Photosynthesis is the largest flux of carbon between the atmosphere and Earth's surface and is driven by enzymes that require nitrogen, namely, ribulose‐1,5‐bisphosphate (RuBisCO). Thus, photosynthesis is a key link between the terrestrial carbon and nitrogen cycle, and the representation of this link is critical for coupled carbon‐nitrogen land surface models. Models and observations suggest that soil nitrogen availability can limit plant productivity increases under elevated CO2. Plants acclimate to elevated CO2 by downregulating RuBisCO and thus nitrogen in leaves, but this acclimation response is not currently included in land surface models. Acclimation of photosynthesis to CO2 can be simulated by the photosynthetic optimality theory in a way that matches observations. Here, we incorporated this theory into the land surface component of the Energy Exascale Earth System Model (ELM). We simulated land surface carbon and nitrogen processes under future elevated CO2 conditions to 2100 using the RCP8.5 high emission scenario. Our simulations showed that when photosynthetic acclimation is considered, photosynthesis increases under future conditions, but maximum RuBisCO carboxylation and thus photosynthetic nitrogen demand decline. We analyzed two simulations that differed as to whether the saved nitrogen could be used in other parts of the plant. The allocation of saved leaf nitrogen to other parts of the plant led to (1) a direct alleviation of plant nitrogen limitation through reduced leaf nitrogen requirements and (2) an indirect reduction in plant nitrogen limitation through an enhancement of root growth that led to increased plant nitrogen uptake. As a result, reallocation of saved leaf nitrogen increased ecosystem carbon stocks by 50.3% in 2100 as compared to a simulation without reallocation of saved leaf nitrogen. These results suggest that land surface models may overestimate future ecosystem nitrogen limitation if they do not incorporate leaf nitrogen savings resulting from photosynthetic acclimation to elevated CO2.

中文翻译：

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由于叶片氮的节省，光合作用对 CO2 的驯化增加了生态系统的碳储存


光合作用是大气和地球表面之间最大的碳通量，由需要氮的酶驱动，即 1,5-二磷酸核酮糖 （RuBisCO）。因此，光合作用是陆地碳和氮循环之间的关键环节，这种联系的表示对于耦合碳氮地表模型至关重要。模型和观察表明，土壤氮的可用性会限制植物在 CO2 升高的情况下生产力的提高。植物通过下调 RuBisCO 来适应升高的 CO2，从而下调叶片中的氮，但这种驯化反应目前不包括在地表模型中。光合作用对 CO2 的适应可以通过光合最优性理论以与观测结果相匹配的方式进行模拟。在这里，我们将该理论纳入了能源百万兆次级地球系统模型 （ELM） 的地表组件中。我们使用 RCP8.5 高排放情景模拟了未来 CO2 升高至 2100 年条件下的地表碳和氮过程。我们的模拟表明，当考虑光合驯化时，光合作用在未来条件下增加，但最大的 RuBisCO 羧化，因此光合氮需求下降。我们分析了两个模拟，这两个模拟在节省的氮是否可以用于工厂的其他部分方面有所不同。将节省的叶片氮分配到植物的其他部分导致 （1） 通过减少叶片氮需求直接缓解植物氮限制，以及 （2） 通过增强根系生长间接减少植物氮限制，导致植物氮吸收增加。因此，重新分配保存的叶片氮使生态系统碳储量增加了 50。3 年为 2100%，与没有重新分配保存的叶片氮的模拟相比。这些结果表明，如果地表模型没有考虑光合作用适应升高的 CO2 而导致的叶片氮节省，它们可能会高估未来的生态系统氮限制。