Nitrogen Solutes in an Adirondack Forested Watershed: Importance of Dissolved Organic Nitrogen

Nitrogen (N) dynamics were evaluated from 1 June 1995 through 31 May 1996 within the Arbutus Lake watershed in the Adirondack Mountains of New York State, U.S.A. At the Arbutus Lake outlet dissolved organic nitrogen (DON), NO3- and NH4+ contributed 61%, 33%, and 6% respectively, to the total dissolved nitrogen (TDN) flux (259 mol ha-1 yr-1). At the lake inlet DON, NO3-, and NH4+ constituted 36%, 61%, and 3% respectively, of TDN flux (349 mol ha-1 yr-1). Differences between the factors that control DON, NO3-, and NH4+ stream water concentrations were evaluated using two methods for estimating annual N flux at the lake inlet. Using biweekly sampling NO3- and NH4+ flux was 10 and 4 mol ha-1 yr-1 respectively, less than flux estimates using biweekly plus storm and snowmelt sampling. DON flux was 18 mol ha-1 yr-1 greater using only biweekly sampling. These differences are probably not of ecological significance relative to the total flux of N from the watershed (349 mol ha-1 yr-1). Dissolved organic N concentrations were positively related to discharge during both the dormant (R2 = 0.31; P < 0.01) and growing season (R2 = 0.09; P < 0.01). There was no significant relationship between NO3- concentration and discharge during the dormant season, but a significant negative relationship was found during the growing season (R2 = 0.29; P < 0.01). Biotic controls in the growing season appeared to have had a larger impact on stream water NO3- concentrations than on DON concentrations. Arbutus Lake had a major impact on stream water N concentrations of the four landscape positions sampled, suggesting the need to quantify within lake processes to interpret N solute losses and patterns in watershed-lake systems.