Straw management stabilizes the chemical composition ofSoil Organic Carbon (SOC): the relationship with aggregate‐associated C in a rice‐rape cropping system

Abstract Soil organic matter stored in soil aggregate fractions is a major soil organic carbon (SOC) stabilization process. The distribution of SOC within soil aggregate fractions and their association with SOC chemical compounds in the agroecosystem is not well understood. Our long‐term study (6 and 9 years) in central China examined the effects of tillage and straw management practices on chemical functional components of SOC, SOC contents of soil aggregates, and the relationship of the SOC composition with aggregate‐associated C in a rice‐rape cropping system. The soil samples from 2 sites were separated into six aggregate classes (>5, 5–2, 2–1, 1–0.5, 0.5–0.25, and <0.25 mm) by a wet‐sieving method. Chemical composition of SOC was assessed by CPMAS 13 C NMR, FTIR and SEM techniques. Among all chemical components of the SOC, O‐alkyl C was the most important C type, which increased (about 54–110%) at both our sites, although aromatic C increased (about 30%) and carbonyl C decreased at one site ‐ Jingzhou after straw was returned. While, aromatic, carboxylic, aliphatic and amide components were stabilized after straw return. Alkyl C and O‐alkyl C ratio were enhanced after straw return but the aromaticity index and hydrophobicity index altered in the studied sites. Among all defined aggregates, approximately 10–18% of SOC was stored in >5 mm, 5–2 mm, 2–1 mm aggregates and 5–10% was stored in other aggregate fractions (1–0.5 mm, 0.5–0.25 mm and <0.25 mm) after straw return. SOC Chemical components were positively and negatively correlated with aggregate‐associated C fractions. This study concluded that straw management improved the chemical composition of SOC and increased the SOC contents of bulk soil as well as helped store more organic matter in macroaggregates.