A Field Study on the Fate of 15N-Ammonium to Demonstrate Nitrification of Atmospheric Ammonium in an Acid Forest Soil

To demonstrate the contribution of atmospheric ammonium to soil acidification in acid forest soils, a field study with 15N-ammonium as tracer was performed in an oak-birch forest soil. Monitoring and analysis of soil solutions from various depths on the 15N-ammonium and 15N-nitrate contents, showed that about 54% of the applied 15N-ammonium was oxidized to nitrate in the forest floor. Over a period of one year about 20% of the 15N remained as organic nitrogen in this layer. The percentage 15N enrichment in ammonium and nitrate were in the same range in all the forest floor percolates, indicating that even in extremely acid forest soils (pH < 4) nitrate formation from ammonium can occur. Clearly, atmospheric ammonium can contribute to soil acidification even at low soil pH.