The influence of the type of crop residue on soil organic carbon fractions: An 11-year field study of rice-based cropping systems in southeast China

Retaining crop residue is an important agronomic practice in sustainable agriculture, particularly to maintain soil organic carbon (SOC). To evaluate the effect of the retention of different types of crop residues on SOC and labile fractions, a long-term rice-based crop rotation experiment was established with five different winter cropping practices: (1) rice-fallow (RF), (2) rice–wheat (RW), (3) rice–potato with rice straw mulch (RP), (4) rice–green manure (Chinese milk vetch; RG), and (5) rice–oilseed rape (RO). The results showed that the annual rice yields in the RP, RO, RG, and RW treatments were not significantly different from each other, but 13.3%, 10.2%, 10.3%, and 8.4% higher, respectively, than in the RF treatment. Furthermore, the soil properties in the RP system improved significantly compared with those at the initiation of the study, with increases of 14.7% in total N, 17.5% in total P, 77.2% in available K, and 13.9% in SOC content. Considering the slight rice yield increase and superiority in the annual net income, the RP treatment seems to be a promising rotation system to meet the sustainability requirements for the agro-ecosystem. The responses of the labile SOC fractions (i.e., microbial biomass C (MBC), dissolved organic C (DOC), hot-water extractable C (HWC), permanganate-oxidizable C (KMnO4-C), and particulate organic C in the 0–20 cm layer) to the crop rotation systems with the retention of different crop residues over the period of 11 years were specific to the types of the rotation system and closely related to the soil fertility properties. The RP treatment had comparatively higher content of DOC, HWC, MBC, and KMnO4-C than the other treatments in both 0–10 cm and 10–20 cm depths. The RW treatment had higher KMnO4-C content but lower MBC and POC content than the other treatments. The RO treatment had lower MBC content compared to the other treatments, while the opposite results were found in the RG treatment. Furthermore, the DOC content was improved by the winter crop growth. The changes in labile SOC might be attributable to the types of residues retained.