We measured soil respiration rates to study effects of tree-grass interactions and climate change on root and soil microbial activity in southern oak savanna. Quercus stellata (post oak, deciduous) and Juniperus virginiana (juniper, evergreen) are the dominant C₃ trees and Schizachyrium scoparium (little bluestem) the dominant C₄ grass. Contrasting photosynthetic pathways and leaf habits of these species may strongly affect respiratory carbon efflux from soils in this system. In addition, climate warming and rainfall redistribution may differentially alter autotrophic and heterotrophic components of soil respiration. We warmed monoculture and tree-grass plots using infrared heaters (100 W m^-2 from 1.5 m) and manipulated rainfall events to intensify summer drought and augment cool season rainfall compared to the long-term mean. Soil CO₂ efflux was measured approximately every month from May 2005. Additionally, we used root and hyphae exclusion tubes to estimate the relative contribution of roots, microbes, and hyphae to soil respiration. Exclusion tubes were installed in June 2008 and data were collected on October 1, November 21, and January 22.

Results/Conclusions

Soil respiration was generally higher in plots populated with grass and lowest in oak monocultures. The warming treatment did not affect soil CO₂ efflux on any date. Soil CO₂ efflux was decreased by intensified summer drought and by increased spring rainfall. A targeted campaign to measure root length density (RLD, km_root m^-3 _soil) and soil respiration before and after a summer rainfall event revealed a strong link between RLD and soil respiration in dry soils that disappeared following the rainfall event. This suggests that the relative contribution of roots and microbes to soil respiration fluctuates and that the microbial contribution increases with a sudden increase in soil water content. Exclusion of roots and hyphae from soil reduced soil respiration rates. Rainfall redistribution decreased the contribution of root and hyphae to soil respiration on October 1, when plots in the redistributed treatment were drier. Juniper monoculture plots had greater contribution of roots and hyphae in January compared to the other plots, and there were no treatment or species effects in November. Across all species mixtures and treatments, the relative contribution of roots and hyphae was 33% in October and November and 20% in January. Overall, these results suggest that soil CO₂ efflux in oak savanna will respond more strongly to changes in species composition and related root length density than to direct effects of altered rainfall distribution or warming.