IDENTIFICATION OF CONIFEROUS FORESTS WITH INCIPIENT NITROGEN SATURATION THROUGH ANALYSIS OF ARGININE AND N-15 ABUNDANCE OF TREES

Stands of Norway spruce [Picea abies (L.) Karst.] differing in through fall N deposition and NO3- leaching were investigated to determine if arginine concentrations and delta(15)N abundances of needles could be used to predict NO3- leaching from stands. Twenty-three stands, all lo cated in southern Sweden, were sampled and five of these had average N O3- concentrations > 0.1 mg NO3--N L(-1) in soil water at a 50-cm dept h. Arginine concentrations and delta(15)N abundances of needles were s ignificantly higher for sites that exhibited NO3- leaching. The delta( 15)N enrichment (epsilon), i.e., difference between foliage and soil o rganic matter in delta(15)N, showed less variation than delta(15)N of needles alone in regressions against NO3- leaching and was, thus, a be tter predictor of leaching. All sites that exhibited N leaching had ar ginine concentrations > 5 mu mol g dw(-1) and epsilon > -0.5 (parts pe r thousand) and thus, there was a strong correlation between arginine and epsilon values. To test if arginine accumulation, as such, can aff ect the delta(15)N of needles, arginine and delta(15)N were determined for Scots pine (Pinus sylvestris L.) trees given different amounts of N fertilizer. Although fertilization caused a dramatic increase in ar ginine concentrations of needles, no immediate change in delta(15)N wa s observed. A hypothesis is proposed to explain the causal linkage bet ween elevated arginine and delta(15)N of needles and leaching of N. Th us, arginine accumulation in trees corresponds to decreased uptake rat es of NH4+ leaving NH4+ ions available for nitrification. During nitri fication, discrimination against the heavier N-15 takes place resultin g in a depleted NO3- pool and enriched NH4+ pool. Preferential uptake of NH4+ by conifer trees will then lead to elevated N-15 abundance of needles. It is concluded that N leaching coniferous forests can be ide ntified through analysis of arginine and delta(15)N of needles.