Nitrogen mineralization and gaseous nitrogen losses from waterlogged and drained organic soils in a black alder (iAlnus glutinosa/i (L.) Gaertn.) forest

摘要

pstrongAbstract./strong Black alder (iAlnus glutinosa/i (L.) Gaertn.) forests on peat soils have been reported to be hotspots for high nitrous oxide (Nsub2/subO) 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 (Nsub2/sub)-fixation of alder trees. Our study addressed the question to what extent drainage enhances the emissions of Nsub2/subO from black alder forests and how N turnover processes and physical factors influence the production of Nsub2/subO 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 Nsub2/subO 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 Nsub2/sub 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 Nsub2/subO fluxes in the field but did in the laboratory experiment. Nsub2/subO 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 Nsub2/subOa??N hasupa??1/sup yrsup−1/sup (D-2). Only 37% of the spatiotemporal variation in field Nsub2/subO fluxes could be explained by peat temperature and groundwater level, demonstrating the complex interlinking of the controlling factors for Nsub2/subO emissions. However, temperature was one of the key variables of Nsub2/subO fluxes in the incubation experiment conducted. Increasing soil moisture content was found to enhance total denitrification losses during the incubation experiment, whereas Nsub2/subO fluxes remained constant. At the undrained site, permanently high groundwater level was found to prevent net nitrification, resulting in a limitation of available nitrate (NOsub3/subsup−/sup) and negligible gaseous N losses. Nsub2/subO flux rates that were up to four times higher were measured in the incubation experiment. They reveal the potential of high Nsub2/subO losses under changing soil physical conditions at the drained alder sites. The high net nitrification rates observed and high NOsub3/subsup−/sup contents bear the risk of considerable NOsub3/subsup−/sup leaching at the drained sites./p.