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

Determination of tipping points in nitrogen (N) isotope (delta 15N) natural abundance, especially soil delta 15N, 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 delta 15N to increases in aridity and if these responses correspond well with soil N cycling remains largely unknown. In this study, we investigated soil delta 15N 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 delta 15N 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 delta 15N 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 delta 15N positively correlated with most soil N-cycling traits, suggesting that high soil delta 15N may result from the "openness" of soil N cycling. Conversely, in drier regions, soil delta 15N showed insignificant relationships with soil N-cycling traits and correlated well with factors, such as soil-available phosphorus and foliage delta 15N, demonstrating that pathways other than typical soil N cycling may dominate soil delta 15N under drier conditions. Overall, these results highlight that different ecosystem N-cycling processes may drive soil delta 15N along the aridity gradient, broadening our understanding of N cycling as indicated by soil delta 15N under changing drought regimes. The aridity threshold of soil delta 15N should be considered in terrestrial N-cycling models when incorporating 15N isotope signals to predict N cycling and availability under climatic dryness. Using an aridity gradient across a 3000-km transect of drylands on the Qinghai-Tibetan Plateau, we found that the effect of increasing aridity on soil delta 15N values shifted from negative to positive in both topsoil and subsoil layers. We further found different mechanisms driving soil 15N isotopes in different aridity regions. In wetter regions, high soil delta 15N mainly result from 15N fractionation during soil N cycling, whereas, in drier regions, high soil delta 15N might be attributed to other mechanisms.image