Collaborative Proposal: Impact of stochastic soil moisture dynamics on vegetation water stress and nutrient cycling

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0236621D'OdoricoIn arid and semiarid ecosystems there is a strong dependence of the rates of evapotranspiration, photosynthesis, and net carbon assimilation on the soil water content. Conditions of limited soil water availability lead vegetation into a state of water stress, which, if prolonged, is responsible for loss of turgidity, wilting and death of plants. Soil water content also affects nitrogen cycling, due to the direct or indirect dependence of the processes of decomposition, mineralization, nitrification, denitrification, root uptake, and leaching on the soil moisture conditions. The quantity of available nitrogen in the root zone in turn affects vegetation processes. Thus, through the soil water balance, the hydrological cycle exerts a significant control on terrestrial ecosystems, in particular in arid and semiarid environments, where the rates of evapotranspiration, nutrient mineralization and nutrient uptake are strongly limited by soil water availability. Even though many physiological and biogeochemical mechanisms relating water stress and the nitrogen cycle to soil moisture conditions are understood, a quantitative process-based framework for the analysis and prediction of how hydrologic processes may impact terrestrial ecosystems is still lacking. This type of framework is crucial to the enhancement of the understanding of the effect of climate variability on terrestrial plant communities and ecosystems. Through a stochastic mechanistic model of the soil water balance at the daily time scale we have related hydrologic processes (including evapotranspiration and the intermittent stochastic character of precipitation) to soil water content and expressed this dependence as a function of the characteristics of climate, soil, and vegetation. For this project, we plan to use this framework to investigate the impact of hydrologic conditions on the soil nitrogen budget. In particular we will: Review data on the nitrogen and carbon cycles in arid and semiarid ecosystems and at a short (daily-to-weekly) time scale and identify possible (and sensible) simplifications of the plant-nutrient relationships. Develop a conceptual framework for the modeling of the fluctuations in soil nitrogen content driven by soil moisture (and temperature) dynamics. This framework will be based on mechanistic biophysical models, on a process-based analysis of the nitrogen cycle, and on a stochastic soil water balance.The model will be used to analyze the impact of seasonal and interannual fluctuations in precipitation on the nitrogen budget of arid and semiarid ecosystems. The development of this framework will indicate the type of data that is needed in the testing and validation of high-resolution biogeochemical models in arid and semi-arid ecosystems. Some data will be collected at a local field site (Blandy Experiment Farm owned and operated by the University of Virginia). However, the collection of more extensive (and more appropriate) data will not be funded by this project but will be part of future research which we wish to propose to NSF at the end of this ex