Elevational Patterns of Soil Nitrogen Forms and Transformations in the Southeastern Qinghai–Xizang Plateau

Alpine forest soil in the southeastern Qinghai–Xizang Plateau plays a crucial role in regional and global climate and biogeochemical cycles, yet the elevational distribution of soil nitrogen (N) availability and losing risk is unresolved. In this study, we characterized soil N composition and key N transformation processes across different elevations in 3 typical mounts of the Qinghai–Xizang Plateau. In contrast to previous suggestions, soil total N and ammonium are found to accumulate in high elevation zones. This accumulation of N at higher altitudes is due to a consistent soil net N mineralization rate coupled with an extremely low net nitrification rate, which is suppressed by low pH and high soil moisture in high elevation zones. Moreover, the elevated rates of biological N fixation along the elevation further contribute to N accumulation in high elevation regions in which the acid-tolerant Bradyrhizobium, plant-associated Herbaspirillum, and Klebsiella are identified as the key diazotrophic microbial taxa responsible for active N fixation. Collectively, our results suggest that total N and NH4+-N accumulation in higher altitude zone is a ubiquitous phenomenon in the southeast Qinghai–Xizang Plateau, with lower nitrification rates and higher biological nitrogen fixation being key processes enabling this occurrence.