The boreal forest is an important global carbon (C) sink. Since low soil nitrogen (N) availability is commonly a key constraint on forest productivity, the prevalent view is that increased N input enhances its C sink-strength. This understanding however relies primarily on observations of increased aboveground tree biomass and soil C stock following N fertilization, whereas empirical data evaluating the effects on the whole ecosystem-scale C balance are lacking. Here we use a unique long-term experiment consisting of paired forest stands with eddy covariance measurements to explore the effect of ecosystem-scale N fertilization on the C balance of a managed boreal pine forest. We find that the annual C uptake (i.e. net ecosystem production, NEP) at the fertilized stand was 16 ± 2% greater relative to the control stand by the end of the first decade of N addition. Subsequently, the ratio of NEP between the fertilized and control stand remained at a stable level during the following five years with an average NEP to N response of 7 ± 1 g C per g N. Our study reveals that this non-linear response of NEP to long-term N fertilization was the result of a cross-seasonal feedback between the N-induced increases in both growing-season C uptake and subsequent winter C emission. We further find that one decade of N addition altered the sensitivity of ecosystem C fluxes to key environmental drivers resulting in divergent responses to weather patterns. Thus, our study highlights the need to account for ecosystem-scale responses to perturbations to improve our understanding of nitrogen-carbon-climate feedbacks in boreal forests.

北方森林是重要的全球碳 (C) 汇。由于低土壤氮 (N) 可用性通常是森林生产力的一个关键限制因素，因此普遍的观点是增加氮输入会增强其碳汇强度。然而，这种理解主要依赖于对施氮后地上树木生物量和土壤碳库增加的观察，而缺乏评估对整个生态系统规模碳平衡影响的经验数据。在这里，我们使用了一个独特的长期实验，该实验由带有涡流协方差测量的成对林分组成，以探索生态系统规模的 N 施肥对管理的北方松林碳平衡的影响。我们发现每年的 C 吸收量（即净生态系统产量，在施氮的第一个十年结束时，施肥林的 NEP) 相对于对照林高 16 ± 2%。随后，在接下来的五年中，施肥林和对照林之间的 NEP 比率保持在稳定水平，平均 NEP 与 N 响应为 7 ± 1 g C/g N。我们的研究表明，NEP 的这种非线性响应对长期施氮肥的影响是氮诱导的生长季节碳吸收增加和随后的冬季碳排放增加之间的跨季节反馈的结果。我们进一步发现，10 年的 N 添加改变了生态系统 C 通量对关键环境驱动因素的敏感性，导致对天气模式的不同反应。因此，