GASEOUS NITROGEN LOSSES FROM TWO FORESTED WATERSHEDS VIA NITRIFICATION AND DENITRIFICATION (NITROUS OXIDE, NITRATE REDUCTION, COWEETA, NUTRIENT CYCLING, SOIL MICROBIOLOGY)

Gaseous nitrogen (N) losses from two forested watersheds (WS) at the Coweeta Hydrologic Laboratory in western North Carolina were studied by in situ N(,2)O diffusion measurements and laboratory soil incubations. A WS with a history of disturbance, including clearcutting, liming, fertilization, conversion to grassland, and herbicide treatment, was dominated by black locust (Robinia pseudoacacia L., Fabaceae) and was further disturbed by a recent stem borer infestation (Megacyllene robiniae Forester). A reference WS, supporting an aggrading mixed hardwood forest, had not undergone major disturbance since the chestnut blight in the 1920's.; A technique for distinguishing between N(,2)O from Nitrification and denitrification by incubating soils under 3 levels of acetylene revealed that nitrification was a relatively unimportant pathway for N(,2)O loss. Denitrification appeared to be regulated primarily by soil moisture and NO(,3)('-) availability. A precipitation event dramatically increased in situ N(,2)O diffusion from a disturbed WS site, but the effects of precipitation were temporary. Gaseous-N loss from well-drained soils is probably relatively unimportant. Riparian zone soils were consistently wetter than upslope soils and exhibited the highest denitrification rates in laboratory incubations. Denitrification in the riparian zone and subsequent degassing of N(,2)O in stream water is suggested as an important pathway for N loss from the disturbed WS.; Low ambient NO(,3)('-) concentrations and low nitrification potentials in the reference WS soils limited denitrification. Nitrification was probably affected by both acidic conditions and competition for NH(,4)('+) in the reference WS. Estimates of N(,2)O diffusion from soils indicated that less than 1 kg N ha('-1) yr('-1) is lost from the reference WS via gaseous-N production.; Laboratory incubations with glucose amendments indicated that available carbon may limit denitrification at some upslope positions and probably becomes limiting with increasing soil depth, but that most surface soils studied were not carbon limited. Acidity may affect relative rates of denitrification and indirectly influence denitrification by inhibiting autotrophic nitrification, but denitrification appears possible in these acidic forest soils (pH 4.6).