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)的生物地球化学循环如何相互作用是至关重要的。氮有效性影响碳在三个主要陆地库中的移动：植物、土壤和大气，其中二氧化碳浓度正在增加。同样，人类活动使生物圈中活性氮的数量增加了一倍。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施氮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，过量的氮加速了土壤碳的分解，促进了二氧化碳的释放；而在其他地点，施氮似乎起到了相反的作用，并促进了土壤碳的稳定。这项研究将提供第一个实验，测试一个新的理论来解释这些看似矛盾的反应。这项工作与陆地生态系统模型的发展直接相关，陆地生态系统模型代表了我们预测人类对氮循环的破坏将在何时何地放大土壤c -气候反馈的最重要工具。这项研究还将提供一个平台，让传统上在生态研究中代表性不足的群体参与进来，为第一代美国原住民大学生提供动手研究的机会。在这项概念验证研究中测试的概念框架基于这样一种观点，即氮肥对两个具有不同生物地球化学控制的土壤有机质库具有相反的影响：未受保护的颗粒有机质（POM）与微生物无法接近的矿物相关有机质（MAOM）。氮肥应以可预测的方式影响POM损失和mam收益之间的平衡，这取决于土壤的土壤和生物特性。本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对氮供应和土壤pH（受氮肥的强烈影响）变化的响应。实验方法将利用稳定同位素示踪剂方法和互补的标准化“合成土壤”方法来量化微生物生物量、POM和MAOM池中C和N的通量。研究人员将综合短期实验室研究和长期田间施肥试验中观察到的响应，量化在局部到区域尺度上运行的环境驱动因素如何调节氮肥施用下土壤C循环的变化。该奖项反映了美国国家科学基金会的法定使命，并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，被认为值得支持。

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.