Eric A. Strauss

Quantification of the Nitrogen Cycle in a Prairie Stream

Nitrogen (N) was added for 35 days in the form of 15NH4Cl 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 approximately 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 15N 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.

Nitrification in the Upper Mississippi River: patterns, controls, and contribution to the NO3− budget

Abstract We measured nitrification rates in sediment samples collected from a variety of aquatic habitats in Navigation Pool 8 of the Upper Mississippi River (UMR) 7 times between May 2000 and October 2001. We also conducted nutrient-enrichment experiments and analyzed vertical profiles of sediment to determine factors regulating nitrification. Nitrification rates were relatively high compared to other ecosystems (ranging from 0–8.25 μg N cm−2 h−1) and exhibited significant temporal and spatial patterns. Nitrification rates were greatest during the summer and spring compared to autumn and winter (ANOVA, p < 0.05) and were greater in contiguous backwater and impounded habitats compared to main and side-channel habitats (p < 0.05). Regression analysis indicated that nitrification rates were weakly (r2 = 0.18, p < 0.0001) related to temperature and exchangeable NH4+ of the sediment. However, nutrient-enrichment experiments showed that NH4+ availability did not limit nitrification in 3 sediment types with variable organic matter. Vertical profiles of sediment cores demonstrated that oxygen concentration and nitrification had similar patterns suggesting that nitrification may be limited by oxygen penetration into sediments. We conclude that temperature and sediment NH4+ can be useful for predicting broad-scale temporal and spatial nitrification patterns, respectively, but oxygen penetration into the sediments likely regulates nitrification rates in much of the UMR. Overall, we estimated that nitrification produces 6982 mt N/y of NO3− or 7% of the total annual NO3− budget.

Influence of Protozoa and Nutrient Availability on Nitrification Rates in Subsurface Sediments

A bstractProtozoan abundance, nitrification potential, and related factors in saturated subsurface sediments and the overlying soil were compared at a nonfertilized grassland and an agricultural cropland site. In a 6-week laboratory experiment, DOC, ammonium, and protozoan abundance were manipulated in flasks containing groundwater-sediment slurries. Microbial abundance (protozoa, actively respiring bacteria, and total bacteria) and nutrient concentrations (extractable ammonium and nitrate) were measured.Results from the soil profile analysis showed that protozoan abundance declined with depth at both sites, but significant numbers (392 cells g−1dw) were found in groundwater sediments at the cropland site. Nitrification potential declined with depth at the grassland site and increased with depth at the cropland site. In the laboratory experiment, treatment responses generally were observed within 3 weeks, but had diminished by 6 weeks. Protozoa reduced bacterial populations through the first 3 weeks, but this effect was not significant by week 6. In the cropland sediments, increased net nitrate production occurred in the two reduced protozoa treatments that received ammonium, suggesting that nitrification was occurring and was limited by ammonium. High protozoan abundance in the cropland sediments increased the nitrate flux response, unless DOC was added; in this case, no response occurred. No such responses were recorded in the grassland sediments.Apparently, appreciable nitrification can occur in some groundwater sediments, if sufficient ammonium is present and DOC availability is low. Furthermore, nitrification can be enhanced when protozoan abundance is elevated. Finally, our results suggest that surface land use practices can alter subsurface nitrification rates and microbial community structure.

Biological properties of soil and subsurface sediments under abandoned pasture and cropland

Abstract Little is known about the effects of most surface land-use practices on shallow subsurface microbial communities. We analyzed duplicate cores taken aseptically from up to 10 m depth from unconsolidated valley sediments (soils) beneath an abandoned pasture reverting to tall grass prairie and cropland. Both profiles had similar soil texture, with moderately higher silt under cropland and a slight decrease in clay with depth. Soluble organic C was about two times higher in the grassland site and dissolved O 2 was about 8% lower compared with the cropland site. Water content and C-to-N ratios were greatest at the grassland surface but were less in the grassland than the cropland site within 2 m depth. In general, numbers of aerobic heterotrophic bacteria and protozoa decreased with depth until the saturated zone (4.3 m in grassland and 5.3 m in the cropland site). Bacterial numbers as determined by plate counts were about 10-fold less at the groundwater interface than in the surface soils at both sites. Direct microscopic counts of total bacteria were approximately the same in the surface soil and the sediments at the top of the water table at both sites. The top of the water table generally did not exhibit elevated microbial biomass or activity relative to deeper sediments. There was no significant relationship between protozoan numbers and microbial thymidine uptake at the cropland site, but a negative relationship was observed at the grassland site. The data suggest that cultivation may affect microbial biomass and activity in the subsurface, as well as community interactions between protozoa and bacteria.

Does Displaying the Class Results Affect Student Discussion during Peer Instruction

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 students second vote on a question and may be misinterpreted as an increase in performance due to student discussion alone.

Variability and regulation of denitrification in an Upper Mississippi River backwater

Abstract Sediments in the backwaters of the Upper Mississippi River (UMR) are highly organic and provide an optimal environment for N removal. We monitored an 8.6-ha UMR backwater site near La Crosse, Wisconsin, for nearly 3 y to assess temporal variability, seasonal trends, and the factors regulating denitrification. We measured rates of unamended denitrification (DEN) and denitrification enzyme activity (DEA) rates at ambient temperature and DEA at 30°C (DEA30). Seasonal mean (±1 SE) DEN rates ranged from 0.041 ± 0.015 to 0.47 ± 0.23 μg N cm−2 h−1 and were highest in winter and lowest in autumn. Seasonal rates of DEA exhibited a different pattern with the highest rates in summer (25.6 ± 3.4 μg N cm−2 h−1) and the lowest rates in winter (10.6 ± 2.1 μg N cm−2 h−1). The overall mean DEA30 rate was 31.0 ± 1.9 μg N cm−2 h−1 but showed no significant seasonal pattern. Short-term (weekly) and seasonal variability exhibited by rates of DEN and DEA were best explained by water-column NO3− concentration and temperature, respectively. No environmental variables explained a significant amount of variability in DEA30. Our results suggest that nutrient (i.e., NO3−) availability and temperature are both regulators of denitrification, with NO3− concentration being the most important limiting factor in this system. The high DEN rates during winter were in response to elevated NO3− concentrations resulting from a chain reaction beginning with algal blooms creating oxic conditions that stimulated nitrification. Increasing hydrological connectivity in large rivers as a river management tool to reduce N flux to downstream areas may be beneficial.

