile P fractions than PM and F alone (Table 2). The LPM treatment increased NH₄Cl–P and NaHCO₃–Pi by up to 282% and 605%, respectively, while LF treatment achieved maximum increases of 225% and 553%. Generally, all treatments produced more pronounced increases in NaHCO₃–Pi and Po than NH₄Cl–P, with all labile P fractions diminishing toward the end of incubation, but smaller declines were observed for biochar treatments. Overall, the post-incubation levels of NH₄Cl–P and NaHCO₃–Pi ranked as follows: 2BPM > 2BF > 1BPM > 1BF > LPM > LF > PM > F > CK. A similar pattern was observed for NaHCO₃–Po, except PM surpassing LF.

Table 2

Table 2. Soil phosphorus fractions after 3, 30, and 90 days of incubation, as influenced by integrating water hyacinth biochar with organic and synthetic phosphorus fertilizers.

In contrast to its effect on labile P fractions, water hyacinth biochar application significantly (P < 0.05) decreased Al/Fe-bound P extracted with NaOH (Table 2). The 2BPM treatment achieved the greatest reductions in NaOH–Pi across all sampling days, recording a 74% decrease after incubation. PM-based treatments consistently showed higher NaOH–Po than NaOH–Pi. Although less effective than biochar, lime application also reduced Al/Fe-bound P, with LPM and LF lowering the NaOH–Pi by up to 29.8% and 5.51%, respectively. The F treatment produced the highest Al/Fe-fixed P across all sampling days, without significant differences from CK in NaOH–Po. Overall, Al/Fe-bound P increased toward the end of incubation across all treatments, but significantly less in biochar-amended soils. Based on NaOH–Pi, the treatments ranked as follows: F > CK > LF > PM > LPM > 1BF > 1BPM > 2BF > 2BPM (Table 2). For NaOH–Po, the treatments ranked as follows: PM > LPM > F ≈ CK > LF > 1BM > 1BF > 2BPM > 2BF.

Additionally, the applied treatments significantly (P < 0.05) changed the Ca-associated P (HCl–P; Table 2). Biochar amendments produced substantially higher Ca-associated P than those without biochar, with B–PM integrations consistently outperforming equivalent B–F combinations. The 2BPM treatment recorded the highest Ca-associated P at all sampling days, whereas CK exhibited the lowest Ca-associated P, without significant variation from F. LPM also displayed significantly higher Ca-bound P than LF and PM. Overall, the treatments followed the same hierarchy across all sampling time points: 2BPM > 2BF > 1BPM > 1BF > LPM > LF > PM > F ≈ CK.

Biochar applications significantly (P < 0.05) decreased the residual P (R–P) compared to those without biochar, with 2% biochar application showing higher residual P compared to 1% (Table 2). First, the B–PM integrations showed significantly higher residual P than the corresponding B–F combinations, but this trend reversed in the later periods. Likewise, LPM exhibited markedly higher residual P than LF in the early stage, although they did not differ significantly after incubation. Both LPM and LF consistently showed lower residual P than CK, F, and PM. Applying F alone did not significantly affect the residual P.

3.3.2 Effects of the amendments on proportions of phosphorus fractions and correlations among fractions and other soil properties

Expressed as the proportion of each P fraction relative to the total P fractions within each treatment after incubation (Figure 4), NH₄Cl–P was the smallest in proportion (0.39%–2.31%), followed by NaHCO₃–Pi (0.44%–4.94%). The F treatment contributed only 0.88% of NH₄Cl–P, 2.10% of NaHCO₃–Pi, and 0.96% of NaHCO₃–Po. In contrast, B–PM and B–F treatments accounted for 1.52%–2.32% of NH₄Cl–P, 4.00%–4.94% of NaHCO₃–Pi, 9.60%–13.0% of NaHCO₃–Po, and 26.5%–36.0% of Ca-bound P. Residual P had the highest content, followed by NaOH–Pi, in the treatments without biochar, with maximum proportions of 42.0% for residual P in CK and 30.5% for NaOH–Pi in F. Conversely, the lowest NaOH–Pi (6.23%) and NaOH–Po (8.70%) were recorded from 2BPM and 2BF, respectively. The decreases in Al/Fe-bound P from CK to 2BPM were accompanied by increases in Ca-associated P. In contrast, increases in labile P and Ca-bound P fractions from CK to 2BPM occurred alongside reductions in residual P. Overall, the efficacy of biochar treatments outperformed that of PM and F alone, as well as their combinations with lime, in both labile and Ca-associated P proportions, whereas the trend was reversed for Al/Fe-bound P and recalcitrant P proportions.

