Effects of Mineral Additives on Biochar Formation: Carbon Retention

Article pubs.acs.org/est

Effects of Mineral Additives on Biochar Formation: Carbon Retention, Stability, and Properties Feiyue Li,†,‡ Xinde Cao,*,† Ling Zhao,† Jianfei Wang,‡ and Zhenliang Ding† †

School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China College of Urban Construction and Environment Science, Anhui Science and Technology University, Anhui 233100, People’s Republic of China

‡

ABSTRACT: Biochar is being recognized as a promising tool for longterm carbon sequestration, and biochar with high carbon retention and strong stability is supposed to be explored for that purpose. In this study, three minerals, including kaolin, calcite (CaCO3), and calcium dihydrogen phosphate [Ca(H2PO4)2], were added to rice straw feedstock at the ratio of 20% (w/w) for biochar formation through pyrolysis treatment, aiming to improve carbon retention and stabilization in biochar. Kaolin and CaCO3 had little effect on the carbon retention, whereas Ca(H2PO4)2 increased the carbon retention by up to 29% compared to untreated biochar. Although the carbon loss from the kaolin-modified biochar with hydrogen peroxide oxidation was enhanced, CaCO3 and Ca(H2PO4)2 modification reduced the carbon loss by 18.6 and 58.5%, respectively. Moreover, all three minerals reduced carbon loss of biochar with potassium dichromate oxidation from 0.3 to 38.8%. The microbial mineralization as CO2 emission in all three modified biochars was reduced by 22.2−88.7% under aerobic incubation and 5−61% under anaerobic incubation. Enhanced carbon retention and stability of biochar with mineral treatment might be caused by the enhanced formation of aromatic C, which was evidenced by cross-polarization magic angle spinning 13C nuclear magnetic resonance spectra and Fourier transform infrared spectroscopy analysis. Our results indicated that the three minerals, especially Ca(H2PO4)2, were effective in increasing carbon retention and strengthening biochar stabilization, which provided a novel idea that people could explore and produce the designated biochar with high carbon sequestration capacity and stability.

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INTRODUCTION The challenge of climate change (caused by the greenhouse effect) has led to the development of new technologies with either enhancement of carbon sinks or the reduction of fossil fuel emission to mitigate increasing concentrations of greenhouse gases in the atmosphere.1,2 One proposed solution is pyrolysis of biomass in which bio-oil and noncondensable gas are available to replace part of fossil fuel consumption; the solid residue product is a recalcitrant carbon-rich material and referred to as biochar.3 It is being increasingly recognized that application of biochar into soil can be an effective and valuable way for long-term carbon sequestration.4,5 Besides, biochar input to soil can have additional agricultural and environment benefits, including improvement of soil quality and remediation of soil pollution.6,7 Biochar is a heterogeneous carbonaceous material. Its properties, including carbon content, composition, and stability that determines the length of its contribution to the mitigation of greenhouse gas emission, depend upon feedstock types and pyrolysis conditions.8,9 Hence, it is important to create biochar with a high yield, high carbon content, and strong stability for carbon sequestration. The feedstock biomasses often contain high contents of minerals, such as Si, Al, Ca, K, Na, and Mg, © XXXX American Chemical Society

together with a smaller amount of S, P, Cl, and Mn present as oxides, silicates, carbonates, sulfates, chlorides, and phosphates.10 These intrinsic minerals can amount from less than 1 wt % to up to 25 wt %11 and are known to catalyze several thermal reactions and greatly alter the product distribution.12 The influence of exogenous minerals on the biochar formation has also been reported in previous work.13,14 Jensen et al.13 observed an increase in the yields of gas and char against a decrease in the liquid in the slow pyrolysis of wheat straw by impregnation of KCl solution. Wang et al.14 indicated a similar trend with the addition of 17.7% K2CO3. In contrast, the addition of 22.2% Ca(OH)2 somewhat raised the yield of liquid with a decrease in the yield of char at 700 °C. Hence, the effect of minerals on biomass pyrolysis behavior depends upon mineral types, mineral quantity, feedstock types, and pyrolysis conditions. Most of the studies on mineral matter addition on biomass pyrolysis behavior were focused on the pyrolysis product distribution and the individual product composition, Received: April 16, 2014 Revised: July 14, 2014 Accepted: September 9, 2014

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dx.doi.org/10.1021/es501885n | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

