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Review

Impacts of Biochar Application on Inorganic Phosphorus Fractions in Agricultural Soils

by
Liwen Lin
1,
Yutao Peng
1,2,
Lin Zhou
2,
Baige Zhang
3,
Qing Chen
2 and
Hao Chen
1,*
1
School of Agriculture and Biotechnology, Shenzhen Campus of Sun Yat-sen University, Shenzhen 518107, China
2
Beijing Key Laboratory of Farmyard Soil Pollution Prevention-Control and Remediation, College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, China
3
Vegetable Research Institute, Guangdong Academy of Agricultural Sciences, Guangzhou 510640, China
*
Author to whom correspondence should be addressed.
Agriculture 2025, 15(1), 103; https://doi.org/10.3390/agriculture15010103
Submission received: 2 December 2024 / Revised: 24 December 2024 / Accepted: 3 January 2025 / Published: 5 January 2025
(This article belongs to the Special Issue Feature Review in Agricultural Soils—Intensification of Soil Health)
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Abstract

:
Inorganic phosphorus (P) is a key component of soil P pools, influencing their availability and mobility. Although studies on biochar’s effect on inorganic P fractions in various soils are growing, a critical review of these findings is lacking. Herein, we conducted a quantitative meta-analysis of 74 peer-reviewed datasets, drawing general conclusions and confirming the absence of publication bias through funnel plot statistics. The results showed that biochars can influence soil inorganic P fractions, with their effects depending on biochar (i.e., feedstock, pyrolysis temperature and time, C:N ratio, pH, ash and P content) and soil-related properties (i.e., pH, texture, P content). Specifically, the addition of biochar significantly enhanced the diverse soil inorganic P fractions and P availability (as indicated by Olsen-P). Only biochars produced from wood residues and having high C/N ratios (>200) did not significantly increase the labile P fractions (water extracted soil phosphorus (H2O-P), Olsen-P, and soil calcium compounds bound phosphorus (Ca2-P)). The application of biochars derived from crop residues significantly increased the soil P associated with iron and aluminum oxides, while there was no significant effect on manure- and wood residue-derived biochars. In addition, applications of low temperature biochars and manure residue-derived biochars could increase the proportions of soil highly stable P. We identified knowledge gaps in biochar production and its potential for soil phosphorus regulation. Due to the complex processes by which biochar affects soils, more systematic evaluations and predictive methods (e.g., modeling, machine learning) are needed to support sustainable agriculture and environmental practices.