Figure 4

Figure 4. The proportions of soil phosphorus fractions relative to total phosphorus fractions after 90 days of incubation. Treatment abbreviations are the same as in Figure 1.

Linear regression and correlation analyses showed a very strong and significant positive relationship between available P and labile P (r = 0.97, P < 0.0001; Figure 5a and Table 3), and both were very strongly positively associated with soil pH (r = 0.96 and 0.98, respectively, P < 0.001). In contrast, labile P showed a strong negative relationship with the residual P (r = −0.84, P < 0.01; Figure 5b). Residual P also exhibited strong negative associations with soil pH, available P, and Ca-associated P, while being strongly positively correlated with Al/Fe-bound P, Ex. Ac, and Ex. Al³⁺. NaOH–Pi and Po were significantly negatively correlated with Ca-associated P (r = −0.95 and −0.86, P < 0.01), pH (r = −0.97 and −0.83), available P (r = −0.92 and −0.78), and labile P (r = −0.96 and −0.85). Both were markedly positively correlated with Ex. Ac and Ex. Al³⁺ (r = 0.79–0.93, P < 0.01). Moreover, Ca-associated P showed a very strong positive correlation with soil pH (r = 0.99, P < 0.001) while being very strongly negatively correlated with Ex. Ac and Ex. Al³⁺ (r = 0.97, P < 0.001).

Figure 5

Figure 5. Relationships between available and labile phosphorus (a) and residual and labile phosphorus (b) after 3, 30, and 90 days of soil incubation as affected by the integrated application of water hyacinth biochar and lime with poultry manure and synthetic phosphorus fertilizers (138 kg P₂O₅ ha^–1).

Table 3

Table 3. Pearson correlations among available and fractional phosphorus and other soil properties after 90 days of incubation.

4 Discussion

4.1 Soil pH and exchangeable acidity

Biochar ameliorates soil acidity through different reaction mechanisms: first, its alkaline constituents—primarily carbonates and bicarbonates—neutralize soil acidity by consuming acidic H⁺ (Guo et al., 2016); second, negatively charged functional groups on the biochar surface (phenolics, carboxylics, and hydroxyls) precipitate or complex acidic cations, such as Al³⁺ and Fe³⁺ (Lewoyehu et al., 2024; 2025); and finally, ion-exchange reactions between biochar-derived organic anions (e.g., malate and citrate) and Al³⁺/Fe³⁺ in acidic soils release OH⁻ ions into the soil solution, thereby increasing soil pH (Mufwanzala and Dikinya, 2010). Additionally, biochar’s negatively charged functional groups enhance soil CEC, thereby increasing the replacement of acidic H⁺, Al³⁺, and Fe³⁺ from soil exchange sites with basic cations (K⁺, Ca²⁺, and Mg²⁺) supplied by the biochar. Moreover, acidic cations can be adsorbed on the improved soil porosity resulting from biochar application (Glaser and Lehr, 2019).

For the above reasons, the application of alkaline water hyacinth biochar with a strong liming potential, high CEC, and abundant phenolic, carboxylic, and hydroxyl functional groups (Table 1) showed a substantial ameliorative effect on soil acidity, significantly increasing soil pH and reducing Ex. Ac and Ex. Al³⁺ (Figures 1a,b, 2a,b; Supplementary Table S1). The absence of significant differences between the B–PM and B–F treatments in Ex. Ac and Ex. Al³⁺ reflects that biochar’s strong buffering capacity neutralized acidity, regardless of PM, whereas lime’s weaker buffering led to greater variations between LPM and LF. The increased precipitation or complexation of Al³⁺ with more functional groups due to a higher biochar application rate (2%), along with co-precipitation of Al with silicate particles (as KAlSi₃O₈), resulted in the complete elimination of Ex. Al³⁺ in the 2BPM and 2BF treatments.

Conversely, lime lacks functional groups that sustain acidity neutralization, and its transient effect reduces soil acidity only for a short period of time (Lewoyehu et al., 2024; Lewoyehu et al., 2025). Consequently, the LPM and LF treatments resulted in significantly lower pH increases and Ex. Ac and Ex. Al³⁺ reductions than the B–PM and B–F treatments (Figures 1a,b, 2a,b; Supplementary Table S1). Additionally, the modest increases in pH and decreases in Ex. Ac and Ex. Al³⁺ with LPM and LF treatments dissipated quickly during incubation, unlike the more persistent effects observed in the biochar-amended soils. The observed increase in the efficacy of water hyacinth biochar over lime in increasing soil pH and decreasing Ex. Ac and Ex. Al³⁺ aligns with the findings of Mosharrof et al. (2022), where rice husk biochar outperformed dolomite.

