DISSERTATION RESEARCH: Where Plant Litter Ends and Soil Carbon Begins: The Role of Microbial Physiology in Stabilizing Soil Organic Matter

土壤中的碳含量比大气和植物中的碳含量加起来还要多。长期以来，人们一直认为储存在土壤中的碳代表了“剩余物”——植物衍生的化合物，不能被土壤微生物进一步分解。最近的研究表明，当土壤微生物分解植物材料时，它们可以产生增加土壤中碳储量的化合物。因此，土壤微生物对土壤碳的分解和积累都很重要。然而，许多问题仍未得到解答。是什么控制着微生物的生长，微生物生长的哪些方面应该导致长期的土壤碳储存？这些都是通过本博士论文改进资助研究要解决的问题。实验将利用一种新技术，在受控的实验室实验和沿着森林自然环境梯度的研究中测量微生物的生成率。该项目的结果将进一步加深我们对土壤碳含量的了解，并将提高我们预测土壤中碳储量最大的时间和地点的能力。调查人员将与高中教师合作制定课程计划和课程材料，并直接与高中学生合作，围绕项目目标提供培训。有机投入品转化为微生物产品从根本上受微生物生长速率、微生物生长效率和微生物周转率三个过程控制。然而，尽管这些参数在理论上对稳定土壤有机质（SOM）的形成具有重要意义，但人们对这些参数如何随资源可用性和环境条件而变化，以及哪些参数对土壤有机质动态控制最强烈，知之甚少。本研究将解决两个问题：1)凋落物质量和土壤性质如何影响微生物的生长速度、效率和周转？2)这些参数的变化在多大程度上影响土壤有机质的形成和土壤碳储量？迄今为止，我们对微生物生长生理学的理解受到无法在现实底物上量化生长的常见方法的阻碍。为了克服这一挑战，研究人员将使用一种新的、与底物无关的方法——跟踪18o标记的水进入微生物DNA——来量化温带森林土壤中的微生物生长参数。首先，将对6个温带森林的微生物生长速率、效率和周转量进行量化，以确定这些参数是否与之前从这些相同地点收集的SOM数据一致。然后，利用同位素标记的凋落物腐烂实验，评估凋落物质量与土壤性质在控制微生物生长参数和植物输入转化为稳定SOM方面的重要性。

More carbon is found in soils than that in the atmosphere and plant life combined. It has long been assumed that the carbon stored in soil represents the "leftovers" - plant-derived compounds that cannot be broken down any further by soil microorganisms. Recent work has shown that, as soil microorganisms decompose plant materials, they can produce compounds that increase carbon storage in soil. Thus, soil microorganisms are important to both the breakdown and the buildup of soil carbon. Many questions, however, remain unanswered. What controls microbial growth, and which aspects of microbial growth should lead to long-term soil carbon storage? These are the questions to be addressed by research through this Doctoral Dissertation Improvement Grant. Experiments will make use of a novel technique to measure rates of microbial production in both a controlled laboratory experiment, and through studies along natural environmental gradients in forests. Results of this project will further our understanding of soil carbon content, and will enhance our ability to predict when and where carbon storage in soils should be greatest. The investigators will work with high school teachers to develop lesson plans and course materials, as well as with high school students directly, to provide training centered around the project objectives. The conversion of organic inputs into microbial products is fundamentally controlled by three processes: microbial growth rate, microbial growth efficiency, and microbial turnover rate. Yet, despite the theoretical importance of these parameters for stable soil organic matter (SOM) formation, little is known about how these parameters vary with resource availability and environmental conditions, and which of these parameters most strongly controls SOM dynamics. This research will address two questions: 1) How do litter quality and soil properties affect microbial growth rate, efficiency, and turnover? 2) To what extent do changes in these parameters influence SOM formation and soil carbon storage? To date, our understanding of microbial growth physiology has been hindered by the inability of common methods to quantify growth on realistic substrates. To overcome this challenge, the researchers will use a new, substrate-independent method - tracking 18O-labeled water into microbial DNA - to quantify microbial growth parameters in temperate forest soils. First, microbial growth rate, efficiency, and turnover will be quantified across natural gradients of resource availability in six temperate forests in order to determine whether these parameters are consistent with previously-collected SOM data from these same sites. Then, using two isotopically labeled litter decay experiments, the importance of litter quality versus soil properties in controlling microbial growth parameters and the conversion of plant inputs into stable SOM will be evaluated.