1 Introduction

Phosphorus (P) is an essential element for plant growth and metabolism (Wang et al. 2014; Zhang et al. 2016). However, the low solubility of phosphates in soils makes it difficult for plants to use phosphates directly or indirectly (Ma et al. 2016; Shen et al. 2011). The deficiency of available P in soil has become one of the most important factors of limiting crop yields, especially in tropical and subtropical areas (Zhang et al. 2016; Blake et al. 2000). In order to cope with P deficiency in soil, it is necessary to activate P and provide available P for plant uptake. In recent years, the application of biochar has been considered as an effective amendment to provide the P nutrient and increase the availability of P in soil (Santos et al. 2019; Schneider and Haderlein 2016; Li et al. 2020). 

Biochar is a product of thermal combustion of biomass feedstocks under anaerobic conditions at relatively low temperatures (300–700 °C) (Yang et al. 2020). The crop straw, as the largest agricultural biomass in subtropical area, has a huge potential as feedstocks for biochar production. The phosphorus in biomass feedstock almost completely retains in biochar in the pyrolysis process. Kim et al. (2018) found that the feedstocks with abundant P could generate the P-rich biochar. Gaskin et al. (2008) found that the mineral contents in biochar produced from different feedstocks increased in the order of poultry litters > herbs > woody plants. Therefore, the content of P in feedstock largely determines the content of P in biochar. In addition, the pyrolysis temperature of feedstock is another important parameter of affecting the nutrient contents of biochar (de Figueiredo et al. 2020; Hong and Lu 2018; Zhang et al. 2016). Previous studies indicated that the content of total P in the biochar increased with the pyrolysis temperature (Sun et al. 2018; Ahmad et al. 2012). When the pyrolysis temperature is above 700–800 °C, the P in biochar will lose and stabilize as insoluble forms. Besides, the duration time and heating rate of pyrolysis also affect the content and availability of P in biochar (Adhikari et al. 2019). Biochar is expected to act as a supply of available P when adding to the soil. To better manage the P nutrient in biochar, it is necessary to understand how the pyrolysis temperature affects the content and availability of P in biochar. 

The addition of biochar contained a large amount of available P that supplies the exogenous P to the soil directly (Eduah et al. 2019; Dai et al. 2017). However, Yang et al. (2020) found that the increase of available P in the soil was mainly due to the endogenous P, which was activated by the addition of biochar. Biochar can increase the available P content in soil by promoting the dissolution of phosphate, which results from the change of soil chemical properties (Glaser et al. 2002; Chintala et al. 2014; Santos et al. 2019). It is concluded that the change of P availability in the biochar-amended soil is a comprehensive result of the increase of soil pH value, the increase of P saturation, and the strong interaction between P and biochar (Peng et al. 2021a; Morales et al. 2013). The effect of biochar on soil P availability varies with biochar feedstock, pyrolysis temperature, and soil types. A meta-analysis showed that the averaged relative change of soil available P with the addition of biochar with different feedstocks followed the order of the manure residue > crop residue > wood residue (Gao et al. 2019). Eduah et al. (2019) reported that biochar produced at 300–450 °C could effectively improve the bioavailability of P, especially in acidic soil. In acidic soils, the addition of biochar enhances P availability by reducing the adsorption of P on iron and aluminum oxides, which is because the increased pH induced by biochar leads to the precipitation of Fe3+ and Al3+ ions (Gérard 2016; Chintala et al. 2014; Hong and Lu 2018; Solaiman et al. 2019; Oladele et al. 2019). On the contrary, the addition of biochar increases the adsorption of P and reduces P availability in neutral or alkaline soils, which is due to a large number of cations (such as calcium and magnesium) that can complex with phosphate (Gustafsson et al. 2012; DeLuca et al. 2015; Harrington and Humphreys 2004).

