Shifts in microbial community and carbon sequestration in farmland soil under long-term conservation tillage and straw returning

Soil microorganisms participate in soil carbon cycling and they are affected by tillage and straw returning. In this experiment, a split-plot design was adopted beginning in 2002. The whole-plot treatment had three tillage modes, conventional tillage (CT), subsoil tillage (ST) and rotary tillage (RT), and the split-plot treatment had two levels of maize straw returning, full straw returning (F) and no straw returning (0). The soil organic carbon (SOC) contents during 2012–2017 were measured, whereas the microbial community function and structure were assessed by Biolog EcoPlates® and Illumina sequencing technology. The results showed that the SOC increased under all treatments from 2012 to 2017. However, the growth rate for SOC content under STF was the highest. Subsoil tillage and full straw returning significantly increased the metabolic function and metabolic diversity of the soil microbial carbon sources. A principal component analysis (PCA) showed that except for CT0 and RT0, the microbial metabolic functions under the various treatments were different. Amino acids and amines were the main carbon sources that caused changes in the soil microbial communities. A redundancy analysis (RDA) showed that the soil water content, bulk density, total nitrogen (TN) content, SOC and pH significantly accounted for the variance in the microbial community function and structure; however, the bulk density was not conducive to variation in the microbial communities. Subsoil tillage with full straw returning could increase the gene abundances in the metabolic pathways for CO2 fixation and organic acids but decrease the gene abundance in the di- and oligosaccharide metabolic pathway. In conclusion, a combination of subsoil tillage and full straw returning improve the basic soil physicochemical and biological properties, affect soil microbial communities, and promote an increase in the SOC content, which can eventually resolve soil degradation issues.