DISSERTATION RESEARCH: Direct and indirect effects of winter climate change on microbes: implications for nitrogen retention in temperate hardwood forests

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Forests in the northeastern United States are likely to experience a reduction in the depth and duration of snow cover during the next century as the climate changes. Because snow insulates soils from below-freezing air temperatures, loss of snow cover will increase the frequency of freeze - thaw events in winter. Past studies show that soil freezing increases movement of nitrogen from forest soils to nearby waterways and the atmosphere, but the causes of these patterns are not well understood, especially the role of soil microbes. The goal of this project is to gain a deeper understanding of the response of soil microbes to these changes, and the consequences for nitrogen loss downstream. This project will also produce a number of societal benefits. First, understanding connections among soils, plants, and climate will improve predictions of the response of temperate forests to climate change, critical information to better understand feedbacks between the biosphere and a changing climate. In addition, this research will provide new educational activities for students at multiple grade levels. High-school students in the Boston public school system will gain valuable hands-on, independent research experience and training in a range of field and laboratory methods. Undergraduate students from Boston University, an urban campus, will have a rare opportunity to take field trips to the Harvard Forest Long-Term Ecological Research site to learn about winter climate change. Finally, data generated by this research will be incorporated into an undergraduate microbiology class and used to train undergraduate students in current methods in this growing field.Past studies have shown that reductions in snow cover increase the intensity of winter soil freezing, which can increase ecosystem nitrogen loss via leaching and gas loss to the atmosphere. Because the abundance and diversity of soil bacterial and fungal communities can have significant effects on soil nitrogen cycling, sustained changes in microbial community structure might explain the positive effect of winter soil freezing on N losses from forest soils. This project will determine whether soil freezing in winter negatively affects soil microbes, and whether these effects are direct, via disruption of soil structure and induced physiological stress, or indirect, through effects on plant roots. This research will pair past measurements of soil nitrogen cycling at two winter climate field experiments at Hubbard Brook Experimental Forest with new measurements of the abundance and diversity of soil bacteria and fungi using archived soils. Soil, microbial and plant samples from a natural climate gradient will also be used to determine the effects of winter climate on soil nitrogen cycling, plant-microbial interactions, and microbial community composition.