ISOTOPE RATIOS AND CONCENTRATIONS OF SULFUR AND NITROGEN IN NEEDLES AND SOILS OF PICEA-ABIES STANDS AS INFLUENCED BY ATMOSPHERIC DEPOSITION OF SULFUR AND NITROGEN-COMPOUNDS

Concentrations and natural isotope abundance of total sulfur and nitrogen as well as sulfate and nitrate concentrations were measured in needles of different age classes and in soil samples of different horizons from a healthy and a declining Norway spruce (Picea abies (L.) Karat.) forest in the Fichtelgebirge (NE Bavaria, Germany), in order to study the fate of atmospheric depositions of sulfur and nitrogen compounds. The mean delta(15)N of the needles ranged between -3.7 and -2.1 parts per thousand and for delta(34)S a range between -0.4 and +0.9 parts per thousand was observed. delta(34)S and sulfur concentrations in the needles of both stands increased continuously with needle age and thus, were closely correlated. The delta(15)N values of the needles showed an initial decrease followed by an increase with needle age. The healthy stand showed more negative 6'5N values in old needles than the declining stand. Nitrogen concentrations decreased with needle age. For soil samples at both sites the mean delta(15)N and delta(34)S values increased from -3 parts per thousand (delta(15)N) or +0.9 parts per thousand (delta(34)S) in the uppermost organic layer to about +4 parts per thousand (delta(15)N) or +4.5 parts per thousand (delta(34)S) in the mineral soil. This depth-dependent increase in abundance of N-15 and S-34 was accompanied by a decrease in total nitrogen and sulfur concentrations in the soil. delta(15)N values and nitrogen concentrations were closely correlated (slope -0.0061 parts per thousand delta(15)N per mu mol eq N g(dw)(-1)), and delta(34)S values were linearly correlated with sulfur concentrations (slope -0.0576 parts per thousand delta(34)S per mu mol eq S g(dw)(-1)). It follows that in the same soil samples sulfur concentrations were linearly correlated with the nitrogen concentrations (slope 0.0527), and delta(34)S values were linearly correlated with delta(15)N values (slope 0.459). A correlation of the sulfur and nitrogen isotope abundances on a Delta basis (which considers the different relative frequencies of N-15 and S-34), however, revealed an isotope fractionation that was higher by a factor of 5 for sulfur than for nitrogen (slope 5.292). These correlations indicate a long term synchronous mineralization of organic nitrogen and sulfur compounds in the soil accompanied by element-specific isotope fractionations. Based on different sulfur isotope abundance of the soil (delta(34)S = 0.9 parts per thousand for total sulfur of the organic layer was assumed to be equivalent to about -1.0 parts per thousand for soil sulfate) and of the atmospheric SO2 deposition (delta(34)S = 2.0 parts per thousand at the healthy site and 2.3 parts per thousand at the declining site) the contribution of atmospheric SO2 to total sulfur of the needles was estimated. This contribution increased from about 20 % in current-year needles to more than 50 % in 3-year-old needles. The proportion of sulfur from atmospheric deposition was equivalent to the age dependent sulfate accumulation in the needles. In contrast to the accumulation of atmospheric sulfur compounds nitrogen compounds from atmospheric deposition were metabolized and were used for growth. The implications of both responses to atmospheric deposition are discussed.