Heterotrophic soil respiration (HSR) is the largest terrestrial, non-anthropogenic source of CO2 contributing to climate change. A result of the decomposition of organic matter, HSR increases with rising temperature, contributing to a positive feedback on atmospheric CO2. Mycorrhizal fungi may mediate the sensitivity of HSR to rising temperatures. Ectomycorrhizal (EM) fungi depress decomposition by limiting nitrogen availability to saprotrophs, while arbuscular mycorrhizal (AM) fungi stimulate decomposition as a nutrient acquisition strategy. We hypothesized that ECM fungi make HSR less responsive to increasing temperatures, while AM fungi further stimulate carbon loss. We sampled soils from temperate forests along a latitudinal gradient (Wisconsin to Georgia) where each forest contained a range of AM and ECM species presence. We incubated soils from plots along mycorrhizal gradients at four temperatures (5, 10, 17, and 25°C), and measured HSR over time and conducted stoichiometric analyses on soils from all plots to determine C:N. Currently, we are using next generation sequencing and bioinformatics techniques to analyze the microbial composition of soil samples. Additionally, we will characterize physical and chemical structure of soil organic matter (SOM) in soil samples. An additional incubation experiment will investigate the combined effects of predicted shifts in temperature and moisture regime.
Results/Conclusions
Preliminary results from temperature incubation trials demonstrate that soils from forests dominated by ECM trees hold a lower temperature sensitivity of HSR than soils from forests dominated by AM trees. This suggests that ECM forest soil carbon stocks may be less vulnerable to warming than those in AM forests. Data analysis and additional experimentation will allow us to understand the biotic and abiotic mechanisms governing this pattern, improving predictions about future CO2 emissions and management plans for forests. Genetic sequencing of soils will determine if the biotic driver of microbial composition does vary across soils from ECM to AM tree dominance. Analyses of SOM and additional incubation will demonstrate whether abiotic drivers are responsible for trends in HSR sensitivity from temperature incubation trials. Because we cannot control the abiotic factors driving CO2 fluxes, and can only marginally influence biotic drivers, it is critical to understand the biogeochemical underpinnings of the forest carbon cycle. Further, since soils contain the largest stock of carbon in biosphere and atmosphere, it is of fundamental importance to understand how, when, and why HSR will convert this carbon into atmospheric CO2 in order to predict, and hopefully prevent, catastrophic positive feedbacks to climate warming.