CHANGES IN SOIL ORGANIC CARBON AND ITS FRACTIONS AFTER 13 YEARS OF CONTINUOUS STRAW RETURN IN A SOYBEAN-MAIZE CROPPING SYSTEM

Straw return to the soil is proposed as an effective practice to increase soil organic carbon (SOC) storage in croplands. Based on a 13-year field experiment with soybean-maize cropping system, we studied the changes of total SOC and SOC fractions under no fertilizer (NF), mineral fertilizers (NPK) and mineral fertilizers with straw return (NPKS). Compared with the initial soil, SOC storage in the bulk soil significantly increased by 7.19% in the NPKS treatment, with an annual increase of 0.32 Mg ha(-1); while the SOC storage significantly decreased by 3.47% in the NF treatment, and no significant change was recorded in NPK treatment after 13 years. The NPKS treatment significantly increased the storage of free light fraction carbon (fLFC), occluded light fraction carbon (oLFC), heavy fraction (HFC), humic acid carbon (HAC) and fulvic acid carbon (FAC) by 44.4, 31.8, 5.47 10.5 and 3.92%, respectively. The HAC contributed the highest percentage (47.0%) of carbon to the improvement of SOC after 13 years of straw return. Therefore, straw return was conducive to the accumulation of labile fractions (fLFC and oLFC) which were in favor of soil fertility. Simultaneously, the increased HAC after straw return are beneficial to carbon sequestration.