Grassland ecosystems comprise over 30% of the Earth's terrestrial surface, and provide a resource base for extensive agricultural activities such as ranching. Grasslands are sensitive to climate change because they exist where hot, dry weather is common and rainfall is unpredictable, and because their plant communities may rapidly shift in response to rising climate and atmospheric CO2 concentrations. Grassland soils contain large stores of long-lived carbon, which could either buffer climate change, or accelerate it if this carbon is released to the atmosphere. Soil microbial communities are the ultimate drivers of soil carbon uptake or loss. This project investigates how plant-microbe interactions regulate soil carbon cycling within an ongoing, state-of-the-art, manipulative climate change experiment in grasslands near Cheyenne, Wyoming, the Prairie Heating and CO2 Enrichment (PHACE) experiment. Thirty plots are exposed to combinations of climate conditions, including warming and CO2 conditions expected to occur before the end of the 21st century, and altered precipitation. An important component of the experiment is the comparison of carbon and nutrient cycling between native and disturbed grasslands with distinct plant communities, including invasive species. Laboratory and growth chamber experiments applying molecular and isotopic methods will test specific hypotheses generated from observations and measurements in the field. This project is expected to reduce uncertainties related to interactions between soil nutrients, biological communities and climate change, leading to improved predictions of future atmospheric CO2 concentrations and associated warming effects.
This project will reach out to agricultural resource management agencies, ranchers and landowners who are concerned about impacts of climate change, disturbance and weed invasion on rangeland productivity, by conducting annual Field Days at the PHACE site and publishing articles in the popular press. Cross-site collaboration and data synthesis will be promoted by incorporating soil data into a comprehensive database. The project will provide strong interdisciplinary training to two graduate students, several undergraduates, and two postdoctoral scientists from under-represented groups. In-service middle school science teachers participating in the University of Wyoming Master's of Science in Natural Science degree program will be invited to the field site for hands-on lesson development, and minority high school students will be mentored. The new Summer Soil Institute at Colorado State University (http://soilinstitute.nrel.colostate.edu) will bring students to the PHACE field site for sampling and analyses.
Variability in grassland primary production was stabilized under elevated CO2 conditions, but dominant grasses declined in production while other less abundant grasses increased. This means that the grassland ecosystem became more resilient to variable precipitation conditions when exposed to elevated CO2. The growing season for this important grazing ecosystem was extended by the climate change treatments, because warming made spring arrive earlier, while elevated CO2 extended growth in autumn by increasing water availability. Productivity responses to drought may be buffered by elevated CO2 and exacerbated by warming. In practical terms, these findings are relevant for ranchers and land managers as they plan their grazing activities and on a broader scale, grassland production stability can be useful for improved predictions of carbon uptake and sequestration. We found that a deep-rooted, invasive forb becomes very competitive under elevated CO2, as its rapid growth and less conservative water use allowed it to increase its biomass several-fold. This finding has important implications for management of invasive weeds with climate change, which may become more dominant in future conditions. Future climate conditions may enhance soil organic matter decomposition, which could further aggravate climate change. This is explained by plant and microbial traits interacting at the root-soil interface, through adjustments in microbial community composition. These adjustments are likely to depend upon changes in nutrient availability, which appears to regulate the extent of organic matter decomposition. The potential for soils to store carbon may be compromised by these interactions between plants and microbes as they compete for nutrients.
