Biochar had effects on phosphorus sorption and desorption in three soils with differing acidity

Ecological Engineering 62 (2014) 54–60

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Biochar had effects on phosphorus sorption and desorption in three soils with differing acidity Gang Xu a,1 , JunNa Sun a,c,1 , HongBo Shao a,b,∗ , Scott X. Chang a,d a Key Laboratory of Coastal Biology & Bioresources Utilization, Yantai Institute of Coastal Zone Research (YIC), Chinese Academy of Sciences (CAS), Yantai 264003, PR China b Institute for Life Sciences, Qingdao University of Science & Technology(QUST), Qingdao 266042, PR China c Graduate University of Chinese Academy of Sciences (CAS), Beijing 100049, PR China d Department of Renewable Resources, 4-42 Earth Sciences Building, University of Alberta, Edmonton, Alberta, Canada T6G 2E3

a r t i c l e

i n f o

Article history: Received 14 June 2013 Received in revised form 10 October 2013 Accepted 21 October 2013 Keywords: Biochar P availability Sorption Desorption Soil fertility

a b s t r a c t Changes in soil phosphorus (P) availability after biochar application have been reported in a number of glasshouse and field trials. However, the mechanisms underlying these changes remain poorly understood. This study evaluated the effects of four biochar application rates (0, 1%, 5%, and 10%, w/w) on P sorption and desorption in three soil types with different levels of acidity. Results showed that the effects of biochar application on P sorption were highly influenced by soil acidity. As the rate of biochar application increased, P sorption increased in the acidic soil but slightly decreased in the alkaline soil. Desorbed P significantly increased at all levels of biochar application in the studied soils. Inorganic P fractionation revealed that biochar addition sharply increased the Ca-bounded P and slightly enhanced the Al-retained P. However, biochar addition decreased the Fe-bounded P. These changes suggest that the increase in P sorption with biochar addition is attributed to Ca-induced P sorption or precipitation and is less affected by Fe and Al oxides. Biochar application is found to have altered P availability by changing the P sorption and desorption capacities of the soils, and these biochar effects were dependent on soil acidity, which have important implications for improving soil productivity on large scale. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Biochar application can potentially combat climate change by carbon sequestration (Lehmann, 2007a,b; Woolf et al., 2010; Liu et al., 2012) as well as improve soil properties such as nutrient availabilities (Glaser et al., 2002; Sohi et al., 2010). Addition of biochar into soils has reportedly enhanced P bioavailability and plant growth (Lehmann et al., 2003; DeLuca et al., 2009). Biochar application to soils has also increased the extractable P within the soil solution regardless of the temperature used for biochar production. Tryon (1948) reported a significant improvement in available P after biochar application to sandy or loamy soils. However, biochar addition did not always increase soil P availability. In a soil column experiment, biochar application significantly increased P retention in soils and decreased P levels in leachate solutions (Novak et al., 2009). Biochar addition also temporarily reduced the

∗ Corresponding author at: Institute of Life Sciences, Qingdao University of Science & Technology, Qingdao 266042, PR China. Tel.: +86 532 84023984. E-mail address: [email protected] (H. Shao). 1 These authors contributed to the paper equally. 0925-8574/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ecoleng.2013.10.027

available P in two soil types: P availability improved in one soil and did not significantly affect P availability in the other soil with extra P added (Nelson et al., 2011; Sandeep et al., 2013). Thus, the effect of biochar on P availability is generally inconsistent. Notably, the mechanisms underlying the altered P availability with biochar application to soil remain poorly understood (Lehmann, 2007a,b). DeLuca et al. (2009) reviewed a number of possible mechanisms by which biochar may directly or indirectly influence the biotic and abiotic components of the P cycle (DeLuca et al., 2009). Biochar contains a large amount of P; thus, direct release of soluble P may be necessary to enhance P availability, especially for short-term uses (Chan et al., 2007; Atkinson et al., 2010). In addition, biochar application reduces soil acidity and subsequently alters P complexing with metals (Al3+ , Fe3+ , and Ca2+ ), which is important for determining P availability by P sorption and desorption reactions in soils (Wang et al., 2012; Yuan et al., 2011a,b). Biochar can also directly adsorb cations such as Al3+ , Fe3+ , and Ca2+ , resulting in delayed P adsorption or precipitation in soil. Sorption of organic molecules on biochar surfaces can reduce their ability to chelate Al3+ , Fe3+ , and Ca2+ in soil. The direct or indirect influence of biochar on the P cycle in soil has rarely been reported, especially P sorption and desorption after biochar addition to soils

