Nitrogen mineralization and gaseous nitrogen losses from waterloggedand drained organic soils in a black alder (Alnus glutinosa (L.) Gaertn.) forest

[摘要] Black alder (Alnus glutinosa (L.) Gaertn.) forests on peat soilshave been reported to be hotspots for high nitrous oxide (N₂O) losses.High emissions may be attributed to alternating water tables of peatlands andto the incorporation of high amounts of easily decomposable nitrogen (N) intothe ecosystem by symbiotic dinitrogen (N₂)-fixation of alder trees. Ourstudy addressed the question to what extent drainage enhances the emissionsof N₂O from black alder forests and how N turnover processes andphysical factors influence the production of N₂O and totaldenitrification. The study was conducted in a drained black alder forest withvariable groundwater tables at a southern German fen peatland. Fluxes ofN₂O 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 Nmineralization rates (NNM) were determined. Additionally a laboratoryincubation experiment was carried out to investigate greenhouse gas andN₂ 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 N₂O fluxes in thefield but did in the laboratory experiment. N₂O fluxes measured were lowfor all sites, with total annual emissions of 0.51 ± 0.07 (U),0.97 ± 0.13 (D-1) and0.93 ± 0.08 kg N₂O–N ha⁻¹ yr⁻¹ (D-2). Only 37%of the spatiotemporal variation in field N₂O fluxes could be explainedby peat temperature and groundwater level, demonstrating the complexinterlinking of the controlling factors for N₂O emissions. However,temperature was one of the key variables of N₂O fluxes in the incubationexperiment conducted. Increasing soil moisture content was found to enhancetotal denitrification losses during the incubation experiment, whereasN₂O fluxes remained constant. At the undrained site, permanently highgroundwater level was found to prevent net nitrification, resulting in alimitation of available nitrate (NO₃⁻) and negligible gaseous Nlosses. N₂O flux rates that were up to four times higher were measuredin the incubation experiment. They reveal the potential of high N₂Olosses under changing soil physical conditions at the drained alder sites.The high net nitrification rates observed and high NO₃⁻ contents bearthe risk of considerable NO₃⁻ leaching at the drained sites.

[学科分类] 地球化学与岩石