ABOVEGROUND N-UPTAKE FROM WET DEPOSITION BY NORWAY SPRUCE (PICEA-ABIES KARST)

Nitrogen-uptake from wet deposition, by above-ground plant parts of 10-year-old Norway spruce trees, was studied as a function of the N-form, N-concentration in precipitation and intensity of irrigation by using N-15-labelled solutions. The irrigated solutions contained all major cations and anions as present in normal forest rainwater but with varying NH4 and NO3 contents. After 5 months of treatment, the trees were harvested and needles, stems, main stem and roots were analysed for N-15 content. Nitrogen-uptake was calculated from the sum of N-15 in all plant compartments. Significant amounts of N were taken up by the above-ground parts of the plants in all treatments. NH4 uptake was higher than NO3 uptake if applied in the same concentration. Nitrogen-uptake increased with increasing N-concentrations and irrigation intensity. Extrapolating these results to mature stands (by extrapolating needle biomass and with the assumption that mature spruces are able to take up N from wet deposition during the winter months at the same rate as determined for the young trees in this experiment) suggests annual uptake rates of N in the range of 9 kg ha-1 under field conditions. Measuring atmospheric N input by throughfall collection may substantially underestimate actual N inputs to forest ecosystems. Uptake of N by above-ground parts may cause nutritional disturbances indicated by changes of the N:Ca and N:Mg ratios in the needles and thus may contribute to the occurrence of Mg-deficiency symptoms of Norway spruce.