Collaborative Research:Impacts of Vegetation Change on Stabilization and Microbial Accessibility of Soil Organic Matter: A Microbiological, Isotopic, and Molecular Study

土壤有机质（SOM）是陆地环境中最大的活跃循环有机碳(C)和氮(N)库。然而，对植物、土壤和微生物之间存在的复杂相互作用的不完全理解限制了我们定量解释这些元素在全球预算中的储存和动态的能力。在土地覆盖/土地利用变化的背景下量化SOM的变化将有助于更好地理解人为扰动和自然变率对生态系统变化和气候变化的影响。木本植物/草地的转变对记录现代碳预算和古环境解释非常重要，因为大约45%至52%的陆地表面被草地、稀树草原、灌丛和半干旱林地覆盖，这些以草为主的生态系统储存了全球30%的土壤有机碳（SOC）。该提案旨在记录和量化各种生物、化学和物理过程如何在从草地到林地的主要植被变化后作为SOC的保护机制。具体而言，将利用木本植物入侵亚热带草原的时间序列（120年）作为模型系统，研究特定土壤物理和化学组分中有机物的储存或释放（以呼吸CO2的形式）。该项目的总体目标是将微生物群落结构和酶活性、植物输入化学和土壤微结构与有机碳和氮的特定化学形式联系起来，这些有机碳和氮通过时间顺序稳定和释放。本研究提出了四个问题：1)木本植物入侵后，土壤物理结构如何决定C随时间累积的程度？2)稳定的植物和微生物碳的化学成分和来源是什么？3)土壤微生物种群和酶活性的变化在凋落物和SOM组分呼吸中的作用及其对聚集动力学的影响？4)木本植物衍生的“新”SOM对微生物腐烂的相对可及性是什么？我们能否将接种实验中物理可识别的SOM组分与计算的平均停留时间与潜在呼吸联系起来？这些问题将通过应用新的分子、同位素和微生物学方法来解决，从而对控制土壤碳储存和动态的过程有一个基本的了解。这一建议的智力价值在于有可能揭示一些最基本的化学和物理机制，这些机制控制着全球碳循环中最大和最动态的组成部分之一。此外，这项工作将有助于为SOM动力学、生态系统过程和全球碳循环建模提供更强大的科学基础。研究小组的多学科组成（生态学、生物地球化学、微生物学）将有助于发展基础广泛的观点，这将使科学界的各个部门受益。这项工作的更广泛的影响包括加强对土壤过程在生物地球化学循环和地球系统中的作用的理解，这将对科学家和决策者具有直接意义，因为人类考虑操纵碳循环的潜力，以减轻潜在的全球气候变化。此外，该项目将为两名博士生提供教育机会，并有望通过普渡大学的国家科学基金会资助的研究生教育和教授联盟（AGEP）项目吸引代表性不足的学生。这些学生将接受生物地球化学和全球变化研究方面最先进方法的培训。

Filly0525346Soil organic matter (SOM) represents the largest pool of actively cycling organic carbon (C) and nitrogen (N) in the terrestrial environment. However, an incomplete understanding of the complex interactions that exist between plants, soils, and microbes limits our ability to quantitatively account for the storage and dynamics of these elements in global budgets. Quantifying changes to SOM within the context of land cover/land use changes will help understand better the effects of human-induced perturbations and natural variability on ecosystem shifts and climate change. An ecosystem scenario that is important for documenting modern carbon budgets as well as paleoenvironmental interpretation is woody plant/grassland transitions as approximately 45 to 52% of the terrestrial surface is covered with grasslands, savannas, shrublands, and semiarid woodlands and these grass-dominated ecosystems store 30% of global soil organic carbon (SOC). This proposal seeks to document and quantify how various biological, chemical, and physical processes act as protective mechanisms for SOC following a major vegetation change from grassland to woodland. Specifically, a chronosequence (120 yrs) of woody plant invasion into a subtropical grassland will be utilized as a model system to investigate the storage or release (as respired CO2) of organic matter from specific soil physical and chemical fractions. The overall project goal is to relate microbial community structure and enzymatic activity, plant input chemistry, and soil microfabric to the specific chemical forms of organic C and N that are stabilized and released through the chronosequence. Four questions drive the research in this proposal: 1) How does soil physical structure determine the extent of C accrual over time following woody plant invasion? 2) What is the chemical composition and source of the plant and microbial carbon that is stabilized? 3) What is the role of shifting populations of soil microbes and enzyme activity in the respiration of litter and SOM fractions and how do they impact aggregation dynamics? 4) What is the relative accessibility of the "new" SOM derived from woody plants to microbial decay, and can we relate physically identifiable SOM fractions with calculated mean residence time to potential respiration in inoculation experiments? These questions will be answered through application of novel molecular, isotopic, and microbiological methods to develop a fundamental understanding of the processes that control soil carbon storage and dynamics. The intellectual merit of this proposal rests on the potential for uncovering some of the most fundamental chemical and physical mechanisms that control one of the largest and most dynamic components of the global carbon cycle, Additionally, this work will contribute to the develop of a stronger scientific basis for modeling SOM dynamics, ecosystem processes, and the global carbon cycle. The multidisciplinary composition of the research team (ecology, biogeochemistry, microbiology) will contribute to the development of broad-based perspectives that will benefit a wide cross-section of the scientific community. The broader impacts of this work include the enhanced understanding of the role of soil processes in biogeochemical cycles and the earth system which will be of immediate significance to both scientists and policy-makers as mankind considers the potential for manipulating the carbon cycle in order to mitigate potential global climate change. Additionally, the project will generate educational opportunities for two Ph.D. students, with great promise to attract underrepresented students through Purdue's NSF Funded Alliance for Graduate Education and the Professoriate (AGEP) program. These students will receive training in state-of-the-art methodologies in biogeochemistry and global change research.