CAREER: Understanding Nitrogen Limitation of Decomposition

现代农业和化石燃料燃烧产生的氮(N)可以从农业或工业区顺风向下运输，并沉积在历史上可能经历过低水平N投入的生态系统中。虽然N是植物和土壤有机体都需要的一种重要养分，但目前尚不清楚这些增加的N投入将如何影响有机物的分解。由于分解是植物在光合作用过程中吸收的碳最终以二氧化碳(CO2)的形式返回大气的主要方式，因此了解N对分解的影响对于预测未来人类对大气CO2浓度的影响是必要的。最近的研究表明，在低氮场所，N含量较高的植物碎屑(凋落物)分解得更快，这表明分解微生物需要N来分解植物凋落物。然而，添加无机氮肥只增加了八个地点中的两个地点的分解。这些自相矛盾的结果质疑了这样的假设，即枯枝落叶N浓度与分解之间的正相关关系之所以出现，是因为N限制了分解。这项拟议的研究将使用数学模型和田间试验来阐明N对分解的影响，重点是N对负责分解过程的土壤微生物的影响。这一具体的研究目标符合研究人员提出的研究和教育计划的更广泛目标，即开发新的技能和方法，以便更有效地在可应用于全球环境变化的生态系统生态学基本问题上对学生和公众进行研究和教育。这项研究将包括在明尼苏达州雪松溪自然历史区进行的一项为期4年的研究，其中提供给分解者的氮的数量和形式将受到实验控制。将监测微生物群落的反应。此外，还将开发数学模型来探索分解率变化的后果。拟议的教育活动包括让本科生参与研究，开发两门新课程，加强现有课程，以及发展有效的公共宣传技能，目的是提高对全球环境变化的认识。拟议的研究和教育计划将产生更广泛的影响，途径是：加深对大气氮沉降如何改变生态系统中碳循环的了解；通过开发氮对分解影响的数学模型；通过发展教员在理论和微生物技术方面的技能；通过开发两门课程(一门本科生，一门研究生)来教育学生环境科学以及全球变化的科学和政策；通过纳入主动学习技术来加强现有的课程；通过支持和指导一名少数族裔研究生；以及通过一个现有的项目支持和指导三名本科生进行研究，该项目成功地从代表性不足的群体中招募了参与者。

Nitrogen (N) produced by modern agriculture and fossil fuel combustion can be transported downwind from agricultural or industrial areas and deposited on ecosystems that historically may have experienced low levels of N inputs. Although N is an important nutrient required by both plants and soil organisms, it is presently unknown how these elevated inputs of N will affect decomposition of organic matter. Because decomposition is the primary way that carbon taken up by plants during photosynthesis is ultimately returned to the atmosphere as carbon dioxide (CO2), understanding effects of N on decomposition is necessary for predicting future human impacts on atmospheric concentrations of CO2. Recent work showed that in low-N sites, plant detritus (litter) with a greater N content decomposed more quickly, suggesting that decomposing microorganisms require N to break down plant litter. However, addition of inorganic N fertilizer increased decomposition in only two of eight sites. These paradoxical results call into question the assumption that the often-observed positive relationship between litter N concentration and decomposition arises because N limits decomposition. The proposed research will use both mathematical models and field experiments to clarify how N affects decomposition, focusing on N effects on the soil microorganisms responsible for the process of decomposition. This specific research goal fits into the investigator's broader objective for the proposed research and education plan to develop new skills and approaches for more effective pursuit of research and education of students and the public in fundamental questions in ecosystem ecology that have applications to global environmental change. The research will involve a 4-y study at the Cedar Creek Natural History Area, Minnesota, in which the amount and form of N supplied to decomposers will be experimentally controlled. The response of the microbial community will be monitored. In addition, mathematical models will be developed to explore the consequences of variation in decomposition rate. Proposed educational activities include involving undergraduates in research, developing two new courses, enhancing an existing course, and developing effective public outreach skills with the aim of improving literacy regarding global environmental change. The proposed research and education plan will achieve broader impacts by increasing understanding of how atmospheric N deposition alters carbon cycling in ecosystems; through development of mathematical models of N effects on decomposition; through the development of a faculty member's skills in theoretical and microbial techniques; through the development of two courses (one undergraduate, one graduate) to educate students in environmental science and the science and policy of global change; through enhancement of an existing course via incorporation of active-learning techniques; through support and mentorship of a minority graduate student; and through support and mentorship of three undergraduates in research via an existing program that has successfully recruited participants from underrepresented groups.