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Table 1 The physical and chemical properties of experimental soils and biochar (soil classification according to USDA soil taxonomy (2010)). Soil/biochar

Soil classification

pH (H2 O)

OC

Sand

Slit

Clay

−1

Inceptisols Inceptisols Alfisols

Exch. Mg −1

g kg Brown soil (BWS) Black soil (BAS) Fluvo-aquic soil (FAS) Biochar

Exch. Ca (cmol kg

TP

Alo

Ald

Feo

Fed

−1

)

mg kg

3.83 5.27

22 53

701 621

279 254

20 125

5.8 8.6

3.6 3.6

581 412

730 955

1528 1657

854 602

5477 8339

8.33 10.4

14 467

172 –

765 –

63 –

20.1 25.4

11.1 4.7

532 2773

400 2796

701 59

744 2048

5849 1051

(Nelson et al., 2011; Sohi et al., 2009). The nature of interactions between the biochar and soil, as well as the mechanism of the increase in P availability, must be elucidated (Atkinson et al., 2010). This study proposed that biochar application increases P availability by decreasing P sorption or increasing P desorption in soil by increasing soil pH and changing the activity or availability of cations (Al3+ , Fe3+ , and Ca2+ ) that interact with P. Batch sorption–desorption experiments were conducted to support our hypothesis. The sorbed P was sequentially fractionated to examine the factors controlling the retention of applied P in these soils. 2. Materials and methods 2.1. Soil and biochar Soils with various pH were collected to a depth of 10 cm on the east side of Shandong Province, China: brown soil (BWS), black soil (BAS), and fluvo-aquic soil (FAS). Two soil samples (BWS and BAS) with relatively low pH were collected from a 10-year old cycleplanted wheat and peanut plantation in Yantai (121◦ 26 E, 37◦ 28 N). The other soil (FAS) with higher pH was sampled from grassland (Suaeda salsa) in a newly formed wetland in the Yellow River Delta (118◦ 07 E, 36◦ 55 N). The physical and chemical properties of the soils are described in detail in Table 1. In general, BWS and BAS were sandy loam inceptisols with relatively higher organic matter content and available P. In comparison, FAS is loam to silt loam with low organic matter content and available P but high exchangeable Ca and Mg (Table 1). The biochar was produced by Sanli New Energy Company, Henan, China. It was made from wheat straw at 350–550 ◦ C in a vertical kiln made of refractory bricks in Sanli New Energy Company, Henan, China. The physical and chemical properties of soil and biochar are shown in Table 1. Infrared spectra (FTIR) was measured using 2 mg grounded sample in a KBr pellet on a FTIR-4100 (Japan Jasco) by scanning from 4000 to 400 reciprocal centimeters, averaging 10 scans at 1 cm−1 intervals with a resolution of 4 cm−1 . A simple method was used to separate biochar pieces from soil for scanning electron micrographs (SEM) and coupled with energy dispersive X-ray (EDX) analysis. For our study, the biochar was ground to pass through a 0.15 mm sieve, and mixed uniformly with the soil. Biochar application rates were set at 10, 50 and 100 g kg−1 of soil weight.

molybdenum blue method (Murphy and Riley, 1962). The sorbed P (Ps) was determined as the difference between the initial amount of P added and the amount in the equilibrium solution. All of the sorption data presented as arithmetic means in the present study. The P sorption data were fitted to the Langmuir equation as follows: S=

