Key factors driving soil diazotrophic community assembly and nitrogenase activity in Qinghai-Tibet alpine meadows

Abstract

Biological nitrogen (N) fixation by soil diazotrophs is the primary source of nitrogen for natural grasslands, especially at high altitudes such as alpine meadow ecosystems. However, the mechanisms shaping the compositions and assembly processes of diazotrophic communities, as well as soil nitrogenase activity in alpine meadows remain poorly understood. In this study, 241 soil samples were collected from alpine meadows on the Qinghai-Tibet Plateau to investigate the distribution patterns and driving factors of diazotrophic communities and nitrogenase activities. Our results showed that soil N fixation potential across all samples ranged from 28.8 to 110 nmol C₂H₄ g^-1h^-1, and Nitrospirillum and Bradyrhizobium were the most dominant diazotrophic genera. The composition and diversity of soil diazotrophic communities were mainly influenced by soil pH, and to a lesser extent by aridity index and mean annual precipitation. The abundance of nifH gene decreased linearly with soil pH, whereas the ɑ-diversity increased linearly with soil total phosphorus. Null model analysis indicated that deterministic processes governed diazotrophic community assembly, and more acidic soil conditions led to more phylogenetically clustered diazotrophic communities. Partial least squares path modeling and linear regression analyses identified diazotrophic community composition, followed by climatic factors, nifH gene abundance and total phosphorus, as the dominant regulators of soil N fixation potential in alpine meadows. Unexpectedly, soil N fixation potential was not closely related to dominant diazotrophic genera, diversity, or assembly, but highly related to the less abundant diazotrophs. This study provides a novel insight into the ecological mechanisms shaping soil biological nitrogen fixation in alpine meadow ecosystems.