The fate of 15NH4 + labeled deposition in a Scots pine forest in the Netherlands under high and lowered NH4+ deposition, eight years after application

To study the long-term fate of deposited ammonium (NH4 +) in a Scots pine forest stand under high nitrogen (N) deposition in the Netherlands we re-sampled the plots of a 15N tracer experiment with high (i.e. ambient) and lowered N deposition in this stand 8 years after application of the tracer. The results were compared with results obtained 7 years earlier. In the 7 years between the samplings the 15N deltas of needles, twigs and upper organic soil layer had converged to similar values still above the natural 15N abundance, suggesting equilibration as a result of intensive cycling of N among these pools. Bark and wood had lower deltas than needles and twigs, but if the label found was attributed to tissue synthesized since the start of the labeling only, bark values were similar to needles and twigs, whereas wood values were higher indicating retranslocation of N into older wood. Mineral soil lost all 15N label it had accumulated after 1 year indicating that this label had not been strongly bound. The first year the low N treatment had retained more of the labeled NH4 + deposition than the high N treatment, but in the seven subsequent years relatively more label was retained in the latter. This better retention after 7 years was ascribed to a larger fraction of label taken up by the vegetation in the high N treatment. This shows that the vegetation can affect the label dynamics despite the fact that only a relatively small amount of label was present in the aboveground vegetation.