KL Sm C 1 + KL C

where C is the concentration of P remaining in solution after the 24 h equilibrium (mg L−1 ), S is the total amount of P sorbed (mg kg−1 ), Sm is the sorption maximum (mg kg−1 ), KL is a constant related to the binding strength (Lair et al., 2009). The maximum P buffer capacity (MBC) of the soil was calculated from the product of Langmuir constants Sm and KL . MBC gives an index of the resistance to change in P concentration of the soil solution of the labile solid phase (Lair et al., 2009). As for the desorption experiment, one more batch of replicated samples were processed as described above. After sorption, three levels of P (20, 100, and 240 mg L−1 ) samples were sacrificed to desorption experiment. After removing the supernatants of the sorption experiment, each tube was weighed to estimate the volume of the residual solution and account for P entrapped in that solution. Then the residuals were mixed with 20 ml of 0.01 M KCl solution for P desorption. The tubes were shaken for 24 h, centrifuged and the supernatants were determined for P content. The desorption process was repeated 3 times and the amount of P remaining in the soil was determined each time. 2.3. Inorganic P sequential fractionation In order to study the mechanisms accounting for the altered P sorption with biochar application, inorganic P retained by Al, Fe and Ca were fractionated after the soil samples were treated with a 240 mg L−1 P solution. The sequential fractionation was preformed according to the methods described by Jiang and Gu (1989). In this procedure, triplicated soil samples were extracted step by step using (1) 0.25 M NaHCO3 , (2) 0.5 M CH3 COONH4 , (3) 0.5 M NH4 F,

2.2. P sorption and desorption on soils or soil–biochar Duplicate 2-g soil or soil–biochar mixture samples were placed in 50 ml centrifuge tubes and suspended in 20 ml of 0.01 mol L−1 KCl solution containing 0, 20, 40, 60, 80, 100, 120, 160, and 240 mg L−1 P added as KH2 PO4 . Two drops of chloroform were added to the samples to inhibit microbial growth. Equilibration was done in an end-over end shaker for 24 h at room temperature. The samples were centrifuged at 3620 g for 10 min, filtered through a 0.45-mm membrane filter, and analyzed for P on Tu-1810 Spectrophotometer (PERSEE, Beijing, China), using the ascorbic acid

Fig. 1. Scanning electron micrographs image for the biochar in the present study.

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Fig. 2. Infrared spectroscopy for the biochar in the present study.

Table 2 P sorption parameters of the isotherms described by Langmuir equation. Soil

BWS

BAS

FAS

Biochar application rate (%)

0 1 5 10 0 1 5 10 0 1 5 10

pH

Estimated by Langmuir equation

3.83 5.13 7.39 7.92 5.27 6.2 7.66 8.01 8.24 8.33 8.54 8.66

KL (L mg−1 )

Sm (mg kg−1 )

MBC (L kg−1 )

R2

0.042 0.069 0.024 0.015 0.055 0.074 0.047 0.046 0.14 0.15 0.12 0.1

400 333 625 769 476 454 625 713 714 667 667 667

16.8 23.0 15.0 11.5 26.2 33.6 29.4 32.8 100.0 100.0 80.0 66.7

0.97 0.99 0.93 0.98 0.99 0.99 0.97 0.99 0.99 0.99 0.99 0.99

Table 3 Amount of P sorbed and desorbed relative to control with biochar application at three P loadings of 20,100 and 240 mg L−1 . soil

Biochar application rates (%)

P load = 20 mg L−1 Psorbed (mg kg

BWS

BAS

FAS

1 5 10 1 5 10 1 5 10

−1

3.0 9.0 19.0 5.0 19.0 16.0 0.0 −5.0 −13.0

)

P load = 100 mg L−1 Pdesorbed

Psorbed

(mg kg

(mg kg

7.2 18.9 35.7 0.7 1.2 6.6 2.4 6.7 13.2

−1

)

