Multi-scale measurements and modeling of denitrification in streams with varying flow and nitrate concentration in the upper Mississippi River basin, USA

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 (U denit, 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°C) and with minimal benthic disturbance, our aggregated data indicated an overall positive relation between U denit (~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 U denit 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, U denit ≈26 * [NO3 −]0.44 or U denit ≈640 * [NO3 −]/[180 + NO3 −]; for a partially weighted dataset, U denit ≈14 * [NO3 −]0.54 or U denit ≈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) U denit was inversely related to denitrification rate constant (k1denit, in day−1) and vertical transfer velocity (v f,denit, in m day−1) at seasonal and possibly event time scales; (2) although k1denit 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 U denit; 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 k1denit and v f,denit are linked implicitly with stream depth, NO3 − concentration, and(or) NO3 − load, estimates of U denit 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 U denit and stream NO3 − concentration and accounting for temporal variation.