Experimental response of peatland carbon dynamics to a water table fluctuation

Abstract

Water table fluctuations influence the carbon balance of wetlands, but the effects are difficult to isolate and quantify in field investigations. Thus, we compared C mineralization in a peatland mesocosm exposed to a water table fluctuation (from 5 to 67 cm beneath the surface) with that in mesocosms with a stable high water table (2 to 6 cm depth) and with production rates obtained from flask incubations. Net turnover rates were calculated from concentration data by diffusive-advective mass-balances. Under stable, high water table conditions, net production of CO2 (6.1 mmol m-2 d-1, CH4 (2.1 mmol m-2 d-1) and DOC (15.4 mmol m-2 d-1) were vertically stratified and production and fluxes equilibrated. Lowering and raising the water table from 5 to 67 cm resulted in complex patterns of net CH4 and CO2 production. Response and equilibration times of processes upon drainage and flooding ranged from days (aerobic CO2 production, CH4 oxidation, fluxes under unsaturated conditions) to months (CH4 production, fluxes under saturated conditions). Averaged over the water table fluctuation, net production of CH4 decreased to 0.36 mmol m-2 d-1 and that of CO2 increased to 140 mmol m-2 d-1. Physical disturbance through the incubation of peat strongly increased production rates of CO2, CH4 and DOC compared to in situ, steady state rates. The decoupling of production and fluxes to the atmosphere under conditions with variable water table depths potentially explains part of the frequently reported lack of correlation between environmental variables and trace gas fluxes in field investigations, and questions the applicability of predictions of gas flux based on empirical relationships established under stable average conditions.