Exchange of N-gases at the Hoglwald Forest - A summary

During 4 years continuous measurements of N-trace gas exchange were carried out at the forest floor-atmosphere interface at the Hoglwald Forest that is highly affected by atmospheric N-deposition. The measurements included spruce control, spruce limed and beech sites. Based on these field measurements and on intensive laboratory measurements of N(2)-emissions from the soils of the beech and spruce control sites, a total balance of N-gas emissions was calculated. NO(2)-deposition was in a range of -1.6 -2.9 kg N ha(-1) yr(-1) and no huge differences between the different sites could be demonstrated. In contrast to NO(2)-deposition, NO- and N(2)O-emissions showed a huge variability among the different sites. NO emissions were highest at the spruce control site (6.4-9.1 kg N ha(-1) yr(-1)), lowest at the beech site (2.3-3.5 kg N ha(-1) yr(-1)) and intermediate at the limed spruce site (3.4-5.4 kg N ha(-1) yr(-1)). With regard to N(2)O-emissions, the following ranking between the sites was found: beech (1.6-6.6 kg N ha(-1) yr(-1)) >> spruce limed (0.7-4.0 kg N ha(-1) yr(-1)) > spruce control (0.4-3.1 kg N ha(-1) yr(-1)). Average N-trace gas emissions (NO, NO(2), N(2)O) for the years 1994-1997 were 6.8 kg N ha(-1) yr(-1) at the spruce control site, 3.6 kg N ha(-1) yr(-1) at the limed spruce site and 4.5 kg N ha(-1) yr(-1) at the beech site. Considering N(2)-losses, which were significantly higher at the beech (12.4 kg N ha(-1) yr(-1)) than at the spruce control site (7.2 kg N ha(-1) yr(-1)), the magnitude of total gaseous N losses, i.e. N(2)-N + NO-N + NO(2)-N + N(2)O-N, could be calculated for the first time for a forest ecosystem. Total gaseous N-losses were 14.0 kg N ha(-1) yr(-1) at the spruce control site and 15.5 kg N ha(-1) yr(-1) at the beech site, respectively. In view of the huge interannual variability of N-trace gas fluxes and the pronounced site differences in N-gas emissions it is concluded that more research is needed in order to fully understand patterns of microbial N-cycling and N-gas production/emission in forest ecosystems and mechanisms of reactions of forest ecosystems to the ecological stress factor of atmospheric N-input.