Effects of long-term maize stovers mulching on maize yield and microbial necromass carbon accumulation

Mulching of maize stovers can be widely used to reduce soil evaporation, thereby to prevent soil degradation, particularly for the conservation of tillage management in northeast China. Continuous mulching with maize stovers can also significantly increase organic carbon in soil. However, the underlying mechanisms on long-term mulching of maize stovers remain unclear, due to microorganism can serve as a critical driving force of transformation to organic carbon in soil. In this study, an eight-year field experiment with/without stovers mulching treatment was conducted in the National Field Observation and Research Station of Shenyang. Samples of plant and soil (0-10 cm and 10-20 cm) were collected after the annual harvest. Biomarker-amino sugars, together with the yield of maize and organic carbon in soil, were measured to calculated microbial necromass in soil. The results showed that there was no significant increase in the yield of maize, organic carbon in soil, and microbial necromass carbon, when mulching stovers in the initial 2-5 years. The reason was probably that the maize stovers cannot be completely decomposed during initial five years in this area. When fitting 8-year data, a first–order model indicated that the change of organic carbon and microbial necromass carbon in soil followed a nonlinear pattern under specific tillage systems, showing an exponential increase, and then a steady state. In the first–order nonlinear parameters for the surface soil, the maximums organic carbon of soil in with/without mulching treatments were 11.35 g/kg and 12.67 g/kg, respectively, and the time to reach a steady state was 2.1 years and 15.4 years, respectively. In microbial necromass carbon, the maximums in two treatments were 8.11, 11.35 g/kg, respectively, and the time to reach steady state was 14.2 years and 26.3 years, respectively. Specifically, the mulching of maize stovers can efficiently increase the maximum of soil organic carbon and microbial necromass carbon, while to extend the time to reach steady state in the surface soil. It infers that the mulching of maize stovers can enhance the retention ability of soil organic carbon and microbial necromass, to improve their accumulation capacity in the surface soil. Meanwhile, the maximum ratios of microbial necromass carbon to organic carbon in the surface layer in two treatments were 72.57% and 76.94%, respectively, and the time to reach a steady state was 24.1 years and 26.7 years, respectively. There was a great contribution of microbial necromass to soil organic carbon in a long-term when maize stovers mulching. In the surface soil, the mulching of maize stovers did not increase soil organic carbon, microbial necromass carbon and their proportions, indicating the average maximums were 10.37 g/kg, 6.82 g/kg and 67.61 %, respectively, and the average time to steady state were 1.7 years, 8.8 years and 16.2 years, respectively. The reason was that the continuous mulching with maize stovers in the surface soil can produce “hotspots” to provide a readily available source of carbon, nutrient and energy for microorganisms, but there was no significant difference of root mass in the subsurface soil. This finding demonstrated that the mulching with maize stovers can effectively increase soil organic carbon via the accumulation of microbial necromass in the whole soil layer, thereby to reduce the risk of carbon loss in soil