Nitrogen biogeochemistry of a mature Scots pine forest subjected to high nitrogen loads

Nitrogen (N) biogeochemistry of a mature Scots pine (Pinus sylvestris L.) stand subjected to an average total atmospheric N deposition of 48 kg ha-1 yr-1 was studied during the period 1992-2007. The annual amount of dissolved inorganic nitrogen (DIN) in throughfall (TF) averaged 34 kg ha-1 yr-1 over the 16-year monitoring period. The throughfall fluxes contained also considerable amounts of dissolved organic nitrogen (DON) (5-8.5 kg N ha-1 yr-1), which should be incorporated in the estimate of N flux using throughfall collectors. Throughfall DIN fluxes declined at a rate of -0.9 kg N ha-1 yr-1, mainly due to the decreasing TF fluxes of ammonium (NH4), which accounted for 70% to TF DIN. The decrease in TF DIN was accompanied by a decrease in DIN leaching in the seepage water (-1.6 kg N ha-1 yr-1), which occurred exclusively as nitrate (NO3-). Nitrate losses in the leachate of the forest floor (LFH) equalled the TF NO3- delivered to the LFH-layer. On the contrary, about half of the TF NH4+ was retained within the LFH-layer. Approximately 60 % of the TF DIN fluxes were leached indicating that N inputs were far in excess of the N requirements of the forest. For DON, losses were only substantial from the LFH-layer, but no DON was leached in the seepage water. Despite the high N losses through nitrate leaching and NOx emission, the forest was still accumulating N, especially in the aggrading LFH-layer. The forest stand, on the contrary, was found to be a poor N sink.