N uptake as a function of concentration in streams

Detailed studies of stream N uptake were conducted in a prairie reach and gallery forest reach of Kings Creek on the Konza Prairie Biological Station. Nutrient uptake rates were measured with multiple short-term enrichments of NO3- and NH4+ at constant addition rates in the spring and summer of 1998. NH4+ uptake was also measured with N-15-NH4+ tracer additions and short-term unlabeled NH4+ additions at 12 stream sites across North America. Concurrent addition of a conservative tracer was used to account for dilution in all experiments. NH4+ uptake rate per unit area (U-t) was positively correlated to nutrient concentration across all sites (r(2) = 0.41, log-log relationship). Relationships between concentration and U-t were used to determine whether the uptake was nonlinear (i.e., kinetic uptake primarily limited by the biotic capacity of microorganisms to accumulate nutrients) or linear (e.g., limited by mass transport into stream biofilms). In all systems, U-t was lower at ambient concentrations than at elevated concentrations. Extrapolation from uptake measured from a series of increasing enrichments could be used to estimate ambient U-t Linear extrapolation of U-t assuming the relationship passes through the origin and rates measured at 1 elevated nutrient concentration underestimated ambient U-t by similar to3-fold. Uptake rates were saturated under some but not all conditions of enrichment; in some cases there was no saturation up to 50 mumol/L. The absolute concentration at which U-t was saturated in Kings Creek varied among reaches and nutrients. Uptake rates of NH4+ at ambient concentrations in all streams were higher than would be expected, assuming U-t does not saturate with increasing concentrations. At ambient nutrient concentrations in unpolluted streams, U-t is probably limited to some degree by the kinetic uptake capacity of stream biota. Mass transfer velocity from the water column is generally greater than would be expected given typical diffusion rates, underscoring the importance of advective transport. Given the short-term spikes in nutrient concentrations that can occur in streams (e.g., in response to storm events), U-t may not saturate, even at high concentrations.