Plant -microbe interactions in oak savanna: Controls on carbon and nitrogen cycling

Microbes perform the biological transformations of organic material that is responsible for soil carbon cycling and the availability of nutrients essential for plant growth. Perhaps the most important (and least understood) factor in influencing rates of ecosystem processes is the interaction between aboveground plant communities and belowground microbial communities. However, the mechanisms controlling this relationship between plant and microbial communities are still poorly understood. I conducted both field and laboratory experiments to address the mechanisms of plant-microbe interactions and their importance in ecosystem processes. Both in a field study and a controlled substrate-addition experiment in the laboratory, I found that the composition, quantity, and diversity of organic inputs to microbial communities significantly affect microbial community structure and function in soil. First, I conducted a field study at Cedar Creek Natural History Area and found that increased fire frequency caused declines in both soil nitrogen availability and litter nitrogen concentrations. However, rates of litter decomposition and litter nitrogen dynamics were primarily controlled by litter characteristics, not differences in soil nitrogen availability. Thus, fire has a significant indirect effect on microbial activity through changes in litter nitrogen concentrations. Second, I conducted a substrate addition experiment in soil microcosms to determine the mechanisms by which organic inputs to soil affect microbial community structure and function. I found that the composition, quantity, and diversity of substrates added all significantly affected extracellular enzyme activity. However, the response of enzyme activity to substrate variation was dependent on the enzyme assayed. Finally, I explored the response of microbial community structure to added substrates and found that the composition of the community was significantly influenced by substrate composition and diversity. There were significant correlations between microbial community structure and function that were strongly correlated with labile C acquisition. I conclude that variation in inputs to microbial communities does significantly affect the structure and function of the microbial community and can have important consequences for ecosystem processes.