Aridity threshold for alpine soil nitrogen isotope signature and ecosystem nitrogen cycling.

Determination of tipping points in nitrogen (N) isotope (δN) natural abundance, especially soil δN, 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 δN to increases in aridity and if these responses correspond well with soil N cycling remains largely unknown. In this study, we investigated soil δN 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 δN 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 δN 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 δN positively correlated with most soil N-cycling traits, suggesting that high soil δN may result from the "openness" of soil N cycling. Conversely, in drier regions, soil δN showed insignificant relationships with soil N-cycling traits and correlated well with factors, such as soil-available phosphorus and foliage δN, demonstrating that pathways other than typical soil N cycling may dominate soil δN under drier conditions. Overall, these results highlight that different ecosystem N-cycling processes may drive soil δN along the aridity gradient, broadening our understanding of N cycling as indicated by soil δN under changing drought regimes. The aridity threshold of soil δN should be considered in terrestrial N-cycling models when incorporating N isotope signals to predict N cycling and availability under climatic dryness.