Wetlands account for nearly 75% of natural global methane (CH4) emissions; however, large uncertainty exists around this estimate. Understanding how hydrologic fluctuations influence CH4production may be a key factor in predicting fluxes from these systems, especially as wetlands experience greater hydrologic variability brought on by more extreme drought-flood cycles.
Hydrology influences most of the biogeochemical controls on CH4 production, such as terminal electron acceptors and donors as well as microbial habitat. In hydrologically dynamic systems, a delay between soil submergence and net emissions of CH4 is complex. The length of this “lag time” between submergence and net CH4 production could potentially determine whether increasing the number of submersion events has any effect on overall wetland CH4 emissions, and provide insight into how extreme wet-dry cycling may effect global wetland CH4emissions.
To investigate this relationship, we looked at CH4 fluxes from a network of sites that undergo periods of drying and rewetting, as determined by water level sensor readings. Static chamber measurements of CH4 fluxes were taken daily over the summer of 2016 to determine fluxes at 14 topographically distinct sites. These data were used to determine the effect of submersion time on CH4 emissions and to identify potential delays in net CH4production.
A total of 530 flux measurements were recorded over a period of 5 months. CH4fluxes showed a large amount of variability, ranging from -3.11 to 30.5 (mg/m2/hr). The study site also exhibited a wide range of hydrologic fluctuations, with an average of 45% of sites submerged daily and a standard deviation of 28%. Compared to upland and lowland sites, mid-elevation sites experienced greater hydrologic variability, with an average of 76% of sites submerged and a standard deviation of 37%. These sites also exhibited the highest variability in fluxes over time.
At all sites, CH4 production from sediments began within 24 hours after flooding and peaked within several days. Overall, a decrease in net CH4flux over time was observed during these individual flooding events. As time since submersion increases over the course of several days, net CH4 production appears to level out and decrease.
The observed rapid spikes in emissions after submergence indicate that wetlands quickly become net sources of CH4 after a rise in water level, with minimal delay. This suggests that increasing the frequency at which these systems dry out and become resubmerged could potentially impact overall wetland emissions over time.