Although there have been many studies on the content of P in biochars and their effect on the availability of P in soils, these results are not consistent (Yang et al. 2019; Peng et al. 2021b). For example, Parvage et al. (2013) found that the addition of wheat residue char had no effect, or even reduced the content of available P in soil. Thus, a more systematic research is needed to reveal how pyrolysis temperature and feedstock affect the availability of P in biochars and biochar-amended soils. In our study, the biochars were produced from two biomass feedstocks (rice straw and canola stalk) under five pyrolysis temperatures (250, 350, 450, 550, and 650 °C). The obtained biochars were applied to two soils (Ultisol and Alfisol) to investigate the effect of biochar on the availability of P in soil. The SEM–EDS observation was used to reveal the element distribution and binding state of biochar and soil mineral particles. The objectives of this study are to: (1) examine how the pyrolysis temperature and feedstock affect the availability of P in biochar, (2) compare the effects of biochar derived from different pyrolysis temperatures and feedstocks on the availability of P in biochar-amended soils, and (3) reveal the mechanisms of the influence of biochar on the availability of P in soil.

2 Materials and methods

2.1 Experimental materials

Rice straw (RS) and canola stalk (CS), the largest agricultural biomass in the subtropical region of China, were collected from Yueqing, Zhejiang Province, China. Rice straw was collected in November 2017 and canola stalk in May 2018. Both straws were cleaned with deionized water, dried at 105 °C for 24 h, and ground to pass a 2 mm sieve. Two soils (Ultisol and Alfisol) used in this study are the most representative agricultural soils in China. They were collected from the surface (0 ~ 20 cm) of soil. Ultisol was collected from the low-hilly region located in Yuyao (N29°45.6′, E121°07.2′), and Alfisol from paddy soil located in Dongyang, Zhejiang province (N29°17.4′, E120°14.4′), China. The basic properties of straws and soils are given in Table 1. 

Table 1 Basic physicochemical properties of soil, straw, and biochars produced at different pyrolysis temperatures
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2.2 Biochar preparation and incubation experiment

Two straws (RS and CS) were placed in a separate porcelain crucible and covered tightly with a lid. The porcelain crucible was sealed with tin foil to keep out oxygen. Rice straw biochars (RB) and canola stalk biochars (CB) were produced under an anaerobic condition in a muffle furnace at 250, 350, 450, 550, and 650 °C, respectively. The heating rate was 10 °C min−1, and after reaching the target temperature, samples were kept in the furnace for 2 h. Then the biochar was weighed to calculate the production yield and finely ground to pass 2-mm and 0.15-mm sieves, respectively. RB and CB produced at pyrolysis temperatures of 250, 350, 450, 550, and 650 °C were labeled as RB250, RB350, RB450, RB550, RB650, CB250, CB350, CB450, CB550, and CB650, respectively. 

The soils were air-dried at room temperature and then ground to pass a 2-mm sieve. The straws and biochars were thoroughly mixed with soils (Ultisol and Alfisol) with a proportion of 1% (w/w). Soils without straw or biochar were used as the control (CK). Each treatment was replicated three times. The straw/biochar-amended soils were packed into polyethylene cups with a bulk density of 1.2 g cm−3 by artificial compaction. The mixtures were wetted up to field water capacity using deionized water and incubated at constant temperature (25 °C). Soils were kept at constant moisture during the whole incubation process by adjusting the weight. After 90 days’ incubation, soils were air-dried and ground to pass 2-mm and 0.15-mm sieves for further analyses.