Soil acidification from organic acids and acidic H⁺ released during organic matter decomposition in PM caused only slight increases in soil pH and smaller reductions in Ex. Ac and Ex. Al³⁺ when PM was applied alone. However, co-application of PM with water hyacinth biochar mitigated this acidification by enhancing the soil buffering capacity and neutralizing the released H⁺. Consequently, soil pH in the B–PM treatments increased by 0.88–1.47 units, while Ex. Ac and Ex. Al³⁺ decreased by 75%–94% and 92%–100%, respectively, relative to PM alone. This suggests the strong ability of water hyacinth biochar to counteract fertilizer-induced soil acidification. The slight acidification induced by the F-only treatment, attributed to H⁺ from superphosphate and fertilizer-stimulated microbial activity, underscores the inherent acidification effect of synthetic P fertilizers, albeit generally weaker than that of N-based fertilizers (Bolan and Hedley, 2003). The significant reduction in soil Ex. Al³⁺ reported for pig-manure biochar and raw manure (Paz-Ferreiro et al., 2020), attributed to the formation of organo-Al complexes, reinforces the noteworthy reductions in Ex. Al³⁺ in the biochar- and PM-amended soils of our study.

4.2 Available phosphorus

P fixation, which renders more than 80% of synthetic fertilizer P unavailable, is a major limitation in acidic soils that reduces crop yields (Tian et al., 2021). Biochar mitigates this problem by increasing P bioavailability through multiple pathways: 1) its alkalinity elevates soil pH, desorbing Al/Fe-bound phosphate (Kamran et al., 2018); 2) negatively charged functional groups on biochar complex Al³⁺/Fe³⁺ and compete for adsorption sites, reducing P sorption and freeing fixed phosphate (Wang et al., 2012; Jiang et al., 2015); and 3) biochar can directly supply P to soil, with the amount depending on the feedstock type and production conditions (Glaser and Lehr, 2019). Moreover, biochar fosters microbial and enzymatic activities that enhance P cycling (Ghodszad et al., 2021).

Thus, co-application of an alkaline water hyacinth biochar containing substantial P (Table 1) with PM or F markedly enhanced soil-available P (Figure 3; Supplementary Table S1), with indirect effects—increase in pH and reduction in Al³⁺—being more pronounced when compared to those produced by direct P supply. The quicker P release in the B–PM treatments than in PM alone suggests biochar-stimulated microbial mineralization. Readily available P from superphosphate allowed the B–F, LF, and F treatments to peak in available P during the first week (Figure 3). The subsequent decrease in available P in LF and F reflects the rapid conversion of soluble P to insoluble Al/Fe-bound forms at low pH. After 2 weeks, the efficacy of B–PM treatment surpassed that of B–F, highlighting biochar-enhanced P release from PM decomposition. Biochar-induced pH increases further optimized P availability, with 2BPM—having the highest pH and lowest Ex. Al³⁺—maintaining the highest available P until the end of incubation, highlighting the synergistic effects of water hyacinth biochar and P sources. Conversely, F alone led to the lowest available P, after the control, coinciding with its lowest pH and highest Ex. Al³⁺.

The delayed increase in available P in the PM treatment alone indicates that organic fertilizers/soil amendments require time for effective mineralization and should, therefore, be applied well before planting (Figure 3). On the other hand, co-application of PM with biochar accelerated P availability in the B–PM treatments, suggesting that water hyacinth biochar has effectively promoted microbial mineralization of organic matter in PM. Generally, the greater improvements in soil pH and available P, along with the reductions in Ex. Ac and Ex. Al³⁺ under the combined application of water hyacinth biochar and PM, relative to the other treatments, demonstrate that integrating these waste materials can simultaneously achieve both waste valorization and soil acidity amelioration.

Higher available P in the biochar-treated soils than in the lime-treated soils (Figure 3) reflects biochar’s superiority in increasing soil pH, reducing Ex. Al³⁺, and supplying P—an ability that lime lacks—with microbial stimulation further enhancing P release from organic matter decomposition. Our results align with those of Zhang et al. (2022), who reported the superiority of biochar over lime in increasing available P, citing its additional capacity to directly contribute nutrients. Additionally, the enhancement in available P by the integrated use of leaf biochar and P fertilizer, attributed to the deactivation of P-fixing Al³⁺ and Fe³⁺ following biochar application, supports our findings (Zhou et al., 2020). The 102%–260% increase in available P from wood biochar–PM integration, compared with only 97% from PM alone, further corroborates our findings by demonstrating that combining biochar with organic amendments enhances P availability far more effectively than using the amendments alone (Adekiya et al., 2020).

