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山脉高山和灌木林生态系统氮限制程度的生物地球化学和水文机制。这些生态系统显示出硝酸盐的大量、间歇性损失，其未来的完整性受到气候变化和附近人类活动的威胁。研究人员推测，控制这些生态系统氮损失的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。地中海气候的季节性转变，如加利福尼亚的气候，其特点是从干燥到潮湿、从温暖到寒冷、从低到高径流期的突然转变。这些转变引起土壤湿度和温度的变化，从而介导硝酸盐的冲刷。从长期来看，季节转换期间的硝酸盐损失可能是维持高寒和灌木林生态系统氮限制的主要机制。该项目将采用流域方法来研究高山和灌木林生态系统中的氮动态，并将利用创新技术的组合，包括地块尺度研究、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：为什么植物群落的氮含量有限？此外，这项工作将提供对季节转换期间水文和生物地球化学之间联系的理解，这是预测加州山地生态系统将如何应对预期的全球变化所需要的。

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.