1. Introduction

Phosphorus (P) plays a role in the cell development of all organisms as an indispensable element [1]. P supply in soil is essential for natural as well as managed ecosystems because P often limits primary productivity [2,3]. This is due to the fact that only a small fraction (<1%, predominantly as orthophosphate anion form) of the total P in the soil solution is readily available for plant assimilation [3,4,5]. To meet the plant’s P requirements, a variety of P-containing materials have been used as soil amendments to improve soil P availability, among which the use of P fertilizer derived from non-renewable phosphate rock has increased rapidly and has become dominant since the 1950s [6,7,8]. While around 15 million tons of P fertilizer is globally applied to agricultural soils each year [9], less than 30% of the P in fertilizers can be taken up by crops in the year after application [10]. Thus, more efficient P management in agroecosystems is required because P deposits are finite and losses of excess P to natural environments can cause adverse effects such as eutrophication [11] and biodiversity variation [12].
Soil P can be supplied not only by mineral fertilizers but also by organic materials from sources such as agroforestry, the breeding industry, and human habitats. Recycling these P resources has great potential to supplement or replace traditional fertilizers, promoting more sustainable agriculture [9,13]. Among these potential new strategies, biochar has gained significant interest in recent years because of its improvements in P management and many other merits for soil quality enhancement [14,15,16]. Biochar is a carbon-rich material produced through the process of pyrolysis, which involves heating organic biomass—such as crop residues, wood, manure, or other plant materials—in a low-oxygen environment. It is also worth mentioning that as carbon-neutral and environmental-friendly technologies, biochar production and application also meet the trend of using wastes following the concept of “waste to wealth” and are closely related to the themes of the sustainable development goals (SDGs) [17,18].
Numerous studies have demonstrated that biochar has the potential to increase P availability in soils [19,20,21,22,23]. Biochar itself is a relatively P-rich product because of the recovery and enrichment of P during thermochemical treatment and thus shows a similar fertilization effect as slow-release P fertilizer [24]. It was reported that the total P in a biochar produced from animal bones could be 152.0 g kg−1 and that the water-extractable P was 6.6 g kg−1 [25]. On the other hand, soil P availability is controlled by dynamics of various transformation processes (e.g., sorption/desorption, precipitation/dissolution, immobilization/mineralization) among different P forms that biochar application can affect via direct and indirect ways [26,27,28,29,30]. Biochar may release P from Fe and Al oxides, hydroxides, calcium carbonate (CaCO3), and Ca, Fe, and Al phosphates [31]. Phosphates that are bound to free cations like Ca2+, Mg2+, Fe3+, and Al3+ also have great potentials to feed the available P pool via dissolution processes, as stimulated by biochar application [9]. Although the enhancement of the soil available P pool by biochar has been well documented and reviewed [7,9,23,32,33], a better understanding of P dynamics governing P availability under biochar application requires separate investigations into the different forms of P in soils.
Soil P occurs in inorganic and organic forms, both of which are associated with the above-mentioned elements or compounds [28]. Compared to natural soils such as grassland and forest, inorganic P pools and their transformation processes are dominant for P supply in agricultural soils [4]. Thus, the investigation of diverse inorganic P fractions in agricultural soils under biochar applications has received more attention. For example, previous studies have indicated that straw-derived biochar enhances the proportion of labile P in lateritic soils [34,35], while straw biochar promotes the transformation of labile P into moderate soil P in calcareous soil [36]. A similar phenomenon was also found in manure biochar application cases. Troy et al. [37] stated that 10 g kg−1 swine manure biochar can increase the soil Morgan’s P content and total P leaching, but Laird et al. [38] demonstrated that 20 g kg−1 swine manure biochar treatments enhanced moderately stable P proportion and reduced total dissolved P leaching by 69%. The above cases implied that the effect of biochar application on soil inorganic P may vary depending on biochar and soil properties. It is noteworthy that biochars are fabricated from a variety of feedstocks using diverse thermochemical conditions (e.g., duration time, pyrolysis temperature, atmosphere) and have diverse properties (e.g., pH, hydrophilicity, aromaticity, and nutrient content) [39]. Also, soils greatly differ in basic properties (i.e., pH, texture) and native P status, which can substantially mediate the biochar effect on inorganic P fractions.
To date, two meta-analysis articles have revealed biochars’ effects on soil P availability [7,23], while results regarding biochar’s effect on diverse inorganic P fractions in different soils have not been critically reviewed yet. Herein, we propose the scientific question, which is how do biochar properties and feedstocks affect soil fractions and ultimately influence P availability? Therefore, a quantitative review is necessary to fill this knowledge gap. Meta-analysis is an effective tool to achieve general conclusions as it is a statistical method used to systematically integrate the published results. By using this method, with comprehensive data analyses derived from 74 collected articles (incl. 673 independent observations) published from 1980 to 2022, we aim to (i) identify the impacts of biochar on the concentration of P in different P fractions; (ii) explain the relationships among P availability, fraction content, and soil properties; and (iii) reveal the potential mechanisms of soil P fraction transformation processes as affected by biochar application. This research is important for optimizing biochar’s role in improving soil P availability, enhancing nutrient efficiency, and promoting sustainable agricultural practices while reducing environmental impact.

