Abstract: The objective of this study was to analyze the interannual and seasonal changes in soil microbial biomass and community composition in a vegetable-pasture rotation field in a temperate agro-ecosystem. The pasture 1 to 5 years old after low-cost vegetable production were identified for sampling on an organic farm, compared with permanent pasture. The signature fatty acids in microorganisms were determined using a high-throughput phospholipid fatty acid identification method. Nonmetric multidimensional scaling ordination was used to analyze the structure of microbial communities. A multi-response permutation procedure (nonparametric multivariate analysis) was used to examine pairwise comparisons and group significant differences in the proportional abundance of fatty acid methyl esters among rotation year and season. The results showed that: 1) Soil total microbial biomass and the diversity of phospholipid fatty acids increased during the pasture rotation years, with a decrease in the Gram positive to Gram negative ratio, and an increase in the fungi to bacteria ratio. 2) Season significantly affected the ratio of special fatty acid methyl esters and the biomass of saprophytic fungi and anaerobes. The soil microbial community composition also varied among seasons. 3) The microbial community composition changed after 4 years of pasture growth and became more similar to that of permanent pasture. Carbon mineralization and particulate organic matter drove the structural differentiation of microbial communities.

Key words: soil microbes, community composition, phospholipid fatty acid, pasture growing years