| Nitrate concentrations in recently infiltrated groundwater
in forested areas in the Netherlands are slowly increasing towards the EU limit
of 50 mg NO3 l–1. The origin of this nitrate
is thought to be leaching from nitrogen (N) saturated semi-natural ecosystems
in these areas. To simulate nitrate leaching on a regional scale, the empirical
model WANDA (a regional nitrogen model With Aggregated Nitrogen
DynAmics) is introduced. The model is built around the concept
that in forests the C:N ratio of the organic layer is indicative of the amount
of nitrate leaving the system in drainage. WANDA was tested on a regional dataset
of an infiltration area of 10 km2. This infiltration area consisted
of 350 forest stands draining to the catchment "Edese Bos" used by a drinking
water company. In 75 of the 350 forest stands the C:N ratio of the organic layer
was measured. In 30 of these 75 stands, the nitrate concentrations below the
rooting zone were measured. A hydrological model calculated water fluxes. The
C:N ratio of the 75 stands ranged from 15.7 to 31.3 g C g–1
N. Scots pine stands had the highest C:N ratios in the organic layer. Nitrate
concentrations varied in the 30 stands from 0.6 to 70 mg NO3 l–1.
The nitrate concentrations in the Douglas fir stands were higher, and in the
beech stands lower, than those observed in the locations with other tree species.
WANDA predicted nitrate concentrations well, but improvements are required to
simulate nitrate fluxes. The simulation of the water fluxes in WANDA is probably
where more focussed modelling effort is now required.
Keywords: nitrogen, nitrate, forest, model, WANDA, C:N ratio, organic layer |