−1

0.0 78.0 101.0 19.0 74.0 117.0 −14.0 −48.0 −64.0

)

P load = 240 mg L−1 Pdesorbed

Psorbed

(mg kg

(mg kg

−1

0.0 31.0 63.9 −8.4 −9.3 2.5 −1.6 11.0 23.0

)

−1

13 194 194 0.0 170.0 219.0 −32.5 −65.1 −73.3

Pdesorbed )

(mg kg

0.0 51.1 82.5 −2.9 7.9 24.6 −5.1 6.4 27.7

−1

)

G. Xu et al. / Ecological Engineering 62 (2014) 54–60

800

(4) 0.1 M NaOH, and (5) 0.5 M H2 SO4 to extract Ca2 -P, Ca8 -P, Al-P, Fe-P, and Ca10 -P, respectively.

3.2. Phosphorus sorption The P sorption data of each sample can be described by the Langmuir equation (r > 0.98, P < 0.01). As shown in Fig. 3, P sorption was enhanced by the increasing initial P concentration in all samples, with or without biochar. At lower concentrations of added P (<40 mg L−1 ), the sorbed P did not vary significantly with biochar addition. However, when P concentration was increased (>60 mg L−1 ), the sorbed P became more sensitive to biochar addition, especially at higher biochar application rates of 5% and 10%. The comparison suggested that the influence of biochar application on P sorption is more pronounced at high P loadings and high rates of biochar addition. Furthermore, the responses of P sorption to biochar addition seem highly dependent on the initial level of soil acidity. As shown in Fig. 3, the adsorbed P increases as the biochar application rate rises in the more acidic soil (pH < 7). However, P sorption declined with the addition of biochar at low P dose in the BAS. As shown in Table 2, the Langmuir sorption maximum (Sm ) significantly increases from 476.2 mg kg−1 to 713.3 mg kg−1 in the BAS as the biochar application rate increases from 0 to 10%. Meanwhile, the value increases from 400 mg kg−1 to 769.2 mg kg−1 in the BWS. Previous studies suggest that the increase in P sorption is due to the precipitation of exchangeable Al as new highly active Padsorbing surfaces in soils or possible co-precipitation with Al and Fe oxides when soil pH is increased (Haynes, 1982; Agbenin, 1994). Studies also indicate that exchangeable Ca significantly affects P sorption because of Ca precipitation or co-sorption with the added P (Agbenin, 1995). In addition, biochar application increases the ionic strength and Ca concentration in soil solutions, which can increase P adsorption (Murphy and Stevens, 2010). In general, increased P sorption results from the changes in Al and Fe oxides or the increase in Ca with biochar application (Xu et al., 2013). The inorganic P fractionation should provide additional information, as discussed in Section 3.4. In contrast, FAS showed a relatively small increase in pH and a decrease in P adsorption with biochar application. The biochar addition decreased the Sm values from 714.3 mg kg−1 for soil alone to 666.7 mg kg−1 with 10% biochar rate. The decreases in P sorption with biochar application may be influenced by pH (Haynes, 1982; Westermann, 1992). As pH increases,

P Sorbed (mg kg -1 soil)

Generally, the biochar had C and N contents of 46.7 and 0.6%, respectively, a total ash content of 20.8%, and a pH (H2O) of 10.4. In addition, the biochar contained 1% Ca, 0.3% P, 0.6% Mg, 0.4% Fe and 2.6% K (Table 1). Fig. 1 presents the SEM images of the biochar. As seen in Fig. 1, the biochar were composed of irregular forms and sizes of the particles which had very coarse and heterogeneous surfaces. The images also showed hollow channels of various diameters that originated from tracheid cells that are parallel to the axis of the wheat straw. These structures may be important for the high internal surface area and adsorption ability as an excellent absorbent. The IR spectra (Fig. 2) demonstrated many bands at 3432 cm−1 (O H), 2923 cm−1 and 2854 cm−1 (C H), 1627 cm−1 (C C), 1420 cm−1 (aliphatic and aromatic groups), and 1080 cm−1 (C O). The band at 875 cm−1 was attributed to aromatic C H outof-plane bending, which indicates the greater degree of aromaticity of this sample.