2. Materials and Methods

2.1. Database and Data Collection Criteria

The target publications were collected using the Web of Science (WOS), Google Scholar (GS), and the Chinese National Knowledge Infrastructure (CNKI). Papers published from 1980 to 2022 were included in these databases. The search keywords included soil AND phosph* AND avail* AND *char. The collected publications were further refined by adopting the following categories: (i) articles which set un-amended soil as a control; (ii) articles where all treatments were without P fertilizer addition; (iii) articles which featured agricultural soils only; and (iv) articles where each treatment had at least three replications and the data had standard errors. As a result, in total, 74 articles were collected for further analysis. The software of GetData (version 2.24) was adopted for extracting the data shown in the figure columns. Data on the soil variables measured in these studies (Olsen-P, H2O-P, Al-P, Fe-P, Ca2-P, Ca8-P, Ca10-P, Residual P (R-P)) were obtained from publications and included mean and standard errors of all treatments. “Olsen-P” refers to NaHCO3 extractable P. Olsen-P primarily represents available P, which refers to P in the soil that is loosely bound to soil particles and readily available for plant uptake. The Olsen-P test extracts calcium-bound P (Ca-P), aluminum-bound P (Al-P), and some iron-bound P (Fe-P), which are active forms of inorganic P that can be easily taken up by plants [36]. For labile P, “H2O-P” refers to water-soluble P; “Ca2-P” refers to P bound to calcium in a low-oxidation state. For moderate labile P, “Al-P” refers to P bound to aluminum; “Fe-P” refers to P bound to iron; and “Ca8-P” refers to P bound to calcium in a medium-oxidation state. For highly stable P, “Ca10-P” refers to P bound to calcium in a high-oxidation state, and “Residual P (R-P)” refers to P that is not easily extracted by the methods listed above [40]. Specifically, a total of 6 P fractions representing low stable soil P (H2O-P, Ca2-P), moderately stable soil P (Al-P, Fe-P, Ca8-P), and highly stable soil P (Ca10-P, R-P) were categorized in this meta-analysis. Besides the soil P availability and diverse P fraction contents, the soil properties (i.e., soil texture, pH, Olsen-P), experimental duration, and biochar properties (i.e., pyrolysis duration, pyrolysis temperature, feedstock type, C/N ratio, pH, ash content, biochar application rate and duration) were carefully recorded in the meta-analysis database. While general biochar properties, such as feedstock type and pyrolysis conditions, were reported across the studies, specific chemical composition data were not available for biochar types. Notably, the P quantities in this meta-analysis refer to the P contents in the soils amended with(out) biochar. P changes reflect both the original soil content and the P added through biochar, capturing the overall impact of biochar on P levels. Its effect on P dynamics is assessed by considering both soil and biochar-derived P.
Data were normalized to the same units for comparison. By using the bulk density of the soil and the depth of the soil where biochar was applied, the data of biochar application rate were converted into mass percentage content (%). The subgroups were grouped as follows. The classification criteria used were primarily based on previous meta-analyses (e.g., Glaser et al. [7]; Yuan et al. [39]; Jeffery et al. [41]; Gao et al. [42]) and adapted to align with the dataset obtained in this study. The biochar raw materials were (1) crop residue; (2) manure residue; and (3) wood residue. The pyrolysis temperatures were (1) <400 °C; (2) 400–500 °C; and (3) >500 °C. The biochar C:N was (1) <50; (2) 50–200; and (3) >200. The soil pH was (1) <5; (2) 5–7; and (3) >7. The application rate was (1) <1%; (2) 1–2.5%; and (3) >2.5%. The experiment’s duration was (1) <3 month; (2) 3 months to 2 years; and (3) >2 years. The pyrolysis time was (1) <1 h; (2) 1 h–2 h; and (3) >2 h. The biochar pH was (1) <6; (2) 6–8; and (3) >8. The biochar ash content was (1) <15%; (2) 5–30%; and (3) >30%. The soil Olsen-P was (1) <9 ppm; (2) 9–40 ppm; and (3) >40 ppm. The soil texture was (1) coarse (sandy loam, sandy clay loam, loam); (2) medium (clay loam, loam, silty clay loam, silty or silty loam); and (3) fine (clay, silty clay, sandy clay).

2.2. Meta-Analysis

The response ratio (RR) was employed to appraise the impacts of biochar amendment on P availability and diverse soil P fractions’ content, which is the mean of the biochar-treated soil divided by the mean of the control group without biochar. In addition, to avoid the poor statistical properties of ratios, transformation of RR into the natural log of the response ratio (ln(RR)) was performed as
l n R R = l n X E X C
where XE and XC represent the means of the treatment (with biochar application) and control (without biochar application) groups, respectively.
The effect size was converted to %Change by using the following equation [39]:
% C h a n g e = e l n R R − 1 × 100 %
The effect sizes and 95% confidence intervals (CIs) were calculated using the Metawin 2.1 software of all the categorical groups with the random effect model. In total, 9999 iterations were performed in Resampling tests [41]. We assessed the potential publication bias and the stability of our meta-analysis results using the Egger test and fail-safe N test, as detailed in Table S1 [39,43]. The importance and interactions of the variables explaining the changes in each soil P fraction (shown in Tables S3–S10) were calculated using the boosted regression tree (BRT) model, implemented with the ‘gbm’ package in R Studio version 1.2.5042 [44].

