NITRIFICATION AND NITROUS-OXIDE PRODUCTION POTENTIALS IN AEROBIC SOIL SAMPLES FROM THE SOIL-PROFILE OF A FINNISH CONIFEROUS SITE RECEIVING HIGH AMMONIUM DEPOSITION

Using aerobic soil slurry technique nitrification and nitrous oxide production were studied in samples from a pine site in Western Finland. The site received atmospheric ammonium deposition of 7-33 kg N ha(-1) a(-1) from a mink farm. The experiments with soil slurries showed that the nitrification potential in the litter layer was higher at pH 6 than at pH 4. However, the nitrification potentials in the samples from the organic and mineral horizons at pH 6 and 4 were almost equal. Also N2O was produced at a higher rate at pH 6 than at pH 4 in slurries of the litter layer samples. The reverse was true for samples from the organic and mineral horizons. The highest N2O production and nitrification rates were measured in the suspensions of litter layer samples. Nitrification activity in field-moist soil samples was lower than the activity in the slurries indicating that the availability of ammonium limited nitrification in these soils. Acetylene (2.5 kPa) retarded nitrification activity (70-100%) and N2O production (40-90%) in soil slurries. Acetylene inhibited the N2O production by 40-60% during the first 3 days after its addition to field-moist samples incubated in aerobic atmosphere. After 3 days the inhibition became much lower (4-5%). The results indicate that, in soil profiles of boreal coniferous forests receiving ammonium deposition, chemolithotrophic nitrification may have importance in the N2O production, and that changes in soil pH affect differently nitrification as well as N2O production in litter and deeper soil layers.