Methane Fluxes from a Rich Fen: Relations with the Hydrochemistry and the Dissolved Carbon Isotopic Composition

Abstract

In the framework of the eutrophic Ob fen study, methane fluxes were measured by the chamber method, and water was sampled for the analysis of general chemical composition, content of biophilic elements (C, N, and P) and δ¹³C of the dissolved inorganic carbon (δ¹³C-DIC). Samples were taken from open and forested areas within the fen, with the latter having long received discharged domestic wastewaters. The methane emissions positively correlate with the concentrations of nitrogen compounds and dissolved organic carbon (DOC). In both areas, higher methane emissions were observed from waterlogged microdepressions than from dry elevated microlandscape features. The largest emission was observed from the microdepressions in the forested area near the wastewater discharge point, where the extreme values of CH₄ fluxes were almost 30 times higher than the background ones. However, the methane fluxes decrease to the background level at 160 m from the wastewater discharge point, together with the concentrations of almost all nitrogen and carbon compounds dissolved in the fen water. This led us to conclude that wastewater pollution significantly affects the intensity of methane fluxes by increasing ebullitive methane emission near the pollution source. The isotopic composition of the dissolved inorganic carbon (DIC) was heavier in the forested area (δ¹³C-DIC = –9.64 to –9.21‰) than in the open one (–12.83 to –11.24‰). In the open area of the fen, DIC isotopic composition became heavier away from the dry upland, correlating with the increase in the methane fluxes. The obtained data indicate that methane-generating processes are more active in the forested area of the fen than in the open one and become more active in the open area with increasing distance from the dry upland. This highlights the potentialities of using δ¹³C-DIC as a fingerprint of wastewater pollution, at least in the Ob fen area.