The nitrogen cycle was studied in selected Adirondack northern hardwood forest and lake ecosystems using a 15N natural abundance and a low level enrichment approach. The three forest study sites (Woods Lake, Pancake-Hall, and Huntington Forest) differed in N availability. The relationship between 15N natural abundance and N availability was investigated. Within a site, delta (del) 15N values in plant tissues were lower than observed for soils. Del 15N values of soil increased with depth. Comparisons of 15N natural abundance of soil and plant tissues among the sites were consistent with the hypothesis that higher del 15N values were associated with greater N availability.

In two Adirondack lakes (Dart's and Arbutus) that differed in nitrate concentration, del 15N values of seston and sediment were compared. In Arbutus Lake, seston del 15N values averaged 2.5 per thousand (mil) and 2.7 per mil at 2 and 6 meters depth, respectively. In Dart's Lake, seston del 15N values averaged 1.3 and 1.1 per mil at 6 and 14 meters depth.

In the epilimnion of Arbutus Lake, nitrate concentrations were generally low (range 0 to 13 microeq/liter) with decreased nitrate concentration during the the summer; summer seston del 15N and water column nitrate concentrations were inversely correlated (r=-0.82, p=0.011). In Dart's Lake, where concentrations were higher (range 12.9 to 22.4 microeq/L) and the magnitude of spring depletion lower, there was no significant correlation between seston del 15N and nitrate concentration. These observations support the hypothesis that differences in seston del 15N values were related to differences in relative utilization of nitrate between these lakes.