3. Results and Discussion

3.1. General Trend

Overall, biochar application remarkably enhanced the content of diverse soil inorganic P fractions, except R-P, Ca8-P, and Ca10-P (Figure 1). As R-P, Ca8-P, and Ca10-P are relatively stable forms of P in soil, as they usually combine with calcium to form insoluble minerals, they are less susceptible to biochar addition in the short term. Biochar may affect more water-soluble P and metal oxides loosely bound P [30,31]. To garner deeper insights into critical factors influencing soil availability and fractions, BRT model results were used to processes the In RR data (Table 1). The results implied that biochar P content and soil pH were important factors influencing the soil Olsen-P and H2O-P, respectively. Further, in terms of moderate labile soil P, Al-P was mainly influenced by application duration, while Fe-P and Ca2-P were greatly influenced by pyrolysis time and biochar pH, respectively. Lastly, pyrolysis temperature, biochar pH, and soil pH were the most important factors inducing changes in stable P (Ca8-P, Ca10-P, and R-P, respectively).
In addition, the results of funnel plot statistics showed that there was generally no publication bias, suggesting that the results were not distorted by selective reporting. Additionally, the fail-safe numbers greater than 5N + 10 for all subgroups further verified the robustness and reliability of the meta-analysis (Table S1) [43].
Specifically, the percentage (%) of H2O-P was significantly improved (Figure 1), as biochar has certain alkalinity and can introduce anions such as hydroxide and chloride into soil to compete with P on soil P adsorption sites [35]. On the other hand, the activity of P-soluble microorganisms (e.g., Lysinobacteria) enhanced by biochar application could promote the conversion of organic P to water-soluble phosphate [6,35]. As shown in Figure 1, the percentage change in moderately stable soil P (Al-P, Fe-P, Ca8-P) and highly stable soil P (Ca10-P, R-P) was smaller than that of more labile fractions like H2O-P and Olsen-P, with the change generally decreasing as the stability of the P fraction increased. Olsen-P is the most common indicator of soil P availability. Our results show that biochar significantly enhances soil P availability, consistent with previous meta-analyses [23,34].
Although some studies have focused on biochar amendments used to improve plant available P content [36,45], current research has emphasized the potential transformations among inorganic P fractions. Biochar affects soil P both directly and indirectly. Directly, it influences P availability through its nutrient content, which may result from mineral changes during production (e.g., the formation of white lockite) and be gradually released into the soil. Indirectly, biochar can alter soil properties like pH, cation exchange capacity (CEC), and metal concentrations, which in turn affect P availability. However, these impact processes could be implicated depending on experimental conditions including biochar and soil properties and experimental durations (Figure 2, Table S2), which will be further discussed in the following sections.

3.2. Biochar Properties

3.2.1. Feedstock

Our results showed that wood residue-derived biochar, compared to crop and manure residue-derived biochars, produced no significant improvement in H2O-P and Olsen-P (Figure 2). This may be attributed to the lower ash content and higher specific surface area of wood residue-derived biochar, meaning that it can exhibit more adsorption and fixation capacities of water-soluble phosphate [33]. Among these three types of biochar feedstocks, manure residue-derived biochar enhanced H2O-P and Olsen-P to the greatest extent. This may be due to the high P content of manure, which can be retained in biochar during the pyrolysis process [46]. Other studies have also reported that, compared to cotton straw- and corncob-derived biochars, chicken manure biochar has a significantly greater effect on improving soil H2O-P and Olsen-P content [47,48]. Wang et al. [49] pointed out that manure-derived biochar itself contains a large amount of soluble P, and the application of 1% pig manure biochar can increase soil Olsen-P by 1.89 times. Furthermore, they found that the content of Olsen-P gradually increases during the 30-day cultivation period, which is related to the release of P from biochar.
Crop residue-derived biochar significantly improved Al-P and Fe-P, likely due to its higher content of inorganic elements like Al, Fe, and Mg, which promote the formation of these compounds in soil [50,51,52]. It was also found that all types of biochar can significantly improve residue P (R-P) content, with the most obvious improvement effect of the manure residue-derived biochar (Figure 2). This could be attributed to the relatively high P concentration and more sites for P sorption within the manure residue-derived biochar. Further, the application of manure residue-derived biochars may stimulate microbial P fixations in soil, as they could provide favorable conditions of pH and nutrients, as well as more habitat positions for microorganisms [53,54].
The above results indicate that the feedstock indeed has great impacts on soil P fractions. This would happen even within a certain feedstock type. For instance, Han et al. [55] showed that biochar produced from soybean pods and straw, but not corncob biochar, could significantly improve soil H2O-P content, because corncob biochar has a larger specific surface area and stronger anion exchange performance. Given the diversity of organic waste and biochar’s impact on soil P, future biochar production for agriculture should balance both economic and ecological benefits.

