Atmospheric deposition and watershed nitrogen export along an elevational gradient in the Catskill Mountains, New York

Cumulative effects of atmospheric N deposition may increase N export from w atersheds and contribute to the acidification of surface waters, but natura l factors (such as forest productivity and soil drainage) that affect fores t N cycling can also control watershed N export. To identify factors that a re related to stream-water export of N, elevational gradients in atmospheri c deposition and natural processes were evaluated in a steep, first-order w atershed in the Catskill Mountains of New York, from 1991 to 1994. Atmospheric deposition of SO42-, and probably N, increased with increasing elevation within this watershed. Stream-water concentrations of SO42- incre ased with increasing elevation throughout the year, whereas stream-water co ncentrations of NO3- decreased with increasing elevation during the winter and spring snowmelt period, and showed no relation with elevation during th e growing season or the fall. Annual export of N in stream water for the ov erall watershed equaled 12% to 17% of the total atmospheric input on the ba sis of two methods of estimation. This percentage decreased with increasing elevation, from about 25% in the lowest subwatershed to 7% in the highest subwatershed; a probable result of an upslope increase in the thickness of the surface organic horizon, attributable to an elevational gradient in tem perature that slows decomposition rates at upper elevations. Balsam fir sta nds, more prevalent at upper elevations than lower elevations, may also aff ect the gradient of subwatershed N export by altering nitrification rates i n the soil. Variations in climate and vegetation must be considered to dete rmine how future trends in atmospheric deposition will effect watershed exp ort of nitrogen.