Mechanisms for Increasing the pH Buffering Capacity of an Acidic

Aug 28, 2017 - The objectives of the study were to compare the effects of different biochars on the pHbuff of an acidic Ultisol with a low pHbuff, to ...
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Mechanisms for Increasing the pH Buffering Capacity of an Acidic Ultisol by Crop Residue-Derived Biochars Ren-yong Shi,†,‡ Zhi-neng Hong,† Jiu-yu Li,† Jun Jiang,† M. Abdulaha-Al Baquy,†,‡ Ren-kou Xu,*,† and Wei Qian† †

State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, P.O. Box 821, Nanjing 210008, P.R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P.R. China S Supporting Information *

ABSTRACT: The effects and underlying mechanisms of crop residue-derived biochars on the pH buffering capacity (pHbuff) of an acidic Ultisol, with low pHbuff, were investigated through indoor incubation and simulated acidification experiments. The incorporation of biochars significantly increased soil pHbuff with the magnitude of the increase dependent on acid buffering capacity of the biochar incorporated to the soil. Cation release, resulting from the protonation of carboxyl groups on biochar surfaces and the dissolution of carbonates, was the predominant mechanism responsible for the increase in soil pHbuff at pH 4.0−7.0 and accounted for >67% of the increased pHbuff. The reaction of protons with soluble silica (Si) in biochars derived from rice straw and corn stover also accounted for ∼20% of the pHbuff increase due to H3SiO4− precipitation. In conclusion, the incorporation of crop residue-derived biochars into acidic soils increased soil pHbuff with peanut stover biochar being the most effective biochar tested. KEYWORDS: biochar, acidic soil, pH buffering capacity, cation release, carboxyl protonation, soluble Si

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INTRODUCTION Biochar is a carbon (C)-rich solid made by pyrolyzing biomass under conditions with limited oxygen and relatively low temperatures ( corn stover biochar > rice straw biochar > peanut stover biochar. The decrease in soil pH induced by the addition of HNO3 in the treatment with Ca(OH)2 was much greater than in the treatments with biochars added, and the pH decreased the least 8114

DOI: 10.1021/acs.jafc.7b02266 J. Agric. Food Chem. 2017, 65, 8111−8119

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Journal of Agricultural and Food Chemistry

twice that of the other biochars, and the amount of protons consumed by rice straw biochar was slightly larger than the amount consumed by corn and canola stover biochars. These results were consistent with the effects of these biochars on soil pHbuff and the results obtained from short-term acid−base titrations of these biochars with HNO3 as shown in Figure S1. Therefore, the increasing extent of soil pHbuff and the resistance of soil to acidification by the biochars was mainly dependent on the buffering capacity of the biochars to protons. The solid carbonates in soils reacted with protons at pH >6.2 and disappeared from the soils at pH 10 mL, which was consistent with the higher buffering capacity of the biochar compared with other biochars (Figure 2). The release of base cations from the other three biochars followed the order of corn stover biochar > rice straw biochar > canola stover biochar. To determine the contribution of the base cation release to H+ consumption by the biochars, correlation analyses between the equivalent base cations (sum of K+, Na+, Ca2+, and Mg2+) released and H+ consumption by the biochars from the initial pH to pH 4.0 were conducted. The results obtained were presented in Figure 3. A linear correlation was observed between base cations released from the biochars and H+ consumption by all biochars, and the correlation coefficient (R2) of the linear regression was higher than 0.95 in all cases (Figure 3). The slope of the lines in Figure 3 represents the relative contribution of base cations released to proton consumption by the biochars. The slopes for all biochars were higher than 0.65, which suggests that base cation release from the biochars was the predominant mechanism for proton consumption by the biochars. The base cations released from canola stover biochar accounted for 94% of the proton consumption. Therefore, the release of base cations was almost the only mechanism controlling its buffering ability to H+. The contributions of base cations released to proton consumption by the biochars from corn stover, rice straw, and peanut stover were 80, 72, and 68%, respectively. Therefore, the dissolution of carbonates and exchange reactions of exchangeable base cations with H+ in the biochars played major roles in consuming the protons by the biochars and pHbuff of these biochars. However, still around 20−30% of the proton consumption by these three biochars from corn stover, rice straw, and peanut stover could be attributed to other mechanisms. Silicate minerals are inorganic components of crop residue biochars.38,39 Aluminosilicate minerals in soils make a contribution to soil pHbuff at pH >5.0 through their weathering.36 Therefore, silicate minerals in these biochars were expected to make a contribution to the buffering ability of the biochars to protons. However, the increase of soluble Si in 8116

DOI: 10.1021/acs.jafc.7b02266 J. Agric. Food Chem. 2017, 65, 8111−8119

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Journal of Agricultural and Food Chemistry

