Changes in nitrogen transformations in forest soil as a result of sprinkling infiltration

Sprinkling infiltration through forest soil is a relatively new technique for generating artificial groundwater reserves to supplement urban water supplies in Finland. The aim of this study was to determine the effects of infiltration and surface runoff, resulting from sprinkling infiltration, on the acidity status and nitrogen transformations in forest soil and in percolation water. The amount of infiltrated water applied annually during the study period was more than 2000 times greater than natural annual recharge in the area. The study area is located on an esker in southern Finland (Ahvenisto, Hämeenlinna). The tree cover on the relatively fertile site consisted of Scots pine and Norway spruce. The results obtained during the first two years of the project indicated that the pH(H2O) of the organic layer of the soil had increased strongly due to sprinkling infiltration. The original pH of the organic layer was 4.7–5.1, and infiltration increased the pH to 6.5. The pH of percolation water collected below the organic layer increased strongly as a result of infiltration (>6.7 during infiltration). The pH of the infiltration water is about 7.0, which is higher than that of precipitation in the area (4.5). Sprinkling infiltration initiated net nitrification due to the elevated pH and increased ammonium availability. Infiltration increased emissions of N2O from the soil. During breaks in the infiltration treatment the leaching of NO3 from the topsoil was considerable compared to that on the control plots.