Seasonal fluctuations and diurnal oscillations in nitrate of a heathland brook

A 5-year study on nitrogen dynamics were performed in a pristine heathland catchment (2.55 km2) in southern Norway. The catchment received 1.6 g total N m−2 yr−1 from wet and dry deposition, which is nearly tenfold more than that of northern Norway. Compared to most Norwegian catchments and forested areas, there is only a modest depression in mid-summer nitrate in runoff from winter maxima of nearly 250 μg litre−1 to late summer minima of 50–100 μg NO3-N litre−1. N:P-ratios in runoff was strongly skewed towards P-limitation, and concentrations of both phosphate and total-P were normally below detection limits (<1 μg litre−1). The nitrate response of a three days P-enrichment in late May in the main brook draining the area was monitored continuously with a 10-min resolution for two months. The data confirmed a strong P-limitation and revealed a strong nitrate uptake and diurnal oscillations in nitrate concentrations over a period of more than one month following the P-enrichment. Over this period, mid-day concentrations were reduced by a factor of 2–3 relative to night maxima. Such reductions in nitrate during day relative to night-time may have profound effect on transport or budget estimates based on day-time samples only. Nitrate contributed normally 20–30% to anion acidity, and over the summer period there was a significant negative correlation between nitrate concentrations and pH, suggesting that increased N-uptake could also affect pH of such low-alkalinity systems.