Denitrification in the Upper Mississippi River: rates, controls, and contribution to nitrate flux

Abstract

Abstract:We evaluated patterns of denitrification and factors effecting denitrification in the upper Mississippi River. Measurements were taken over 2 years, during which river discharge ranged from record flooding to base flow conditions. Over the period of study, average denitrification enzyme activity was highest in backwater lakes and lowest in the main channel. Throughout the study reach, highest denitrification enzyme activity occurred during fall and lowest occurred in winter. Rates during spring floods (2001) were only slightly higher than during the preceding winter. Mean unamended denitrification rates ranged from 0.02 (fall 2001 in backwaters) to 0.40 µg N·cm -2 ·h -1 (spring 2001 in backwaters). Laboratory experiments showed that denitrification rates increased significantly with addition of NO 3 -regardless of sediment C content, while rates increased little with addition of labile C (glucose). Denitrification in this reach of the upper Mississippi River appears to be NO 3 -limited throughout the growing season and the delivery of NO 3 -is strongly controlled by river discharge and hydrologic connectivity across the floodplain. We estimate that denitrification removes 6939 t N·year -1 or 6.9% of the total annual NO 3 -input to the reach. Hydrologic connectivity and resultant NO 3 -delivery to high-C sediments is a critical determinant of reach-scale processing of N in this floodplain system. Résumé : Nous avons évalué les patterns de dénitrification et les facteurs qui opèrent la dénitrification dans le Mississippi supérieur. Les mesures ont été réalisées sur 2 années, pendant lesquelles le débit de la rivière a varié d'inondations record à des conditions d'étiage. Durant la période d'étude, l'activité enzymatique moyenne de dénitrifi-cation était maximale dans les lacs de la plaine de débordement et minimale dans le chenal principal. Dans toute la zone d'étude, les valeurs maximales de l'activité enzymatique moyenne de dénitrification ont été mesurées à l'automne et les valeurs minimales en hiver. Les taux durant les inondations du printemps (2001) étaient tout juste un peu plus élevés que l'hiver précédent. Les taux moyens non corrigés de dénitrification variaient de 0,02 (automne 2001 dans des eaux de la plaine de débordement) à 0,40 µg N·cm -2 ·h -1 (printemps dans des eaux de la plaine de débordement). Des expériences en laboratoire montrent que les taux de dénitrification augmentent de façon significative après l'addition de NO 3 -, quel que soit le contenu des sédiments en C; ces taux augmentent peu après l'addition de C labile (glucose). La dénitrification dans cette section du Mississippi semble être limitée par NO 3 -durant la saison de croissance et l'apport de NO 3 -est fortement contrôlé par le débit de la rivière et la connectivité hydrologique à travers la plaine de déborde-ment. Nous estimons que la dénitrification retire 6 939 t N·an -1 , soit 6,95 % de l'apport annuel de NO 3 -dans la section. La connectivité hydrologique et l'apport de NO 3 -aux sédiments riches en C qui en résult...

Distribution of typical denitrifying functional genes and diversity of the <i>nirS</i>-encoding bacterial community related to environmental characteristics of river sediments

“…As shown in Table 1, on average, DIN contents were rather high, especially NH + 4 , compared to other river sediments (Richardson et al, 2004;Laverman et al, 2010). DIN concentrations of the sediment surfaces (0-5 cm) increased from locations ZB to E, such that NO concentrations at location E showed insignificant variations with the sediment depth.…”

Dynamic modeling of nitrogen losses in river networks unravels the coupled effects of hydrological and biogeochemical processes

“…New sampling also needs to include a broader range of discharge regimes, stream sizes, ratecontrolling variables (e.g., organic matter, biotic uptake, respiration; Smith et al 2006;Opdyke et al 2006;Opdyke and David 2007;Mulholland et al 2008Mulholland et al , 2009, and aquatic environments including floodplains (Richardson et al 2004;Wollheim et al 2008b) and lakes and reservoirs (Harrison et al 2008). Sampling in streams larger than those in the case study watersheds is technically challenging, but has particular importance for improving measurements of the cumulative effects of nitrogen removal in river networks.…”

“…Moreover, we are interested in evaluating the effects of stream properties that can be more readily generalized for watersheds and the NHD river network. The field measurements reflect withinchannel processes and do not include the effects of floodplains and backwater areas that can become important during high flows (e.g., Richardson et al 2004).…”

Enhanced hyporheic exchange flow around woody debris does not increase nitrate reduction in a sandy streambed

“…To maximise the removal of nitrate as it flows through a river, diversion of the flow into the carbon-rich and oxygendepleted parts of the streambed is vital. Depending on the porosity, organic matter content and biogeochemical activity in the streambed, different flow-path lengths will be required in order for the nitrate to be fully reduced (Mulholland et al 2008;Richardson et al 2004;Trimmer et al 2012;Zarnetske et al 2011). In this sandy river we expected denitrification to be the dominant form of nitrate reduction, with lesser contributions for anammox and DNRA.…”