Within landscapes, specific areas (‘hot spots’) and time periods (‘hot moments’) can contribute disproportionately to some biogeochemical processes. We hypothesize that vernal pools are hot spots, and that seasonal changes in water availability and soil water content creates hot moments, which can lead to disproportionately high nitrous oxide (N2O, from incomplete denitrification) and methane (CH4) emissions. Moreover, we hypothesize that vernal pool microbial communities responsible for these emissions are linked spatially via exchange of resources, and are organized along hydrologic flow paths across multiple scales. In other words, hydrology brings together appropriate chemicals that can be converted into greenhouse gases only when a microbial community that has the potential for the transformation is functionally able to respond. In this study, we investigated N2O and CH4 emissions from vernal pools in a forested ecosystems along a topographic gradient (bottom slope and upslope) across seasons. Bacterial communities were examined based on abundance and expression of nitrate reduction (denitrification) and methanogenesis genes.
Based on preliminary analyses, there were seasonal and spatial differences in greenhouse gas production and physiochemical conditions, as predicted. From spring to fall there was significant decrease in soil moisture content and increase in nitrate content. There was no significant difference in nitrate or soil moisture content between upslope (US) and bottom slope (BS) vernal pools. However, nitrate content in US pools in the fall was significantly higher than nitrate content in the spring, as spring and fall nitrate contents in BS pools, which were not significantly different from each other. N2O and CH4 emissions did not differ significantly between US and BS pools, although CH4 emissions increased significantly from spring to fall, but N2O emissions did not. This is attributed to contrasting correlations between N2O emissions and nitrate contents in US and BS pools in spring and fall. N2O emissions were positively correlated with soil moisture in the spring in both US and BS pools, but negatively correlated with soil moisture in the fall. CH4 emissions were positively correlated with nitrate content in BS pools in both periods, and negatively correlated with nitrate content in US pools in both periods, but were positively correlated with soil moisture across US and BS pools during both periods. Overall, CH4 and N2O emissions were negatively correlated with each other. Ongoing work is focused on acquiring microbial gene expression data and additional sampling periods, to investigate seasonal differences in CH4 and N2O emissions.