Cotton Grown in Rotation with Soybean Alters Soil Organic Matter over Time

Crop rotation could effectively alleviate continuous cropping problems, but how it affects soil organic carbon (SOC) components and chemical structures in case of different duration of continuous cropping (CC) remains unclear. In this study, we selected cotton fields that had been continuously planted for 5, 10, 15, and 20 years for separate soybean rotations (SR). The mechanism of SR to improve soil quality in different duration of CC was determined by SOC components and their chemical structure. The results showed that the content of soil available nutrients, SOC, microbial biomass carbon and soluble organic carbon in the SR treatments significantly increased compared with those in the CC treatment (p < 0.05), but particulate organic carbon content decreased by 24.89–81.10%. Compared with CC in different years, SR treatments significantly reduced soil humus content. SR in 5, 10 and 15 years of CC cotton fields significantly increased the content of sucrase and decreased that of polyphenol oxidase. Compared with CC fields in different years (5-y, 10-y, 15-y, 20-y), SR increased the relative peak intensity of polysaccharides and decreased the relative peak intensity of alcohols and phenols. Among them, the relative peak intensity of R5-y, R10-y and R20-y aliphatic compounds increased, while that of R5-y, R15-y and R20-y aromatic compounds decreased, and R10-y recalcitrant components (Aliphatic + Aromatic) increased.