Collaborative Proposal: MSA: Predicting the effects of nitrogen deposition on the soil carbon sink with a continental-scale experiment

为了预测生态系统将如何应对持续的环境变化，了解碳（C）和氮（N）的地球化学循环如何相互作用至关重要。氮的有效性影响C在其三个主要陆地库：植物，土壤和大气中的运动，其中CO2的浓度正在增加。同样，人类活动使生物圈中活性氮的数量翻了一番。为了了解全球氮和碳循环之间的相互作用，生态学家需要能够预测氮肥对土壤有机质的影响，土壤有机质是陆地碳的最大储存库。在某些地点，过量的N加速土壤C的分解，促进CO2的释放;而在其他地点，N施肥似乎做相反的事情，并促进土壤C的稳定。这项研究将提供第一个实验，将测试一个新的理论来解释这些看似矛盾的反应。这项工作是直接相关的陆地生态系统模型的发展，这是我们最重要的工具来预测何时何地人类破坏的N循环将放大土壤C-气候反馈。这项研究还将提供一个平台，与传统上在生态研究中代表性不足的群体接触，为第一代美洲原住民大学生提供实践研究的机会。 在这个概念验证研究中测试的概念框架基于这样的想法，即氮肥对两个具有不同生物地球化学控制的土壤有机质池具有相反的影响：未受保护的颗粒有机质（POM）与微生物不可及的矿物相关有机质（MAOM）。施氮应以可预测的方式影响POM损失和MAOM收益之间的平衡，这取决于土壤和生物特性。这项研究将利用土壤地球化学和气候的陡峭梯度来量化土壤C对改变N供应和土壤pH值（受N施肥的强烈影响）的响应。实验方法将利用稳定同位素示踪方法和一个互补的，标准化的“合成土壤”的方法来量化的微生物生物量，POM和MAOM池之间的C和N通量。研究人员将综合在短期实验室研究和长期实地施肥实验中观察到的反应，以量化环境驱动因素如何在地方到区域尺度上调节氮施肥下土壤碳循环的变化。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估来支持。

To predict how ecosystems will respond to ongoing environmental change, it is critical to understand how biogeochemical cycles of carbon (C) and nitrogen (N) interact with one another. N availability influences the movement of C among its three major terrestrial pools: plants, soils, and the atmosphere, where concentrations of CO2 are increasing. Similarly, human activities have doubled the quantity of reactive N in the biosphere. To understand interactions between global N and C cycles, ecologists need to be able to predict the effects of N fertilization on soil organic matter, which is the largest reservoir of terrestrial C. At some sites, excess N accelerates the decomposition of soil C, promoting release of CO2; whereas at other sites, N fertilization appears to do the reverse, and promote soil C stabilization. This research will provide the first experiment that will test a new theory to explain these seemingly contradictory responses. This work is directly relevant to the development of terrestrial ecosystem models, which represent our most important tools to forecast when and where human disruption of the N cycle will amplify soil C-climate feedbacks. The research will also provide a platform to engage with groups traditionally underrepresented in ecological research, supporting hands-on research opportunities for first-generation Native American college students. The conceptual framework tested in this proof-of-concept study rests on the idea that N fertilization has opposing effects on two pools of soil organic matter with distinct biogeochemical controls: unprotected particulate organic matter (POM) vs. microbially inaccessible mineral-associated organic matter (MAOM). N fertilization should affect the balance between POM losses and MAOM gains in a predictable manner that is contingent on soil edaphic and biotic properties. This research will leverage steep gradients of soil geochemistry and climate to quantify soil C responses to altered N supply and soil pH (which is strongly influenced by N fertilization). The experimental approach will utilize both stable isotope tracer approaches and a complementary, standardized "synthetic soil" method to quantify fluxes of C and N among the microbial biomass, POM, and MAOM pools. The researchers will synthesize responses observed in short-term laboratory studies and longer-term field-based fertilization experiments to quantify how environmental drivers operating at local to regional scales mediate changes in soil C cycling under N fertilization.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.