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Dynamic and Migration Characteristics of Soil Free Amino Acids in Paddy Soil Applied with Milk Vetch

To explore the attribution factors and migration characteristics of free amino acids (FAAs) in paddy soils after green manure application during the entire growth period of rice. Amino acid analyzer, quantitative PCR, and high-throughput sequencing were used to analyze the effects of different application rates of milk vetch on FAAs in paddy soil under equal nitrogen, phosphorus, and potassium conditions. Soil FAAs concentration at different growth stages was highest at the seedling stage and lowest at the tillering stage. The concentration of threonine, alanine, valine, isoleucine, leucine, and phenylalanine was most abundant during the growth period, accounting for 59.42–76.46% of the respective FAAs pool. The application of milk vetch was shown to increase the soil FAAs concentration, especially glutamic acid, which increased by 368.17–680.78%, but the excessive application had an inhibitory effect. Soil bacteria were critical factors affecting soil FAAs dynamics, among which Bacteroidetes, Firmicutes, and Nitrospirae contributed 56.89% to the FAAs dynamics. FAAs displayed significant vertical profile characteristics, and the mobility of serine, glycine, and proline was high. Conclusively, the application of milk vetch was able to significantly change the concentration and composition of soil FAAs, which were affected by soil bacteria.