2.3 Biochar and soil analyses

The pH of biochar and soil was determined by a pH meter (PB-21, Sartorius) with the suspension ratio of 1:20 and 1:2.5, respectively. Total organic carbon (TOC) was determined by oxidation method with potassium dichromate. Total potassium (TK) was determined by digesting biochar with HClO4-HF solution and measured with a flame spectrophotometer (Bao 2000). The elemental composition (C, H, N, and S) of biochar and soil was determined, using an elemental analyzer (Elementar Analysensysteme GmbH, Germany). Around 1 mg of biochar samples was ground with 100 mg KBr for the preparation of pellets, and the IR spectra of samples were recorded in a wavenumber range of 4000–400 cm−1 by the Fourier-transform infrared spectroscopy (FTIR) (Shimadzu Ltd., Japan). The band assignments in the infrared spectra were done according to the literature (Peng et al. 2011; Uchimiya et al. 2011). The X-ray diffraction (XRD) measurements were made using an X-ray diffractometer (Rigaku Corporation, Japan). The X-ray patterns were recorded in the range of 2θ = 10–90° at a scan rate of 3° min−1, with a generating voltage of 40 kV and a current of 40 mA. The morphology and chemical composition of samples were determined by using the scanning electron microscope (SEM) (SIRION-100; FEI, The Netherlands) with a dispersive spectrometer (EDS) (EDAX Co., USA), with the beam energy of 15 kV, a spot size of 3 mm, and a dwell time of 100 ms. The contents of oxygenic functional groups on the surface of biochar were determined by Boehm (1994) method. The Boehm titration method was used to conduct a qualitative and quantitative analysis of oxides according to the possibility of reaction between alkaline and acidic surface oxides with different strengths. Soil exchangeable acidity was extracted with KCl solution and determined by titration with NaOH (Bao 2000). Soil cation exchangeable capacity (CEC) was measured by ammonium acetate method, and the exchangeable base by atomic absorption spectrophotometer (exchangeable Ca2+ and Mg2+) and a flame photometer (exchangeable K+ and Na+) (Bao 2000). The basic physicochemical properties of soil, straw, and biomass charcoal are shown in Table 1.

2.4 Phosphorus measurements of biochar and soil

The total P (TP) content of biochar and soil was determined, using the molybdate-ascorbic acid procedure after digesting samples with HClO4-HF solution (Murphy and Riley 1962). The available P in biochar was extracted with deionized water, 0.5 M H2SO4 and 0.5 M NaHCO3 (pH = 8.5) solution, respectively. Water-extractable P was readily plant-available P. The NaHCO3-extractable P (Olsen-P) included water-extractable P and labile P adsorbed on the surface of Fe and Al oxides or hydroxides or calcium carbonate. The sulfuric acid extracted readily plant-available P and available P in Ca, Fe, and Al phosphates (Mehlich 1978). The extractable P was determined by the phosphomolybdate colorimetric method. The P fractionation procedure modified from Hedley et al. (1982) was used to identify soil inorganic phosphates with different solubilities. In brief, soil samples were sequentially extracted by each of the following extractants: 1.0 M NH4Cl for the loosely bound phosphate (NH4Cl-P), 0.5 M NH4F (pH 8.2) for the aluminum phosphate (Al-P), 0.1 M NaOH and 0.1 M Na2CO3 for the iron phosphate (Fe–P), and 0.5 M H2SO4 for the calcium phosphate (Ca-P). The concentrations of P in the extracts were immediately determined by the phosphomolybdate colorimetric method. All the extractions were repeated three times. The average data and standard deviations were reported.

2.5 Data analysis

SPSS 17.0 (SPSS, Inc., Chicago, IL, USA) was used for the statistical analysis of data. A one-way analysis of variance (ANOVA) was carried out to determine significant differences between treatments, using LSD’s test with a significant level of p < 0.05. The Jupyter Lab of Python IDE was used for correlation characterization.

