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，人类显著增加了生物可利用氮(所谓的活性氮)的数量。释放的大部分活性氮最终进入了雨雪中，从而无意中使森林和草原土壤变得肥沃。这种氮肥对土壤中碳的储存和碳从土壤中释放回大气的影响尚不清楚。这项研究的目的是研究增加的氮输入对土壤有机质被微生物腐烂的影响，微生物是向大气中释放二氧化碳的过程。这项研究将使用正在进行的氮素添加实验的全球网络(养分网络)来测试增加的氮沉积对草原土壤的影响。这些信息对于为全球生态系统模型提供信息以及对土壤中碳的长期储存做出预测是必要的。该项目还包括一项由项目调查人员开展的教育、外联和培训活动计划。他们将通过雪松溪学校长期生态研究计划以及雪松溪年度开放参观活动向公众传播他们的研究成果。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Macalester学院的研究实习生计划的本科生进行独立研究。快速循环的土壤碳(C)池很容易被微生物分解，而更慢的循环池则不受微生物分解的影响，对氮(N)的增加可能会有不同的反应。初步数据表明，施氮量增加了土壤速效碳库的分解速度(平均停留时间为1年)，降低了土壤慢速碳库的分解速度(平均停留时间为1~10年)。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。为了预测氮素添加对土壤固碳的影响，必须建立评价机制。本研究的目的是研究控制土壤有机质分解对氮素添加的响应的化学和生物学机制。这项研究是对正在进行的学位论文研究的改进，该研究考察了氮肥对多个土壤有机质库的影响，包括非封闭、团聚体封闭和矿物封闭的土壤有机质。具体地说，这项研究将利用一系列正在进行的氮素添加试验，在美国中大平原的六个草地地点重复进行，以量化氮素添加对底物化学、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氮素的响应机制。研究将检验以下假设：1)氮素添加增加微生物生长、水解酶活性和植物组织(底物)氮含量，有助于加速土壤快速碳库的分解；2)氮素添加降低了氧化酶活性，提高了微生物生长效率，有助于减缓土壤缓慢碳库的分解；3)与一次性一次性添加氮素相比，长期添加氮素对微生物过程(酶活性和生长效率)的影响更大。

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.