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

Abstract

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.

Wildfire-Derived Nitrogen Aerosols Threaten the Fragile Ecosystem in Himalayas and Tibetan Plateau

“…Numerous field simulation experiments have been conducted to explore the ecological effects of N deposition. However, most of these experiments mimicked N deposition by adding NH 4 NO 3 , especially the simulation studies conducted in the HTP. , During the wildfire episodes, the abundance of WSON in aerosols (accounting for 26.1% of TN) already exceeded the proportions of NH 4 + -N and NO 3 – -N (Figure ), emphasizing the need to consider WSON when assessing the ecological impact of N deposition. For example, amino acids in WSON are highly bioavailable and play an important role in the nutrient budgets of low-N ecosystems …”

Nonlinear responses of community diversity, carbon and nitrogen cycles of grassland ecosystems to external nitrogen input

“…, 降 低物种多样性 (Isbell et al, 2013;Tian et al, 2016b; 张世虎等, 2022)。同时, 氮富集会提高植被生产力 (LeBauer & Treseder, 2008;杨晓霞等, 2014), 导致 生态系统碳库增大 (Yue et al, 2016;秦加敏等, 2022)。此外, 外源氮输入增加还会改变土壤氮转化 过程 (Lu et al, 2011;Mao et al, 2021), 促进N 2 O排 放 (Lu et al, 2011;Shcherbak et al, 2014;Peng et al, 2018)。因此, 理解草地生态系统结构和功能对氮富 集的响应及其机制有助于综合评估外源氮输入的生 态效应。 为了揭示草地生态系统结构和功能对外源氮输 入的响应及其机制, 生态学家在全球草地生态系统 开展了大量氮添加控制实验。然而, 早期的实验大 多仅设置氮添加和对照两个处理 (Neff et al, 2002;Dijkstra et al, 2004)。事实上, 持续的氮输入会使生 态系统经历从"氮限制"到"氮饱和"再到"氮过量"的 过程, 进而可能导致生态系统结构和功能呈非线性 变化 (Aber et al, 1989(Aber et al, , 1998吕超群等, 2007;Niu et al, 2016;Peng et al, 2020)。和单个水平的氮添加控 制实验相比, 全球范围内建立的多水平氮添加控制 实验平台为揭示草地生态系统结构和功能对氮输入 的非线性响应机制提供了有效途径。早在20世纪80 年代, 著名生态学家David Tilman教授(1987)在 Cedar Creek建立了草地生态系统多水平氮添加控 制实验平台。 该实验共设置8个氮添加水平, 即0、 1、 2、3.4、5.4、9.5、17、27.2 g•m -2 •a -1 。自此之后, 特 别是近20年来不少学者在热带草地 (Raposo et al, 2020;Silva et al, 2020)、温带草地 (Bai et al, 2010;Tian et al, 2016b;Zhang et al, 2016;McHugh et al, 2017;王玉冰等, 2020;Yang et al, 2022)、高寒草地 (Liu et al, 2013;Peng et al, 2017a; 图1 青藏高原高寒草原多水平氮添加实验平台景观(刘洋 摄)。该实验平台隶属于中国科学院植物研究所杨元合课题 组, 位于青海省刚察县三角城种羊场, 开始于2013年5月, 涉及8个氮添加水平(0, 1, 2, 4, 8, 16, 24, 32 g•m -2 •a -1 ), 所施 氮肥类型为NH 4 NO 3 。 Fig. 1 Multi-level nitrogen (N) manipulation experiment in an alpine steppe on the Qingzang Plateau (photo credit: LIU Yang).…”