Wetland management reduces sediment and nutrient loading to the upper Mississippi river.

Restored riparian wetlands in the Upper Mississippi River basin have potential to remove sediment and nutrients from tributaries before they flow into the Mississippi River. For 3 yr we calculated retention efficiencies of a marsh complex, which consisted of a restored marsh and an adjacent natural marsh that were connected to Halfway Creek, a small tributary of the Mississippi. We measured sediment, N, and P removal through a mass balance budget approach, N removal through denitrification, and N and P removal through mechanical soil excavation. The marsh complex had average retention rates of approximately 30 Mg sediment ha yr, 26 kg total N ha yr, and 20 kg total P ha yr. Water flowed into the restored marsh only during high-discharge events. Although the majority of retention occurred in the natural marsh, portions of the natural marsh were hydrologically disconnected at low discharge due to historical over-bank sedimentation. The natural marsh removed >60% of sediment, >10% of P, and >5% of N loads (except the first year, when it was a N source). The marsh complex was a source of NH and soluble reactive P. The average denitrification rate for the marsh complex was 2.88 mg N m h. Soil excavation removed 3600 Mg of sediment, 5.6 Mg of N, and 2.7 Mg of P from the restored marsh. The marsh complex was effective in removing sediment and nutrients from storm flows; however, retention could be increased if more water was diverted into both restored and natural marshes before entering the river.

Effect of habitat type on in-stream nitrogen loss in the Mississippi River

Eutrophic and hypoxic coastal waters are often associated with high nutrient inputs from riverine systems. For example, nitrogen (N) export from the Mississippi River into the Gulf of Mexico has been identified as an important factor causing eutrophication and seasonal hypoxia. Modelling studies of N flux in large rivers, including the Mississippi River, suggest that much of the N that enters rivers remains in solution and is exported downstream. However, patterns of N cycling in the Mississippi River are complex and vary according to habitat type and season. Here we use spatial habitat data and empirically derived denitrification rates to extrapolate N loss to various reaches in 2,400 km of the Mississippi River from Minneapolis, Minnesota to the Atchafalaya diversion. Our results indicate that 9.5 % of the total N load is lost through denitrification in the river and that reaches containing large areas of impoundments and backwater lakes exhibit elevated rates of N loss. The northern 1,041 km reach of the river contains significant areas of impoundments and backwater lakes and yielded a total N loss from denitrification of 89,172 t N y–1. In comparison, total N loss from the southern 1,352 km open river was 69,872 t N y–1. Our results are consistent with high throughput of N in large rivers, but specify that habitat diversity, channel complexity, and retention time are important factors affecting nitrogen loss in rivers.

The Impact of Nutrient Pulses on Trophic Interactions in a Farm Pond

Abstract We placed eight 1500 L mesocosms in a 0.2 ha eutrophic cattle pond during summer 1991 to determine if zooplankton grazing, nutrients, or both control algal biomass and productivity. The three treatments: + zooplanktivorous fish (39 bluegill, mean total length = 36 mm); + zooplankton (10x ambient); and + N + P (160 μM NH4 + and 10.0 μM PO4 3-) were duplicated and compared to ambient pond conditions and two control mesocosms. In the + N + P treatment, chl a concentrations increased 700% in four days and then decreased to initial levels; further nutrient enrichments failed to create an algal response, probably because of grazing associated with an eightfold increase in large cladocerans. After nutrients were added to the + fish treatment, the NH4 + and soluble reactive phosphorus concentrations rose and then decreased rapidly, whereas chl a concentrations and rotifer numbers increased. When nutrients were added to the + zooplankton treatments, chl a increased, but less than when either fish or nutri...

Effects of Flooding on Ion Exchange Rates in an Upper Mississippi River Floodplain Forest Impacted by Herbivory, Invasion, and Restoration

We examined effects of flooding on supply rates of 14 nutrients in floodplain areas invaded by Phalaris arundinacea (reed canarygrass), areas restored to young successional forests (browsed by white-tailed deer and unbrowsed), and remnant mature forests in the Upper Mississippi River floodplain. Plant Root Simulator ion-exchange probes were deployed for four separate 28-day periods. The first deployment occurred during flooded conditions, while the three subsequent deployments were conducted during progressively drier periods. Time after flooding corresponded with increases in NO3−-N, K+ and Zn+2, decreases in H2PO4−-P, Fe+3, Mn+2, and B(OH)4-B, a decrease followed by an increase in NH4+-N, Ca+2, Mg+2 and Al+3, and an increase followed by a decrease for SO4−2-S. Plant community type had weak to no effects on nutrient supply rates compared to the stronger effects of flooding duration. Our results suggest that seasonal dynamics in floodplain nutrient availability are similarly driven by flood pulses in different community types. However, reed canarygrass invasion has potential to increase availability of some nutrients, while restoration of forest cover may promote recovery of nutrient availability to that observed in reference mature forests.