3 Results

3.1 Characterization of biochars

The characteristics of biochars produced at different pyrolysis temperatures are shown in Tables 1 and 2. Higher pyrolysis temperature resulted in higher pH in biochar, with a decreasing trend of production yield and H content. The CB had higher contents of C, N, and S than RB. The highest TOC was observed in biochar produced at 450 °C. The total oxygenic functional groups increased with the pyrolysis temperature, which was 12.17 and 30.97 mmol g−1 for RB and CB at 650 °C, respectively. The content of acidic functional group decreased with the pyrolysis temperature, while the content of alkaline functional group increased. The pyrolysis temperature had no significant variation of carboxyl group in RB, while it affected the carboxyl group in CB significantly. Biochars produced at the temperature of 250 and 350 °C had more phenolic hydroxyl than the biochars produced at the relatively higher temperature, even though the content of phenolic hydroxyl was generally low in all the biochars. The spectral properties (FTIR) and X-ray diffraction (XRD) patterns are presented in Figs. S1 and S2. Adsorption intensities at the bands of 3400 and 2900 cm−1 decreased with the pyrolysis temperature, indicating the reduction of O–H and aliphatic C–H bonds. The intensified adsorption at 1400 cm1 indicated the increase of aromatic C. The visible peaks at 3407, 2923, 1630, 1578, 1422, and 1055 cm−1 indicated that the considerable amounts of original organic residues were preserved at 450 °C, whereas most of them disappeared at 650 °C. Some visible peaks at 1000–600 cm−1 could be assigned to the metal–oxygen bonds. The main mineral types of biochar were quartz, sylvite, phosphate, and calcite; in addition to this, the RB included arcanite, while CB included halite. There was a trend for the intensity and peak value to increase with the pyrolysis temperature, implying that the carbonization may form some minerals or change some crystalline phases and mineral compositions. 

Table 2 The change of surface oxygenic functional groups (alkali, acid, carboxyl, and phenol) of biochars (RB and CB) with different pyrolysis temperatures
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3.2 Content of total P and available P in biochars

The effects of pyrolysis temperature and feedstock on the content of total P and available P in biochars are shown in Fig. 1. The amount of total P and Olsen-P in biochars significantly (p < 0.05) increased with the pyrolysis temperature. The contents of total P in RB650 and CB650 were as high as 3.94 and 5.06 g kg−1, and Olsen-P 1.09 and 1.18 g kg−1, respectively. There was no significant difference in the content of Olsen-P between the rice straw and canola stalk biochars. Pyrolysis temperature had no significant effect on the contents of H2O-P and H2SO4-P in the biochar. At the same pyrolysis temperature, the content of total P in CB was higher than that of RB, while the H2O-P exhibited an opposite trend. The RB had higher content of H2SO4-P than the CB at the temperature above 250 °C. Among the tested extract solutions, the amount of H2SO4-extractable P in rice straw biochar was much higher than either H2O-P or Olsen-P, which followed in the order of H2SO4-P > Olsen-P > H2O-P. 

Fig. 1
figure 1

Changes of total P, Olsen-P, H2O-, and H2SO4-extractable P of biochars (RB and CB) with different pyrolysis temperatures. Error bars indicate ± standard deviation. Different letters above error bars indicate significant difference between treatments (p < 0.05, LSD test)

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3.3 The acidity, SOM, and exchange cations in biochar-amended soils

The effects of biochars produced with different pyrolysis temperatures and feedstocks on soil properties are shown in Table 3. Compared with the straw, biochar had a greater impact on the soil acidity, soil organic matter (SOM), and exchange cation composition. The addition of straw and biochar increased the pH of the soil. For the Ultisol, both RS and RB significantly (p < 0.05) increased the pH by 0.23–0.37 units, and CS and CB produced at the pyrolysis of 250–450 °C significantly increased the soil pH. On the contrary, the addition of straw and biochar significantly (p < 0.05) decreased the exchangeable acid content. For the Alfisol, the addition of biochar had no significant effect on the exchangeable acid content because of its relatively high organic matter. The original SOM values of the Ultisol and Alfisol were 8.25 and 22.77 g kg−1, which significantly (p < 0.05) increased by 67.7–173% and 36.6–63.9%, with the addition of straws and biochars, respectively. The content of SOM in soils treated with biochars was higher than the straw-treated soils. The soils treated with biochar produced at the temperature of 550 °C had the largest content of organic matter. The content of exchangeable cations in Ultisol increased with the pyrolysis temperature between 250 and 550 °C. The addition of biochar significantly increased the CEC of both soils. The largest increases of CEC in two soils were observed in the CB450 treatment, which increased by 42.0% for Ultisol and 48.1% for Alfisol, respectively. The largest contents of exchangeable Ca2+ and Na+ were also found in the Ultisol treated with the canola stalk biochar, produced at the temperature of 450 °C. On the other hand, the effect of biochar on soil CEC also depended on soil types (Table 3). The CB increased the CEC of Ultisol more than the Alfisol, while RB increased the exchangeable Ca2+ and Mg2+ of the Alfisol more than the Ultisol. 

