1a. Objectives (from AD-416)
Our research objective is to initiate a long-term field experiment to determine the temporal stability of ecosystem service provision from switchgrass monocultures, improved pastures, and low input/high diversity native grassland. Ecosystem services of primary interest are plant biomass production and soil carbon storage, and their responses to three major stressors: 1. Climate change (variability in water availability caused by changes in precipitation patterns), 2. Invasive plant species (invasions by the introduced grass Sorghum halapense, Johnsongrass). The central hypothesis guiding this research is that net ecosystem service provision is greater from high diversity native grassland than monocultures. This result is expected because we predict that: 1. Temporal variability in plant biomass production due to varying climate is lower than monocultures, 2. Net bioenergy potential (biomass produced - inputs) and carbon storage is greater than monocultures, and 3. Susceptibility to invasive plant species is reduced.
1b. Approach (from AD-416)
Our approach focuses on a large-scale field study that will provide information for parameterizing a plant growth model (ALMANAC) and a coupled soil-plant-atmosphere biogeochemistry model (SPAB). Replicated 0.63 ha (1.5 ac) plots will be planted with three treatments: 1. Switchgrass (Panicum virgatum) in monoculture, 2. Coastal bermudagrass (Cynodon dactylon L.) in monoculture, and 3. Native prairie polyculture of 30 native prairie species, including common dominant C4 grasses and numerous forbs. Nitrogen fertilizer will be added to half the switchgrass and coastal bermudagrass plots and subplots within the native prairie plots (150 lb N ac-1). Subplots within each planting treatment will be irrigated to replace evapotranspiration to minimize water limitation of plant and soil processes by minimizing seasonal and interannual variability in water availability. Additional subplots will be planted with seeds of a widespread introduced invasive grass, Sorghum halapense (Johnsongrass), to test invisibility. Measurements of plant biomass and its Bioenergy potential, plant community composition, plant and soil pools and fluxes of carbon and nitrogen, microclimate, and soil moisture will be conducted to quantify differences in ecosystem service provision among these treatments. Field data will be used to parameterize two models: 1. ALMANAC: This plant growth model will simulate plant biomass production and will be calibrated and verified for each planting treatment. Additional simulations will be run to predict production under environmental scenarios beyond those represented in the experiment. 2. SPAB: This coupled soil-plant-atmosphere biogeochemistry model will predict long-term probability distributions of soil carbon storage.
3. Progress Report
The core objective of this project is to evaluate the mean and variability in aboveground biomass productivity in mixed species and monoculture perennial grass systems. In spring 2010 we initiated this research by planting 24 plots (0.6 to 0.9 acres per plot) to 1) a mix of native tallgrass prairie grasses and forbs, 2) monoculture switchgrass (Panicum virgatum), or 3) monoculture coastal Bermuda (Cynodon dactylon). These plots will be allowed to establish for the next two growing seasons (through 2011) before field experimentation begins. The dominant grass species planted in the field plots were evaluated for above- and belowground nitrogen allocation in a greenhouse experiment. Plants were grown in monocultures and mixtures and evaluated for tissue nitrogen and carbon content, leaf area, and photosynthetic and transpiration rates. This study provided important preliminary information on plant competitive interactions and nitrogen budgets that will help explain the controls on plant species composition dynamics and productivity in the field plots.