The responses of nitrogen transformations and nitrate (NO3-) leaching to ex
perimentally increased N deposition were studied in forested sub-catchments
(1500 m(2)) with Gleysols in Central Switzerland. The aim was to investiga
te whether the increase in NO3- leaching, due to elevated N deposition, was
hydrologically driven or resulted from N saturation of the forest ecosyste
m. Three years of continuous N addition at a rate of 30 kg NH4NO3-N ha(-1)
yr(-1) had no effects on bulk soil N, on microbial biomass N, on K2SO4-extr
actable N concentrations in the soil, and on net nitrification rates. In co
ntrast, N losses from the ecosystem through denitrification and NO3- leachi
ng increased significantly. Nitrate leaching was 4 kg N ha(-1) yr(-1) at an
ambient N deposition of 18 kg N ha(-1) yr(-1). Leaching of NO3- at elevate
d N deposition was 8 kg N ha(-1) yr(-1). Highest NO3- leaching occurred dur
ing snowmelt. Ammonium was effectively retained within the uppermost centim
etres of the soil as shown by the absence of NH4+ in the soil solution coll
ected with micro suction cups. Quantifying the N fluxes indicated that 80%
of the added N were retained in the forest ecosystem. Discharge and NO3- co
ncentrations of the outflow from the sub-catchments responded to rainfall w
ithin 30 min. The water chemistry of the sub-catchment outflow showed that
during storms, a large part of the runoff from this Gleysol derived from pr
ecipitation and from water which had interacted only with the topsoil. This
suggests a dominance of near-surface flow and/or preferential transport th
rough this soil. The contact time of the water with the soil matrix was suf
ficient to retain NH4+, but insufficient for a complete retention of NO3-.
At this site with soils close to water saturation, the increase in NO3- lea
ching by 4 kg N ha(-1) yr(-1) through elevated N inputs appeared to be due
to the bypassing of the soil and the root system rather than to a soil-inte
rnal N surplus.