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

Abstract

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 year−1 was studied during the period 1992–2007. The annual amount of dissolved inorganic nitrogen (DIN) in throughfall (TF) averaged 34 kg ha−1 year−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 year−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 year−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 year−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 NO x 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.