1a. Objectives (from AD-416):
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.
1b. Approach (from AD-416):
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.
3. Progress Report:
Scientific effort and fiscal resources from a project terminated in September 2012 were consolidated with the previous version of this project. Objectives 4 and 5 were added. We made substantial progress in addressing each of the five current objectives and related sub-objectives of this project, all of which relate to Objectives identified in National Programs 212 and 215. Objective 1 - We analyzed 7 years of data from an experiment in which assemblages of grassland plants on intact monoliths of each of three soil types have been exposed to a CO2 gradient spanning pre-Industrial to elevated concentrations. One goal is to determine CO2 effects on litter decomposition, soil organic matter pools, and soil C dynamics as mediated by abiotic variables, including soil type and water content, and biotic variables, such as litter quality and the soil microbial community. Litter decomposition rates were increased by increasing soil water content and the frequency of soil drying and re-wetting. Decomposition was greater in clay than sandy soils. Soil type also influenced the response of soil microbial communities to CO2. The number of fungal species and relative abundance of a group of fungi (chytrids) increased linearly with CO2 in a clay soil, whereas the relative abundance of arbuscular mycorrhizal fungi that facilitate plant uptake of soil phosphorus increased linearly with CO2 in a sandy soil. With ARS collaborators at Lincoln, NE, we are evaluating CO2 effects on productivity and bioenergy-relevant tissue constituents in switchgrass (Panicum virgatum). Objective 2 - Using data from 4 long-term experiments, we and university collaborators found that variability in grassland production is reduced by increasing the number of plant species per unit of land area. Production varied less in species-rich than species-poor communities because increasing species richness increased the amount by which assemblages over-yielded relative to yields expected based on production of monocultures of component species. Objective 3 - With university collaborators, we began to develop and test the ALMANAC model for describing CO2 effects on grasslands. ALMANAC was parameterized and validated for a native prairie community of six plant species growing on three soil types at ambient CO2. Daily weather input was created for each monolith along the experimental subambient to elevated CO2 gradient. We are evaluating the capacity of ALMANAC to simulate CO2-caused changes in aboveground production. Objective 4A - Data collection was continued at the USDA-ARS Riesel Watersheds to evaluate runoff water quality impacts of fertilizer recommendations based on enhanced soil testing methods. Objective 4B - Data collection was completed for evaluation of liquid fertilizer injection guided by GPS auto-steer technology. Objective 5 - The enhanced method to rapidly determine soil microbial activity based on CO2 evolution after drying/rewetting soil was completed. This will empower commercial and research labs to produce cost-effective and more accurate quantitative estimates of N mineralization potential and biological soil quality.
Daneshgar, P.P., Wilsey, B.J., Polley, H.W. 2013. Simple plant traits explain functional group diversity decline in novel grassland communities of Texas. Plant Ecology. 214:231-241.