400

200

0

-200

0

50

100

150

200

250

P in Equilibrium Soluon (mg L -1)

Soil(no biochar)

1%CA

5%CA

10%CA

Biochar alone

800

BAS

P Sorbed (mg kg -1 soil)

3.1. Biochar characterization

BWS

600

600

400

200

0

0

50

100

150

200

250

50

100

150

200

250

P in Equilibrium Soluon (mg L -1)

800

FAS P Sorbed (mg kg -1 soil)

3. Results and discussion

57

600

400

200

0

0

P in Equilibrium Soluon (mg L -1) Fig. 3. Phosphorus sorption isotherms for brown, black and fluvo-aquic soils. Biochar (CA); brown soil (BWS); black soil (BAS); fluvo-aquic soil (FAS)

the soil surface becomes more negatively charged, thus increasing anion repulsion and decreasing P sorption (Murphy and Stevens, 2010). Higher pH depresses the formation of HP04 2− , which is preferentially adsorbed by soil colloids (Haynes, 1982). The bonding energy (KL ) increased at 1% biochar rate and then decreased at 5% and 10% of biochar in all soils. The results indicate that a lower dose of biochar amendments fixed the adsorbed P more strongly but did not easily desorb it. By contrast, relatively higher

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G. Xu et al. / Ecological Engineering 62 (2014) 54–60

Fig. 4. Amount of P sorbed on soils and three desorption steps (Des 1, 2, 3, respectively) with 0.01 M KCl when the samples were loaded with solutions contained 20, 100, and 240 mg P L−1 , respectively. The error bars indicate standard deviation. Different letters above the same columns with same color indicate significant difference at P < 0.05.

biochar application rates decrease the binding intensity, suggesting that more adsorbed P may be easily desorbed. The maximum buffer capacity (MBC) of P values decreased with biochar amendment except at a lower dose (1%) in BWS and FAS. In the BAS, the relatively higher values of MBC with biochar addition indicate that higher P is required when biochar is added to the soil. In the present study, biochar cannot independently sorb P because it solubilizes P at all additional P concentrations, as shown in Fig. 3. This observation is consistent with a previous report on biochar derived from sugar beet tailing, which shows very low P adsorption capacity (Yao et al., 2011). Evidence indicates that the cation exchange capacity of biochar is markedly higher than its anion exchange capacity (Silber et al., 2010; Mukherjee et al., 2011). The difference suggests that biochar surfaces are mainly characterized by negatively charged functional groups and attracted cations instead of anions such as phosphate (PO4 3− ) or nitrate (NO3 − ) in amended soils (Cheng et al., 2008; Mukherjee et al., 2011).

average percentage of desorbed P over lower P loads (20 mg L−1 ) were 36%, 37%, 39%, and 41% for the 0, 1%, 5%, and 10% biochar rates after three successive extractions. By contrast, more than 60% of the newly sorbed P was released into the solution at higher P loads (100 and 240 mg L−1 ). The enhanced P desorption indicates high mobility and leaching potential of the newly sorbed P in these soils (Singh and Gilkes, 1991). Meanwhile, desorbed P increased with the introduction of biochar to the soils. The desorbed P seemed more sensitive to biochar addition in BWS than those in BAS and FAS. This difference may be due to the significant changes in binding energy in BWS because the values (KL ) first increased from 0.042 L mg−1 in the soil alone to 0.069 L mg−1 at 1% biochar rate and then decreased to 0.024 and 0.015 L mg−1 at 5% and 10% of biochar rates, respectively. The decrease in binding energy suggests higher P desorption. The decrease in binding energy is attributed to the increase in pH with biochar application.