3.2.2. Pyrolysis Temperature and Duration

Biochar pyrolyzed at relatively low and medium temperatures (<500 °C) was more conducive to improving soil H2O-P, and medium temperature (400–500 °C) biochar had the best improvement effect on Olsen-P (Figure 2). Since P volatilizes around 700 °C and biochar is typically produced at lower temperatures, the P contained in biochar remains similar to its original feedstock [56]. It is noteworthy that these P transformation processes under different pyrolysis temperatures are also time-dependent (Figure 2). Our results showed that as pyrolysis time increased, the percentage of soil Al-bound P decreased, while soil H2O-P and R-P increased. This suggests that the transformation processes of P contained in biochar may occur between different pools, and these processes are dependent on pyrolysis temperature and time. Our results revealed that biochar prepared at a relatively low temperature can enhance soil available P more effectively, which may be because relatively stable P compounds were formed from other ‘labile’ P fractions at higher pyrolysis temperatures. In addition, biochar fabricated at high pyrolysis temperatures may exhibit high ion-binding strength through chemical adsorption, which may immobilize nutrients into unusable forms [57]. Biochars produced at low to medium temperatures have more oxygen-containing functional groups (e.g., hydroxyl, carboxyl, carbonyl) and higher cation exchange capacity (CEC), making it easier for them to release P into the soil’s available P pool [55,58]. Our study emphasizes a significant role of the pyrolysis conditions of biochar in mediating P availability in soil since they greatly affect biochar properties and in turn affect soil P pools and dynamics after application via direct and indirect pathways.

3.2.3. C/N Ratio

Biochar with a lower C/N ratio showed a more profound improvement effect on soil H2O-P and Olsen-P, which may be related to the unstable C and relatively sufficient N supply within biochar (Figure 2). Biochars with more available C and N compounds are beneficial to soil microbe growth. Many studies have pointed out that the soil P solubilizing bacteria (e.g., Pseudomonas and Bacillus species) can be improved after biochar addition [35,59]. Biochars with a higher C/N ratio are usually produced at a higher temperature and have higher contents of mineral compounds, which would strengthen P immobilization and therefore have greater potentials for decreasing soil P availability [55,59]. In contrast, biochars with lower C/N ratios, often derived from non-woody materials (e.g., manure, straw), contain relatively higher soluble N and P [60].

3.2.4. pH

Biochar with alkaline pH (>8) had a greater effect in improving H2O-P and Olsen-P than biochar with acidic (<6) and neutral pHs (6–8). Most biochars are alkaline, as the surface acidic groups (e.g., carboxyl, hydroxyl, and phenolic groups) will decrease and the surface basic groups (e.g., lactones) will increase with increasing pyrolysis temperature [61]. In addition, mineral elements such as Na, K, Mg, and Ca remain as oxides or carbonates during biochar production [62]. Zhou et al. [63] produced sawdust biochar at 300 °C and 600 °C, resulting in a pH of 4.05 and 7.96, respectively. They applied the same dosage of biochar to soil (pH 4.34) and found similar increases in available P at 7 days. However, after 80 days, the available P in the alkaline biochar treatment was significantly lower than in the acid biochar treatment. This may be due to the alkaline biochar promoting P precipitation by minerals and/or P fixation by microorganisms in the acidic soil.

3.2.5. Ash Content

The high-ash-content biochar showed significantly higher improvement effects on Al- and Fe-bound P, as well as Olsen-P (Figure 2). In addition to phosphate, the ash in biochar mainly consists of minerals such as sulfate, silicate, and chloride [64]. Thus, the ash content in biochar may serve as a direct contributor to the availability of P; otherwise, the remaining P would have high potential to be bound to Al- and Fe-containing minerals, as indicated by the results of the present meta-analysis. Further, wood residue-derived biochar usually has a low ash content. It was reported that the ash content of corn straw and rice husk biochars is about 40%, which is much higher than the ash content of pine biochar (11.6%) [65]. Thus, high-ash-content biochar has a similar effect on inorganic P fractions as biochars pyrolyzed from cellulose-rich materials, rather than lignin.

