Straw amendment and soil tillage alter soil organic carbon chemical composition and are associated with microbial community structure

Soil organic carbon (SOC) accumulation and its components are influenced by agricultural practices, which are interfered by soil microorganisms. A field experiment was conducted to investigate how straw amendment and soil tillage influenced SOC fractions and chemical components, and their interrelations with microbial communities evaluated through phospholipid fatty acids (PLFAs) in Mollisols. Density fractionation and Fourier-transform Infrared Spectroscopy were combined to characterize OC properties, including free and occluded light fractions (fLFC, oLFC), heavy fraction (HFC), aliphatic-C and aromatic-C functional groups. Compared with control soil, straw amendment caused surface SOC increased by 6.7%–12.4% (P < 0.05). The dissolved OC, microbial biomass carbon (SMBC), fLFC, oLFC, aliphatic-C, and total PLFAs also increased after straw amendment. Whereas the aromatic-C proportions in bulk soil, fLFC, and oLFC decreased. The OC components and microbial properties didn't obviously change in soil with straw ash amendment. Straw amendment to the subsurface layer only increased subsurface SOC and OC fractions (P < 0.05). Principal component analysis revealed that soil microbial community structure changed with the amounts of straw amendment, soil tillage further underpinned the separation. The separation via PC1 was associated with labile OC components, including SMBC, fLFC, oLFC, and aliphatic-C pointing to soils with higher amounts of straw amendment, and with recalcitrant aromatic-C and HFC pointing to soils without straw or straw ash. Higher amounts of straw amendment plus soil tillage affected soil microbial community structure, and consequently influenced OC sequestration by altering labile OC fractions and components in Mollisols.