4.3 Soil total and fractional phosphorus

The significant increases in total P observed in the biochar-treated soils, with 2BPM adding 196 mg kg^–1 by the end of incubation, suggest that water biochar application can serve as an important P reserve in soil (Supplementary Table S2). Because PM supplies more P, the B–PM combinations had a higher soil total P content than the corresponding B–F integrations, and PM treatment exceeded the efficacy of that of F. The lack of P contribution by lime resulted in lower total P in the lime-treated soils than in the biochar-treated soils and is similar total P between F and LF and between PM and LPM. This finding aligns with that of Kiflu et al. (2017), who reported that lime application did not affect the total P in Ethiopian acidic soil. Conversely, the 91% increase in soil total P from the co-application of bamboo biochar with organic fertilizer (Zhang et al., 2024) and the 63.4% and 45.6% increases after 7 and 180 days of incubation from bamboo biochar–fertilizer integration (Xu et al., 2024) reinforce the total P increases observed in the biochar–PM and biochar–fertilizer treatments.

4.3.1 Phosphorus fractions

Soil P availability largely hinges on the distribution of P fractions, with Al/Fe-bound P controlling the available P pool in acidic and weathered soils (Zhang et al., 2021). Biochar application can significantly boost labile P by increasing the pH and decreasing Ex. Al³⁺ and P sorption (Kamran et al., 2019). Accordingly, the higher labile P fractions in biochar-treated soils (Table 2; Figure 4) than those without biochar reflect the ability of water hyacinth biochar to saturate soil sorption sites, reduce P fixation, and enhance its availability in the soil solution. Conversely, lime’s limited effect in increasing soil pH, reducing Ex. Al³⁺, and its inability to contribute P led to markedly lower labile P fractions in the lime-treated soils than in the biochar-treated soils.

The first higher NH₄Cl–P and NaHCO₃–Pi levels in the fertilizer-based treatments than in the PM-based treatments reflect the rapid P supply from synthetic fertilizers (Table 2). In contrast, the dominance of PM-based treatments in the later periods explains slower P release from PM. As PM requires time for effective decomposition and nutrient release, the benefits of the biochar such as being a strong acidity buffer and producing microhabitats likely stimulated microbial mineralization, producing markedly higher NH₄Cl–P and NaHCO₃–Pi in B–PM than in PM alone. On the other hand, the consistently higher NaHCO₃–Po in the B–PM, LPM, and PM treatments than in the corresponding fertilizer-based treatments reflects the greater organic P contribution from PM. Additionally, increased NaHCO₃–Po observed at the early stage of incubation in the biochar-amended soils indicates that water hyacinth biochar application stimulated microbial activity and enhanced Po build-up (DeLuca et al., 2015).

The subsequent decrease in NaHCO₃–Po in the later stages across all treatments demonstrates progressive Po mineralization, which correspondingly produced higher NH₄Cl–P and NaHCO₃–Pi in the B–PM, LPM, and PM treatments than in the fertilizer-based counterparts, underscoring their stronger stimulation of Po transformation into more labile P fractions (Table 2). The parallel decrease in labile P with decreasing pH and increasing Ex. Ac and Ex. Al³⁺ reflects increased P fixation. However, the smaller losses in B–PM and B–F highlight the ability of water hyacinth biochar to curb P fixation and sustain its availability, consistent with the observations of de Figueiredo et al. (2020), who noted that biosolid-derived biochar, unlike the P fertilizer, sustained sufficient P for corn over 4 consecutive years. Similarly, Lutfunnahar et al. (2021) observed that manure-derived biochar combined with P fertilizers significantly elevated NH₄Cl–P and NaHCO₃–P relative to the fertilizer alone, with subsequent decreases over time but consistently higher levels with biochar–fertilizer treatments—indicating the patterns observed in B–PM and B–F treatments in our study. Likewise, Kamran et al. (2019) reported that integrating peat and chicken-manure biochars with P fertilizers in acidic soil increased H₂O–P and NaHCO₃–Pi and Po beyond the fertilizer alone, reinforcing the enhanced labile P fractions observed in the biochar-amended soils in our study.

