Quantitative estimation on the carbon sequestration potential of agricultural by-products biochar using chemical oxidation

Abstract

The application of biochar to soil is widely considered for carbon sequestration, primarily due to the higher stability of biochar compared to biomass and soil organic matter. However, carbon is partially lost during pyrolysis by releasing gases such as CO and CH4. In addition, biochar contains labile carbon, which soil microbes can mineralise. Therefore, quantifying unstable carbon in biochar is essential to accurately evaluate biochar’s carbon sequestration potential. The labile carbon of biochar has typically been quantified using incubation, yet it is time-consuming. Chemical oxidation has been proposed as a rapid method to quantify the unstable carbon in biochar. Although many studies on the quantification of unstable carbon have used chemical oxidation, it is unclear whether the unstable carbon fraction by chemical oxidation is mineralised in soil. This study aims to establish a chemical oxidation method for rapidly quantifying labile carbon in biochar by matching the chemically oxidised carbon of biochar with the fraction mineralised in soil. Firstly, rice straw biochar was oxidised using KMnO4, H2O2, and K2Cr2O7/H2SO4. Fourier transform infrared spectroscopy analysis confirmed that aliphatic groups remained after oxidation, suggesting that KMnO4 has weak oxidation potential. Additionally, H2O2 underwent self-decomposition due to the high pH of biochar, resulting in a consistently low unstable carbon fraction in 600 ◦C biochar at all concentrations. The result indicates that the characteristics of biochar can significantly influence the oxidant. Therefore, both oxidants are inappropriate for chemical oxidation. K2Cr2O7/H2SO4 was selected as an optimal oxidant due to its high oxidation potential and minimal influence on the properties of biochar. Thereafter, chemical oxidation using K2Cr2O7/H2SO4 with different concentrations was applied to the agricultural by-products of biochar. The unstable carbon fraction of biochar commonly remained constant within 30-40 mmol g−1 biochar. The result infers that the oxidation potential was insufficient to oxidise stable carbon, leading to the selective oxidation of only unstable carbon. Fourier transform infrared spectroscopy analysis confirmed that aliphatic groups in biochar were removed by oxidation. To verify the optimal condition, incubation of biochar before and after oxidation is needed to identify the association between chemical oxidation and incubation.