Aridity threshold for alpine soil nitrogen isotope signature and ecosystem nitrogen cycling
Jinhua Mao, Junxiao Pan, Lei Song, Ruiyang Zhang, Jinsong Wang, Dashuan Tian, Quancheng Wang, Jiaqiang Liao, Jinlong Peng, Shuli Niu
Abstract
Abstract Determination of tipping points in nitrogen (N) isotope (δ 15 N) natural abundance, especially soil δ 15 N, with increasing aridity, is critical for estimating N‐cycling dynamics and N limitation in terrestrial ecosystems. However, whether there are linear or nonlinear responses of soil δ 15 N to increases in aridity and if these responses correspond well with soil N cycling remains largely unknown. In this study, we investigated soil δ 15 N and soil N‐cycling characteristics in both topsoil and subsoil layers along a drought gradient across a 3000‐km transect of drylands on the Qinghai–Tibetan Plateau. We found that the effect of increasing aridity on soil δ 15 N values shifted from negative to positive with thresholds at aridity index (AI) = 0.27 and 0.29 for the topsoil and subsoil, respectively, although soil N pools and N transformation rates linearly decreased with increasing aridity in both soil layers. Furthermore, we identified markedly different correlations between soil δ 15 N and soil N‐cycling traits above and below the AI thresholds (0.27 and 0.29 for topsoil and subsoil, respectively). Specifically, in wetter regions, soil δ 15 N positively correlated with most soil N‐cycling traits, suggesting that high soil δ 15 N may result from the “openness” of soil N cycling. Conversely, in drier regions, soil δ 15 N showed insignificant relationships with soil N‐cycling traits and correlated well with factors, such as soil‐available phosphorus and foliage δ 15 N, demonstrating that pathways other than typical soil N cycling may dominate soil δ 15 N under drier conditions. Overall, these results highlight that different ecosystem N‐cycling processes may drive soil δ 15 N along the aridity gradient, broadening our understanding of N cycling as indicated by soil δ 15 N under changing drought regimes. The aridity threshold of soil δ 15 N should be considered in terrestrial N‐cycling models when incorporating 15 N isotope signals to predict N cycling and availability under climatic dryness.