Al/Fe-bound P is the overriding P fraction in acidic soils, indicating an unavailable P fraction chemisorbed to Al/Fe oxides, including Po associated with humic substances (Kiflu et al., 2017; Kamran et al., 2019). By optimizing soil pH, reducing Ex. Al³⁺, and releasing competing organic anions, biochar curtails P precipitation and fixation, thereby enhancing its availability in the soil solution and decreasing Al/Fe-bound P (Khan et al., 2025; Zhang et al., 2025). Accordingly, water hyacinth biochar integrated with PM or fertilizer markedly decreased Al/Fe-bound P relative to PM and F alone and with lime, increasing both labile and Ca-associated P fractions (Table 2; Figure 4). This pattern accords with that observed by Wu et al. (2022), who reported a 75% attenuation of Al/Fe-bound P and a 159%–255% augmentation of bioavailable P following rice-straw biochar application.

The lowest NaOH–Pi in the 2BPM treatment across all sampling days aligned with its highest pH and lowest Ex. Ac and Ex. Al³⁺, whereas the reverse trend was observed for the F treatment (Table 2; Supplementary Table S2). Soil pH below 5.5 increases the solubility of Al compounds, releasing Al³⁺ that precipitates as insoluble AlPO₄, thereby increasing NaOH–P. The strong negative correlations of soil pH with Ex. Al³⁺ and NaOH–P also support this finding (Table 3). Accordingly, the less effectiveness of lime in increasing soil pH and reducing Ex. Al³⁺ led to significantly higher Al/Fe-bound P fractions in the LPM and LF treatments than in the biochar-amended soils (Table 2). The level of NaOH–Po exceeded that of NaOH–Pi in the B–PM, LPM, and PM treatments because of greater Po release from PM, while Pi from superphosphate reversed it in the fertilizer counterparts. The increases in Al/Fe-bound P in the late incubation period—opposite to the trend in labile P—reflect intensified P fixation driven by decreasing pH and increasing Ex. Ac and Al³⁺. However, biochar treatments maintained markedly lower Al/Fe-bound P, underscoring the role of water hyacinth biochar in limiting Al/Fe-bound P formation and preserving P availability, consistent with the patterns reported by Lutfunnahar et al. (2021).

HCl–P represents sparingly available P associated with Ca, while R–P indicates the most recalcitrant P pool (Hedley et al., 1982). Biochar application promotes Ca–P formation by increasing pH, ionic strength, and Ca concentration in the soil solution (Murphy and Stevens, 2010). Accordingly, the augmented Ca-associated P in biochar treatments indicates increased Ca and P association upon the transformation of organic matter (Adhikari et al., 2019), matching both the substantial Ca-associated P content of the biochar itself (3517 mg kg^–1; Table 1) and the pH increases that further enhanced Ca–P association (Table 2; Figure 4). The increase in HCl–P from CK to 2BPM, along with the decrease in NaOH–P, demonstrates biochar-induced transformation of Al/Fe-bound P into Ca-associated P, leading to the highest Ca-bound P in 2BPM across all sampling days (Table 2). Similarly, LPM exceeded LF, PM, and F in Ca-associated P due to its higher pH and Ca release from lime and PM. In contrast, the F treatment with the lowest pH had a negligible effect on Ca-associated P and did not differ significantly from the control because the interaction of Ca and P is poor at low pH.

Because soil pH, even with biochar, remained below 7, the association between Ca and phosphate might not be strong enough to form stable calcium phosphate [Ca₃(PO₄)₂] precipitates, keeping Ca-associated P in the biochar-treated soils potentially plant-available—consistent with the findings of Khan et al. (2025), who classified diluted-HCl-extractable P as available. Additionally, carboxyl and carbonyl groups in low-temperature-produced biochar reacted with Ca, inhibiting the formation of insoluble P forms (Adhikari et al., 2019). The superior effect of water hyacinth biochar over lime in increasing Ca-associated P aligns with the findings of Zhang et al. (2022), who reported higher Ca-bound P in rice straw biochar-treated soil than in lime-treated soils. Moreover, the rise in Ca-bound P with biochar–fertilizer treatment and the negligible response to F alone align with that reported by Lutfunnahar et al. (2021) and Xu et al. (2016), respectively.

