Recent observations document that long-term soil warming in a temperate deciduous forest leads to significant soil carbon loss, whereas chronic soil nitrogen enrichment leads to significant soil carbon gain. Most global change experiments like these are single factor, investigating the impacts of one stressor in isolation of others. Because warming and ecosystem nitrogen enrichment are happening concurrently in many parts of the world, we designed a field experiment to test how these two factors, alone and in combination, impact soil carbon cycling. Here, we show that long-term continuous soil warming or nitrogen enrichment when applied alone followed the predicted response, with warming resulting in significant soil carbon loss and nitrogen fertilization tending towards soil carbon gain. The combination treatment showed an unanticipated response, whereby soil respiratory carbon loss was significantly higher than either single factor alone, but without a concomitant decline in soil carbon storage. Observations suggest that when soils are exposed to both factors simultaneously, plant carbon inputs to the soil are enhanced, counterbalancing soil carbon loss and helping maintain soil carbon stocks near control levels. This has implications for both atmospheric CO₂ emissions and soil fertility and shows that coupling two important global change drivers results in a distinctive response that was not predicted by the behaviour of the single factors in isolation.

最近的观测表明，温带落叶林的长期土壤变暖会导致土壤显著的碳损失，而长期的土壤氮富集会导致土壤显著的碳增加。大多数像这样的全球变化实验都是单因素的，孤立地研究一个压力源的影响。由于变暖和生态系统氮富集在世界许多地方同时发生，我们设计了一个田间实验来测试这两个因素单独和组合如何影响土壤碳循环。在这里，我们表明，单独施用时，长期持续的土壤变暖或氮富集遵循预测的反应，变暖导致土壤显著的碳损失，氮肥趋于土壤碳增加。联合处理显示出意想不到的反应，其中土壤呼吸碳损失显著高于单独的单个因素，但没有随之而来的土壤碳储量下降。观察表明，当土壤同时暴露于这两个因素时，植物对土壤的碳输入会增加，从而抵消土壤碳流失并有助于将土壤碳储量维持在控制水平附近。这对大气中的 CO₂ 排放和土壤肥力都有影响，并表明将两个重要的全球变化驱动因素耦合会产生独特的响应，而这种响应是孤立的单一因素的行为无法预测的。