Patterns of nitrate loss from a chronosequence of clear-cut watersheds

Three clear-cuts at the Hubbard Brook Experimental Forest (NH) have resulted in a chronosequence of forest watersheds in close proximity. Following clear-cutting, the stands, now 12, 21, 27, and 78 years old, have different species composition, nutrient capital, and biogeochemistry. In this study, we compared seasonal patterns of NO₃^- in streamwater, changes in N capital, and N retention in watersheds of differing stand age. All of the watersheds showed elevated losses of NO₃^-, H⁺ and nutrient cations (Ca²⁺, Mg²⁺, K⁺) during the first few years following clear-cutting. Increased retention of N occurred during vegetation regrowth compared to the reference watershed (W6). Nitrate concentrations were low during the summer growing season, increased in the late fall and peaked in March during spring snowmelt. Concentrations of NO₃^- were lower in the regrowing watersheds than in W6 during all months. In W6, there was considerable year-to-year variability in N retention, which was not initially observed in the manipulated watersheds. However, two cut watersheds exhibited higher export of NO₃^- in 1989 and 1990, corresponding to a 10-year high value in annual NO₃^- loss in W6. These results demonstrate the importance of land use and cutting history in assessments of N saturation and loss from forest watersheds.