Identification of Coniferous Forests with Incipient Nitrogen Saturation through Analysis of Arginine and Nitrogen‐15 Abundance of Trees

Stands of Norway spruce [ Picea abies (L.) Karst.] differing in throughfall N deposition and NO 3 − leaching were investigated to determine if arginine concentrations and δ 15 N abundances of needles could be used to predict NO 3 − leaching from stands. Twenty‐three stands, all located in southern Sweden, were sampled and five of these had average NO 3 − concentrations > 0.1 mg NO 3 − ‐N L −1 in soil water at a 50‐cm depth. Arginine concentrations and δ 15 N abundances of needles were significantly higher for sites that exhibited NO 3 − leaching. The δ 15 N enrichment (ε), i.e., difference between foliage and soil organic matter in δ 15 N, showed less variation than δ 15 N of needles alone in regressions against NO 3 − leaching and was, thus, a better predictor of leaching. All sites that exhibited N leaching had arginine concentrations > 5 µmol g dw −1 and ε > −0.5 (‰) and thus, there was strong correlation between arginine and ε values. To test if arginine accumulation, as such, can affect the δ 15 N of needles, arginine and δ 15 N were determined for Scots pine ( Pinus sylvestris L.) trees given different amounts of N fertilizer. Although fertilization caused a dramatic increase in arginine concentrations of needles, no immediate change in δ 15 N was observed. A hypothesis is proposed to explain the causal linkage between elevated arginine and δ 15 N of needles and leaching of N. Thus, arginine accumulation in trees corresponds to decreased uptake rates of NH 4 + leaving NH 4 + ions available for nitrification. During nitrification, discrimination against the heavier takes place resulting in a depleted NO 3 − pool and enriched NH 4 + pool. Preferential uptake of NH 4 + by conifer trees will then lead to elevated 15 N abundance of needles. It is concluded that N leaching coniferous forests can be identified through analysis of arginine and δ 15 N of needles.