Sybil P. Seitzinger | USGS Science

USGS2009

Dynamic modeling of nitrogen losses in river networks unravels the coupled effects of hydrological and biogeochemical processes

The importance of lotic systems as sinks for nitrogen inputs is well recognized. A fraction of nitrogen in streamflow is removed to the atmosphere via denitrification with the remainder exported in streamflow as nitrogen loads. At the watershed scale, there is a keen interest in understanding the factors that control the fate of nitrogen throughout the stream channel network, with particular attention to the processes that deliver large nitrogen loads to sensitive coastal ecosystems. We use a dynamic stream transport model to assess biogeochemical (nitrate loadings, concentration, temperature) and hydrological (discharge, depth, velocity) effects on reach-scale denitrification and nitrate removal in the river networks of two watersheds having widely differing levels of nitrate enrichment but nearly identical discharges. Stream denitrification is estimated by regression as a nonlinear function of nitrate concentration, streamflow, and temperature, using more than 300 published measurements from a variety of US streams. These relations are used in the stream transport model to characterize nitrate dynamics related to denitrification at a monthly time scale in the stream reaches of the two watersheds. Results indicate that the nitrate removal efficiency of streams, as measured by the percentage of the stream nitrate flux removed via denitrification per unit length of channel, is appreciably reduced during months with high discharge and nitrate flux and increases during months of low-discharge and flux. Biogeochemical factors, including land use, nitrate inputs, and stream concentrations, are a major control on reach-scale denitrification, evidenced by the disproportionately lower nitrate removal efficiency in streams of the highly nitrate-enriched watershed as compared with that in similarly sized streams in the less nitrate-enriched watershed. Sensitivity analyses reveal that these important biogeochemical factors and physical hydrological factors contribute nearly equally to seasonal and stream-size related variations in the percentage of the stream nitrate flux removed in each watershed.

Biogeochemistry↗

USGS1993

Denitrification measurements in aquatic sediments: A comparison of three methods

Measurements of denitrification using the acetylene inhibition, 15 N isotope tracer, and N 2 flux methods were carried out concurrently using sediment cores from Vilhelmsborg sø, Denmark, in an attempt to clarify some of the limitations of each technique. Three experimental treatments of overlying water were used: control, nitrate enriched, and ammonia enriched water. The N 2 flux and 15 N tracer experiments showed high rates of coupled nitrification/denitrification in the sediments. The acetylene inhibition method did not capture any coupled nitrification/denitrification. This could be explained by acetylene inhibition of nitrification. A combined 15 N tracer/acetylene inhibition experiment demonstrated that acetylene inhibition of N 2 O reduction was incomplete and the method, therefore, only measured approximately 50% of the denitrification due to nitrate from the overlying water. Similar rates of denitrification due to nitrate in the overlying water were measured by the N 2 flux method and the acetylene inhibition method, after correcting for the 50% efficiency of acetylene inhibition. Rates of denitrification due to nitrate from the overlying water measured by the 15 N tracer method, however, were only approximately 35% or less of those measured by the acetylene inhibition or N 2 flux methods.

Biogeochemistry↗