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especially for the liquid product.15,16 However, reports on the influence of mineral addition on the carbon retention and stability of biomass pyrolysis solid product (biochar) in terms of carbon sequestration are fairly limited. In this study, three common minerals, including kaolin, calcite (CaCO3), and calcium dihydrogen phosphate [Ca(H2PO4)2], were used as additives and mixed with rice straw biomass for biochar production. These three minerals are often soil amendment materials for improvement of soil quality and remediation of soil pollution.17,18 The overall objective of this study was to investigate the influence of the three minerals on biochar formation behavior. Specifically, this study will (1) determine the effect of the three mineral additions on the physical and chemical properties of biochar, (2) assess the carbon retention and stability of biochar, and (3) elucidate the potential mechanism for carbon retention and stability of biochar.

Biochar pH was measured in the 24 h equilibrated solution of biochar and deionized water with a solid/liquid ratio of 1:20 (w/v) (EUTECH pH 510, Vernon Hills, IL). Dissolved organic carbon (DOC) was extracted using deionized water with a solid/liquid ratio of 1:50 (w/v) after 24 h equilibrium, filtered through a filter 0.45 μm membrane, and analyzed with a total carbon analyzer (TOC-VCPH, Shimadzu, Japan). Total C, H, and N content analysis of biochar was conducted on an element analyzer (Vario ELIII, Elementar, Germany). Specific surface area and pore size distribution of biochars were determined using a BET-N2 SA analyzer (JW-BK222, China). Surface functional group distributions of biochar were determined by Fourier transform infrared (FTIR) spectroscopy (IR Prestige 21 FTIR, Shimadzu, Japan) and cross-polarization magic angle spinning 13C nuclear magnetic resonance spectra (CP-MAS 13C NMR). The NMR spectra were obtained at a frequency of 100.6 MHz using a Varian Unity Inova 400 NMR spectrometer (AVANCEIII 400, Bruker, Switzerland). Measurement of Biochar Stability. Two methods were applied to test biochar stability: One was a simulated long-term stability method using chemical oxidation treatment;19,20 this method was to determine the chemical stability of biochar. The other one was a simulated mineralization experiment to test the microbe-resistance stability of the biochar;21 this method was to evaluate the biological stability of biochar. To determine chemical oxidation, hydrogen peroxide (H2O2) and potassium dichromate (K2Cr2O7) were used to assess the labile fraction of C in biochar samples. For the H2O2 treatment, 0.1 g of C of biochar was treated in a glass test tube with 7 mL of 5% H2O2 at 80 °C for 2 days and stable C was expressed as the percentage of the initial C that remained after oxidation and assessed from the mass loss and determination of the C content before and after oxidation.19 For the K2Cr2O7 treatment, 0.1 g of C of biochar was treated in a glass test tube with 40 mL of 0.1 M K2Cr2O7/2 M H2SO4 solution at 55 °C for 60 h.20 Results were expressed as the fraction of total C oxidized by K2Cr2O7. The biological stability of biochar was evaluated in terms of the microbial mineralization rate under simulated soil/sediment microbial conditions in a lab-scale experiment, which was widely used in previous reports.21,22 Generally, the biological stability of biochar is assessed under the aerobic conditions. To the best of our knowledge, there is a lack of work on the biological stability of biochar conducted under anaerobic conditions. In this study, both aerobic and anaerobic microbial incubation of biochar were carried out. The anaerobic microbial experiment was designed with two hypotheses: one was that CH4 emission would be detected because of the anaerobic conditions; the other one was that the biochar under anaerobic conditions was much more stable than that under aerobic conditions. Both aerobic and anaerobic microbial incubation of biochar were carried out in sterilized 20 mL borosilicate vials with rubber septa. Briefly, for aerobic incubation, three replicate incubations of 0.2 g of biochar, 2 g of cleaned quartz sand, and 0.3 mL of aqueous nutrient solution [60 g L−1 (NH4)2SO4 + 6 g L−1 KH2PO4] were prepared.21 The addition of quartz sand was to increase permeability, thus increasing the water and oxygen accessibility for the biochar. A total of 0.3 mL of microbial inoculate, the supernatant of the university forest soil after 24 h of shaking in deionized water at the 1:5 solid/liquid (m/v) and centrifugation, was added to the biochar + sand system. The vials were incubated at 30 °C. Before the