3.2.6. Biochar Application Rate and Duration

Biochar application rate can generally magnify the effects of other biochar properties mentioned above on soil inorganic P fractions. For example, Ippolito et al. [66] conducted a 6-month incubation experiment by applying switchgrass biochar (pH 5.8) to a soil with a pH of 7.6, and the soil pH decreased by 0.2–0.4 units and the soil available P, as well as the soil microbial growth, increased with increasing biochar application rate. The biochar application improved the availability of Olsen-P, which decreased over time. In contrast, stable P fractions (e.g., Al-P, Fe-P, Ca2-P, Ca8-P) increased from short-term (<3 months) to medium-term (3 months–2 years), suggesting that precipitation and sorption processes may occur over time. It was evidenced in another study that biochar application increased the contents of Ca2-P and Al-P in soil after one rice season [67]. A longer application duration may strengthen the biochar effect, which was supported by findings that 5-year biochar application increased the contents of Ca2-P, Ca8-P, Al-P, Fe-P, and Ca10-P by 1.92 times, 2.61 times,1.87 times, and 0.43 times, respectively [68]. In addition to the time effect, it is noteworthy that agronomy activities such as planting, irrigation, fertilization, and tillage may also substantially affect the soil P dynamics and fractions.

3.3. Soil Properties

Soil properties can substantially mediate the biochar effect on P fractions. It was found that soils with neutral pH showed higher soil water-soluble P content after biochar application than alkaline and acid soils. This is probably due to the fact that high alkalinity stimulates P fixation by metal oxides [42,69]. In addition, the precipitation of Ca-P in alkaline soils could form series of products that reduce P solubility, resulting in low P availability. Under relatively acidic conditions, the P newly introduced by biochar application may be adsorbed on soil minerals or precipitate with aluminum and iron oxides [34]. There is a significant difference in soil P retention in acidic and alkaline soils amended with biochar as a P carrier [8,70]. Also, biochar can mediate other physicochemical and biological processes that affect P availability in soils with different pHs. For instance, the competition between dissolved organic matter from biochar and P in soil solution at soil P adsorption sites will vary with soil pH. On the one hand, biochar can induce changes in soil enzyme activities and/or microbial population dynamics that are sensitive to soil pH change [71]. These together suggest that biochar should be used cautiously to retain P in soils with different acidity. It is also noteworthy that there are exceptions to this situation. For example, in fertile soils, when the soil is already saturated for P adsorption sites, biochar application can cause the release of P from the fertile soil, thereby rapidly increasing soil P availability [35]. While studying the pH effects of biochar on soil P, it is necessary to consider this in conjunction with other relevant factors (e.g., soil P adsorption saturation degree, soil and biochar characteristics, and the duration of biochar in soil).
The application of biochar in fine-textured soils can significantly increase Olsen-P content by promoting aggregate formation, enhancing P adsorption and fixation, and reducing the risk of P loss [72]. The introduction of biochar can improve the stability of soil aggregates and increase the content of large aggregates in soil [73].

3.4. Soil P Dynamics Influenced by Biochar

Previous studies have proved that most P is present as orthophosphate, pyrophosphate, and hydroxyapatite (Ca10-P) [74]. Studies have noted that biochar application could commonly induce a rapid increase in the soil labile P and Ca10-P, which is in line with our results (Figure 1). However, the less significant increase in moderate labile P (Fe/Al/Ca8-P) is probably induced by indirect methods after biochar application. Manure-derived biochars are particularly effective at enhancing soil available P fractions such as H2O-P and Olsen-P, owing to their high intrinsic P content [20,37]. This biochar type also fosters microbial activity that can facilitate P mineralization and solubilization [46]. In contrast, wood-derived biochars, characterized by their low ash content, large surface area, and higher porosity, exhibit superior P adsorption capacity [36]. Crop residue-derived biochars contribute to improvements in aluminum-bound (Al-P) and iron-bound (Fe-P) P fractions, due to their relatively high inorganic element content, which interacts with soil minerals to stabilize P [35].
Pyrolysis conditions further modulate biochar’s effects. Low to medium pyrolysis temperatures (<500 °C) produce biochars rich in oxygen-containing functional groups, higher cation exchange capacity (CEC), and soluble P, which enhance soil P availability [33,37]. Conversely, biochars generated at higher temperatures (>500 °C) result in the formation of more stable and less soluble P forms, favoring long-term P sequestration but potentially reducing short-term availability [35]. Biochars with lower C/N ratios release more soluble nutrients, promoting microbial growth and enzymatic activity, which facilitate P solubilization [27]. The biochar pH also plays a critical role. Alkaline biochars (>pH 8) can increase H2O-P and facilitate P release in neutral to acidic soils but may cause P precipitation or immobilization over time in highly acidic soils [51].
High-ash-content biochars contribute to the retention of P in mineral-bound forms, such as calcium phosphates, particularly in alkaline soil environments [35,36]. Application rates and durations also influence these effects. In the short term, biochar addition tends to increase soil available P by enhancing desorption and solubilization processes. Over time, however, the dynamics shift toward the stabilization of P in forms that are less prone to leaching, improving P retention [52]. The interplay between biochar and soil properties, such as soil pH, texture, and mineral composition, further dictates the outcomes. For example, in sandy soils with low P buffering capacity, biochar can significantly improve P retention, while in clay-rich soils, its effects may be moderated by the native P dynamics and existing mineral interactions [30,31].