Table 3 Effects of biochar produced at different pyrolysis temperatures on the acidity, SOM, and cation exchange performance of two soils (Ultisol and Alfisol)
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3.4 Content of available P in biochar-amended soils

The effects of biochars prepared at different pyrolysis temperatures on the total and available P in soils are shown in Fig. 2. The content of total P in biochar-amended soils was significantly higher than the control. The soils treated with the RB450 and CB550 had much higher values of total P, which were 0.74 and 0.67 g kg−1 for the Ultisol and 1.37 and 1.57 g kg−1 for the Alfisol, respectively. The contents of available P extracted by different extraction methods followed the order of Olsen-P > H2SO4-P > H2O-P, which were higher in the Alfisol than the Ultisol. Biochar-amended soils had higher content of available P than the straw-amended soils. All biochar treatments significantly increased the content of Olsen-P in the Ultisol. The maximum contents of Olsen-P in the Alfisol were observed in the RB450 and CB450 treatments, respectively. The RB450 and CB450 treatments increased the content of Olsen-P by 115 and 110% for Ultisol and 149 and 170% for Alfisol, respectively, compared to the control. All treatments had no significant impact on the H2O-extrable P content of the Ultisol, but significantly increased the H2O-extractable P content of the Alfisol. The H2O-extractable P contents of the Alfisol were 2.12 and 1.73 mg kg−1 under RB450 and CB450 treatments, increasing by 105% and 68.0%, respectively. The effect of biochar treatments on the H2SO4-extractable content of two soils was consistent with the change of Olsen-P. The soils treated with RB450 and CB450 had the maximum content of H2SO4-extractable P, which increased by 94.5% and 110% for Ultisol and 18.4% and 17.7% for Alfisol, respectively, compared to the control.

Fig. 2
figure 2

The total P, Olsen-P, H2O-, and H2SO4-extractable P of two soils (Ultisol and Alfisol) amended with biochars produced in the different pyrolysis temperatures. Error bars indicate ± standard deviation. Different letters above error bars indicate significant difference between treatments (p < 0.05, LSD test)

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3.5 Inorganic P in biochar-amended soils

The effects of biochars prepared at different pyrolysis temperatures on the soil inorganic P are shown in Fig. 3. The contents of various inorganic P in soils followed the order of Ca-P > Fe–P > Al-P. The biochar-amended soils had more inorganic P than straw-treated soils. However, the effect of biochar on the inorganic P in the soils varied with the pyrolysis temperature and the feedstock type. The rice straw biochar significantly (p < 0.05) increased the contents of Al-P and Fe–P, while it decreased the content of Ca-P in the Ultisol. The CB450 treatment significantly increased the contents of Al-P and Fe–P, while it had no significant effect on the content of Ca-P in the Ultisol. For the biochar-amended Alfisol, only RB450 and CB450 treatments significantly increased the contents of Al-P and Ca-P. Under RB450 and CB450 treatments, the content of Al-P increased by 124% and 190% in the Ultisol and 55.2% and 42.4% in the Alfisol, and the content of Fe–P increased by 61.4% and 72.1% in the Ultisol and 23.8% and 16.2% in the Alfisol, respectively, compared to the control. RB450 and CB450 treatments increased the Ca-P contents in the Alfisol by 26.5% and 32.5%, respectively. Therefore, the RB450 treatment had the largest effect on increasing the contents of Al-P and Fe–P in the Alfisol, while the CB450 treatment had the largest effect on increasing Al-P and Fe–P in the Ultisol and Ca-P in the Alfisol.

