Nitrogen mineralization and gaseous nitrogen losses from waterlogged and drained organic soils in a black alder ( Alnus glutinosa (L.) Gaertn.) forest
Black alder ( Alnus glutinosa (L.) Gaertn.) forests on peat soils have been reported to be hotspots for high nitrous oxide (N 2O) losses. High emissions may be attributed to alternating water tables of peatlands and to the incorporation of high amounts of easily decomposable nitrogen (N) into the ecosystem by symbiotic dinitrogen (N 2)-fixation of alder trees. Our study addressed the question to what extent drainage enhances the emissions of N 2O from black alder forests and how N turnover processes and physical factors influence the production of N 2O and total denitrification. The study was conducted in a drained black alder forest with variable groundwater tables at a southern German fen peatland. Fluxes of N 2O were measured using the closed chamber method at two drained sites (D-1 and D-2) and one undrained site (U). Inorganic N contents and net N mineralization rates (NNM) were determined. Additionally a laboratory incubation experiment was carried out to investigate greenhouse gas and N 2 fluxes at different temperature and soil moisture conditions. Significantly different inorganic N contents and NNM rates were observed, which however did not result in significantly different N 2O fluxes in the field but did in the laboratory experiment. N 2O fluxes measured were low for all sites, with total annual emissions of 0.51 ± 0.07 (U), 0.97 ± 0.13 (D-1) and 0.93 ± 0.08 kg N 2O–N ha −1 yr −1 (D-2). Only 37% of the spatiotemporal variation in field N 2O fluxes could be explained by peat temperature and groundwater level, demonstrating the complex interlinking of the controlling factors for N 2O emissions. However, temperature was one of the key variables of N 2O fluxes in the incubation experiment conducted. Increasing soil moisture content was found to enhance total denitrification losses during the incubation experiment, whereas N 2O fluxes remained constant. At the undrained site, permanently high groundwater level was found to prevent net nitrification, resulting in a limitation of available nitrate (NO 3−) and negligible gaseous N losses. N 2O flux rates that were up to four times higher were measured in the incubation experiment. They reveal the potential of high N 2O losses under changing soil physical conditions at the drained alder sites. The high net nitrification rates observed and high NO 3− contents bear the risk of considerable NO 3− leaching at the drained sites.