Rising CO2 and warming reduce global canopy demand for nitrogen

Nitrogen (N) limitation has been considered as a constraint on terrestrial carbon uptake in response to rising CO₂and climate change. By extension, it has been suggested that declining carboxylation capacity (<em>V</em>_cmax) and leaf N content in enhanced-CO₂­ experiments and satellite records signify increasing N limitation of primary production. We predicted <em>V</em>_cmaxusing the coordination hypothesis, and estimated changes in leaf-level photosynthetic N for 1982–2016 assuming proportionality with leaf-level <em>V</em>_cmax at 25˚C. Whole-canopy photosynthetic N waas derived using satellite-based leaf area index (LAI) data and an empirical extinction coefficient for <em>V</em>_cmax, and converted to annual N demand using estimated leaf turnover times. The predicted spatial pattern of <em>V</em>_cmaxshares key features with an independent reconstruction from remotely-sensed leaf chlorophyll content. Predicted leaf photosynthetic N declined by 0.28 %/year, while observed leaf (total) N declined by 0.2–0.25 %/year. Predicted global canopy N (and N demand) declined from 1997 onwards, despite increasing LAI. Leaf-level responses to rising CO₂, and to a lesser extent temperature, may have reduced the canopy requirement for N by more than rising LAI has increased it. This finding provides an alternative explanation for declining leaf N that does not depend on increasing N limitation.氮（N）限制被认为是制约陆地生态系统碳吸收以响应大气CO₂浓度升高与气候变化的核心限制因子。据此进一步推导，高CO₂浓度操控实验与卫星观测记录中观测到的羧化能力（V_cmax）与叶片氮含量下降现象，表明陆地初级生产受到的氮限制正在加剧。本研究基于协调假说对V_cmax进行预测，并假设25℃下叶片水平V_cmax与叶片光合氮含量呈比例关系，据此估算了1982-2016年的叶片光合氮含量变化。冠层总光合氮含量通过基于卫星的叶面积指数（LAI）数据与V_cmax的经验消光系数推导得到，并结合估算的叶片周转时间转换为年氮需求总量。预测得到的V_cmax空间分布格局，与基于遥感叶片叶绿素含量得到的独立重建结果具有关键共性特征。预测的叶片光合氮含量以每年0.28%的速率下降，而观测得到的叶片（总）氮含量以每年0.2%~0.25%的速率下降。尽管叶面积指数（LAI）持续升高，但预测得到的全球冠层氮总量（及氮需求总量）自1997年起呈下降趋势。叶片对大气CO₂浓度升高的响应，以及影响程度相对较弱的温度响应，可能使冠层氮需求的减少幅度超过了叶面积指数升高所带来的氮需求增加幅度。这一发现为叶片氮含量下降现象提供了一种无需依赖“氮限制加剧”假设的全新解释路径。