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 two major stressors: 1. Climate change (variability in water availability caused by changes in precipitation patterns) and 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 invasibility. 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:
1. During the previous fiscal year we installed important infrastructure to support future experiments, replanted switchgrass plots because of prior poor establishment, and designed and deployed irrigation equipment to conduct 'evapotranspiration replacement' (ET) treatments. 2. Postdoc completed an experiment determining the controls on early growth dynamics in several native and invasive grasses present in the field plots, to explain why the invasive species out-competes the natives. Manuscript in prep from this work will show mechanisms by which Johnsongrass outcompetes native grasses in early phases of growth. This will be useful for designing management strategies to favor native grass species in grassland restoration or bioenergy production contexts. 3. Postdoc substantially completed a modeling study testing the effects of varying rainfall seasonality on switchgrass biomass production, and the dependence of those effects on fertilizer levels and soil type. This will inform nutrient management efforts in switchgrass planted for bioenergy production. Results were presented at a meeting of USDA-NIFA Principal Investigators, at the Ecological Society of America meetings, and at the Switchgrass II Conference. 4. Recruited temporary field technician to assist with fieldwork and sampling processing.