Winter as a driver of nitrogen cycle responses to environmental change: responses of leguminous plants to variability in freezing

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Nitrogen is a key soil nutrient for plant growth. Therefore, a thorough understanding of the factors that affect nitrogen availability is crucial for predicting plant responses to environmental variation. Changes in soil freezing resulting from variation in winter conditions can have a substantial influence on the activities of soil microbes and plant roots, and thus can alter nitrogen availability and uptake by plants over the following growing season. However, an important question is whether soil nitrogen responses to severe freezing events can persist over the longer term. Although populations of soil microbes and plant root systems disrupted by freezing can recover by the next year, freezing-driven changes in the types and abundances of different perennial plant species could have lasting effects on soil nitrogen, because the nitrogen content of decomposing plant material differs among species. Legumes, which are plants that can obtain nitrogen directly from the air by partnering with specialized bacteria, may be more sensitive to freezing than other plant species. Given that the decomposition of legumes can increase soil nitrogen substantially, changes in legume numbers in response to changing winters could have strong effects on soil fertility. Research on this topic is particularly timely and important given that warmer winters are resulting in decreased snow cover, which can increase the freezing of plants that overwinter in soil. The overarching objective of my research program is to address how limits to the freezing tolerances of legumes can influence the extent to which winter processes control soil nitrogen availability both in natural plant communities (e.g. grassland) and managed ecosystems (cropland). Using a combination of experiments under controlled growth conditions and in the field, I will test the novel hypothesis that increased freezing intensity reduces the survival and growth of legumes disproportionately relative to non-leguminous plants. I will then use larger scale, multi-year field experiments in combination with nitrogen tracer techniques to examine how lethal or sublethal freezing damage to legumes affect the amount and timing of nitrogen availability over subsequent plant growing seasons. In addition to providing important knowledge regarding changes in the soil fertility of natural environments in response to variation in weather and climate change, this research will contribute significantly to the effective use of legumes in applied scenarios, such as in agricultural cover crop mixtures and in the selection of species for the restoration of native plant communities. Although winter biology has been explored most extensively in alpine and arctic systems, my research will focus on plants in northern temperate regions, which include the most highly populated and agricultural regions of Canada.