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

要预测生态系统将如何应对持续的环境变化，关键是要了解碳(C)和氮(N)的生物地球化学循环如何相互作用。氮的有效性影响着碳在三个主要的陆源库之间的移动：植物、土壤和大气，那里的二氧化碳浓度正在增加。同样，人类活动使生物圈中的活性氮数量翻了一番。为了了解全球N和C循环之间的相互作用，生态学家需要能够预测N肥对土壤有机质的影响，土壤有机质是陆地C的最大储存库。在某些地点，过量的N加速土壤C的分解，促进二氧化碳的释放；而在其他地点，N肥似乎相反，促进土壤C的稳定。这项研究将提供第一个实验，测试一种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，该模型是我们预测人类破坏N循环的时间和地点将放大土壤C气候反馈的最重要工具。这项研究还将提供一个平台，让传统上在生态学研究中代表性较低的群体参与进来，为第一代美洲原住民大学生提供实践研究机会。在这项概念验证研究中测试的概念框架基于这样一个想法，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)和微生物不可接近的矿质相关有机质(MAOM)。氮肥应以一种可预测的方式影响POM损失和MAOM收获之间的平衡，这取决于土壤的土壤和生物性质。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对变化的N供应和土壤pH(受N肥的强烈影响)的响应。该实验方法将利用稳定的同位素示踪方法和一种补充的、标准化的“合成土壤”方法来量化微生物生物量、POM和MAOM池中的C和N通量。研究人员将综合在短期实验室研究和长期田间施肥实验中观察到的反应，以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下土壤碳循环的变化。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.