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Rare species can switch between abundant and rare, respond to various external disturbances, enhance community resilience, and reduce the lag in ecosystem recovery. However, the impact of N enrichment on microbial community dynamic changes in rare species during the growing season is still poorly understood. To clarify which taxa contribute most to the community variation under N enrichment and whether the responses of these taxa vary at different periods of the growing season, we set up a 6-years multi-level field N addition experiment in an alpine meadow on the Tibetan Plateau. The variation in soil bacterial community diversity, composition, species interactions, and assembly process of different abundance profiles were investigated in the early, mid, and late-growing seasons. Approximately 80% of bacteria were classified as rare taxa (including permanently and conditionally rare taxa). The influence of N enrichment on the bacterial community was seasonally dependent. Bacterial community diversity and composition dissimilarity significantly increased only in the early growing season, especially in permanently rare taxa and conditionally rare taxa, but remained stable in the mid-and late-growing seasons. Seasonal fluctuations led to a shift in community composition, and conditionally rare taxa were the primary component of seasonal variation in community composition. Simultaneously, microbial network complexity and stability gradually increased over time, as evidenced by decreased modularity and vulnerability and increased robustness, clustering coefficient, connectivity, and keystone nodes. The assembly of conditionally rare taxa showed a significant difference under N addition, presented as decreased drift and increased dispersal limitation, which are closely related to the composition of the entire bacterial community. Our findings emphasize that the conditionally rare taxa decisive contribute to changes in the bacterial community under N enrichment, but seasonal fluctuations can make bacterial communities resistant to the influence of N enrichment. [ABSTRACT FROM AUTHOR]

To understand effects of soil microbes on soil biochemistry in alpine grassland ecosystems under environmental changes, we explored relationships between soil microbial diversity and soil total nitrogen, organic carbon, available nitrogen and phosphorus, soil microbial biomass and soil enzyme activities in alpine meadow, alpine steppe and cultivated grassland on the Qinghai-Tibetan plateau under three-year warming, enhanced precipitation and yak overgrazing. Soil total nitrogen, organic carbon and NH4-N were little affected by overgrazing, warming or enhanced precipitation in three types of alpine grasslands. Soil microbial biomass carbon and phosphorus along with the sucrase and phosphatase activities were generally stable under different treatments. Soil NO3-N, available phosphorus, urease activity and microbial biomass nitrogen were increased by overgrazing in the cultivated grassland. Soil bacterial diversity was positively correlated with, while soil fungal diversity negatively with soil microbial biomass and enzyme activities. Soil bacterial diversity was negatively correlated with, while soil fungal diversity positively with soil available nutrients. Our findings indicated soil bacteria and fungi played different roles in affecting soil nutrients and microbiological activities that might provide an important implication to understand why soil biochemistry was generally stable under environmental changes in alpine grassland ecosystems. [ABSTRACT FROM AUTHOR]