Ronald C. Antweiler Judson W. Harvey Andrew E. Laursen Lesley K. Smith Richard L. Smith Mary A. Voytek J.K. Bohlke 2009 <p><span>Denitrification is an important net sink for NO</span>₃⁻<span> 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</span>₃⁻<span> 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</span>₃⁻<span> 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 (</span><i class="EmphasisTypeItalic ">U</i>_denit<span>, in μmol&nbsp;N&nbsp;m</span>⁻²<span>&nbsp;h</span>⁻¹<span>), 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&nbsp;±&nbsp;4°C) and with minimal benthic disturbance, our aggregated data indicated an overall positive relation between </span><i class="EmphasisTypeItalic ">U</i>_denit<span> (~0–4,000&nbsp;μmol&nbsp;N&nbsp;m</span>⁻²<span>&nbsp;h</span>⁻¹<span>) and stream NO</span>₃⁻<span>concentration (~20–1,100&nbsp;μmol&nbsp;L</span>⁻¹<span>) representing seasonal variation from spring high flow (high NO</span>₃⁻<span>) to late summer low flow (low NO</span>₃⁻<span>). The temporal dependence of </span><i class="EmphasisTypeItalic ">U</i>_denit<span> on NO</span>₃⁻<span>was less than first-order and could be described about equally well with power-law or saturation equations (e.g., for the unweighted dataset, </span><i class="EmphasisTypeItalic ">U</i>_denit<span>&nbsp;≈26&nbsp;*&nbsp;[NO</span>₃⁻<span>]</span>^0.44<span> or </span><i class="EmphasisTypeItalic ">U</i>_denit<span>≈640&nbsp;*&nbsp;[NO</span>₃⁻<span>]/[180&nbsp;+&nbsp;NO</span>₃⁻<span>]; for a partially weighted dataset, </span><i class="EmphasisTypeItalic ">U</i>_denit<span>&nbsp;≈14&nbsp;*&nbsp;[NO</span>₃⁻<span>]</span>^0.54<span> or </span><i class="EmphasisTypeItalic ">U</i>_denit<span>&nbsp;≈700&nbsp;*&nbsp;[NO</span>₃⁻<span>]/[320&nbsp;+&nbsp;NO</span>₃⁻<span>]). Similar parameters were derived from a recent spatial comparison of stream denitrification extending to lower NO</span>₃⁻<span> concentrations (LINX2), and from the combined dataset from both studies over 3 orders of magnitude in NO</span>₃⁻<span>concentration. Hypothetical models based on our results illustrate: (1) </span><i class="EmphasisTypeItalic ">U</i>_denit<span> was inversely related to denitrification rate constant (</span><i class="EmphasisTypeItalic ">k</i><span>1</span>_denit<span>, in day</span>⁻¹<span>) and vertical transfer velocity (</span><i class="EmphasisTypeItalic ">v</i>_f,denit<span>, in m day</span>⁻¹<span>) at seasonal and possibly event time scales; (2) although </span><i class="EmphasisTypeItalic ">k</i><span>1</span>_denit<span> was relatively large at low flow (low NO</span>₃⁻<span>), its impact on annual loads was relatively small because higher concentrations and loads at high flow were not fully compensated by increases in </span><i class="EmphasisTypeItalic ">U</i>_denit<span>; and (3) although NO</span>₃⁻<span> assimilation and denitrification were linked through production of organic reactants, rates of NO</span>₃⁻<span> loss by these processes may have been partially decoupled by changes in flow and sediment transport. Whereas </span><i class="EmphasisTypeItalic ">k</i><span>1</span>_denit<span> and </span><i class="EmphasisTypeItalic ">v</i>_f,denit<span> are linked implicitly with stream depth, NO</span>₃⁻<span> concentration, and(or) NO</span>₃⁻<span> load, estimates of </span><i class="EmphasisTypeItalic ">U</i>_denit<span> may be related more directly to field factors (including NO</span>₃⁻<span> 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 </span><i class="EmphasisTypeItalic ">U</i>_denit<span> and stream NO</span>₃⁻<span>concentration and accounting for temporal variation.</span></p> application/pdf 10.1007/s10533-008-9282-8 en Springer Multi-scale measurements and modeling of denitrification in streams with varying flow and nitrate concentration in the upper Mississippi River basin, USA article