Effect of Warming and Precipitation Distribution on Soil Respiration and Mycorrhizal Abundance in Post Oak Savannah

Projected climate change may alter soil carbon dioxide (CO2) efflux from terrestrial ecosystems; yet disentangling effect of plant species from climate drivers remains a key challenge. We explored the effects of the dominant plant species, warming, and precipitation distribution on soil CO2 efflux, its underlying components, and mycorrhizal abundance in southern post oak savannah. Post oak savannah in the south-central US are dominated by three contrasting plant functional types: Schizachyrium scoparium (Michx.) Nash. (little bluestem) a C4 grass, Quercus stellata Wangenh.(post oak)a C3 deciduous tree, and Juniperus virginiana L. (eastern redcedar) a C3 evergreen tree. Monocultures and tree-grass plots were warmed using infrared heaters and precipitation events were manipulated to intensify summer drought and augment cool season precipitation. Soil CO2 efflux, the root, bacterial and hyphal components of CO2 efflux, and mycorrhizal abundance were measured. Soil CO2 efflux varied with seasonal changes in soil VWC and temperature, with higher soil CO2 efflux rates in the spring and lower rates in both the cooler winter season and at the end of the dry summer period. There was no relationship between root length density or root mass density and soil CO2 efflux during the short term precipitation distribution campaigns. Partitioning of root, fungal, and bacterial component contribution to soil CO2 efflux indicated a substantial contribution of bacterial respiration to soil CO2 efflux within this system. There was no relationship between microbial biomass [microbial dissolved organic carbon (DOC)] and soil CO2 efflux, or root length (or mass) density and microbial biomass. This suggests that species and climatic effects on root and microbial activity drive soil CO2 efflux. As plant species within this system differed in their association with mycorrhizal fungi and had a strong effect on the individual components of soil CO2 efflux, we conclude that shifts in vegetation cover and growth and the response of vegetation to long term warming and potential future extreme precipitation events (e.g., large preciptation events, prolonged drought) will be major drivers of changes in soil carbon (C) dynamics and associated soil CO2 efflux.