Climate and vegetation strongly influence the water cycle on local to regional scales. A change in the surface energy and water balance, especially in dry climatic regions, can have a significant impact on local water availability and, therefore, water resource management. The purpose of this study is to quantify the energy and water balance of a riparian wetland in a subhumid region of the central US, as well as the role of seasonal climate variability and vegetation phenology. The site is located in the Republican River basin in south-central Nebraska, where decreases in streamflow have been observed in recent decades. In an effort to reduce consumptive water use from evapotranspiration (ET), and thereby reclaim surface water, invasive species such as Phragmites australis have been removed throughout the riparian corridor of the river basin. In this study, we used energy/water balance monitoring stations, a Large Aperture Scintillometer (LAS), and numerous water and soil temperature probes to determine the energy and water balance during the 2009 growing season (April 11−October 3). Sensible heat flux was measured using the LAS, while ET was calculated as a residual of the energy balance (i.e., net radiation minus sensible heat flux and heat storage rates in the canopy, water, and soil). Rigorous quality control and uncertainty analyses were performed, and comparisons were also made with ET rates calculated via the simpler Priestley-Taylor method. Results of the energy budget analysis indicate that the average ET rate for the wetland during the growing season was 4.4 mm day −1 , with a maximum daily rate of 8.2 mm day −1 (occurring on June 29). Precipitation during the same 176-day period averaged 2.7 mm day −1. Net radiation and vegetation phenology were found to be the two largest drivers of seasonal variability in ET. Sensible heat flux was significantly larger than latent heat flux early in the season, when standing vegetation in the wetland was still dry and brown. By late May and early June, however, Bowen ratios had declined well below 0.5 in response to greener and more abundant vegetation, higher transpiration rates, and reduced sensible heat flux. Heat storage rates in the wetland were dominated by changes in water temperature (as compared to soil or canopy heat storage) and comprised a significant portion of the hourly energy balance. On daily mean timescales, changes in the rate of heat storage corresponded to ~13% of the variability in net radiation, while for the season-long average, the heat storage term was found to be essentially negligible. The Priestley-Taylor equation provided a reasonable estimate of ET during the height of the growing season but significantly overestimated ET during the beginning of the season (since it could not account for large sensible heat fluxes from the dry vegetation). Analysis of the wetland water balance showed seasonal variations in water level that were similar to changes in cumulative water inputs (i.e., precipitation minus ET). Portions of the season when the...

As a result of increased anthropogenic nitrogen (N) loading in surface waters of agricultural watersheds, there is enhanced interest to understand and quantify N removal mechanisms. Denitrification, an important N removal mechanism in aquatic systems, may contribute to reducing N pollution in agricultural headwater streams. However, the key factors controlling this process in lotic systems remain unclear. The objective of our study was to examine the factors regulating rates of denitrification in the sediments of agricultural headwater streams in the midwestern USA. Denitrification rates were variable among streams and treatments (\0.1-28.0 lg N g AFDM -1 h -1 ) and on average, were higher than those reported for similar headwater streams. Carbon quantity and quality, and pH had no effect on denitrification, while temperature and nitrate (NO À 3 ) concentrations had a positive effect on rates of denitrification. Specifically, NO À 3 controlled denitrification followingMichaelis-Menten kinetics. We calculated a value of k m (1.0 mg NO À 3 -N L -1 ) that was comparable to other studies in aquatic sediments but was well below the median in-stream NO À 3 concentrations (5.2-17.4 mg NO À 3 -N L -1 ) observed at the study sites. Despite high rates of denitrification, this removal mechanism is most likely NO À 3 saturated in the agricultural headwater streams we examined, suggesting that these systems are not effective at removing in-stream N.

