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Nitrogen Transformations and Losses in Forest Soil Subjected to Sprinkling Infiltration

Vantaa Research Centre, Finnish Forest Research Institute, Box 18, FIN-01301 Vantaa, Finland;
Rovaniemi Research Station, Finnish Forest Research Institute, Box 16, FIN-96301, Rovaniemi, Finland.

^* Corresponding author (laura.paavolainen{at}metla.fi).

ABSTRACT

Artificial ground water has been produced in Ahvenisto esker in southern Finland by sprinkling lake water directly onto the forest soil. The aim of this study was to investigate N transformations in the soil, especially nitrification, 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 started. Intensive nitrification continued throughout the study period. Although infiltration increased N₂O emissions from the soil, the emissions were very small compared with N losses via leaching. The esker retained organic N. The mean (NO₂ + NO₃)–N concentration in percolation water during infiltration was close to that of the infiltration water (about 0.2 mg L^–1) but, during 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 + NO₃)–N, produced in the soil, by the large volume of infiltration water. The results of this 3-yr study 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 risk of nitrate leaching into the ground water. The actual risk this poses to ground water quality depends on the size of the infiltration area in relation to the whole aquifer.