Quantification of the nitrogen cycle in a prairie stream

Nitrogen (N) was added for 35 days in the form of (NH4Cl)-N-15 to Kings Cre ek on Konza Prairie, Kansas. Standing stocks of N in key compartments (that is, nutrients, detritus, organisms) were quantified, and the amount of lab eled N entering the compartments was analyzed. These data were used to calc ulate turnover and nux rates of N cycling through the food web, as well as nutrient transformation rates. Inorganic N pools turned over much more rapi dly in the water column of this stream than in pelagic systems where compar able measurements have been made. As with Other systems, the mass of ammoni um was low but it was the key compartment mediating nutrient Bur through th e ecosystem, whereas dissolved organic N, the primary component of N flux t hrough the system, is not actively cycled. Nitrification was also a signifi cant flux of N in the stream, with rates in the water column and surface of benthos accounting for approximately 10% Of the total ammonium uptake. Pri mary consumers assimilated 67% of the inorganic N that entered benthic alga e and microbes. Predators acquired 23% of the N that consumers obtained. In vertebrate collectors, omnivorous crayfish (Orconectes spp.), and invertebr ate shredders dominated the N flux associated with primary consumers. Mass balance calculations indicated that at least 23% of the 309 mg of N-15 adde d during the 35 days of release was retained within the 210-m stream reach during the release. Overall, the rates of turnover of N in organisms and or ganic substrata were significantly greater when C:N was low. This ratio may be a surrogate for biological activity with regard to N flux in streams.