Abstract
Abstract. Black alder (Alnus glutinosa (L.) Gaertn.) forests on peat soils have been reported to be hotspots for high nitrous oxide (N2O) 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 (N2)-fixation of alder trees. Our study addressed the question to what extent drainage enhances the emissions of N2O from black alder forests and how N turnover processes and physical factors influence the production of N2O 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 N2O 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 N2 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 the field but did in the laboratory experiment. N2O 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 N2O–N ha−1 yr−1 (D-2). Only 37% of the spatiotemporal variation in field N2O fluxes could be explained by peat temperature and groundwater level, demonstrating the complex interlinking of the controlling factors for N2O emissions. However, temperature was one of the key variables of N2O fluxes in the incubation experiment conducted. Increasing soil moisture content was found to enhance total denitrification losses during the incubation experiment, whereas N2O fluxes remained constant. At the undrained site, permanently high groundwater level was found to prevent net nitrification, resulting in a limitation of available nitrate (NO3−) and negligible gaseous N losses. N2O flux rates that were up to four times higher were measured in the incubation experiment. They reveal the potential of high N2O losses under changing soil physical conditions at the drained alder sites. The high net nitrification rates observed and high NO3− contents bear the risk of considerable NO3− leaching at the drained sites.
Highlights
Black alder (Alnus glutinosa (L.) Gaertn.) forests represent the natural vegetation in many semi-wet and wet regions, mostly on soils with high organic carbon contents (Dilly et al, 1999; Schäfer and Joosten, 2005; Kätzel, 2003)
Our study addressed the question to what extent drainage enhances the emissions of N2O from black alder forests and how N turnover processes and physical factors influence the production of N2O and total denitrification
The topsoil at the undrained site U consisted of a mud sediment while the topsoil at the drained sites D-1 and D-2 consisted of highly degraded fen peat
Summary
Black alder (Alnus glutinosa (L.) Gaertn.) forests represent the natural vegetation in many semi-wet and wet regions, mostly on soils with high organic carbon contents (Dilly et al, 1999; Schäfer and Joosten, 2005; Kätzel, 2003). The few annual observations of N2O emissions from Alnus forest ecosystems on drained or undrained peatlands range between 0.1 and 72.0 kg N ha−1 yr−1 (Augustin et al, 1998; Brumme et al, 1999; Merbach et al, 2001; von Arnold et al, 2005). The high range requires in-depth process understanding to narrow down the uncertainty and better understand N2O formation, not least because of the attempt to reduce the climate impact of drained fen peatland by rewetting and afforestation with black alder trees (Alnus glutinosa (L.) Gaertn.) (Schäfer and Joosten, 2005; Wichtmann and Joosten, 2007)
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