Competition of spruce trees for substrates of microbial N2O-production and -emission in a forest ecosystem

Uptake of ammonium and nitrate by the roots of spruce trees at a N-limited control site and a site subjected to ammonium sulphate fertilisation with 150 kg N ha(-1) in one dose were determined shortly after and two years subsequent to the treatment. The changes in the rates of nitrogen uptake by the roots were compared with changes in microbial nitrogen conversion that are connected with the production of N2O in, and the emission of these atmospheric trace gases from soils by nitrification and denitrification. Uptake of ammonium and nitrate by the roots of the N-limited trees were low and initially were strongly enhanced by fertiliser application. This increase was mainly due to an increase in ammonium and nitrate available to the roots in. the soil-water. Two years subsequent to the fertiliser application, ammonium and nitrate contents of the soil-water were still enhanced compared to the untreated control site, but uptake of ammonium and nitrate by the spruce roots was completely shut down. At the N-limited control site competition between root uptake of inorganic N and its microbial conversion was largely in favour of root uptake by the spruce trees. The rates of nitrification and denitrification of inorganic N were minute under these conditions and appreciable N2O emission was not observed. Application of ammonium sulphate transiently enhanced both root uptake and microbial N conversion, and was connected with increased emission of N2O. In the long term, ammonium sulphate application completely inhibited ammonium and nitrate uptake by the trees, irrespective of enhanced concentrations of ammonium and nitrate in the soil-water. As a consequence, considerable amounts of inorganic N were still available for microbial conversion 2 years after fertiliser application. At this time nitrification rates were still high at the fertilised site, but the nitrate produced was not further converted by denitrification. It is assumed that nitrate produced by nitrification is subjected to microbial immobilisation. Under these conditions N2O emission from the control site and the fertilised site were low and did not differ significantly.