Microbial and Hydrological Controls of Nitrogen Losses From Alpine and Chaparral Ecosystems During Seasonal Transitions

该项目将研究影响加州内华达州山脉高山和查帕拉尔生态系统氮限制程度的地球化学和水文机制。这些生态系统表现出大量的、偶发性的硝酸盐损失，其未来的完整性受到气候变化和附近人类活动的威胁。研究人员假设，控制这些生态系统中氮损失的一个关键因素是生长季节和非生长季节之间的过渡对非生长季节土壤中微生物种群和生物地球化学过程的影响。地中海气候的季节转换，如加州的季节转换，其特征是从干燥到潮湿的条件，温暖到寒冷的温度和从低到高径流期的突然转变。这些转变引起土壤水分和温度的变化，介导冲洗的硝酸盐。从长远来看，在季节转换过程中的硝酸盐损失可能是一个主要的机制，氮限制是保持在高山和查帕拉尔生态系统。该项目将采取流域办法研究高山和查帕拉尔生态系统中的氮动态，并将利用各种创新技术，包括地块规模研究、同位素和化学示踪剂、流域质量平衡和生态系统建模。 这些调查的结果将有助于解释陆地生态学中的一个主要问题：为什么植物群落中的氮是有限的？ 此外，这项工作将提供一个了解水文和地球化学之间的连接在季节性转变，这是需要预测如何加州的山地生态系统将响应预期的全球变化。

The project will examine biogeochemical and hydrological mechanisms that influence the extent of N limitation in alpine and chaparral ecosystems of the Sierra Nevada, California. These ecosystems exhibit large, episodic losses of nitrate and their future integrity is threatened by climate change and nearby human activities. The investigators hypothesize that a key factor controlling N-losses in these ecosystems is the effect of transitions between growing and non-growing seasons on microbial populations and biogeochemical processes in soils during non-growing seasons. Seasonal transitions in Mediterranean climates, like those of California, are characterized by abrupt shifts from dry to wet conditions, warm to cold temperatures and from low to high runoff periods. These transitions induce changes in soil moisture and temperature that mediate flushing of nitrate. Over the long-term, nitrate losses during seasonal transitions may be a primary mechanism by which N limitation is maintained in alpine and chaparral ecosystems. The project will take a watershed approach to studying N dynamics in alpine and chaparral ecosystems and will utilize a combination of innovative techniques, including plot-scale studies, isotopic and chemical tracers, watershed mass balances and ecosystem modeling. Results from these investigations will help explain a major question in terrestrial ecology: Why are plant communities N limited? Additionally, the work will provide an understanding of the connections between hydrology and biogeochemistry during seasonal transitions, which is needed to predict how California's montane ecosystems will respond to anticipated global change.