Fig. 3
figure 3

The content of inorganic P in the soils (Ultisol and Alfisol) amended with biochars produced in different pyrolysis temperatures. Error bars indicate ± standard deviation. Different letters above error bars indicate significant difference between treatments (p < 0.05, LSD test)

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3.6 SEM–EDS observation on biochar-amended soils

Our results indicated that biochar made at 450 °C had the largest effect on the availability of P and inorganic P forms in soils. Thus, the soils treated by RB450 and CB450 were observed and their SEM images and EDS spectra are shown in Fig. 4. SEM images (A, B, C, and D) of biochar-treated soils showed that soil mineral particles were not only attached to the surface of biochar (sites b, e, h, and k), but also entered into the pores of biochar (sites e, f, i, and l). The EDS maps of biochar-treated soils showed that biochar was almost composed of C element, while soil mineral particles were mainly composed of Si and Al elements, indicating the distribution and binding state of biochar and soil particles. The characterization of P on biochar (site j) indicated that the P contained in biochar could be released into the soil directly. Meanwhile, fine mineral particles could be inserted into the pores of biochar and produced aggregation effect in the biochar-amended soils. These interactions promoted the release and conversion of P in both biochar and soil mineral particles. 

Fig. 4
figure 4

SEM images and EDS maps of soils amended with biochars produced at pyrolysis temperature of 450 °C. A Ultisol + RB; B Ultisol + CB; C Alfisol RB; D Alfisol CB. The lowercase letters “a” to “l” represent the site of EDS 

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4 Discussion

4.1 Effect of pyrolysis temperature on P availability in biochar

Our results indicated that the pyrolysis temperature and feedstock type, as the important factors in determining the potential application of biochar, could largely affect the contents of total P and available P in biochar. On the whole, the total P and Olsen-P in biochar were positively correlated with the pyrolysis temperature due to the C volatilization and P preservation. The higher temperature yielded the greater decomposition of biomass structure, which resulted in higher residual phosphate (Figueiredo et al. 2018). The correlation analysis showed that there were significant positive correlations between pyrolysis temperatures and total-P (R2 = 0.86, p < 0.01) and Olsen-P (R2 = 0.95, p < 0.01) of biochar (Table 4). Studies have shown that K and P were lost at above 800 °C, while Mg and Ca at above 1200 °C. Therefore, the enrichment of the base cations and P with the pyrolysis temperature was due to their higher vaporization energy (Dai et al. 2017; Knicker 2007; Zheng et al. 2013). The IR spectra revealed that the functional groups in the biochar decreased with the pyrolysis temperature while the carbonization had an opposite trend due to the loss of oxygen-containing species (Ahmad et al. 2012). The XRD patterns also showed that there were more calcite and phosphate at the higher pyrolysis temperature (Fig. S2). Qian and Jiang (2014) found that P could transfer to available form with the increasing pyrolysis temperature, especially from 400 to 500 °C. Uzoma et al. (2011) reported that the content of Olsen-P in a woody biochar produced at 500 °C was as high as 23 g kg−1, while the Olsen-P in the biochar produced at 300 °C was only 1.2 g kg−1. 

