Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY, USA; National Center for Atmospheric Research, Boulder, CO, USA.
Glob Chang Biol. 2013 Oct;19(10):2986-98. doi: 10.1111/gcb.12281. Epub 2013 Aug 8.
Projections of future changes in land carbon (C) storage using biogeochemical models depend on accurately modeling the interactions between the C and nitrogen (N) cycles. Here, we present a framework for analyzing N limitation in global biogeochemical models to explore how C-N interactions of current models compare to field observations, identify the processes causing model divergence, and identify future observation and experiment needs. We used a set of N-fertilization simulations from two global biogeochemical models (CLM-CN and O-CN) that use different approaches to modeling C-N interactions. On the global scale, net primary productivity (NPP) in the CLM-CN model was substantially more responsive to N fertilization than in the O-CN model. The most striking difference between the two models occurred for humid tropical forests, where the CLM-CN simulated a 62% increase in NPP at high N addition levels (30 g N m(-2) yr(-1)), while the O-CN predicted a 2% decrease in NPP due to N fertilization increasing plant respiration more than photosynthesis. Across 35 temperate and boreal forest sites with field N-fertilization experiments, we show that the CLM-CN simulated a 46% increase in aboveground NPP in response to N, which exceeded the observed increase of 25%. In contrast, the O-CN only simulated a 6% increase in aboveground NPP at the N-fertilization sites. Despite the small response of NPP to N fertilization, the O-CN model accurately simulated ecosystem retention of N and the fate of added N to vegetation when compared to empirical (15) N tracer application studies. In contrast, the CLM-CN predicted lower total ecosystem N retention and partitioned more losses to volatilization than estimated from observed N budgets of small catchments. These results point to the need for model improvements in both models in order to enhance the accuracy with which global C-N cycle feedbacks are simulated.
利用生物地球化学模型预测未来土地碳（C）储存的变化取决于准确模拟 C 和氮（N）循环之间的相互作用。在这里，我们提出了一个分析全球生物地球化学模型中 N 限制的框架，以探讨当前模型的 C-N 相互作用如何与野外观测结果相比较，确定导致模型分歧的过程，并确定未来观测和实验的需求。我们使用了来自两个使用不同方法模拟 C-N 相互作用的全球生物地球化学模型（CLM-CN 和 O-CN）的一组 N 施肥模拟。在全球范围内，CLM-CN 模型的净初级生产力（NPP）对 N 施肥的响应明显大于 O-CN 模型。两个模型之间最显著的差异发生在潮湿的热带森林中，在那里 CLM-CN 模拟在高 N 添加水平（30 g N m(-2) yr(-1)）下 NPP 增加了 62%，而 O-CN 预测由于 N 施肥增加植物呼吸的作用大于光合作用，NPP 将减少 2%。在 35 个具有野外 N 施肥实验的温带和北方森林站点中，我们表明 CLM-CN 模拟了 N 响应导致的地上 NPP 增加了 46%，这超过了观察到的增加 25%。相比之下，O-CN 仅在 N 施肥地点模拟了地上 NPP 的 6%增加。尽管 NPP 对 N 施肥的响应很小，但与经验（15）N 示踪剂应用研究相比，O-CN 模型准确地模拟了生态系统对 N 的保留和添加 N 到植被中的命运。相比之下，CLM-CN 预测的总生态系统 N 保留较低，并且将更多的损失分配给挥发，这与从小流域观测到的 N 预算估计的损失不同。这些结果表明，需要改进这两个模型，以提高模拟全球 C-N 循环反馈的准确性。
