15N study of the reactivity of atmospheric nitrogen in four mountain forest catchments (Czech Republic, central Europe)

Reactivity of atmospherically deposited nitrate (NO3-) and ammonium (NH4+) was investigated in three mountain forest catchments and one ombrotrophic peat bog located in the northern Czech Republic. The study sites are characterized by moderate to high N pollution rates that are currently decreasing. Monitoring of hydrodynamic data and catchment hydrochemistry (precipitation, soil solutions and runoff) was complemented by delta N-15 analyses of dissolved inorganic N (NO3-, NH4+). Measured delta N-15 data were used to calculate the extent of biogeochemical reactions and to construct N mass balances. Two-component models of runoff generation were developed using hydrological and delta O-18-H2O data on local precipitation, runoff, and soil waters collected by lysimeters. Catchment discharge was formed by groundwater (60-80%) and storm precipitation (40-20%), with minimal time lags. Groundwater had a diluting effect on reactions proceeding in the soil zone, including nitrification and denitrification. The rates of nitrification were estimated by comparing the delta N-15 values of atmospheric input and soil water, while the rates of denitrification were derived from the differences in the delta N-15 values of soil water and stream discharge. The Nu isotope effect of mineralisation of organic N was assessed by comparing delta N-15 values of soil and soil extracts. Nitrogen release from the catchments was controlled by temperature-dependent seasonality of soil reactions. Atmospheric N entered the runoff directly only during a short late winter - early spring period, accounting for approximately 10% of the total N input. Important inputs of mineralised organically cycled soil N were observed at the beginning and the end of the growing season, with measurable denitrification occurring during the same time periods. The measured apparent N isotope fractionations were significantly lower than previously published values (-3 to -14 parts per thousand vs. -30 parts per thousand for nitrification, and -3 vs. -20 parts per thousand for denitrification). Lower N-15 fractionations originated both from the dilution effect of groundwater on stream discharge and from the depletion of available ammonium during nitrification reactions. Denitrification proceeding during recharge of the groundwater body was estimated from the difference in the delta N-15 values of NO(3)(- )in precipitation and groundwater.