Magnitude and mechanisms of nitrogen-mediated responses of tree biomass production to elevated CO2: a global synthesis

1. Elevated atmospheric CO2 concentration (eCO2) typically stimulates tree growth, which is mediated by nitrogen (N) availability; but how N regulates tree biomass responses to eCO2 remains uncertain, which limits our prediction of forest carbon (C) cycling under future global change scenarios. 2. A meta-analysis of a global dataset including 3399 observations from 283 papers published from 1980s to February 2021 was conducted with the aim of elucidating N-mediated responses of tree biomass production to eCO2 and the underlying mechanisms. 3. We found that eCO2 stimulated tree biomass production (+32.0%), while it induced accumulation of nonstructural carbohydrates in leaves rather than in woods and roots, suggesting that the production may be C-limited but depend on the sink strength of organs. Biomass responses to eCO2 of N-fertilized trees (+39.6%) were 68.4% greater than those of non-fertilized trees (+25%), confirming that tree growth is also N-limited. Such N limitation was alleviated by the eCO2-induced increases in N uptake and N-use efficiency (NUE), with the former being more important. Increases in tree N pool arose from the enhanced production of fine roots with a lower specific root length, whereas increases in NUE resulted from the flexibility in tissue C:N ratios instead of N resorption efficiency. The positive responses of tree biomass production to eCO2 were greater for ectomycorrhizal trees and conifers than for arbuscular mycorrhizal trees and angiosperms, respectively. 4. Synthesis: Our findings suggest that eCO2 stimulates tree biomass production by increasing C availability, and alleviating N limitation in a feedback way via enhancing N uptake and NUE; and they improve our mechanistic understanding of responses of forest productivity and C sequestration to eCO2 under global change.

1. 大气CO2浓度升高（elevated atmospheric CO2 concentration, eCO2）通常可促进树木生长，这一过程受氮（nitrogen, N）有效性调控；但氮如何调控树木生物量对eCO2的响应仍不明确，这限制了我们对未来全球变化情景下森林碳（carbon, C）循环的预测。
2. 本研究针对1980年代至2021年2月间发表的283篇文献、包含3399组观测数据的全球数据集开展了荟萃分析（meta-analysis），旨在阐明树木生物量生产对eCO2的氮介导响应及其潜在作用机制。
3. 研究结果显示，eCO2可刺激树木生物量生产（增幅达32.0%），同时会引发叶片而非木质部与根系中非结构性碳水化合物的积累，这表明树木生物量生产可能受碳限制，但受各器官汇强度的调控。施氮树木的生物量对eCO2的响应增幅（+39.6%）较未施氮树木（+25%）高出68.4%，证实树木生长同时受到氮限制。eCO2诱导的氮吸收与氮利用效率（nitrogen use efficiency, NUE）提升可缓解这种氮限制，其中前者的作用更为关键。树木氮库的增加源于比根长更低的细根产量提升，而NUE的提升则源于组织碳氮比的适应性调整，而非氮回收效率的改变。外生菌根树木与针叶树的生物量生产对eCO2的正响应分别强于丛枝菌根树木与被子植物。
4. 综合分析表明：本研究结果证实，eCO2可通过提升碳有效性，并通过增强氮吸收与NUE以反馈途径缓解氮限制，从而促进树木生物量生产；本研究深化了我们对全球变化背景下森林生产力与碳固存响应eCO2的机制认知。