Dynamics and composition of soil organic carbon in response to 15 years of straw return in a Mollisol

Crop straw return is being widely applied as a sustainable soil management practice to improve soil organic carbon (SOC) storage in intensive agricultural ecosystems. However, the dynamics and chemical composition of SOC under long-term straw return are not fully understood. Based on a 15-year field experiment with soybean (Glycine max (L.) Merrill.)–maize (Zea mays L.) cropping system, we studied SOC temporal dynamics under no fertilization (NF), mineral fertilizers (NPK) and mineral fertilizers with straw return (NPKS). Meanwhile, we determined five labile carbon (C) pools, i.e., microbial biomass C (MBC), water-soluble organic C (WSOC), light fraction C (LFC), readily oxidizable organic C (ROC), and particulate organic C (POC), as well as δ13C values and organic functional groups of soil organic matter (SOM). After 15-year application of continuous straw return with mineral fertilizer application, SOC content increased by 14.2%. The δ13C values of SOC increased with time under all treatments, and its positive relationship with cumulative maize C input indicated a larger C contribution to SOC from maize than soybean residues. The NPKS treatment significantly increased the contents of MBC, WSOC, LFC, ROC, and POC and their proportion in SOC, compared with the NF treatment. The aliphatic and aromatic relative peak areas were significantly affected by straw return, with an increasing relative peak area at 2930 cm−1 and decreasing at 1620 cm−1, respectively. Our results demonstrate that straw return could continuously increase SOC in an inter-annual rotation of soybean and maize cropping system in Mollisol. Meanwhile, long-term fertilization can alter SOM chemical composition, with lower humification degree under the combined application of straw and mineral NPK fertilizers.