Alterations of soil labile organic carbon fractions and biological properties under different residue-management methods with equivalent carbon input

Crop residue return to soil can greatly affect soil carbon (C) cycling. However, the differences in soil organic C (SOC) pool among different residue-management methods, including residue-biochar, are still unclear. We investigated the changes in SOC content, SOC stock, labile organic C fractions, C pool management index (CPMI), microbial abundance (bacteria, actinomycetes, and fungi), and C-cycle enzyme activities (invertase, cellulose, and beta-glucosidase) after four years of applying residue and residue-biochar in a barley-cotton cropping system in Yancheng, China. Five treatments were developed based on equivalent C input: residue-biochar (RB, 3.5 Mg ha(-1)), residue plus microbial inoculant (RI, 5.0 Mg ha(-1)), residue (R, 5.0 Mg ha(-1)), residue mulching (RM, 5.0 Mg ha(-1)), and no residue or residue-biochar (CK). The results showed that SOC content, SOC stock, and the CPMI in RB were the highest among all treatments with residue and residue-biochar, being 16.3%, 8.7%, and 66.2% higher than that in CK, respectively. Regarding the labile organic C fractions, RB primarily increased the contents of particulate organic C (POC), light fraction organic C (LFOC), and permanganate oxidizable C (KMnO4-C), RI and R primarily increased the water-extractable organic C (WEOC) and microbial biomass C (MBC), whereas RM had a relatively small effect on all fractions. Soil microbial abundance and C-cycle enzyme activities were greatly enhanced by RI and R, particularly by RI. The principal component analysis revealed that POC, LFOC, and KMnO4-C were the major factors causing the changes in SOC, SOC stock, and CPMI. Soil microbial abundance and C-cycle enzyme activities were significantly correlated with WEOC and MBC. In conclusion, applying residue-biochar could effectively increase soil C sequestration by enhancing the POC, LFOC, and KMnO4-C contents in comparison with residue return.

Automated summary

The application of crop residue-biochar effectively increases soil carbon sequestration, enhances soil organic C content and C pool management index, optimizes soil carbon cycle enzyme activities, and has a positive impact on soil microbial abundance.