Effects of green manure returning on soil microbial biomass carbon and mineralization of organic carbon in smash ridging paddy field

Returning green manure is an important method for controlling soil adversity in modern intensive paddy fields. To evaluate the influence of smash ridging under green manure returning on soil microbial biomass carbon and carbon mineralization in paddy fields, annual field application tests of double-cropping rice were conducted with two tillage modes in early rice (smash ridging and conventional tillage), four fertilization treatments (no fertilizer, and under the same fertility conditions applying chemical fertilizer, single green manure with chemical fertilizer, and double green manure with chemical fertilizer), and chemical fertilizer application to late rice for no-tillage. The results showed that the content of soil microbial biomass carbon under single green manure returning to paddy fields can be up to two times higher than that of conventional tillage, which can effectively increase the utilization rate of soil carbon by microbes. An increase in the amount of green manure increased the soil organic carbon mineralization and mineralization potential in the smash ridging paddy field. Compared with chemical fertilizer application, the cumulative mineralization of soil organic carbon and mineralization potential of early and late rice increased by 1.6%-32.8% and 0.6%-16.6%, respectively, under single green manure returning, and 58.6%-70.9% and 29.6%-38.4%, respectively, under double green manure returning. Soil respiration intensity of late rice during no-tillage at the full heading and harvest stages was reduced by 33.4% and 38.7%, respectively, compared with conventional tillage; and reduced by 8.5%-31.4% compared with the other smash ridging treatments. The metabolic quotient in rice soil with smash ridging under single green manure returning decreased by 65.5% compared with conventional tillage. Compared with conventional tillage, smash ridging under double green manure returning and under chemical fertilizer increased by 20.3% and 159.2%, respectively; smash ridging under double green manure returning can effectively alleviate the increase in the soil metabolic quotient. There was a negative correlation between the content of microbial biomass carbon and the priming effect of organic carbon mineralization, with a correlation coefficient of 0.44 under green manure returing. There was a significant positive correlation between cumulative mineralization and the metabolic quotient, with a correlation coefficient of 0.59 under smash ridging. In conclusion, a combination of green manure returning and smash ridging can increase the content of soil microbial biomass carbon in paddy soil, reduces soil respiration intensity during the growth stages, and serves as an important technical measure for the regulation and control of soil stability and carbon fixation.