3.3. Phosphorus desorption

3.4. Phosphorus availability with biochar application

The amount and percentage of P desorbed were influenced by the rate of P and biochar application. As shown in Fig. 4, an increased amount of P is desorbed as P addition increases. In the BAS, the

Table 3 contrasts the sorbed and the desorbed P with biochar application at 20, 100, and 240 mg L−1 P addition into the samples. P availability with biochar application is controlled by comparing

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the amount of sorbed and desorbed P relative to the control. In BWS, P sorption depended on P loading; P sorption decreased at a lower P loading and increased at a relatively higher P loading. In addition, biochar application enhanced P desorption with P loading. These observations indicate that biochar application increased P availability at lower P loadings and decreased P availability at higher P loadings in BWS (Table 3). BAS showed an increase in P sorption and desorption. The increase in P sorption was generally greater than the increase in P desorption, suggesting that less P was soluble with biochar application in BAS. FAS exhibited a significant decrease in P sorption and an increase in P desorption with biochar application, suggesting that more P was available when biochar and fresh P was added into the soil. These results indicate that biochar addition increased P availability in alkaline soils, whereas their effect on P availability in acidic soils depended on the amount of the added P. Earlier studies reported that biochar addition increased P availability even without fresh P addition (DeLuca et al., 2009). This increase could be due to the direct release of soluble P from the biochar because of the substantial amount of P in the biochar (DeLuca et al., 2009). In most studies, biochar and fertilizer were simultaneously applied into the soil, but the effect of biochar application on P availability when fresh P is added into the soil has thus far been rarely investigated. 3.5. Inorganic phosphorus fractionations after adsorption with 240 mg L−1 P solution Biochar application increased P sorption in BAS and FAS but decreased P sorption in FAS. The increase or decrease in the soil capacity to retain P is mainly attributed to changes in Fe, Al, and Ca content (Lair et al., 2009). Consequently, altered P sorption may be due to the changes in inorganic P fractions in the soil. Our results demonstrated that biochar application significantly affected the concentrations of inorganic P fractions. Biochar application increased the Ca-bounded P (e.g., Ca2 -P, Ca8 -P, and Ca10 -P) and Al-retained P (slightly) but decreased the Fe-bounded P, especially in BWS and BAS (Fig. 5). In contrast, FAS, which had the highest soil pH and exchangeable Ca concentration, showed almost no changes in Ca-bounded P but slightly decreased the Fe-retained P (Fig. 5). The abundance of crystalline and non-crystalline Fe and Al oxides as well as hydroxides and their organic matter complexes influences P sorption in many soil types (Singh and Gilkes, 1991; Agbenin, 2003). Biochar application is shown to increase Fe as well as Al oxides and hydroxides in the soil (Table 1). Amorphous polymeric Fe and Al cations first precipitate as soil pH increases with biochar application (Murphy and Stevens, 2010; Yuan et al., 2011a,b). In addition to its effects on the soil pH, biochar also directly provides a substantial amount of Fe and Al oxides. However, the abundance of free Fe and Al oxides is reduced with biochar application (unpublished data). The reduction may be caused by the sorption of these oxides onto the biochar surface (unpublished data). More studies suggest that biochar particles are enclosed with mineral fractions to prevent soil decomposition (Brodowski et al., 2005, 2006; Cusack et al., 2012). Consequently, Fe- and Al-retained P was not markedly affected by biochar addition (Fig. 5). The results suggest that precipitation of Fe and Al oxides or phosphates at higher pH levels with biochar application was less significant to P sorption. An increase in exchangeable Ca and Mg was found after biochar application to soil. Ca-induced P sorption or precipitation is another possible mechanism underlying the increase in P sorption after biochar application. These findings agree with the increase in P sorption when soil pH is increased in savanna soils. The increase in P sorption is attributed to the chemistry and retention of Ca rather

Fig. 5. Inorganic P fractionations after sorbing with 240 mg L−1 P solution for soils of brown, black and fluvo-aquic soils.

