2008 Annual Report 1a.Objectives (from AD-416) The model will longitudinally resolve micrometeorological forcing of photosynthetic processes at the half-hourly time scale, enhancing and optimizing control of the LYCOG CO2 gradient while minimizing undesired temperature excursions. The model will then be upscaled to the daily level, and calibrated with LYCOG measurements of biogeochemistry and plant/soil fluxes to resolve daily variation in water, carbon, and nitrogen fluxes along the CO2 gradient. The result will be a calibrated model of CO2 impacts on biogeochemistry. Temperature and precipitation effects will be added to the CO2 model by calibrating photosynthesis, plant water potential, and soil C-N cycling submodels with temperature and soil moisture relationships from the literature or measured in LYCOG. This will enable prediction of the joint effects of temperature and precipitation on the fluxes of water, carbon and nitrogen along the CO2 gradient. 1b.Approach (from AD-416) Use field data collected from LYCOG to calibrate and test a stochastic soil moisture/biogeochemistry model at subhourly to daily time scales. 3.Progress Report We initiated this cooperative agreement with Duke University to conduct research with two primary objectives: . 1)Model the effects of air CO2 concentration on the water, energy, carbon, and nitrogen balance of grassland on three soil types in the LYCOG facility in order to resolve daily variation in water, carbon and nitrogen fluxes along the CO2 gradient. The result will be a calibrated model of CO2 impacts on biogeochemistry. . 2)Model the impacts of precipitation and temperature variability on ecosystem fluxes. The model will thus predict the joint effects on grassland biogeochemistry of precipitation and temperature variability along a progression from pre-industrial to end-of-century CO2. This will guide the development of future field experiments and allow exploration of spatial variation in grassland biogeochemistry across regional temperature and precipitation gradients. The model was developed, with sub-units describing water and carbon mass balances for each component of the soil-plant-atmosphere continuum and a plant-atmosphere gas exchange scheme. The model was tested for its ability to simulate the carbon and water fluxes in plots containing grassland vegetation on soils from three contrasting soil series exposed to a gradient of CO2 (250 to 500 µL L-1) in the Lysimeter CO2 Gradient experiment (LYCOG) facility. Initial results indicated good correspondence between modeled and measured leaf photosynthesis and water potential, canopy transpiration and soil moisture profiles. Progress on this project is regularly assessed through phone calls, ARS visits to Duke University, site visits by the contractor, and meeting at conferences.