Nitrogen transformations and losses in forest soil subjected to sprinklinginfiltration

Artificial ground water has been produced in Ahvenisto esker in southern Fi nland by sprinkling lake water directly onto the forest soil. The aim of th is study was to investigate N transformations in the soil, especially nitri fication, and N losses via leaching and gaseous emissions during the first three years of this sprinkling infiltration. During the first year, the pH of the humus layer increased from about 5 to 6.7 and net nitrification star ted, Intensive nitrification continued throughout the study period. Althoug h infiltration increased N2O emissions from the soil, the emissions were ve ry small compared with N losses via leaching, The esker retained organic N, The mean (NO2 + NO3)-N concentration in percolation water during infiltrat ion was close to that of the infiltration water (about 0.2 mg L-1) but, dur ing breaks in infiltration, the concentrations generally exceeded 10 mg L-1 . The ground water (NO2 + NO3)-N concentration, however, remained very low (mean 0.2 mg L-1) due to dilution of the (NO2 + NO,)-N, produced in the soi l, by the large volume of infiltration water. The results of this 3-yr stud y show that the leaching of nitrate does not presently pose a threat to the quality of ground water as long as infiltration is continuous. As nitrate was still being produced 2 yr after cessation of infiltration, there is a r isk of nitrate leaching into the ground water. The actual risk this poses t o ground water quality depends on the size of the infiltration area in rela tion to the whole aquifer.