Dynamics of Nitrogen and Carbon Mineralization in a Fen Soil Following Water Table Fluctuations

Changes of water table level and oxygen supply affect the nitrogen (N) and carbon (C) mineralization of fen soils with potential consequences for the N and C sink and sources function of fens. Here we studied the response of gross N mineralization and CO2 emissions to water table fluctuations in an acidic minerotrophic fen. In a laboratory study lasting 117 days, undisturbed soil cores were either a) permanently flooded or b) subject to flooding, water table drawdown and reflooding. In the permanently flooded cores the CO2 emissions were constantly low, but gross ammonification and immobilization of NH (4) (+) increased after a lag phase of about 30 and 70 days, respectively. In the fluctuated cores, gross ammonification and NH (4) (+) immobilization first remained constant but then increased after water table drawdown of 30 days. Emission of CO2 peaked immediately after water table drawdown, followed by a decrease and a second maximum after about 30 days. Following re-flooding, gross ammonification and immobilization of NH (4) (+) first decreased but recovered after about 30 days to the level of the permanently flooded cores. In contrast, the CO2 emissions decreased immediately and permanently after re-flooding. The cumulative gross ammonification was larger in the permanently flooded cores than in the fluctuated cores. Rates of gross nitrification and immobilization of NO (3) (-) were generally low and did not respond to the treatments. The ratios of CO2 emission/gross ammonification were in the range of 1 to 4 under anoxic condition which seems to be caused by fast N turnover in the microbial biomass pool and low rates of C-mineralization of soil organic matter. Our results indicate that water table fluctuations in fen soils affect N and C mineralization differently. Changes of water table of a few days likely have a bigger effect on C-mineralization than on gross N mineralization.