Micro-scale CO 2 and CH 4 dynamics in a peat soil during a water fluctuation and sulfate pulse

Abstract

We investigated the response of CO 2 and CH 4 production to a water table fluctuation and a SO 4 2− pulse in a bog mesocosm. Net gas production rates in the mesocosm were calculated from concentration data by diffusive mass-balances. Incubation experiments were used to quantify the effect of SO 4 2− addition and the distribution of potential CO 2 and CH 4 production rates. Flooding of unsaturated peat resulted in rapid depletion of O 2 and complex patterns of net CH 4, CO 2, and H 2S production. Methane production began locally and without a time lag at rates of 3–4 nmol cm −3 d −1 deeper in the peat. Similar rates were determined after a time lag of 10–60 days in the surface layers, whereas rates at lower depths declined. Net CO 2 production was largest immediately after the water table position was altered (100–300 nmol cm −3 d −1) and declined to −50–50 nmol cm −3 d −1 after a few weeks. SO 4 2− addition (500 mM) significantly increased potential CH 4 production rates in the surface layer from an average of 132–201 nmol cm −3 d −1 and reduced it below from an average of 418–256 nmol cm −3 d −1. Our results suggest that deeper in the peat (40–70 cm) under in situ conditions, methanogenic populations are less impaired by unsaturated conditions than in the surface layers, and that at these depths after flooding the substrate availability for CH 4 and DIC production is significantly enhanced. They also suggest that methanogenic and SO 4 2−-reducing activity were non-competitive in the surface layer, which might explain contradictory findings from field studies.