Long-term rice-oilseed rape rotation increases soil organic carbon by improving functional groups of soil organic matter

Aggregate carbon (C) pools, which are closely associated with soil organic C (SOC) stability and agricultural productivity, are greatly affected by the composition of soil organic matter (SOM) functional groups. However, the long-term effect of different rice-based rotations on the composition of SOM functional groups remains elusive. In this work, a 10-year field experiment was conducted to assess the effect of different rice-based rotations (rice-wheat [RW] and rice-oilseed rape rotation [RR]) on the composition of SOM functional groups and soil aggregate C pools. The composition of SOM functional groups was determined by the nuclear magnetic resonance spectroscopy method. Aggregate C pools were isolated through density gradient fractionation. After 10 years of rice-based rotation, compared with RW, RR significantly reduced the proportion of macromolecular functional groups (nuclear magnetic resonance zone (NMR) > 110 mg kg–1) in SOC, especially that of aromatic C (by 25.0%) and carboxyl C (by 26.8%), but significantly increased the contents of SOM compounds, particularly those of polysaccharides (by 30.7%), nitrogen carrier (by 28.4%) and lipids (by 34.7%). RR enhanced the proportions of large macroaggregate (by 56.0%) and intra-microaggregate particulate organic C (iPOC) (by 13.0%), as well as the SOC content (by 12.7%) relative to RW. Structural equation modeling revealed that RR could promote aggregate development and the stock of SOC as iPOC by enhancing the proportion of small functional groups in SOM (NMR < 110 mg kg–1) and promoting SOM compound formation, thereby improving soil C pool stability and content relative to RW in the long-term rotation.