Heffernan and Cohen demonstrated how uptake rates could be calculated from high temporal resolution time-series profiles. The expanding availability of high-frequency, high-resolution stream solute signals from automated sensors, has allowed development of new passive methods of inferring uptake rates which scale across river size. The resulting high degree of uncertainties represented a substantial knowledge gap on real uptake quantities in higher order streams. Thus, until recently, uptake rates in larger rivers have largely been estimated using models which scale up observations from smaller streams.
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Other passive methods, like naturally abundant isotope fractionation may identify presence of removal processes, but not quantitative estimates of uptake rates. Prior to the advent of high-resolution, high frequency sensors, inferring reliable uptake rates from a passive mass balance approach was typically not feasible because analytical precision was too low and effects of errors in discrete samples too high. For rivers, active methods such as isotope addition, plateau enrichment or even pulse enrichment are commonly impracticable due to the large masses needed to achieve detectable signals.
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ĭespite their importance in global N cycling, direct methods of quantifying in-stream removal in defined reaches have largely been limited to small streams. Models suggest that up to 70% of N inputs may be removed during transport through river networks.
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However, far from being nonreactive conduits, river networks exhibit a remarkable capacity to process and remove N in-transit. While the majority returns to the atmosphere via soil denitrification, a substantial fraction of terrestrial N loads, roughly 25%, ultimately enter into river drainage networks, resulting in widespread cultural eutrophication of downstream water bodies. Nearly 200 Tg-N (200 billion kg) of reactive nitrogen (N) is added worldwide to the land surface every year for agricultural purposes, with only a small fraction being assimilated by crops. However, uncertainty in diffuse groundwater inputs and more importantly the effects of alternative nitrogen species, in this case ammonium, pose considerable challenges to this method. From a methodological perspective, we demonstrate that a mass balance approach based on high frequency data can be useful in deriving quantitative uptake estimates, even under dynamic inputs and lateral tributary inflow. Additionally, seasonal variations in temperature and insolation affected the relative contribution of assimilatory versus dissimilatory uptake processes, with the latter exhibiting a stronger positive dependence on temperature. Our results suggest that net mass removal rates of nitrate were markedly higher in the unmodified reach. We use a combination of two station time-series and longitudinal profiling of nitrate to assess differences in nitrogen processing dynamics in a natural versus a channelized impounded reach with WWTP effluent impacted water chemistry. Here we extend these methods to higher order streams with anthropogenically elevated nitrogen levels, substantial tributaries, complex input signals, and multiple N species. The recent advent of automated sensors has allowed high frequency measurements, and the development of new passive methods of quantifying nitrogen uptake which are scalable across river size.
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However direct measurement of in-stream removal in higher order streams and rivers has been extremely limited. River networks exhibit a globally important capacity to retain and process nitrogen.