Table 4 Correlation coefficients between biochar properties and soil acidity and P fractionations. Superscripts 1 and 2 represent the biochar and soil properties, respectively. EX, exchangeable. **Correlation is significant at the 0.01 level, *Correlation is significant at the 0.05 level. The lower left corner represents the correlation of Ultisol, and the upper right corner represents the correlation of Alfisol 
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The main form of P in biochar was solid phosphate. Therefore, the release of soluble P in biochar was mainly regulated by the dissolution reaction, which was significantly affected by the pH value of the solution (Liu et al. 2018; Wei et al. 2013; da Silva and Fitzsimmons 2016). The pH of biochar increased with the pyrolysis temperature because of the separation of alkali salt from the biomass structure and the increase of alkaline functional groups on the surface of biochar (Ahmad et al. 2012; Kameyama et al. 2012). In addition, the cations in straw were pyrolyzed to form carbonate or oxide. The pH of biochar played an important role in P complexation, dissolution, and precipitation. The correlations showed that there were significant positive correlations between pH of biochar and total-P (R2 = 0.63, p < 0.05) and Olsen-P (R2 = 0.77, p < 0.01) (Table 4). However, the water- and H2SO4-extractable P had no significant correlations with the total P in the biochar. Among the three extraction methods, Olsen-P extraction was buffered at a pH of 8.5, H2SO4 extraction was strongly acidic, and water extraction depended upon the pH of biochar. The weak correlation between water-extractable P and total P in the biochar could be explained by the variations of pH in biochar. Studies have shown that the phosphate could be adsorbed on the surface of biochar by anion exchange, which was closely related to basic functional groups in biochar. In addition, the surface deposition of phosphate on metal oxides (such as MgO, Al2O3, and CaO) of biochar surface was also a potential mechanism for biochar adsorption of PO43− (Hollister et al. 2013; Wang et al. 2015). The phosphate extracted by water was easily-soluble P in biochar and H2SO4-extractable P was the slow-soluble P, consisting of P binding Ca (mostly), Mg, and other elements. The increase of H2SO4-P in biochar was due to the enhanced contact between Ca, Mg, and P after the conversion of organic matter in straw and then the dissolution of insoluble Ca-P and Mg-P compounds (Cross and Schlesinger 1995). Therefore, straw biochar could function as a reservoir of P and an important resource of releasing available P. The potential available P in biochar could be extracted by NaHCO3-and H2SO4 solutions. 

4.2 Effect of pyrolysis temperature on P availability in biochar-amended soils

The incubation experiment indicated that the application of biochar could effectively affect the P availability in soil, especially the Ultisol. The increased contents of total-P, available P, and inorganic P in biochar-amended soils were partly due to the direct diffusion of available P in biochar, and partly due to the change of soil physical and chemical properties, which further affected the P availability in soils. The addition of biochars was able to bring available P into soils, but the amount and form of available P were dependent on pyrolysis temperature and feedstock type. The result was consistent with many previous studies (Cole et al. 2019; Gao and DeLuca 2018; Hong and Lu 2018). A meta-study on the effect of biochar amendment on the P availability of various soils around the world showed that application of biochar was able to increase plant-available P in neutral (pH 6.5–7.5) and acidic soils (pH < 6.5), while they found no positive effects on P availability in alkaline soils (Glaser and Lehr 2019). The soil pH was considered as one of the major factors in affecting the bioavailability and transformation of P in soil. The application of biochar indirectly changed the P availability by affecting soil pH (Gustafsson et al. 2012). The maximum availability of P in soil was recorded at the pH range of 5.5–7.2. The application of biochar to acidic soil increased the soil pH and then led to the precipitation of polymeric Fe and Al oxides, thereby increasing the availability of P in soil. In addition, the increased pH could dissolve the phosphates bound with Fe3+ and Al3+ ions and released available P (da Silva and Fitzsimmons 2016; Gérard 2016). Morales et al. (2013) also found that biochar reduced the fixed capacity of P in degraded acidic tropical soils. Higher pyrolysis temperature could enhance the availability of phosphate in biochar-amended soils because of the higher biochar pH. The correlations between the properties of biochar and soil showed that there were positive correlations between biochar pH and Olsen-P (R2 = 0.77, p < 0.01) in two soils (Table 4). The soil pH was positively correlated with the Al-P (R2 = 0.75, p < 0.05), Fe–P (R2 = 0.84 p < 0.01) of the Ultisol and the Al-P (R2 = 0.63 p < 0.01) of the Alfisol (Table 4). The soil organic matter (SOM) was another important factor affecting the availability and forms of phosphate in soil. The SOM would provide a good environment for the survival and metabolism of soil microorganisms and promote the activities of P-solubilizing microbes (Xu et al. 2016). The correlation analysis showed that there were positive correlations between SOM and total-P (R2 = 0.65, p < 0.05), Olsen-P (R2 = 0.92, p < 0.01) of Ultisol (Table 4). Biochar had a more noticeable effect on the contents of Al-P and Fe–P in soil, which could be explained by the changes in pH and other chemical properties in acidic soil. The changes of soil chemical environment could affect the binding state of iron, aluminum, and other metals with P and then promote the release of P from precipitates. The OH– had a desorption effect on adsorbed iron P, thus significantly increasing the content of Fe–P (Hass et al. 2012). Thus, the impacts of biochar on P availability in soil were regulated by changes of soil chemical properties, specifically soil pH. 

