Abstract:Green manure returning is an important way to affect soil organic matter in farmland. In this paper, paddy fields were cultivated under conventional tillage and smash ridging respectively and four fertilization modes were carried out in order to explore the effect of green manure returning on soil labile organic carbon (LOC) in paddy field under smash ridging, the fertilization modes included:1) no fertilizer, 2) single chemical fertilizers, 3) single green manure with chemical fertilizers under same fertility as single chemical fertilizers, and 4) double green manure with chemical fertilizers under same fertility as single chemical fertilizers. The results showed that:1) Smashing ridging reduced the content of soil total organic carbon (TOC). Increasing green manure returning promoted soil TOC content under smash ridging, in which double green manure returning increased soil TOC content by 25.6%-60.1%, while single green manure increased TOC contents in all layers of soil profiles. 2) Green manure returning increased the content of soil labile organic carbon (LOC) and improved soil carbon sequestration level under smash ridging, and increasing green manure also improved soil carbon pool management index (CPMI) under smash ridging. Under smash ridging, the contents of low, medium and high LOC were increased by 28.4%-47.9%, 37.3%-49.9% and 30.1%-49.6% under single green manure returning, and by 47.8%-64.3%, 40.0%-61.3% and 29.6%-59.7% under double green manure returning, respectively. 3) Soil TOC and CPMI under smash ridging changed linearly with the growth stages at and after booting stage (R2 was 0.999). Under smash ridging, with the increase of soil depth, the proportion of soil non-labile organic carbon (NLOC) with green manure returning showed a slow linear decrease (R2 was 0.998-0.999), indicating a regular effect of green manure returning on potential utilization efficiency of the vertical migration and change of soil LOC. Therefore, green manure returning can effectively increase soil organic carbon storage and the stability of labile organic carbon under smash ridging, which is important in regulating soil carbon pool and improving soil quality.