than the hydrolytic reactions of Al (Agbenin, 1995; Soil Survey Staff, 2010; Zhang et al., 2010; Cui et al., 2011). 4. Conclusions More P was sorbed onto acidic soils as biochar application rates were increased, whereas a contrasting trend was observed in alkaline soils. These behaviors suggest that the increase in P sorption was related to the chemical retardation of Ca rather than the hydrolytic reaction of Al and Fe oxides. Biochar could not

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independently sorb the added P. However, substantial Fe3+ , Al3+ , and Ca2+ contents in the biochar were observed. This result indicates that biochar affected P availability by interaction with other organic and inorganic components in the soil, such as organic matter or other base cations in the soil. These aspects deserve further investigation. In addition, biochar contains a large amount of P. Therefore, the main mechanism underlying the enhancement of P availability with biochar application must be determined. Future investigations should focus on the relative contributions to P availability of desorbed P from biochar or soil by isotope (32 P) analysis. Acknowledgments This research was supported by the National Natural Science Foundation of China (No. 41001137; 41171216), One Hundred-Talent Plan of CAS, the CAS/SAFEA International Partnership Program for Creative Research Teams, the Important Direction Project of CAS (KZCX2-YW-JC203), Yantai Science & Technology Development Project (No. 2011016; 2010245), Yantai Double-hundred High-end Talent Plan (XY-003-02), the Science & Technology Development Plan of Shandong Province (010GSF10208)and 135 Development Plan of YIC-CAS. References Agbenin, J.O., 1994. Adsorbed phosphorus partitioning in some benchmark soils from Northeast Brazil. Nutr. Cycl. Agroecosyst. 40, 185–191. Agbenin, J.O., 1995. Phosphorus sorption by three cultivated savanna Alfisols as influenced by pH. Nutr. Cycl. Agroecosyst. 44, 107–112. Agbenin, J.O., 2003. Extractable iron and aluminum effects on phosphate sorption in a savanna alfisol. Soil Sci. Soc. Am. J. 67, 589–595. Atkinson, C.J., Fitzgerald, J.D., Hipps, N.A., 2010. Potential mechanisms for achieving agricultural benefits from biochar application to temperate soils: a review. Plant Soil 337, 1–18. Brodowski, S., Amelung, W., Haumaier, L., Abetz, C., Zech, W., 2005. Morphological and chemical properties of black carbon in physical soil fractions as revealed by scanning electron microscopy and energy-dispersive X-ray spectroscopy. Geoderma 128, 116–129. Brodowski, S., John, B., Flessa, H., Amelung, W., 2006. Aggregate-occluded black carbon in soil. Eur. J. Soil Sci. 57, 539–546. Chan, K.Y., Van Zwieten, L., Meszaros, I., Downie, A., Joseph, S., 2007. Agronomic values of greenwaste biochar as a soil amendment. Aust. J. Soil Res. 45, 629–634. Cheng, C.H., Lehmann, J., Engelhard, M.H., 2008. Natural oxidation of black carbon in soils: changes in molecular form and surface charge along a climosequence. Geochim. Cosmochim. Acta 72, 1598–1610. Cui, H.J., Wang, M.K., Fu, M.L., Ci, E., 2011. Enhancing phosphorus availability in phosphorus-fertilized zones by reducing phosphate adsorbed on ferrihydrite using rice straw-derived biochar. J. Soils Sediments 11, 1135–1141. Cusack, D.F., Chadwick, O.A., Hockaday, W.C., Vitousek, P.M., 2012. Mineralogical controls on soil black carbon preservation. Global Biogeochem. Cycle 26, GB2019. DeLuca, T.H., MacKenzie, M.D., Gundale, M.J., 2009. In: Lehmann, J., Joseph, S. (Eds.), Biochar Effects on Soil Nutrient Transformations. Biochar for Environmental Management: Science and Technology?, pp. 251–270. Glaser, B., Lehmann, J., Zech, W., 2002. Ameliorating physical and chemical properties of highly weathered soils in the tropics with charcoal – a review. Biol. Fertil. Soils 35, 219–230.

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