Effects of substitution of chemical fertilizer by Chinese milk vetch on distribution and composition of aggregates-associated organic carbon fractions in paddy soils

Purpose: Green manure plays a key role in reducing chemical fertilizer applications and increasing soil organic carbon (SOC) stock. The effects of chemical fertilizer substitution by Chinese milk vetch (MV) on the distribution and composition of organic carbon fractions in macroaggregates and microaggregates and SOC stability mechanism were investigated in a paddy soil in southern China. Methods A 11-year (2008–2018) field experiment was conducted, including no fertilizer (CK), 100% NPK fertilizer (F100), MV with different percentages of chemical fertilizer (MV + F100, MV + F80, MV + F60 and MV + F40). The soil was separated into distinct organic carbon fractions using aggregate density fractionation and SOC chemical structure was analyzed by fourier-transform infrared and nuclear magnetic resonance. Results Chemical fertilizer substitution by MV increased SOC contents in the bulk soil by 9.07% (MV + F80), 5.08% (MV + F60), 17.93% (MV + F40) compared to F100. Organic carbon fraction mainly existed in mSOC, accounting for 70.28–83.70% and 69.43–83.97% of the relative mass of macroaggregates and microaggregates, respectively. Compared to F100, aromatic C increased by 11.59% and 29.06% under MV + F60 within mSOC in macroaggregates and microaggregates. Within the mSOC in macroaggregates, compared to CK, MV + F80 and MV + F60 promoted the proportion of alkyl C by 9.61% and 6.69%, and decreased the content of O-alkyl C by 14.10% and 11.03%, respectively, which correspondingly increased alkyl C/O-alkyl C ratio. Conclusions Substitution of chemical fertilizer by MV (especially MV + F80 and MV + F60) improved the stability of SOC via increasing recalcitrant structure in mSOC, and was conducive to the sequestration of SOC in paddy soils.