『Abstract
　Denitrification is an important net sink for NO3^- in streams, but direct measurements are limited and in situ controlling factors are not well known. We measured denitrification at multiple scales over a range of flow conditions and NO3^- concentrations in streams draining agricultural land in the upper Mississippi River basin. Comparisons of reach-scale measurements (in-stream mass transport and tracer tests) with local-scale in situ measurements (pore-water profiles, benthic chambers) and laboratory data (sediment core microcosms) gave evidence for heterogeneity in factors affecting benthic denitrification both temporally (e.g., seasonal variation in NO3^- concentrations and loads, flood-related disruption and re-growth of benthic communities and organic deposits) and spatially (e.g., local stream morphology and sediment characteristics). When expressed as vertical denitrification flux per unit area of streambed (Udenit, in μmol N m^-2 h^-1), results of different methods for a given set of conditions commonly were in agreement within a factor of 2-3. At approximately constant temperature (〜20±4℃) and with minimal benthic disturbance, our aggregated data indicated an overall positive relation between Udenit (〜0-4,000 μmol N m^-2 h^-1) and stream NO3^- concentration (〜20-1,100 μmol L^-1) representing seasonal variation from spring high flow (high NO3^-) to late summer low flow (low NO3^-). The temporal dependence of Udenit on NO3^- was less than first-order and could be described about equally well with power-law or saturation equations (e.g., for the unweighted dataset, Udenit ≒26*[NO3^-]』^0.44 or Udenit ≒640*[NO3^-]』/[180+NO3^-]: for a partially weighted dataset, Udenit ≒14*[NO3^-]^0.54 or NO3^- ≒700*[NO3^-]』/[320+NO3^-]). Similar parameters were derived from a recent spatial comparison of stream denitrification extending to lower NO3^- concentrations (LINX2), and from the combined dataset from both studies over 3 orders of magnitude in NO3^- concentration. Hypothetical models based on our results illustrate: (1) Udenit was inversely related to denitrification rate constant (kldenit, in day^-1) and vertical transfer velocity (νf,denit, in m day^-1) at seasonal and possibly event time scales; (2) although kldenit was relatively large at low flow (low NO3^-), its impact on annual loads was relatively small because higher concentrations and loads at high flow were not fully compensated by increases in Udenit; and (3) although NO3^- assimilation and denitrification were linked through production of organic reactants, rates of NO3^- loss by these processes may have been partially decoupled by changes in flow and sediment transport. Whereas kldenit and νf,denit are linked implicitly with stream depth, NO3^- concentration, and(or) NO3^- load, estimates of Udenit may be related more directly to field factors (including NO3^- concentration) affecting denitrification rates in benthic sediments. Regional regressions and simulations of benthic denitrification in stream networks might be improved by including a non-linear relation between Udenit and stream NO3^- concentration and accounting for temporal variation.

Keywords: Denitrification; Seasonal; Benthic; Hyporheic zone; Isotope tracer; Nitrogen gas; Reach-scale; Microcosm』

Introduction
Study sites
Methods
　Reach-scale methods
　　Reach NO3^-
　　Reach N2
　　Reach ¹⁵N2
　Local in situ methods
　　Hyporheic ¹⁵N2
　　Chamber ¹⁵N2
　Laboratory methods
　　Core N2 and core ¹⁵N2
Summary of results
　Reach-scale results
　　Reach NO3^-
　　Reach N2
　　Reach ¹⁵N2
　Local in situ results
　　Hyporheic ¹⁵N2
　　Chamber ¹⁵N2
　Laboratory results
　　Core N2 and core ¹⁵N2
Discussion
　Comparison of methods and sources of uncertainty
　Comparison of results
　Reach-scale controls on benthic denitrification
　Relation between denitrification and NO3^- concentration
　Spatial and temporal variations of denitrification
　Generalized expressions of nitrogen transmission through streams
　Hypothetical models of temporal variations in denitrification
Conclusions
Acknowledgments
Open Access
References