The pyrolysis temperature of biochar also largely influenced the content of available P and chemical forms of P in biochar-amended soil. The effect of biochar produced at the lower temperature on the availability of P in soil was not obvious, which was due to the low decomposition degree of soluble phosphate. The increasing temperatures could result in higher proportion of available P and pH value in biochar because of the higher decomposition degree of biomass feedstock. Another potential mechanism was related to porous structure of biochar produced at higher temperature. The porous biochar could be the good habitat for microorganism such as P solubilizing bacteria (PSB), and it may be conducive to increase the dissolution of insoluble phosphate in soils (Zhou et al. 2020). On the whole, the effect of biochar on the P availability in soils largely depended on pyrolysis temperature. The biochar produced at the temperature of 450 °C had the largest improvement on the Olsen-P and inorganic P in two soils. The effect of biochar on the availability of P in soils also depended on soil types and biomass feedstocks (Fig. 5). The results showed that the biochar had a more effect on the P availability in acidic soil. The correlation analysis showed that the available P in biochar-amended soils had a better correlation with the available P in the RB than the CB, and biochar exhibited a larger effect on the P availability in the Ultisol than the Alfisol (Fig. 5). The difference may due to the higher acidity and poor nutrients of the Ultisol versus the Alfisol. Therefore, soil types, biochar feedstocks, and pyrolysis temperatures should be considered comprehensively when applying biochar as soil amendments.

Fig. 5
figure 5

Correlation comparison of P availability of biochars and biochar amended soils among different feedstocks and soil types. Superscripts 1 and 2 represent properties of biochars and soils, respectively. PT, pyrolysis temperature; EX, exchangeable. The darker color represents the higher positive correlation (close to 1); the lighter color represents the higher negative correlation (close to -1). Red circles indicate significant correlation at the level of 0.01, while yellow circles indicate at the level of 0.05 

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5 Conclusions

Our results demonstrated that the pyrolysis temperature had a significant effect on the availability of P in biochars and biochar-amended soils. The contents of total P and Olsen-P in biochar increased with the pyrolysis temperature. The canola stalk biochar had higher content of total P than the rice straw biochar. The addition of biochar would improve the availability of P in soils by adding available P directly into the soil. However, the amount and form of available P added were dependent on pyrolysis temperature and feedstock of biochar. Biochar significantly increased the content of total P, Olsen-P, H2O-extractable P (only in Alfisol), and H2SO4-extractable P in soils. The pyrolysis temperature also affected the forms of inorganic P in biochar-treated soils, resulting in the increase of Al-P and Fe–P. The soils treated with biochar produced at the temperature of 450 °C had the largest increase of inorganic P content. The effects of biochar on soil pH and the availability of P were also influenced by soil type. The available P extracted by different extraction solutions in the Alfisol was higher than the Ultisol. The content of available P in biochar-amended soils was significantly correlated with the pH of biochar and biochar-amended soils. On the whole, the biochar produced at the temperature of 450 °C had the largest effect on the availability of P in soils. The selection of biochar in practice should consider the suitable pyrolysis temperature to obtain the better soil improvement.