A recent publication (Mason et al. in Science 376:261, 2022a) suggested that nitrogen (N) availability has declined as a consequence of multiple ongoing components of anthropogenic global change. This suggestion is controversial, because human alteration of the global N cycle is substantial and has driven much-increased fixation of N globally. We used a simple model that has been validated across a climate gradient in Hawai ‘i to test the possibility of a widespread decline in N availability, the evidence supporting it, and the possible mechanisms underlying it. This analysis showed that a decrease in δ¹⁵N is not sufficient evidence for a decline in N availability, because δ¹⁵N in ecosystems reflects both the isotope ratios in inputs of N to the ecosystem AND fractionation of N isotopes as N cycles, with enrichment of the residual N in the ecosystem caused by greater losses of N by the fractionating pathways that are more important in N-rich sites. However, there is other evidence for declining N availability that is independent of ¹⁵N and that suggests a widespread decline in N availability. We evaluated whether and how components of anthropogenic global change could cause declining N availability. Earlier work had demonstrated that both increases in the variability of precipitation due to climate change and ecosystem-level disturbance could drive uncontrollable losses of N that reduce N availability and could cause persistent N limitation at equilibrium. Here we modelled climate-change-driven increases in temperature and increasing atmospheric concentrations of CO₂. We show that increasing atmospheric CO₂ concentrations can drive non-equilibrium decreases in N availability and cause the development of N limitation, while the effects of increased temperature appear to be relatively small and short-lived. These environmental changes may cause reductions in N availability over the vast areas of Earth that are not affected by high rates of atmospheric deposition and/or N enrichment associated with urban and agricultural land use.

最近的一篇出版物（Mason 等人，《Science》376:261, 2022a）表明，由于人为全球变化的多个持续因素，氮 (N) 的可用性已经下降。这一建议是有争议的，因为人类对全球氮循环的改变是巨大的，并导致全球氮固定量大大增加。我们使用了一个已经在夏威夷的气候梯度上得到验证的简单模型来测试氮可用性广泛下降的可能性、支持它的证据以及其背后的可能机制。该分析表明，δ ¹⁵ N的减少并不足以证明氮可用性下降，因为生态系统中的 δ ¹⁵ N 既反映了生态系统氮输入中的同位素比率，也反映了氮同位素作为氮循环的分馏，并且富集生态系统中残留的氮是由于分馏途径造成的更大的氮损失而造成的，这在富含氮的地点更为重要。然而，还有其他证据表明氮利用率下降，与¹⁵ N无关，这表明氮利用率普遍下降。我们评估了人为全球变化的组成部分是否以及如何导致氮可用性下降。早期的研究表明，气候变化和生态系统水平扰动导致的降水变化的增加都可能导致氮的不可控制的损失，从而减少氮的可用性，并可能导致平衡状态下持续的氮限制。在这里，我们模拟了气候变化驱动的温度升高和大气中 CO ₂浓度的增加。我们表明，大气CO ₂浓度的增加会导致氮有效性的非平衡下降，并导致氮限制的发展，而温度升高的影响似乎相对较小且短暂。这些环境变化可能会导致地球上大部分地区的可用氮量减少，而这些区域并未受到与城市和农业土地使用相关的高大气沉降率和/或氮富集率的影响。