Project Number: 6206-11220-005-00
Project Type: Appropriated

Start Date: Oct 01, 2009
End Date: Sep 30, 2014

Objective:
1. Determine CO2 effects on grassland plant production, plant species composition, and soil C dynamics. 1A. Determine responses of leaf gas exchange (C assimilation, stomatal conductance), plant water status, and plant production of tallgrass prairie assemblages to a subambient to elevated gradient in atmospheric CO2 concentration. 1B. Determine responses of soil respiration and soil organic matter pools (soil C dynamics) of tallgrass prairie assemblages to a subambient to elevated gradient in atmospheric CO2 concentration. 1C. Determine the response of species composition of tallgrass prairie vegetation to a subambient to elevated gradient in atmospheric CO2 concentration. 1D. Determine responses of photosynthetic C assimilation, biomass production, and bioenergy-relevant tissue constituents of the native grass species Panicum virgatum (switchgrass) to a subambient to elevated gradient in atmospheric CO2 concentration. 1E. Determine whether CO2 enrichment from subambient to elevated concentrations increases the potential for invasion of tallgrass prairie assemblages by a non-native grass species. 2. Determine effects of inter-annual variability in precipitation on productivity of switchgrass monocultures and mixed-species plantings of tallgrass prairie species. 2A. Compare responses of aboveground net primary productivity (ANPP) of switchgrass monocultures and mixtures of tallgrass prairie species to inter-annual variability in precipitation. 2B. Determine whether the frequency and magnitude of water limitation to ANPP of switchgrass and mixed-species plantings of prairie vegetation differ between a mollisol and vertisol soil. 3. Validate plant growth and biogeochemistry models to enable simulations of the impact of CO2 enrichment and precipitation variability on grassland production. 3A. Parameterize and validate the ALMANAC model with data from the CO2 gradient experiment and field-scale plots of switchgrass and prairie species. 3B. Parameterize and validate a coupled soil-plant-atmosphere-biogeochemistry model with plant and soil data from the CO2 gradient experiment. 4. Develop strategies to assess and manage the consequences for soil productivity, including carbon, of changing crop production strategies. 4A: Conduct evaluations of short-term carbon mineralization and water extractable organic C and N as a predictor of potential nitrogen mineralization in soil under conventional (inorganic) and organic fertilization. 5: Develop new strategies to improve crop fertilizer use efficiency for agronomic, economic and environmental benefits. 5A: Make final determinations of runoff water quality impacts of fertilizer recommendations based on enhanced soil testing methods. 5B: Conduct final evaluations of liquid fertilizer injection guided by GPS auto-steer technology in terms of yield and economics.

Approach:
Expose vegetated monoliths of three soil types to a continuous gradient in atmospheric carbon dioxide ranging from low levels of the pre-industrial period to elevated concentrations predicted within the century. We will measure leaf gas exchange (carbon assimilation, stomatal conductance), plant water status, plant production, and changes in the relative abundances of tallgrass prairie vegetation growing on each soil type. Soil carbon efflux and changes in soil organic carbon content will be measured in each soil as a function of carbon dioxide treatment. We will measure the responses of photosynthetic carbon assimilation and water use efficiency, biomass production, and bioenergy-relevant tissue constituents of the native grass species switchgrass to carbon dioxide and determine whether carbon dioxide enrichment increases the potential for invasion of tallgrass prairie vegetation by a non-native grass species. We also will compare responses of aboveground net primary productivity of field-scale plantings of switchgrass monocultures and mixtures of tallgrass prairie species to inter-annual variability in precipitation on upland and lowland soils. Two simulation models, the Agricultural Land Management Alternative with Numerical Assessment Criteria model and a coupled soil-plant-atmosphere biogeochemistry model, will be validated with data from the carbon dioxide experiment and field-scale plots of switchgrass and prairie species to simulate effects of changes in both atmospheric carbon dioxide concentration and precipitation patterns on grassland ecosystems. To evaluate short-term carbon mineralization and water extractable organic C and N as a predictor of potential nitrogen mineralization, soil samples will be collected from across the country and include samples from the NAPT soil database. Each sample will be analyzed using the ARS-developed Solvita respiration method and other currently used mineralization tests. To determine runoff water quality impacts of fertilizer recommendations based on enhanced soil testing methods, water quality samples from 6 field-scale cultivated watersheds at the Riesel Watersheds will be collected and analyzed. To evaluate the impacts of liquid fertilizer injection guided by GPS auto-steer technology on crop yield and economics, four replicated treatments (0, 20, 30, and 40 gal rates of 24-8-0 liquid fertilizer) will be implemented on two 25-ac fields, and crop yield, cost, and revenue data will be collected and analyzed.