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MATERIALS AND METHODS Materials. A common crop residue, i.e., rice straw, was collected from a farm near the suburb of Shanghai, China. The rice straw was air-dried with the moisture lower than 5% and ground to less than 0.15 mm (100 meshes). Three minerals: kaolin (Al2O3·2SiO2·2H2O), calcite (CaCO3), and calcium dihydrogen phosphate [Ca(H2PO4)2·H2O] were all obtained from Sinoparm Chemical Reagent Company. These minerals are analytical reagents. Biochar Production. First, the minerals were mixed with the ground rice straw at the ratio of 20% (w/w); the high addition rate was chosen to ensure full contact of biomass and additive in the process of biochar formation. The mixtures were then placed into a lab-scale stainless-steel pyrolysis reactor and heated under a N2 atmosphere with a heating rate of 15 °C min−1 to reach four settled gradient temperatures (200, 300, 400, and 500 °C). At each gradient temperature, the heating time was kept for 1 h. This gradient heating process could provide enough time for biomass carbonization and minimize volatile organic decomposition.9 The solid residue in the reactor was designated as biochar. For simplicity, the biochar without mineral addition was referred to as unmodified biochar and those with kaolin, calcite, and calcium dihydrogen phosphate addition were referred to as modified biochar and designated as BC, kaolin−BC, CaCO3−BC, Ca(H2PO4)2−BC, respectively. Characterization of Biochars. The yield of biochar was obtained by weighting the sample and solid residue before and after pyrolysis, with the contribution of minerals being subtracted in calculation of the yield of biochar (eq 1) yield =

Mbc − M mr × 100 Mbm

(1)

where Mbc, Mmr, and Mbm are the weights of solid residue (g), mineral residual after pyrolysis (g), and biomass (g), respectively. The carbon retention in biochar after pyrolysis was calculated on the basis of the yield and carbon content (eq 2) carbon retention =

Mbm × yield × C bc × 100 Mbc × C bm

(2)

where Mbc, Mbm, and yield are described above in eq 1 and Cbc and Cbm are the total carbon contents of biochar and rice straw biomass, respectively. B



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Table 1. Selected Properties of Biomass and Modified and Unmodified Biochars pH C (%) H (%) N (%) H/C ash (%)b DOC (mg g−1) SAc (m2 g−1) PVd (cm3 g−1) APDe (nm) yield (%) carbon retention (%) a

rice straw

BC

kaolin−BC

CaCO3−BC

Ca(H2PO4)2−BC

a 39.1 5.32 0.66 1.63 10.9 a a a a a a

8.60 47.6 2.56 1.21 0.65 28.9 7.61 8.14 0.02 3.71 35.9 43.7

8.39 49.0 2.62 1.19 0.64 29.1 3.53 151 0.10 4.16 33.5 42.0

9.05 48.7 2.52 1.04 0.62 27.2 7.59 8.79 0.02 3.32 33.7 42.0

4.30 52.8 2.59 1.32 0.59 23.8 3.61 64.1 0.03 3.75 41.7 56.3

Not determined. bHeating at 900 °C. cSA = BET-N2 surface area. dPV = pore volume. eAPD = average pore diameter.

times compared to BC, following the order of kaolin−BC > Ca(H2PO4)2−BC > CaCO3−BC > BC (Table 1). The highest surface area in kaolin−BC is probably due to the presence of kaolin, which has a high surface area. There were no obvious differences for the pore volume and pore diameter in the unmodified biochar and modified biochar, except for kaolin− BC, in which both pore volume and diameter were increased, probably resulting from kaolin itself. Carbon Retention of Biochar. Kaolin and CaCO 3 addition had little effect on the biochar yield and carbon retention (Table 1). However, the biochar yield and carbon retention in Ca(H2PO4)2−BC were greatly elevated by 16.2 and 29%, respectively (Table 1). The similar results were observed in many previous studies.16,27,28 Gao et al.27 reported that, with (NH4)2HPO4 pretreatment, the corn straw biochar yield increased up to 42.8%. Therefore, in terms of carbon sequestration, Ca(H2PO4)2 mineral was the most effective additive to the feedstock for biochar formation because of its elevated yield and carbon retention. Phosphate can accelerate the dehydration of biomass to prevent the generation of levoglucosan and reduce the release of elements C, H, and O in the form of small molecules at a subsequent stage of pyrolysis, leading to the increase of the yield and carbon retention.29 Chemical Oxidation of Biochar. Many studies reported that chemical oxidants, such as K2Cr2O7, KMnO4, HNO3, and H2O2, can be used to evaluate the oxidative nature of biochar and reflect the long-term stability of biochar.8,30,31 With H2O2 oxidation, the unmodified BC lost 18.8% carbon (Figure 1). However, the carbon loss in CaCO3−BC and Ca(H2PO4)2− BC was 16.0 and 7.78%, respectively. The reduction of carbon

