DISSERTATION RESEARCH: Effects of nitrogen enrichment on multiple soil organic matter pools

在过去的100年里，由于化石燃料的燃烧，固氮作物的种植和氮肥的创造，人类显着增加了生物可利用氮（所谓的活性氮）的数量。 大部分被释放出来的活性氮最终会变成雨和雪，从而无意中污染了森林和草原的土壤。 这种氮肥对土壤中碳储存的影响，以及碳从土壤释放回大气的影响尚不清楚。 本研究的目的是研究氮素输入增加对微生物分解土壤有机质的影响，这是一个向大气释放二氧化碳的过程。该研究将使用正在进行的氮添加实验的全球网络（营养网络）来测试氮沉积增加对草原土壤的影响。这些信息对于为全球生态系统模型提供信息以及预测土壤中碳的长期储存是必要的。该项目还包括一个由项目调查人员开展的教育、推广和培训活动方案。他们将通过雪松溪校园长期生态研究计划以及雪松溪年度开放日向公众传播他们的研究结果。此外，研究人员将继续指导明尼苏达大学本科生研究机会计划和麦卡莱斯特学院研究实习生计划的独立研究。土壤碳（C）的快速循环池容易被微生物分解，而对微生物分解稳定的较慢循环池可能对氮（N）的反应不同。初步数据表明，N添加增加了快速土壤C库（平均停留时间1年）的分解速率，降低了缓慢土壤C库（平均停留时间1-10年）的分解速率。这些结果与落叶分解研究的结果相一致，并提出了一些生物和化学机制来解释这种模式。评价机制是预测施氮对土壤固碳影响的必要条件。本研究的目的是探讨控制土壤有机质分解的化学和生物学机制的反应，氮添加。本研究改进了正在进行的论文研究，探讨氮富集对多个土壤有机质池，包括未封闭，聚集闭塞，和矿物闭塞的土壤有机质的影响。具体而言，该研究将使用一个正在进行的N添加实验网络，在美国中部大平原的六个草地站点复制，以量化N富集对基质化学，微生物酶活性和微生物生长效率的影响，以阐明土壤有机质分解对添加N的响应机制。本研究将验证以下假设：1）施氮促进微生物生长、水解酶活性和植物组织（2）施氮降低了土壤氧化酶活性，提高了微生物生长效率，减少了土壤慢速C库的分解;和3）与立即、一次性N添加相比，N添加对微生物过程（酶活性和生长效率）的影响在长期N添加之后将更大。

Humans have significantly increased the amount of biologically available nitrogen (so-called reactive nitrogen) over the past 100 years due to fossil fuel combustion, cultivation of nitrogen-fixing crops, and creation of nitrogen fertilizers. Much of the reactive nitrogen that has been released ends up in rain and snow, thus inadvertently fertilizing forest and grassland soils. The effects of this nitrogen fertilization on storage of carbon in soils, and release of carbon from soils back to the atmosphere, are unclear. The objective of this research is to examine the effects of elevated nitrogen inputs on the decay of soil organic matter by microbes, a process that releases carbon dioxide to the atmosphere. The research will use a global network of ongoing nitrogen addition experiments (the Nutrient Network) to test the effects of increased nitrogen deposition on grassland soils. This information is necessary to inform global ecosystem models, as well as make predictions about the long-term storage of carbon in soils. The project also includes a program of education, outreach, and training activities pursued by the project's investigators. They will disseminate their results to the general public through the Cedar Creek Schoolyard Long-Term Ecological Research program, as well as Cedar Creek's Annual Open House. Furthermore, the investigators will continue to mentor undergraduates in independent research in the University of Minnesota Undergraduate Research Opportunity program and Macalester College's Research Intern Program.Rapidly cycling pools of soil carbon (C) that are readily decomposed by microbes, and more slowly cycling pools that are stabilized against microbial decomposition, likely respond differently to nitrogen (N) enrichment. Preliminary data shows that N addition increases the rate of decomposition of the fast soil C pool (mean residence time 1 year) and decreases the rate of decomposition of the slow soil C pool (mean residence time 1-10 years). These results parallel those from leaf litter decomposition studies and a number of biological and chemical mechanisms have been proposed to explain the pattern. Evaluating mechanism is necessary to make predictions about the effects of N addition on soil C sequestration. The objective of this research is to examine chemical and biological mechanisms controlling the response of soil organic matter decomposition to N addition. This research improves upon ongoing dissertation research that examines the effects of N enrichment on multiple soil organic matter pools, including unoccluded, aggregate-occluded, and mineral-occluded soil organic matter. Specifically, the research will use a network of ongoing N addition experiments, replicated in six grassland sites across the Central Great Plains, USA, to quantify the effects of N enrichment on substrate chemistry, microbial enzyme activity, and microbial growth efficiency towards elucidating the mechanisms underlying the response of soil organic matter decomposition to added N. The research will test the hypotheses that: 1) N addition increases microbial growth, the activity of hydrolytic enzymes, and plant tissue (substrate) N content, contributing to increased decomposition of the fast soil C pool; 2) N addition decreases activity of oxidative enzymes and increases microbial growth efficiency, contributing to decreased decomposition of the slow soil C pool; and 3) N addition effects on microbial processes (enzyme activity and growth efficiency) will be larger following long-term N addition compared to an immediate, one-time N addition.