Influence of tillage and straw retention on soil carbon pool and maize-wheat yield in Shajiang black soil

Long-term no-tillage mulching with straw retention have led to the shallowness of plough layer, the increase of bulk density at the depth of 10-30 cm layer, and the imbalance of soil nutrients in Shajiang black soil along Huai River, which have limited the improvement of productivity of summer maize (Zea mays) and winter wheat (Triticum aestivum). Tillage practice and residue management play important roles in carbon pool and crop yields in soils. The reasonable combination of tillage with straw retention is an effective method to solve this problem. After 8-years field experiment in wheat - maize cropping systems, four treatments were set up, 1) no-tillage with straw removal for summer maize and winter wheat (N); 2) tillage with straw removal for summer maize and winter wheat (D); 3) no-tillage with straw mulching for summer maize and winter wheat (NS); and (4) no-tillage with straw mulching for summer maize and plough tillage with straw incorporation for winter wheat (DS). Soil samples were taken from the 0-10, 10-20, 20-30, 30-40, 40-50 cm and 50-60 cm soil depths. Soil total organic carbon (TOC), particulate organic C (POC), microbial biomass C (MBC), potassium permanganate-oxidizable C (KMnO4-C) and dissolved organic C (DOC) were measured. Carbon pool management index (CPMI) was also calculated. These indexes and the year-round yield of maize and wheat were comprehensively analyzed to gain the optimal mode for enriching the Shajiang black soil. Our tesults showed that DS treatment increased the contents of TOC, POC, MBC, KMnO4-C and CPMI at the depth of 0-30 cm layer, compared with N treatment, but NS treatment only enhanced the contents of above-mentioned indexes at the depth of 0-10 cm layer. The tillage operation incorporated the maize residue (straw, stubble, and root) to a greater soil depth in DS treatment, which meant that DS had a higher subsoil TOC, POC, MBC, KMnO4-C and CPMI content (10-30 cm) than the other treatments. There were no significant (P<0.05) differences in TOC, POC, MBC, KMnO4-C contents in the 30-60 cm layers among the four treatments. In addition, the total yields for summer maize and winter wheat in four treatments fluctuated widely from year to year. Treatment DS significantly (P<0.05) inceased the year-round crop yield, and the mean crop yield of DS was 14.7%, 12.9% and 8.5% greater than that of N, D and NS over 8 years. Moreover, microbial biomass carbon and KMnO4-C was significantly (P<0.05) affected by the tillage and straw retention and was sensitive to different management practices. Total organic carbon was positively correlated with MBC and KMnO4-C, suggesting they were suitable as an early indicator of soil quality. The significant crop yield improvement observed in DS was attributed to the combined effect of tillage practice and crop residue management on improving the soil fertility and enhancing the labile carbon pool in the Shajiang black soil, which promoted the growth of summer maize and winter wheat. Generally, treatment DS was an sustainable and effective management practice to improve the soil carbon pool and enhance the year-round yield of summer maize and winter wheat in Shajiang black soil along Huai River.