The Physical and Chemical Mechanisms Responsible for Carbon Sequestration in Soil Microaggregates

土壤团聚体形成和稳定性的动态变化对理解和加强土壤固碳有着深远的意义。土壤微团聚体对长期固存特别重要，因为它们保护C不被分解，导致C的停留时间更长。然而，为什么土壤微团聚体中的有机碳具有如此长的停留时间还不是很清楚。本研究的目的是确定土壤微团聚体形成和稳定性的结构和化学基础。这一目标将通过研究微团聚体的内部表面形态和C在微团聚体内的分布，以及支撑微团聚体结构和稳定性的纳米级有机矿物缔合来实现。这些特征的演变将在一系列土壤中进行研究，这些土壤代表了影响碳积累的农艺管理系统(耕作与免耕、常规与有机)，以及增强碳储存的土地利用选择(草原恢复、牧草管理系统)。这项建议侧重于微团聚体；然而，拟议研究的价值和意义将通过与几名研究同一田间地点土壤团聚体演变的研究人员的合作来发挥。他们的参与将我们的研究结果与碳储存的数量和质量变化联系起来，受到管理实践的影响。相反，我们的过程水平洞察力将为他们在不同农艺系统或土地利用方式下观察到的土壤碳积累或损失的变化提供机械解释。将使用多种最先进的技术来测试将团聚体结构与碳积累和管理实践联系起来的假设，包括(1)氮气吸附以确定微团聚体内可获得的表面积，并评估暴露表面是有机还是无机的；(2)小角中子和X射线散射以表征总孔隙度的表面积和尺寸分布，并使用对比匹配技术，确定微生物、细胞外酶和养分对内部孔隙度的可及性。(3)扫描电子显微镜，以显示完整的微团聚体，并帮助解释氮气吸附和散射数据，以及(4)扫描透射式X射线显微镜，它允许收集X射线光谱数据，以评估聚集体内OM的化学成分，识别固有顽固分子的存在，或显示不稳定化合物的保存。通过比较影响微团聚体形成和稳定性的机制的差异，以及合作者提供的关于土壤碳库大小和动态的信息，这些新出现的模式将广泛地与识别土壤中碳转移速率的主要机制有关。这一认识的加强将为制定有效的管理战略以加强陆地系统的碳固存提供科学基础，这是碳循环科学计划的主要目标之一。具体地说，该提案侧重于过程层面的研究，以确定和量化土壤中碳转化和保持的关键机制。该项目将通过一名学生和博士后研究员的参与，将研究和教育结合起来。他们的科学视角将通过他们与我们不同的合作者群体的互动而进一步拓宽和加强。学生在这个项目中的经验将使他们成为领导者，为理解全球碳循环的长期努力做出重大贡献。

ABSTRACTThe dynamics of soil aggregate formation and stability have profound implications to understanding and enhancing C sequestration in soil. Soil microaggregates are particularly crucial to long-term sequestration because they protect C against decomposition, resulting in much longer residence times for C. However, the reasons why organic carbon in soil microaggregates has such long residence times is not well understood. The goal of this research is to determine the structural and chemical bases of soil microaggregate formation and stability. This goal will be accomplished by investigating the internal surface morphology of microaggregates and the distribution of C within microaggregates, as well as the nanoscale organomineral associations underlying microaggregate structure and stability. The evolution of these features will be examined in a series of soils representing agronomic management systems that affect C accumulation (till versus no-till, conventional versus organic), as well as land-use options for enhanced C storage (prairie restoration, forage management systems).This proposal focuses on microaggregates; however, the value and significance of the proposed research will be leveraged by collaborations with several investigators who are working on the evolution of soil aggregates at the same field sites. Their participation links the results of our study to quantitative and qualitative changes in carbon storage, as affected by management practices. Conversely, our process-level insights will provide a mechanistic explanation for the changes they observe in accumulation or loss of soil carbon under different agronomic systems or land use options.Hypotheses relating aggregate structure to C accumulation and management practices will be tested using multiple state-of-science techniques including (1) N2 adsorption to determine the accessible surface area within microaggregates and assess whether exposed surfaces are organic or inorganic, (2) Small Angle Neutron and X-Ray Scattering to characterize the surface area and size distribution of the total porosity, and, using contrast matching techniques, determine the accessibility of internal porosity to microbes, exocellular enzymes, and nutrients, (3) Scanning Electron Microscopy to visualize intact microaggregates and help interpret N2 adsorption and scattering data, and (4) Scanning Transmission X-ray Microscopy that allows for collection of X-ray spectral data to evaluate the chemical composition of intra-aggregate OM and identify the presence of inherently recalcitrant molecules, or show preservation of labile compounds. Data from all four techniques will be interpreted in the context of a fractal model of pore space geometries.By comparing differences in the mechanisms influencing microaggregate formation and stability, coupled with information from collaborators on the size and dynamics of soil carbon pools, the emerging patterns will be broadly relevant to identifying major mechanisms controlling rates of carbon transfer in soils. This enhanced understanding will provide a scientific basis for developing effective management strategies to enhance C sequestration in terrestrial systems, which is one of the major goals of the Carbon Cycle Science Plan. Specifically, the proposal focuses on process-level studies to define and quantify key mechanisms for carbon transformation and retention in soil.The project will integrate research and education through participation of a student and postdoctoral fellow. Their scientific perspectives will be further broadened and enhanced by their interactions with our diverse group of collaborators. The students' experiences in this project will position them to be leaders who will make major contributions to the long-term effort to understand the global carbon cycle.