Effects of Land Use Change on the Development of Hot Spots of Methane Flux in a Restored Freshwater Wetland

A big source of uncertainty in the methane budget is the existence of "hot spots", defined as patches in the landscape with disproportionately higher rates of biogeochemical activity. In this study, we characterized hot spots of methane flux at the typical eddy-covariance site level (i.e. within an area of about 500 by 500 m) using micrometeorological, biogeochemical, and microbial analysis. Using a novel approach called Footprint-Weighted Flux Maps and chambers we detected and validated a hot spot of CH4 flux in an oligohaline restored marsh on a farmed island, constructed over a layer of highly compacted, oxidized, and decomposed peat as a result of land use conversion. Through chamber measurements during the months of April and May, we found that fluxes at the hot spot were on average as high as 6589 ± 7889 nmol m-2 s-1 whereas background flux from the open water was on average 15.2 ± 7.5 nmol m-2 s-1. Importantly, we find that the hot spot originates below a compacted clay layer located 30 cm under the surface soil, where high methane concentrations and a high methanogenic to methanotrophic ratio was observed. The stable isotopic signature of the carbon in the methane and in the dissolved inorganic carbon indicated biogenic methane production but also larger methane oxidation of the ancient peat layer. We analyze the patterns in porewater chemical analytes, microbial activity, and isotopic signatures, and suggest that higher methane in the hot spot is a result of the thinning of the surface compacted and oxidized peat layer below which lies an unoxidized old peat layer that contains large quantities of methane.