Impacts of fire on microbial carbon cycling in subtropical wetlands

Abstract

Fire is a major determinant of the global carbon (C) balance. While it is known that C is lost through organic matter combustion, the effect fire has on soil C biogeochemistry is unclear. Studies investigating the role of fire on C greenhouse gas production (CO2 and CH4) have been conducted in forested and grassland ecosystems, yet research in wetlands has been limited. With their high potential for C storage, wetland ecosystems are important in C cycling while simultaneously serving as the largest single CH4 source in the world. Wildfires typically consume a majority of the above-water biomass in wetland systems that result in direct C losses, but the subsequent implications for C processing are unknown. Thus, understanding C cycling in wetlands regularly maintained or influenced by fire is critical to meeting C sequestration management objectives. This study focused on a fire-adapted wetland ecosystem undergoing restoration from agricultural impacts within the Everglades National Park, Florida, USA. Within the site, the effects of prescribed fire on C cycling (organic C, extractable organic C, enzyme activity, CO2, and CH4 production) were monitored in a restored (high-phosphorus [P]) and reference (low-P) wetland at both high and low elevations. Because fire can affect both C and P forms and availability, the objective of this study was to investigate the short- (two-day) and long-term (one-year) effect of fire on C cycling in subtropical wetlands soils of varying soil nutrient concentrations. Initially (two days post fire), C cycling was stimulated in both soils. However, stimulation of CO2 and CH4 production was observed only at the reference (low-P) site. This result suggests that fire may have an adverse effect on C cycling in low-P soils, initially augmenting C greenhouse gas production. Minimal heat transfer coupled with constant microbial biomass suggests that nutrients may have been a regulating factor in this process. After one year, no fire effect was distinguishable on C parameters from reference sites, yet variable effects were observed in restored soils. This suggests that C cycling in reference sites may recover more quickly than restored sites. The ultimate consequences of fire on C cycling in these wetlands systems are dependent on time and are strongly influenced by pre-fire site conditions.