headspace CO2 was determined, the air was replaced by simulated air without CO2 (pure O2 and N2). After incubation for 24 h, the headspace CO2 content was measured. The incubation lasted for 60 days, and headspace CO2 was determined in intermittent time using a gas chromatograph (GC-14B, Shimadzu, Japan). Both the CO2 emission rate and cumulative emission of biochar were calculated according to Li et al.23 The anaerobic incubation was similar to the aerobic incubation. The differences between them were as follows: (1) The microbial inoculate was derived from the supernatant of the university pond sediment after 7 days of shaking in deionized water at the 1:5 solid/liquid (m/v) in anaerobic conditions. (2) Aqueous nutrient solution was prepared according to Chen et al.24 to create the beneficial conditions for anaerobic microbe grow. (3) The vial were vacuumed before 20 mL of pure N2 was injected, and the incubation time also lasted for 60 days, during which time the vials were kept close to ensure the anaerobic conditions.

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RESULTS AND DISCUSSION Physicochemical Properties of Biochar. The addition of the three minerals resulted in changes in the physical−chemical properties of biochar (Table 1). In comparison to the pH at 8.6 in the unmodified BC, the pH value of Ca(H2PO4)2−BC was reduced to 4.30 because of acidic Ca(H2PO4)2, while CaCO3 addition increased pH of CaCO3−BC to 9.05 because of alkalinity of CaCO3. There was little change of pH observed in kaolin−BC compared to the unmodified BC (Table 1). Three minerals slightly increased total C contents of biochars by 2.3− 10.9% compared to the unmodified biochar. However, the DOC content in kaolin−BC and Ca(H2PO4)2−BC greatly decreased by more than 50%. Concentrations of H and N were little affected by the three minerals. The H/C ratio, usually used as an index to estimate carbon aromaticity,25,26 was much lower in all biochars than that in rice straw (Table 1), showing the formation of an enhanced aromatic structure in biochars because of the pyrolysis of the straw biomass. Furthermore, the H/C ratio in the mineral-modified biochar was slightly lower than that in the unmodified biochar (Table 1), because H and C contents were subtracted from those as a result of the contribution from the mineral addition; the ratio decline indicated that the carbon aromaticity degree of the modified biochar was improved by mineral addition. All minerals increased the surface area of biochars, especially for kaolin−BC that increased the surface area by up to 18.6

Figure 1. Carbon loss from the modified and unmodified biochars through chemical oxidation. C



