Effect of Biochar Types and Rates on SOC and Its Active Fractions in Tropical Farmlands of China

To date, most studies have shown that biochar has great potential in carbon sequestration and reduction, as well as soil quality improvement. However, there is limited knowledge of its effect on soil organic carbon (SOC) fractions in tropical farmland. This study aimed to determine the impact of different types and rates of biochar applied in tropical farmlands on so SOC and its active fractions. The SOC, microbial biomass carbon (MBC), dissolved organic carbon (DOC), and soil mineralizable carbon (SMC) in the 0–30 cm soil layers under rice hull (R) and peanut shell (P) biochar treatments were measured. The results showed that the application of R and P biochar increased the contents, stocks, and cumulative stocks of SOC, MBC, and DOC in the 0–10 cm, 10–20 cm, and 20–30 cm soil layers. The contents, stocks, and cumulative stocks increased with increasing biochar application rates. Compared with CK, the ranges of the increased SOC, MBC, and DOC cumulative stocks were 10.76–46.36%, 30.04–195.65%, and 0.02–17.03%, respectively. However, the R60 and P60 had the lowest cumulative stocks of SMC, decreasing by 14.69% and 8.05%, respectively. The biochar treatment of more than 20 t ha−1 reduced the ratio of SMC:SOC and active fractions:SOC. Therefore, it can be inferred that the application of biochar improved the levels of SOC, MBC, and DOC, and the application of more than 20 t ha−1 biochar could decrease soil carbon mineralization, thus improving the stability of SOC in tropical farmlands.