Abstract：
Determination of tipping points in nitrogen (N) isotope (δ15N) natural abundance, especially soil δ15N, 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 δ15N to increases in aridity and if these responses correspond well with soil N cycling remains largely unknown. In this study, we investigated soil δ15N 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 δ15N 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 δ15N 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 δ15N positively correlated with most soil N-cycling traits, suggesting that high soil δ15N may result from the \"openness\" of soil N cycling. Conversely, in drier regions, soil δ15N showed insignificant relationships with soil N-cycling traits and correlated well with factors, such as soil-available phosphorus and foliage δ15N, demonstrating that pathways other than typical soil N cycling may dominate soil δ15N under drier conditions. Overall, these results highlight that different ecosystem N-cycling processes may drive soil δ15N along the aridity gradient, broadening our understanding of N cycling as indicated by soil δ15N under changing drought regimes. The aridity threshold of soil δ15N should be considered in terrestrial N-cycling models when incorporating 15N isotope signals to predict N cycling and availability under climatic dryness.
摘要：
测定氮（N）同位素（δ15N）自然丰度的临界点，特别是土壤δ15N，随着干旱的增加，对于估计陆地生态系统中的氮循环动力学和氮限制至关重要。然而,土壤δ15N对干旱的增加是否存在线性或非线性响应，以及这些响应是否与土壤N循环很好地对应，仍然未知。在这项研究中,我们沿着干旱梯度研究了青藏高原3000公里干旱地区的表层土壤和下层土壤中的土壤δ15N和土壤氮循环特征。我们发现，在表层土壤和底土的干旱指数（AI）=0.27和0.29的阈值下，干旱对土壤δ15N值的影响从负变为正，分别,尽管土壤氮库和氮转化率随着两个土壤层干旱的增加而线性下降。此外，我们确定了高于和低于AI阈值的土壤δ15N和土壤氮循环性状之间的显着不同相关性（表土和底土为0.27和0.29，分别)。具体来说,在较湿润的地区，土壤δ15N与大多数土壤氮循环性状呈正相关，这表明高土壤δ15N可能是土壤氮循环的“开放性”所致。相反，在较干燥的地区，土壤δ15N与土壤氮素循环性状关系不显著，与因子相关性良好，如土壤有效磷和树叶δ15N，证明在较干燥的条件下，除典型的土壤氮循环外，其他途径可能主导土壤δ15N。总的来说,这些结果表明，不同的生态系统氮素循环过程可能沿着干旱梯度驱动土壤δ15N，在干旱变化的情况下，土壤δ15N表明了我们对氮循环的理解。在结合15N同位素信号以预测气候干燥下的氮循环和有效性时，应在陆地氮循环模型中考虑土壤δ15N的干旱阈值。
