Nitrogen Saturation and Retention in Forested Watersheds of the Catskill Mountains, New York

The Catskill Mountains of southeastern New York receive relatively high rates of atmospheric N deposition, and NO3 concentrations in some streams have increased dramatically since the late 1960s. We measured the chemistry of 39 firstand second-order streams with forested watersheds to determine the variability of nitrogen concentrations within the Catskill Mountain area. We found that some streams have low NO3 concentrations throughout the year, some have seasonal cycles of varying amplitude, and some have relatively high concentrations year round. If the concentration and seasonality of NO3 in stream water are used as indices of nitrogen saturation, then most stages of nitrogen saturation are evident in our survey of Catskill watersheds. Organic nitrogen was a small portion of the total nitrogen for streams with high NO3 concentration, but organic N was the dominant form of N (up to 73% of the total) in the streams with lowest nitrate. Estimated retention of N in these watersheds (based on total N in stream water) ranged from 49% to 90% of the atmospheric input. The variation in stream water NO3 concentration and the amplitude of the seasonal fluctuations did not appear to be attributable to differences among watersheds in atmospheric deposition, watershed topography, or groundwater influx to the stream. We hypothesize that differences among watersheds in forest species composition and forest history, which are interrelated, produce most of the variation in NO3 concentration that we observed.