Ambient N2:Ar in surface waters: How useful for understanding dynamics of reactive N?

Abstract:

Surface waters alter the amount of downstream reactive nitrogen fluxes through both N fixation and denitrification. These processes are often difficult to quickly measure at ecosystem scales, but one potential approach is to measure N2:Ar disequilibria using Membrane Inlet Mass Spectrometry (MIMS). We explored patterns of N2:Ar disequilibria in a variety of freshwater systems of the New England coastal plain, including streams, beaver ponds, fluvial wetlands, and reservoirs over diel, seasonal and storm event time scales. We find considerable evidence of denitrification in stream channels based on positive N2:Ar disequilibria during typical day time sample periods. However, stream disequilibria were also commonly negative, especially in low nutrient ecosystems or at low flows when nutrients were supply limited. Negative N2:Ar disequilibria also occur within small reservoirs during the warmest, low flow periods, when algal blooms were present, indicating hotspots of N fixation. Diel sampling in several headwater streams suggested that in some streams, the dominant process shifted from N fixation during the day ro denitrification at night. Flowpaths transitioning from channelized streams to fluvial wetland dominated streams, also transitioned from N fixation to denitrification. More widespread use of N2:Ar in regular stream and lake monitoring could add considerable insight into balance of N fixation and denitrification throughout river systems.