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loss in the both biochars was probably due to reduction of the DOC content (Table 1). The DOC is a readily decomposed substance when it is attacked by oxidants.32The increased carbon loss was observed in kaolin−BC (Figure 1), which probably resulted from the high surface area of kaolin−BC. A high surface area allowed for a considerable quantity of volatiles trapped inside the biochar, which was not readily extracted by deionized water but readily attacked by H2O2, leading to the increase of C loss. With K2Cr2O7 oxidation, the unmodified BC lost 27.4% carbon (Figure 1); however, the carbon loss in the modified biochar was reduced from 0.3 to 38.8%, with the reduction following the order of Ca(H2PO4)2−BC > kaolin− BC > CaCO3−BC, compared to unmodified BC (Figure 1). In comparison to H2O2 oxidation, the carbon loss in all biochars, except for kaolin−BC, was high by K2Cr2O7 oxidation because of its high oxidizing capacity. The carbon loss in kaolin-BC with H2O2 oxidation was even higher than that in the unmodified BC, which was probably attributed to the catalytic activity of kaolin, an aluminum silicate, generating hydroxyl radicals (•OH) from hydrogen peroxide (H2O2), which led to more carbon loss.33 Results from the chemical oxidation experiment indicated that Ca(H2PO4)2 mineral was the most effective additive to the feedstock for the formation of biochar with the elevated chemical oxidation stability. Rosas et al.34 indicated that H3PO4 could facilitate generating thermally stable phosphorus complexes containing metaphosphates, C−O− PO3 groups, and C−PO3 groups. Those groups are suggested to act as a physical barrier against carbon decomposition as well as block the active carbon sites.22 We expected similar mechanisms involved in the Ca(H2PO4)2-modified biochar. Biological Mineralization of Biochar. The incubation of biochar in sand medium has been widely adopted in previous studies to evaluate the microbe-resistance stability of biochar because of its obvious advantage of excluding decomposition of native soil organic matter (evolved CO2 = biochar−C).21,22 For the aerobic incubation (Figure 2a), the CO2 emission rate in Ca(H2PO4)2−BC was much lower and remained stable over 60 days of incubation. However, the other three biochars had a great CO2 emission rate and showed a similar trend that the rapid decline happened within the first 12 days and remained stable in the later 48 days. The observation was in accordance with previous work.23,35 The initial emission of CO2 in the three biochars was most likely due to microbial decomposition of an easily degradable fraction of the biochar, which was related to the DOC content in biochar.36 As shown in Table 1, the DOC content of modified biochar was lower than unmodified biochar, leading to a reduction of CO2 emission. Therefore, all minerals, especially for Ca(H2PO4)2, could significantly inhibit the emission rate of CO2 over the incubation period compared to unmodified biochar (Figure 2a). The cumulative CO2 emission amount during the 60 day aerobic incubation was 23.9 mg of CO2−C g−1 of C for the unmodified biochar (Figure 2b). In other word, about 2.4% of added biochar−C had been mineralized as CO2. Similar results were obtained in other works that biochar−C losses ranged from 0.3 to 3%.3,21,36 In comparison to the unmodified BC, the cumulative CO2 emissions of kaolin−BC, CaCO3−BC, and Ca(H2PO4)2−BC were reduced by 31.0, 22.2, and 88.7%, respectively, indicating that mineral addition was beneficial for the biochar to resist its aerobic biological degradability. It was expected that the CH4 emission could happen during the anaerobic incubation. However, there was no CH4 emission detected, and on the contrary, a certain amount of CO2

Figure 2. (a) CO2 emission rate in the modified and unmodified biochars under aerobic conditions during the 60 day incubation period and (b) cumulative CO2 emission of the modified and unmodified biochars under aerobic conditions during the 60 day incubation period.

released during the anaerobic incubation, though it was much lower than that under aerobic incubation. The reason was likely that biochar may inhibit the methanogen activity and/or biochar as the carbon source was not assimilated by methanogens. This again explained that biochar addition to soils could reduce CH4 emission in previous studies.37,38 The CO2 cumulative emission from the unmodified BC during the 60 day incubation was 2.0 mg of CO2−C g−1 of C (Figure 3). Like the aerobic conditions, kaolin, CaCO3, and Ca(H2PO4)2 minerals also reduced the cumulative CO2 emission by 5.0, 45.0, and 61.0%, respectively, compared to BC, indicating that the mineral addition was also beneficial for biochar to resist its anaerobic biological degradability.

Figure 3. Cumulative CO2 emission of the modified and unmodified biochars under anaerobic conditions over the 60 day incubation period. D



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Biochar Structure and Its Relation with the Carbon Retention and Stability. The carbon cluster size and group distribution in all biochars were identified by CP-MAS 13C NMR. As shown in Figure 4, the aromatic C with a chemical

Figure 5. FTIR spectra of the modified and unmodified biochars.

and carbonyl (185−225 ppm) contents was higher in Ca(H2PO4)2−BC than that in the others (Table 2). Vibration bands at around 1573 cm−1, corresponding to the stretching of CC groups of their aromatic nature,44 were observed in all biochars. The similar observation was indicated by the 13C NMR spectrogram analysis (Table 2). As expected, a strong peak at 1396 cm−1 representing CO32− 45 was observed in CaCO3−BC. The peaks at 1086 and 788 cm−1 corresponding to the stretching of Si−O46 were observed in all biochars, but kaolin−BC showed the highest intensity. The peak at 935 cm−1 corresponding to the stretching of P−O47 was only observed in Ca(H2PO4)2−BC. The peak at 873 cm−1 assigned to the bands of CO32− 48 was only observed in CaCO3−BC.