cm−1 (υ CO) and 1208 cm−1 (υ C−OH or υ C−O−C). The absorption peaks at 1600 and 1375 cm−1 were assigned to the antisymmetric stretching and symmetric stretching carboxylate (COO−). The weak peak at 2918 cm−1 was assigned to -CH2.37,40,41 The peak at 1208 cm−1 only appeared in canola stover biochar. The strong peak at 1069 cm−1 in the ATR-FTIR spectra of rice straw biochar was assigned to Si−O−Si,27,39 demonstrating that it contained more silicate minerals than the other biochars. The infrared absorbance of carboxyl groups on biochar surfaces varied with pH due to the protonation. When COO− on biochar surfaces was protonated with the decrease of solution pH, the infrared absorption at 1650−1540 cm−1 (υ as) and 1420−1300 cm−1 (υ s) for COO− shifted to 1750−1690 cm−1 (υ CO) and 1300−1200 cm−1 (υ C−OH or υ C−O− C) for COOH (Figure S5).37,42 Consequently, the peak height of COOH at 1720 cm−1 increased, whereas the peak heights of COO− at 1600 and 1375 cm−1 decreased (Figure S5). For the extent of protonation of the carboxyl with decreasing pH to be investigated, infrared absorption heights for the peaks of COO− and COOH were normalized to the peak height of -CH2 at 1918 cm−1. The ratio of infrared absorption intensity for peaks of COOH and COO− to the peak intensity of -CH2 at 2918 cm−1 was calculated. The change of the absorption peak ratio with pH reflected the extent of protonation of COO− because -CH2 infrared absorption does not change with pH. As shown in Figure 6, the normalized peak intensities of COOH in all biochars increased with a decrease in the suspension pH. Correspondingly, the normalized peak intensity for COO− decreased. These results indicated that COO− protonation

Figure 5. Relative contribution of base cation release (carbonate and exchangeable cations) and soluble Si loss in suspensions of crop residue biochars to the pH buffering capacity (pHbuff) of the biochars.

The Protonation of Carboxyl Groups during the Biochar Reactions with Protons. The absorption peaks of COOH and COO− are located at different positions in the ATR-FTIR spectra; thus, ATR-FTIR techniques were used to investigate the protonation of carboxyl groups on the biochars. There were obvious absorption peaks at 1700, 1600, and 1373 cm−1 and a weak absorption peak at 2925 cm−1 in the ATRFTIR spectra of all four biochars (Figure S4). The characteristic absorption peaks of COOH or COOC were probed at 1700

Figure 6. Normalized intensity of infrared absorption determined by ATR-FTIR for the bands of COOH and COO− on surfaces of crop residue biochars at various pH values after the biochars reacted with protons for 7 d. The square (■) and rhombus (◆) labels represent the ratios of infrared absorption intensity of COOH at aprroximately 1700 and 1200 cm−1 to that of υ as (CH2) at 2918 cm−1. The circle (●) and triangle (▲) labels represent the ratios of infrared absorption intensity of COO− at approximately 1600 and 1375 cm−1 to that of υ as (CH2) at 2918 cm−1. 8117

DOI: 10.1021/acs.jafc.7b02266 J. Agric. Food Chem. 2017, 65, 8111−8119

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Journal of Agricultural and Food Chemistry

Key Research and Development of China (Grant 2016YFD0200302).

contributed to the pHbuff of all the biochars. A previous study found that the pKa of the carboxyl on the surface of biochars ranged from 5.0 to 6.0,37 and therefore, carboxyl on the biochars could contribute to the pHbuff in the pH range from 3.0 to 8.0 due to the pH buffering range at pKa ±2. These analyses demonstrated that the protonation of carboxyl on the biochar surfaces played the main role in increasing the pHbuff of acidic soils by biochar incorporation. Although the carboxyl content on the biochars from corn stover, canola stover, and rice straw were similar, rice straw biochar and corn stover biochar increased the soil pHbuff to a greater extent than that of canola stover biochar due to the contribution of soluble Si precipitation (Figure 1). In the acid titration curve of peanut stover biochar, the plateau region from 7.0 to 6.2 was caused by the high carbonate content in peanut stover biochar (32.6 cmol kg−1) (Figure 2). Consequently, the carboxyl protonation contributed to the pHbuff at the pH range from 6.2 to 4.0. The normalized peak intensity of Si−O−Si at 1069 cm−1 in rice straw biochar did not change with pH in the pH range studied here, indicating that the contribution of solid silicate minerals in the biochar to pHbuff could be neglected in short-term laboratory acidification experiments. In conclusion, biochars derived from crop residues can increase not only the soil pH but also the soil pHbuff, especially in soils with a low pHbuff. The ability of the biochars to increase soil pHbuff depended on the reactions of the biochars with protons. The protonation of carboxyl on the biochars with the subsequent release of base cations was the predominant mechanism responsible for proton consumption by the biochars in the pH range from 6.2 to 4.0. The soluble Si in rice straw biochar and corn stover biochar also contributed to pHbuff of the biochars through precipitation. Peanut stover biochar was the best of the biochars studied for ameliorating soil acidity and increasing soil pHbuff. The findings obtained in the present study are of significance in practice for the amelioration and management of acidic soils. Crop residue biochars can ameliorate soil acidity to increase soil pH. However, reacidification of ameliorated soils will occur due to continue application of ammonium-based fertilizers in crop lands. Application of the biochars can increase soil pHbuff and the resistance of acidic soils to acidification and thus will inhibit reacidification of ameliorated soils caused by ammonium-based fertilizers. Thus, the biochars derived from crop residues are excellent amendments for acidic soils. However, the long-term effects of biochars on soil pHbuff need to be confirmed under field conditions in the future.

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Notes

The authors declare no competing financial interest.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jafc.7b02266. Proton uptake curves, changes in sums of base cations and soluble Si levels, and ATR-FTIR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 25 86881183; e-mail: [email protected]. ORCID

Ren-kou Xu: 0000-0002-5541-0205 Funding

This study was supported by the National Key Basic Research Program of China (Grant 2014CB441003) and the National 8118

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DOI: 10.1021/acs.jafc.7b02266 J. Agric. Food Chem. 2017, 65, 8111−8119