| Effects of contrasting
climatic conditions and nitrogen (N) deposition levels on streamwater N dynamics
are assessed at two small heathland catchments; Dalelva in northern Norway (69°N)
and Øygard in southwestern Norway (58°N). The study
comprises 11 years of data on climate, hydrology and N inputs/outputs from
Dalelva and 8 years of corresponding data from Øygard. Both sites are
comparable in catchment size, geology and land cover characteristics, but have
large differences in climate and N deposition. Dalelva is characterised by a
cold, arctic climate and low N deposition (2-3 kg N ha–1y–1),
whereas the Øygard site has a more mild, humid climate with much larger N
deposition (13–19 kg N ha–1yr–1). Streamwater
nitrate (NO3‾) concentrations at Dalelva generally were negligible
during the growing season, but showed a steady increase during the dormant
season until a maximum of 40-100 μg N L–1 was reached just before
snowmelt. At onset of the snowmelt flood, NO3‾ concentrations
decreased momentarily to very low levels, suggesting that N eluted from the
seasonal snowpack to a great extent was infiltrated and immobilised in the
soils. At Øygard, flood peaks occurred frequently during all seasons, and
usually there was no distinct spring flood. A lack of clear dilution effects
from floods on streamwater N3‾ concentrations may indicate a
relatively high NO3‾ leaching potential in
this catchment. On average, the annual NO3– export was negligible
at Dalelva (<0.1 kg N ha–1yr–1), while at Øygard
it amounted to 3.0±0.3 (±1 s.d.) kg ha–1yr–1, or
nearly 20% of the annual N deposition. In addition to this relatively high
annual N loss, elevated NO3‾ concentrations during the growing season
further indicate that the N supply at Øygard is in excess of the combined plant
and microbial demand.
Keywords: catchments, surface water, nitrogen deposition, nitrate leaching,
climate, hydrology, snowmelt, Dalelva brook, Øygard brook |