Department of Biological Sciences, Texas Tech University, Lubbock, Texas, USA.
Climate and Ecosystem Sciences, Lawrence Berkeley National Laboratory, Berkeley, California, USA.
Glob Chang Biol. 2024 Nov;30(11):e17558. doi: 10.1111/gcb.17558.
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 CO. Plants acclimate to elevated CO by downregulating RuBisCO and thus nitrogen in leaves, but this acclimation response is not currently included in land surface models. Acclimation of photosynthesis to CO 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 CO 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 CO.
光合作用是大气与地球表面之间最大的碳通量，由需要氮的酶驱动，即核酮糖-1,5-二磷酸（RuBisCO）。因此，光合作用是陆地碳氮循环的关键环节，该环节的表示对于耦合碳氮陆地表面模型至关重要。模型和观测表明，在升高的 CO 下，土壤氮供应可能会限制植物生产力的提高。植物通过下调 RuBisCO 从而减少叶片中的氮来适应升高的 CO，但这种适应响应目前尚未包含在陆地表面模型中。光合作用对 CO 的适应可以通过光合最优化理论以匹配观测的方式进行模拟。在这里，我们将该理论纳入到能量超大规模地球系统模型（ELM）的陆地表面组件中。我们使用 RCP8.5 高排放情景模拟了未来升高的 CO 条件下至 2100 年的陆地表面碳氮过程。我们的模拟表明，当考虑光合作用适应时，未来条件下光合作用会增加，但最大 RuBisCO 羧化作用因此光合作用氮需求下降。我们分析了两个模拟，这两个模拟的区别在于保存的氮是否可以用于植物的其他部分。保存的叶片氮的分配导致（1）通过减少叶片氮需求直接缓解植物氮限制，（2）通过增强根生长间接减少植物氮限制，从而增加植物氮吸收。结果，与没有保存叶片氮再分配的模拟相比，保存叶片氮的再分配使 2100 年生态系统碳储量增加了 50.3%。这些结果表明，如果陆地表面模型不包含光合作用适应升高的 CO 而导致的叶片氮素节约，它们可能会高估未来生态系统氮素限制。
