Environmental factors controlling NO3− leaching, N2O emissions and numbers of NH4+ oxidisers in a coniferous forest soil

Main and interactive effects of temperature, throughfall volume and NH4+ deposition on soil solution NO3− concentrations, N2O emissions and numbers of NH4+ oxidisers were investigated in a controlled laboratory experiment. Large intact soil cores from a Picea abies (L.) Karst. stand were incubated according to an `incomplete factorial designʹ at 4, 12 or 20°C and watered every 2 weeks with 300, 500 or 700 ml (442, 737 and 1032 mm yr−1) of a natural throughfall solution enriched with 0, 37.5 or 75 kg NH4+–N ha−1 yr−1. Watering and sampling were performed every 2 weeks, during a 112 d period. At d 112, a temperature optimum for NO3−–N concentrations in the leachate, NO3−–N fluxes and numbers of NH4+ oxidisers in the mineral soil layer was determined at ca. 11°C. NO3−–N concentrations also decreased with throughfall volume, towards a minimum at 590 ml, with temperature however contributing most to modelling NO3−–N concentrations and the two factors acting independently. The model explained 59% of the variability in the data, and the regression between observed and predicted concentrations was highly significant (P<0.0001, r2=0.93). NO3−–N fluxes increased quadratically with throughfall volume, and throughfall volume and NH4+ deposition interacted significantly in determining the numbers of NH4+ oxidisers in the mineral soil layer. Numbers of NH4+ oxidisers were higher in the humus layer and decreased with increasing temperatures. N2O fluxes increased quadratically with temperature, and the linear and quadratic effects of throughfall volume (maximum at 500 ml). Results suggest that optimum temperatures for net nitrification may have been overestimated in previous studies by the use of disturbed soils.