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

氮是植物生长的关键土壤养分。因此，深入了解影响氮素有效性的因素对于预测植物对环境变化的响应至关重要。冬季条件变化导致的土壤冻结变化对土壤微生物和植物根系的活动有重大影响，从而可以改变植物在下一个生长季节的氮有效性和吸收。然而，一个重要的问题是，土壤氮对严重冰冻事件的反应是否能持续更长时间。虽然被冻结破坏的土壤微生物种群和植物根系可以在第二年恢复，但冻结驱动的不同多年生植物物种类型和丰度的变化可能对土壤氮产生持久的影响，因为植物分解物质的氮含量因物种而异。豆科植物是一种可以通过与特殊细菌合作直接从空气中获取氮的植物，它可能比其他植物物种对冷冻更敏感。考虑到豆科植物的分解可以显著增加土壤氮，豆科植物数量随冬季变化的变化可能对土壤肥力产生强烈影响。考虑到暖冬导致积雪减少，这可能会增加土壤中越冬植物的冻结，对这一主题的研究尤为及时和重要。我的研究计划的总体目标是解决豆科植物耐寒性的限制如何影响冬季过程在自然植物群落（如草地）和管理生态系统（农田）中控制土壤氮有效性的程度。通过在受控生长条件下和田间进行的实验，我将验证一个新的假设，即与非豆科植物相比，增加冷冻强度会不成比例地降低豆科植物的存活和生长。然后，我将使用更大规模的多年田间试验，结合氮示踪技术，研究致命或亚致命的冷冻损害对豆科植物的影响如何在随后的植物生长季节影响氮可用性的数量和时间。除了提供有关自然环境土壤肥力随天气和气候变化而变化的重要知识外，本研究还将对豆科植物在农业覆盖作物混合和本地植物群落恢复的物种选择等应用场景中的有效利用做出重大贡献。虽然冬季生物学已经在高山和北极系统中进行了最广泛的探索，但我的研究将集中在北温带地区的植物上，其中包括加拿大人口最多和农业地区。

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.