Abstract-Nitrification, the microbial conversion of ammonium to nitrate, is an important transformation in the aquatic nitrogen cycle, but the factors regulating nitrification rates in freshwater ecosystems are poorly understood. We investigated the effects of organic carbon quantity and quality on nitrification rates in stream sediments. First, we hypothesized that when environmental C : N ratios are high, heterotrophic bacteria are subject to N limitation and will outcompete nitrifying bacteria for available NH , thereby reducing nitrification increased nitrification by 40% compared with unamended controls (P Ͻ 0.0001). Carbon amendments also increased microbial respiration rates over controls by 4-6 times. Therefore, organic carbon additions significantly decreased nitrification rates but increased total microbial activity. Second, we hypothesized that carbon of high quality would have a stronger negative effect on nitrification than would carbon of low quality. To stream sediments, we added organic carbon as either glucose (higher quality) or sugar maple leaf extract (lower quality). Nitrification rates were reduced by the addition of either organic carbon source but were more severely inhibited by glucose (P ϭ 0.001). Our results suggest that organic carbon is an important regulator of nitrification rates and is of key importance in understanding N dynamics in freshwater ecosystems.

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...

Nitrogen (N) was added for 35 days in the form of 15 NH 4 Cl to Kings Creek on Konza Prairie, Kansas. Standing stocks of N in key compartments (that is, nutrients, detritus, organisms) were quantified, and the amount of labeled N entering the compartments was analyzed. These data were used to calculate turnover and flux rates of N cycling through the food web, as well as nutrient transformation rates. Inorganic N pools turned over much more rapidly in the water column of this stream than in pelagic systems where comparable measurements have been made. As with other systems, the mass of ammonium was low but it was the key compartment mediating nutrient flux through the ecosystem, whereas dissolved organic N, the primary component of N flux through the system, is not actively cycled. Nitrification was also a significant flux of N in the stream, with rates in the water column and surface of benthos accounting for ap-proximately 10% of the total ammonium uptake. Primary consumers assimilated 67% of the inorganic N that entered benthic algae and microbes. Predators acquired 23% of the N that consumers obtained. Invertebrate collectors, omnivorous crayfish (Orconectes spp.), and invertebrate shredders dominated the N flux associated with primary consumers. Mass balance calculations indicated that at least 23% of the 309 mg of 15 N added during the 35 days of release was retained within the 210-m stream reach during the release. Overall, the rates of turnover of N in organisms and organic substrata were significantly greater when C:N was low. This ratio may be a surrogate for biological activity with regard to N flux in streams.

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Physical, chemical, hydrologic, and biologic factors affecting nitrate (NO3−) removal were evaluated in three agricultural streams draining orchard/dairy and row crop settings. Using 3‐d “snapshots” during biotically active periods, we estimated reach‐level NO3− sources, NO3− mass balance, in‐stream processing (nitrification, denitrification, and NO3− uptake), and NO3− retention potential associated with surface water transport and ground water discharge. Ground water contributed 5 to 11% to stream discharge along the study reaches and 8 to 42% of gross NO3− input. Streambed processes potentially reduced 45 to 75% of ground water NO3− before discharge to surface water. In all streams, transient storage was of little importance for surface water NO3− retention. Estimated nitrification (1.6–4.4 mg N m−2 h−1) and unamended denitrification rates (2.0–16.3 mg N m−2 h−1) in sediment slurries were high relative to pristine streams. Denitrification of NO3− was largely independent of nitrification because both stream and ground water were sources of NO3− Unamended denitrification rates extrapolated to the reach‐scale accounted for <5% of NO3− exported from the reaches minimally reducing downstream loads. Nitrate retention as a percentage of gross NO3− inputs was >30% in an organic‐poor, autotrophic stream with the lowest denitrification potentials and highest benthic chlorophyll a, photosynthesis/respiration ratio, pH, dissolved oxygen, and diurnal NO3− variation. Biotic processing potentially removed 75% of ground water NO3− at this site, suggesting an important role for photosynthetic assimilation of ground water NO3− relative to subsurface denitrification as water passed directly through benthic diatom beds.

1. Microbial decomposition of dissolved organic carbon (DOC) contributes to overall stream metabolism and can influence many processes in the nitrogen cycle, including nitrification. Little is known, however, about the relative decomposition rates of different DOC sources and their subsequent effect on nitrification. 2. In this study, labile fraction and overall microbial decomposition of DOC were measured for leaf leachates from 18 temperate forest tree species. Between 61 and 82% (mean, 75%) of the DOC was metabolized in 24 days. Significant differences among leachates were found for labile fraction rates (P < 0.0001) but not for overall rates (P=0.088). 3. Nitrification rates in stream sediments were determined after addition of 10 mg C L–1 of each leachate. Nitrification rates ranged from below detection to 0.49 μg N mL  sediment–1 day–1 and were significantly correlated with two independent measures of leachate DOC quality, overall microbial decomposition rate (r=–0.594, P=0.0093) and specific ultraviolet absorbance (r=0.469, P=0.0497). Both correlations suggest that nitrification rates were lower in the presence of higher quality carbon. 4. Nitrification rates in sediments also were measured after additions of four leachates and glucose at three carbon concentrations (10, 30, and 50 mg C L–1). For all carbon sources, nitrification rates decreased as carbon concentration increased. Glucose and white pine leachate most strongly depressed nitrification. Glucose likely increased the metabolism of heterotrophic bacteria, which then out‐competed nitrifying bacteria for NH4+. White pine leachate probably increased heterotrophic metabolism and directly inhibited nitrification by allelopathy.

We investigated the response in nitrification to organic carbon (C) availability, the interactive effects of the C: nitrogen (N) ratio and organic N availability, and differing pH in sediments from several streams in the upper midwestern United States. In addition, we surveyed 36 streams to assess variability in sediment nitrification rates. Labile dissolved organic carbon (DOC) additions of 30 mg C·L–1 (as acetate) to stream sediments reduced nitrification rates (P < 0.003), but lower concentration additions or dilution of ambient DOC concentration had no effect on nitrification. C:N and organic N availability strongly interacted to affect nitrification (P < 0.0001), with N availability increasing nitrification most at lower C:N. Nitrification was also strongly influenced by pH (P < 0.002), with maximum rates occurring at pH 7.5. A multiple regression model developed from the stream survey consisted of five variables (stream temperature, pH, conductivity, DOC concentration, and total extractable NH4+) and explained 60% of the variation observed in nitrification. Our results suggest that nitrification is regulated by several variables, with NH4+ availability and pH being the most important. Organic C is likely important at regulating nitrification only under high environmental C:N conditions and if most available C is relatively labile.

The use of personal response systems, or clickers, is increasingly common in college classrooms. Although clickers can increase student engagement and discussion, their benefits also can be overstated. A common practice is to ask the class a question, display the responses, allow the students to discuss the question, and then collect the responses a second time. In an introductory biology course, we asked whether showing students the class responses to a question biased their second response. Some sections of the course displayed a bar graph of the student responses and others served as a control group in which discussion occurred without seeing the most common answer chosen by the class. If students saw the bar graph, they were 30% more likely to switch from a less common to the most common response. This trend was more pronounced in true/false questions (38%) than multiple-choice questions (28%). These results suggest that observing the most common response can bias a student's second vote on a question and may be misinterpreted as an increase in performance due to student discussion alone.