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

T. Eickenscheidt,J. Heinichen,J. Augustin,A. Freibauer,M. Drösler

Published 2014 in Biogeosciences

ABSTRACT

Black alder (Alnus glutinosa (L.) Gaertn.) forests on peat soils have been reported to be hotspots for high ni- trous oxide (N2O) losses. High emissions may be attributed to alternating water tables of peatlands and to the incorpo- ration 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 ex- tent drainage enhances the emissions of N2O from black alder forests and how N turnover processes and physical fac- tors influence the production of N 2O and total denitrifica- tion. The study was conducted in a drained black alder for- est 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 mineral- ization rates (NNM) were determined. Additionally a labo- ratory 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 N 2O 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 N 2O fluxes could be explained by peat temperature and ground- water 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 incu- bation 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 (NO 3 ) and negligible gaseous N losses. N2O flux rates that were up to four times higher were mea- sured 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 ob- served and high NO 3 contents bear the risk of considerable NO 3 leaching at the drained sites.

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