4. Future Research Recommendations

The following aspects warrant attention in future research and field applications: (1) the endogenous P contents of biochar produced from manure and sludge residues were higher than that of other biochars, and attention should be given to the potential risks associated with the soil P in manure or sludge biochar-amended agricultural soils. However, studies evaluating P fractions and mobility in soils amended with these biochars are not sufficient yet. (2) The duration of biochar application in soil could significantly affect soil P forms, availability, and mobility through indirect ways. In addition, some reports have claimed that applying biochar to soil is risky because it contains some harmful substances such as heavy metals, organophosphorus, and inorganic pollutants, although the concentrations of these toxins are usually below the permitted levels included in the regulations of various organizations. Therefore, the effect of biochar duration/aging time on soil P fractions and mobility after application into different soils requires more systematic investigations. (3) The physicochemical properties of biochar are highly different depending on feedstock and pyrolysis conditions, and these series of properties play joint roles in biochar’s effects on soil P fractions and dynamics. More comprehensive methods such as model simulations and machine learning, together with verification of field experiments, should be used for better understanding and practice.

5. Conclusions

From the current quantitative meta-analysis results, it can be seen that biochar properties, such as feedstock type, pyrolysis temperature, C:N ratio, pH, and ash content, influence soil P fractions by interacting with soil minerals and organic matter. Biochars from different feedstocks (e.g., wood, crop residues, manure) can affect the availability of labile P (like H2O-P, Olsen-P) and more stable forms (such as P bound to iron, aluminum, or calcium (hydr)oxides) in varying ways. High C:N ratio biochars, for instance, may have limited impacts on labile P fractions, while biochars from crop residues can enhance P associated with iron and aluminum oxides. In summary, the application of biochar has shown an overall increase in the proportion of various P fractions, with the most significant enhancement observed in the active P fraction. However, only slight improvements in the moderately stable and stable P fractions were found. The effects of biochar on the enhancement and transformation of P fractions are driven by a combination of biological and abiotic processes. Further systematic and in-depth research is needed to better understand these mechanisms.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agriculture15010103/s1. Table S1: The results of publication bias used in the present study; Table S2. Summary of the averaged relative change (%) in diverse soil fractions in response to biochar addition. Significance of Wilcoxon signed rank tests: * p < 0.05, no symbol following the number indicates not statistically significant. N/A indicates data not available. The numbers in brackets represent the 95% confidence intervals (CI); Table S3: Pairwise interactions of Olsen-P modeled using the boosted regression tree (BRT) approach; Table S4: Pairwise interactions of H2O-P modeled using the boosted regression tree (BRT) approach; Table S5 Pairwise interactions of Fe-P modeled using the boosted regression tree (BRT) approach; Table S6: Pairwise interactions of Al-P modeled using the boosted regression tree (BRT) approach; Table S7: Pairwise interactions of Ca2-P modeled using the boosted regression tree (BRT) approach; Table S8: Pairwise interactions of Ca8-P modeled using the boosted regression tree (BRT) approach; Table S9: Pairwise interactions of Ca10-P modeled using the boosted regression tree (BRT) approach; Table S10: Pairwise interactions of R-P modeled using the boosted regression tree (BRT) approach.

