Effects of Different Ratio Returning Rice Residue and Milk Vetch on Soil Microbial Communities and Functions

ABSTRACT Residue return is a widespread field management practice known for its environmental benefits, including C storage and soil properties. Milk vetch (MV) is the main green manure used to improve the soil fertility of paddy rice in southern China. Returning MV to the field provides the necessary nutrients for crop growth, reduces soil microbial biomass (including bacillus, actinomycete, and AM‐fungi), and enhances the priming effect of short‐term N (increasing functional genes of gdhA and gdh2, nasB, and amoB). However, the microbial mechanism of the proportions of MV with rice residue returning to paddy fields still needs to be clarified. We mixed rice residue and MV at ratios of 1:0 (RS), 2:1 (RM 2_1), 1:1 (RM 1_1), 1:2 (RM 1_2), and 0:1 (MV), buried them in paddy fields using a little bag method, and analysed the C, N, and microbial community of residues over a period of 97 days from May to August 2022 in Ningde, China. C was released from the residue (−30% to −62%) in all treatments after 97 days. N was released from the residue only under MV and RM 1_2 (−65%, −58%, respectively) from 0 to 97 days. From 0 to 7 days, with the release of straw N under MV and RM 1_2 (−44%, −54%), Simpson index (+5%, +9%), relative abundance of fermentation genes (+169%, +73%) and chemoheterotrophic genes (+210%, +112%) increased, respectively. In all treatments, the Shannon index of the bacterial community increased by 12%–66% on the 97th day, especially from Day 55 to 97 (+9% to +36%) compared to that from Day 0 to 55 (−7.7% to +22%). From Day 55 to 97, the relative abundance of signal transduction (+14% to +42%) and lipid metabolism genes (+19% to +26%) increased, but that of translation (−23% to −31%) and folding sorting and degradation genes (−12% to −16%) decreased. This study found that the abundance of aromatic compound degradation genes increased by 2404%–4608% on Day 50, while the relative abundance of Proteobacteria and Firmicutes increased during 0–55 days and 0–7 days (265%–598% and 57%–100%, respectively) and then decreased (60%–77% and 45%–66%, respectively). Bacteroidota decreased by 46%–67% during the experiment, whereas Acidobacteriota increased by 495%–8811%. MV treatment had the highest relative abundance of Firmicutes (66%) on the 7th day and Proteobacteria (64%) on the 55th day. RM 1_1 had the highest Bacteroidota (24%) on the 97th day. Ascomycota was the fungal community with the highest relative abundance (89%–96%). However, its abundance reduced by 19%–29% on the 97th day in all treatments. Residue returning greatly increased the network of fungal communities from 0 to 90 days but reduced that of bacterial communities on the 55th day. Rice height (91–92 cm) and yield (117–118 kg ha −1 ) were the highest under MV and RM 1_2. In conclusion, the residue with a higher ratio (> 1) of MV and rice decomposition triggered N release through increased microbial function of fermentation and chemoheterotrophy (Firmicutes) on the 7th day and aromatic compound degradation (Proteobacteria) on the 55th day, resulting in the highest rice yield.