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

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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.