Applying nutrient-rich biochar to soil prevents P fixation and residual formation by sustaining labile P (da Silva Carneiro et al., 2021). Accordingly, biochar-treated soils displayed reduced residual P, with B–PM and LPM treatments inducing less residual P than their fertilizer-based counterparts, although the greater P input from 2% biochar elevated residual P in 2BPM and 2BF relative to 1% application (Figure 4; Table 2). Our findings align with those reported by Mukherjee et al. (2020), who found increased labile P along with decreases in Al/Fe-bound P and residual P with rice residue biochar treatment in acidic soil. Other studies likewise showed that biochar, particularly with P fertilizers, enhanced labile P and decreased recalcitrant P more effectively than fertilizers and lime in acidic and tropical soils (Zhou et al., 2020; Wu et al., 2022; Zhang et al., 2025).

4.3.2 Correlations among different phosphorus fractions, available phosphorus, and other soil properties

The strong negative correlations between HCl–P and NaOH–Pi and Po (Table 3) indicate that biochar-induced pH optimization and Ex. Al³⁺ reduction promoted the transformation of Al/Fe-bound P into Ca-associated forms. This is further supported by HCl–P’s very strong positive and negative correlations with pH and Ex. Al³⁺, respectively. Likewise, the strong positive associations between soil pH and both available P and labile P highlight the role of water hyacinth biochar in enhancing P availability through ameliorating soil acidity. The inverse relationships between labile P and NaOH–P fractions demonstrate P desorption from Al/Fe-bound pools into more accessible forms, which simultaneously increased Ca-bound P and decreased residual P, as evidenced by the negative correlations of residual P with labile P and Ca-associated P (Figure 5b; Table 3). The strong positive relationship between available P and labile P (Figure 5a; Table 3) confirms that labile P fractions are the primary contributors of P availability. This aligns with findings of previous studies showing that biochar application enhanced labile P in soil while reducing non-labile forms through P transformation (Mukherjee et al., 2020; Khan et al., 2025).

5 Conclusion

This study was conducted to evaluate the impact of invasive water hyacinth biochar with organic and synthetic P input integration on soil acidity amelioration and P fraction transformation. Both water hyacinth biochar and lime amendments ameliorated soil acidity, significantly increasing soil pH and reducing Ex. Ac and Ex. Al³⁺, with biochar outperforming lime in efficacy. A 2% biochar integrated with PM and fertilizer eliminated Ex. Al³⁺. Additionally, biochar–PM and biochar–fertilizer treatments markedly enhanced labile P fractions (NH₄Cl–P and NaHCO₃–Pi/Po) and Ca-associated P (HCl–P) while significantly decreasing Al/Fe-bound P fractions (NaOH–Pi/Po) and recalcitrant P (R–P).

Biochar–PM combinations have better efficacy than biochar–fertilizer counterparts, PM and fertilizer alone, and their combinations with lime. This suggests that regions lacking mineable P and experiencing high P deficiency could recycle P from organic wastes to achieve greater independence. The huge amount of phosphate-rich organic waste in developing countries could enable them to meet a significant portion of their P needs and promote economic returns.

Therefore, it can be concluded that valorizing invasive water hyacinth into biochar and applying it with organic and synthetic P fertilizers can be a cohesive strategy for repurposing waste materials to overcome P deficiency in agricultural soils, improve soil fertility, and minimize reliance on costly fertilizer, thereby promoting sustainable agricultural production and effective waste management. The study’s results have important implications for crop production and environmental mitigation. As both PM and water hyacinth biochar are waste materials, the integrated use of these problematic wastes for soil amendment is economically important for farmers. However, as this work was a 90-day laboratory incubation study, the findings currently lack validation from plant growth studies or field trials and cost-effectiveness analysis. Therefore, further field-based studies across various soil types, biochar application rates, and agroecological zones are recommended to identify optimal biochar application rates and to assess the long-lasting effects of water hyacinth biochar on soil P transformation and plant growth. Additionally, meaningful cost-effectiveness analysis—typically based on crop response, agronomic efficiency, and economic returns—is very important.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Author contributions

ML: Writing – review and editing, Conceptualization, Investigation, Writing – original draft, Validation, Formal Analysis, Data curation, Methodology, Visualization. YK: Writing – review and editing. SA: Writing – review and editing, Project administration. AG: Writing – review and editing. TW: Writing – review and editing. DF: Writing – review and editing. SS: Methodology, Validation, Investigation, Resources, Writing – review and editing, Supervision, Funding acquisition, Conceptualization.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This research was supported by the SATREPS EARTH project (Grant Number JPMJSA 2005) funded by the Japan Science and Technology Agency (JST)/Japan International Cooperation Agency (JICA).

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fenvs.2025.1742046/full#supplementary-material

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