Soil Nitrogen Transformations Respond Diversely to Multiple Levels of Nitrogen Addition in a Tibetan Alpine Steppe

• Published in: Journal of geophysical research. Biogeosciences 2021-05, Vol.126 (5), p.n/a
• Main Authors: Mao, Chao, Kou, Dan, Peng, Yunfeng, Qin, Shuqi, Zhang, Qiwen, Yang, Yuanhe
• Format: Article
• Language: English
• Subjects: Abundance; Ammonia; Archaea; Biomass; Carbon content; Dilution; Dissolved organic carbon; Dynamics; Ecological function; Ecosystems; Enrichment; Environment models; Evaluation; gross nitrification; gross nitrogen mineralization; Immobilization; microbial immobilization; Microorganisms; Mineralization; Nitrification; Nitrogen; nitrogen cycling; Nitrogen enrichment; nitrogen input; Nitrogen isotopes; Oxidation; Regulatory mechanisms (biology); Saturation; Soil; Soil chemistry; Soil dynamics; Soil pH; Soil stabilization; Soils; Steppes; Transformations; Variation
• ISSN: 2169-8953; 2169-8961
• DOI: 10.1029/2020JG006211

Elevated reactive nitrogen (N) input could modify soil N transformations, regulating ecosystem functions such as soil N retention and loss. Although multiple hypotheses advocate nonlinear variations in soil N transformations with continuous N input, there still lacks empirical evidences for the responses of soil N transformations to multiple N additions. Here, based on a manipulative N addition experiment and a 15N pool dilution approach, we explored changes in soil gross N transformations with eight N addition levels and associated mechanisms in a Tibetan alpine steppe. Our results showed that soil gross N mineralization rate (GNM) increased first and then stabilized with increasing N additions. Meanwhile, soil microbial immobilization rate (MIM) exhibited an initially increased and subsequently declined pattern under various N addition levels. In contrast, soil gross nitrification rate (GN) increased linearly across multiple N addition levels. Our results also revealed that variations in GNM were mainly regulated by aboveground vegetation N pool‐induced changes in dissolved organic N content along the N addition gradient. Meanwhile, changes in GN were dominantly modified by soil pH‐induced variations in ammonia‐oxidizing archaea abundance across multiple N addition levels. Additionally, alterations in MIM under various N input levels were primarily controlled by microbial biomass which was regulated by dissolved organic carbon content under low N input and NH4+‐N content at high N level, respectively. Overall, patterns and drivers of soil N transformations observed in this study provide valuable benchmark for Earth system models to better predict ecosystem N dynamics under global N‐enrichment scenarios. Plain Language Summary Our knowledge about the responses of soil N transformations to additional N input could be beneficial to understand the trajectory of soil N dynamics and predict the variations in ecosystem functions under global N‐enrichment scenarios. However, to date, there still lacks direct experimental studies to explore the responses of soil N transformations to increasing N input​s and examine the potential mechanisms regulating these changes, especially when ecosystems experiencing N limitation to saturation scenarios. In this study, we offered a compressive evaluation about the linear and nonlinear changes in gross N transformation rates as well as the regulatory mechanisms with a manipulative experiment of eight N addition levels in a Tib