Author Contributions

Conceptualization, H.C. and L.L.; methodology, L.L., Y.P., L.Z., B.Z. and Q.C.; validation, L.L.; investigation, L.L. and Y.P.; resources, Y.P.; data curation, Y.P.; writing—original draft preparation, L.L., Y.P. and H.C.; writing—review and editing, L.L., L.Z., B.Z., Q.C. and H.C.; supervision, H.C.; project administration, L.L.; funding acquisition, Y.P. and H.C. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the National Natural Science Foundation of China (No. 42007047, 42207015), post-doctorate research funding to Shenzhen (szbo202207, szbo202323), the modern agricultural innovation center, and the Henan Institute of Sun Yat-sen University (N2021-003, N2021-002).

Data Availability Statement

All data generated or analyzed during this study are included in this published article [and its supplementary information files].

Conflicts of Interest

The authors declare no conflicts of interest.

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Agriculture 15 00103 g001
Figure 1. Grand mean of all cases for diverse inorganic P fractions when biochar was applied regardless of experimental conditions.
Figure 1. Grand mean of all cases for diverse inorganic P fractions when biochar was applied regardless of experimental conditions.
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Figure 2. Effect of explanatory variables on content of Olsen-P (a), H2O-P (b), Al-P (c), Fe-P (d), Ca2-P (e), Ca8-P (f), Ca10-P (g) and R-P (h) in soil. Symbols indicate the mean % change in effect size with 95% confidence interval. The number after the name of group indicates the amount of pairwise comparison. The red dotted line indicates the zero line. Orange, blue, and grey dots represent the positive, negative, and no significant effects.
Figure 2. Effect of explanatory variables on content of Olsen-P (a), H2O-P (b), Al-P (c), Fe-P (d), Ca2-P (e), Ca8-P (f), Ca10-P (g) and R-P (h) in soil. Symbols indicate the mean % change in effect size with 95% confidence interval. The number after the name of group indicates the amount of pairwise comparison. The red dotted line indicates the zero line. Orange, blue, and grey dots represent the positive, negative, and no significant effects.
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Table 1. Contribution rate (%) of explanatory variables in the boosted regression tree (BRT) model for explaining the variation in soil P fractions.
Table 1. Contribution rate (%) of explanatory variables in the boosted regression tree (BRT) model for explaining the variation in soil P fractions.
Explanatory VariableOlsen-PH2O-PAl-PFe-PCa2-PCa8-PCa10-PR-P
Feedstock0.480.000.040.000.090.432.060.00
Pyrolysis temperature10.33.940.6618.90.0321.81.540.00
Pyrolysis time6.000.690.6617.51.346.382.380.00
Biochar C:N ratio4.296.815.749.865.576.315.070.18
Biochar pH7.597.687.111.6930.49.5918.10.07
Biochar ash2.640.000.0012.20.003.090.040.00
Application rate8.398.427.4623.90.0810.815.20.25
Application duration6.7817.618.00.780.127.458.890.14
Biochar total P19.813.610.90.690.040.0010.20.13
Biochar Olsen-P0.001.952.392.104.120.0017.80.02
Soil pH12.722.726.84.6258.314.710.267.7
Soil Olsen-P18.57.646.872.100.0016.63.290.08
Soil texture2.428.989.970.000.002.715.1731.3
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Lin, L.; Peng, Y.; Zhou, L.; Zhang, B.; Chen, Q.; Chen, H. Impacts of Biochar Application on Inorganic Phosphorus Fractions in Agricultural Soils. Agriculture 2025, 15, 103. https://doi.org/10.3390/agriculture15010103

AMA Style

Lin L, Peng Y, Zhou L, Zhang B, Chen Q, Chen H. Impacts of Biochar Application on Inorganic Phosphorus Fractions in Agricultural Soils. Agriculture. 2025; 15(1):103. https://doi.org/10.3390/agriculture15010103

Chicago/Turabian Style

Lin, Liwen, Yutao Peng, Lin Zhou, Baige Zhang, Qing Chen, and Hao Chen. 2025. "Impacts of Biochar Application on Inorganic Phosphorus Fractions in Agricultural Soils" Agriculture 15, no. 1: 103. https://doi.org/10.3390/agriculture15010103

APA Style

Lin, L., Peng, Y., Zhou, L., Zhang, B., Chen, Q., & Chen, H. (2025). Impacts of Biochar Application on Inorganic Phosphorus Fractions in Agricultural Soils. Agriculture, 15(1), 103. https://doi.org/10.3390/agriculture15010103

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