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 NO 3 − 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 NO 3 − 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 NO 3 − 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 NO 3 − concentration (~20–1,100 μmol L −1 ) representing seasonal variation from spring high flow (high NO 3 − ) to late summer low flow (low NO 3 − ). The temporal dependence of U denit on NO 3 − 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 * [NO 3 − ] 0.44 or U denit ≈640 * [NO 3 − ]/[180 + NO 3 − ]; for a partially weighted dataset, U denit ≈14 * [NO 3 − ] 0.54 or U denit ≈700 * [NO 3 − ]/[320 + NO 3 − ]). Similar parameters were derived from a recent spatial comparison of stream denitrification extending to lower NO 3 − concentrations (LINX2), and from the combined dataset from both studies over 3 orders of magnitude in NO 3 − concentration. Hypothetical models based on our results illustrate: (1) U denit was inversely related to denitrification rate constant ( k 1 denit , in day −1 ) and vertical transfer velocity ( v f,denit , in m day −1 ) at seasonal and possibly event time scales; (2) although k 1 denit was relatively large at low flow (low NO 3 − ), 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 NO 3 − assimilation and denitrification were linked through production of organic reactants, rates of NO 3 − loss by these processes may have been partially decoupled by changes in flow and sediment transport. Whereas k 1 denit and v f,denit are linked implicitly with stream depth, NO 3 − concentration, and(or) NO 3 − load, estimates of U denit may be related more directly to field factors (including NO 3 − 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 NO 3 − concentration and accounting for temporal variation.