Figure 4. CP-MAS 13C NMR spectra of the modified and unmodified biochars.

shift of 108−145 ppm was a dominate C form in all of the biochars. Table 2 summarized the relative proportion of C in each chemical functional group for biochars, which were integrated in the chemical shift (ppm) resonance intervals from 0 to 225 ppm according to Jindo et al.39 and Zhao et al.40 Obviously, aromatic C was the main C-containing functional group in all biochars, ranging from 40.2 to 57.3% (Table 2). In comparison to that in the unmodified BC, the aromatic C proportion in kaolin−BC, CaCO3−BC, and Ca(H2PO4)2−BC increased by 42.5, 39.8, and 33.1%, respectively. That was a possible reason that the modified biochar resisted the biological mineralization (Figures 2 and 3) because the aromatic C proportion was considered to reflect stability of carbonaceous materials.9,41 The carbonyls with a chemical shift of 185−225 ppm in kaolin−BC were higher than unmodified BC (Table 2), which may explain the increased carbon loss of kaolin−BC (Figure 1). It has been reported that the carbonyls could be readily disintegrated by hydroxyl radicals (•OH) generated from hydrogen peroxide (H2O2) catalyzed by kaolin.33 The FTIR spectra further evidenced the change in the surface functional groups of biochars with or without modification (Figure 5). The absorption peaks between 4000 and 2000 cm−1 remained unchanged in all biochars. However, the peak at 1702 cm−1 attributed to the carboxyl CO stretching vibration42,43 obviously appeared in Ca(H2PO4)2− BC, which was in accordance with the result from the 13C NMR spectrogram analysis that the sum of carboxyl (160−185 ppm)

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ENVIRONMENTAL SIGNIFICANCE The present study demonstrated that exogenous minerals had influences on the biochar formation during the pyrolysis process in terms of carbon retention, stability, and properties. Although kaolin and CaCO3 were not effective in improving the carbon retention in biochar, they greatly increased biochar stability. The chemical oxidation and biological mineralization were largely reduced in the both modified biochars. The Ca(H2PO4)2 mineral not only greatly improved the carbon retention but also significantly enhanced the carbon stability, which was more beneficial for long-term carbon sequestration. In addition, acidic Ca(H2PO4)2-modified biochar can restore alkaline soils and increase their soil fertility, while its function is carbon sequestration. Overall, our results indicated that the three minerals, especially Ca(H2PO4)2, were an effective additive to biomass feedstock for the formation of the biochar with enhanced carbon retention and stabilization. It should be pointed out that only one biomass of rice straw was used in this study; more different biomass feedstocks should be tested to

Table 2. Relative Proportion (% Biochar−C) of Chemical Functional Groups in the Modified and Unmodified Biochars, Determined by CP-MAS 13C NMRa chemical shift, δ (ppm) BC kaolin−BC CaCO3−BC Ca(H2PO4)2−BC

0−46

46−65

65−90

90−108

108−145

145−160

160−185

185−225

225−250

4.90 2.26

0.98

13.7 15.2 10.8 12.1

0.98

40.2 57.3 56.2 53.5

4.90 3.12

6.86

25.5 30.6 10.8 22.2

1.96

5.91 2.02

3.03

18.7 13.1

1.97 2.02

a The spectra were integrated in the chemical shift (ppm) resonance intervals of 0−46 ppm (alkyl C, mainly CH2 and CH3 sp3 carbon), 46−65 ppm (methoxy and N alkyl C from OCH3, C−N, and complex aliphatic carbon), 65−90 ppm (O−alkyl C, such as alcohol and ether), 90−108 ppm (anomeric carbon in carbohydrate-like structures), 108−145 ppm (aromatic and phenolic carbon), 145−160 ppm (oxygen aromatic carbon and olefinic sp2 carbon), 160−185 ppm (carboxyl, amide, and ester), and 185−225 ppm (carbonyl).

E



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demonstrate the mineral effect. In addition, the biological stability of biochar was only evaluated in a short time of 60 days of incubation laboratory experiment and under a simulated soil environment in the sand−microbe medium conditions without thinking of the effect of aggregation and biochar−clay interactions on biochar stability. The long-term evaluation of biochar in real soils for its carbon sequestration capacity and stability as well as its subsequent impact on the soil property needs to be carried out in the future.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-21-3420-2841. Fax: +86-21-5474-0825. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the National Natural Science Foundation of China (21107070 and 21377081), the Key Project of Shanghai Science and Technology Commission (13231202502), the Key Project of Shanghai Education Commission (14ZZ026), and the State Key Laboratory of Pollution Control and Resource Reuse Foundation (PCRRF12009).

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