Nitrification and denitrification in forest soil subjected to sprinkling infiltration

In Ahvenisto esker, southern Finland, artificial recharging of groundwater has been done by sprinkling infiltration, i.e. by sprinkling lake water dir ectly onto forest soil. Due to infiltration, the pH of the humus layer rose from about 5 to 6.5, nitrification was initiated and the fluxes of N2O and leaching of nitrate from the soil increased. Our aim was to study nitrogen transformations in different soil layers and to determine the response of nitrification to pH. Nitrification in ammonium-enriched soil suspensions wa s pH-dependant in a gradient from 4.7 to 6.7. In the soils subjected to inf iltration, the production of (NO2 + NO3)-N was inhibited by decreasing the pH to 5.3 or lower. Low pH also led to decreased numbers of nitrifiers. In the soils not subjected to infiltration (control soils), (NO2+NO3)-N produc tion initiated at pH 6.7 and the numbers of nitrifiers increased. In incuba tion experiments, with no added ammonium, the adjustment of pH to 6.7 also initiated nitrification in the control soils. Thus, increase in soil pH was the main reason for initiation of nitrification at this site. During infil tration, N2O was produced mainly by denitrification and approximately 75% o f the denitrification products was N-2. In the samples from the humus layer , the concentrations of (NO2 + NO3)-N, the net production of mineral N and net nitrification were in general less, whereas denitrification enzyme acti vity and denitrification potential were higher than in the samples from the mineral soil layer. The mineral soil may therefore contribute substantiall y to the leaching of nitrate. (C) 2000 Elsevier Science Ltd. All rights res erved.