Nasholm T, Nordin A, Edfast AB, Hogberg P
Identification of coniferous forests with incipient nitrogen saturation through analysis of arginine and nitrogen-15 abundance of trees
Journal of Environmental Quality: 1997 26:302-309
Stands of Norway spruce [Picea abies (L.) Karst.] differing in throughfall 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 located in southern Sweden, were sampled and five of these had average NO3- concentrations > 0.1 mg NO3--N L(-1) in soil water at a 50-cm depth. Arginine concentrations and delta(15)N abundances of needles were significantly higher for sites that exhibited NO3- leaching. The delta(15)N enrichment (epsilon), i.e., difference between foliage and soil organic matter in delta(15)N, showed less variation than delta(15)N of needles alone in regressions against NO3- leaching and was, thus, a better predictor of leaching. All sites that exhibited N leaching had arginine concentrations > 5 mu mol g dw(-1) and epsilon > -0.5 (parts per thousand) and thus, there was a strong correlation between arginine and epsilon values. To test if arginine accumulation, as such, can affect 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 arginine concentrations of needles, no immediate change in delta(15)N was observed. A hypothesis is proposed to explain the causal linkage between elevated arginine and delta(15)N of needles and leaching of N. Thus, arginine accumulation in trees corresponds to decreased uptake rates of NH4+ leaving NH4+ ions available for nitrification. During nitrification, discrimination against the heavier N-15 takes place resulting 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 identified through analysis of arginine and delta(15)N of needles.
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