One of the major uncertainties for carbon–climate feedback predictions is an inadequate understanding of the mechanisms governing variations in ecosystem productivity response to warming. Temperature and water availability are regarded as the primary controls over the direction and magnitude of warming effects, but some unexplained results signal that our understanding is incomplete. Using two complementary meta-analyses, we present evidence that soil nitrogen (N) availability drives the warming effects on ecosystem productivity more strongly than thermal and hydrological factors over a broad geographical scale. First, by synthesizing temperature manipulation experiments, a meta-regression model analysis showed that the warming effect on productivity is mainly driven by its effect on soil N availability. Sites with a higher warming-induced increase in N availability were characterized by stronger productivity enhancement and vice versa, suggesting that N is a limiting factor across sites. Second, a synthesis of full-factorial warming × N addition experiments demonstrated that N addition significantly weakened the positive warming effect, because the additional N induced by warming may not further benefit plant growth when N limitation is relieved, providing experimental evidence that N regulates the warming effect. Furthermore, we demonstrated that warming effects on soil N availability were modulated by changes in dissolved organic N and soil microbes. Overall, our findings enrich a new mechanistic understanding of the varying magnitudes of observed productivity response to warming, and the N scaling of warming effects may help to constrain climate projections.

中文翻译：

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氮的可用性决定了生态系统应对气候变暖的生产力

碳-气候反馈预测的主要不确定性之一是对控制生态系统生产力对变暖响应变化的机制的理解不足。温度和水的可用性被认为是控制变暖效应方向和幅度的主要控制因素，但一些无法解释的结果表明我们的理解是不完整的。我们使用两项互补的荟萃分析，提供证据表明，在广泛的地理范围内，土壤氮 (N) 可用性比热力和水文因素更能驱动对生态系统生产力的变暖影响。首先，通过综合温度操纵实验，元回归模型分析表明，变暖对生产力的影响主要是由其对土壤氮有效性的影响驱动的。变暖引起的 N 可用性增加较高的地点的特点是生产力提高更强，反之亦然，这表明 N 是跨地点的限制因素。其次，全因子增温 × N 添加实验的综合表明，N 添加显着削弱了正增温效应，因为当 N 限制解除时，增温诱导的额外 N 可能不会进一步有益于植物生长，提供了 N 调节植物生长的实验证据变暖效果。此外，我们证明了变暖对土壤氮有效性的影响是由溶解的有机氮和土壤微生物的变化调节的。总的来说，我们的发现丰富了对观察到的生产力对变暖的不同程度的反应的新机制理解，