The objective of this project was to assess in-stream nitrogen removal capacity in a fragmented agricultural landscape and to compare removal capacities in streams with agricultural or residential (hereafter referred to as agricultural streams) and forested riparian land use. We also identified what stream characteristics control nitrogen removal in these systems. We examined paired reaches (one agricultural and one forested reach) along five headwater streams in an agricultural watershed (Upper Sugar Creek Watershed) in northeast Ohio. Although denitrification rates were high (,0.1-17.2 mg N m 22 h 21 ), annual nitrogen removal was most likely low because during spring and fall, when in-stream nitrogen loads were high, removal was low, and during summer when instream nitrogen loads were low, removal was high. Between the agricultural and forested reaches removal rates were similar in terms of loss rate and uptake velocity. Removal capacities were similar despite forested reaches having higher hydraulic residence times. Using a redundancy analysis we identified temperature, in-stream nitrate concentration, and relative transient storage as stream characteristics that affect nitrogen removal. Further analysis suggests that nitrogen removal via denitrification in these headwater streams was not limited by the availability of nitrate. In this fragmented agricultural watershed in-stream nitrogen removal was low and riparian land use had no effect on this process, most likely because of nitrate saturation.Since the middle of the 20th century in the United States, reactive nitrogen inputs to landscapes have tripled as the result of agricultural fertilizer applications and cultivation of N-fixing crops (Galloway et al. 2003). Reactive N is considered by some researchers to be the third largest threat to our planet after biodiversity loss and climate change (Giles 2005). Excess N degrades habitat and limits biodiversity in aquatic ecosystems (Vitousek et al. 1997;Howarth et al. 2000). In the midwestern United States, agricultural watersheds are the dominant exporters of reactive N, particularly inorganic N (Goolsby et al. 1999), and have been linked to the eutrophication of the Gulf of Mexico (Rabalais et al. 2001). Because of these negative effects, several studies have examined N removal capacity and removal strategies over large spatial scales (Mitsch et al. 2001;Seitzinger et al. 2002). A crucial process responsible for the removal of N is denitrification. Denitrification is an important biogeochemical process because it removes a mobile, reactive form of N (nitrate) from an ecosystem by converting it to an unreactive gas (dinitrogen). This reaction has been extensively studied since the early 1970s, yet questions still remain about its efficacy and the characteristics that control this process in stream ecosystems (Boyer et al. 2006).In streams and rivers, the transport of nutrients has been coupled to the downstream movement of water by the nutrient spiraling theory (Webster and Patton 1979;Newbold et al. ...

The energy and water balance of a Phragmites australis dominated wetland in south central Nebraska was analyzed to assess consumptive water use and the potential for "water savings" as a result of vegetation eradication via herbicide treatment. Energy balance measurements were made at the field site for two growing seasons (treated and untreated), including observations of net radiation, heat storage, and sensible heat flux, which was measured using a large-aperture scintillometer. Latent heat flux was calculated as a residual of the energy balance, and comparisons were made between the two growing seasons and with model simulations to examine the relative impacts of vegetation removal and climate variability. Observed ET rates dropped by roughly 32% between the two growing seasons, from a mean of 4.4 ± 0.7 mm day-1 in 2009 (with live vegetation) to 3.0 ± 0.8 mm day-1 in 2010 (with dead P. australis). These results are corroborated by the Agro-IBIS model simulations, and the reduction in ET implies a total "water savings" of 245 mm over the course of the growing season. The significant decreases in ET were accompanied by a more-than-doubling of sensible heat flux, as well as a ~60% increase in heat storage due to decreased LAI. Removal of P. australis was also found to cause measurable changes in the local micrometeorology at the wetland. Consistent with the observed increase in sensible heat flux during 2010, warmer, drier, windier conditions were observed in the dead, P. australis section of the wetland, compared to an undisturbed section of live, native vegetation. Modeling results suggest that the elimination of transpiration in 2010 was partially offset by an increase in surface evaporation, thereby reducing the subsequent water savings by roughly 60%. Thus, the impact of vegetation removal depends on the local climate, depth to groundwater, and management decisions related to regrowth of vegetation.

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Abstract:This study examined if riparian land use (forested vs agricultural) affects hydraulic transport in headwater streams located in an agriculturally fragmented watershed. We identified paired 50-m reaches (one reach in agricultural land use and the other in

Background and aims Invasive plants have been associated with alterations to soil properties, functions, and organisms, with the potential to impact ecosystem processes. An observational study was conducted to determine how the invasive plant Frangula alnus affects soil microbial communities and biogeochemical processes in Wisconsin forests. Methods Paired invaded/non-invaded sites (n = 10), including high (n = 5) and low (n = 5) density invasions, were sampled in spring, summer, and fall. Soil was analyzed for extractable and total nitrogen (N), N mineralization rate, total carbon, microbial biomass carbon and N, and microbial community structure using terminal restriction fragment length polymorphisms. Results Linear regression analysis with robust variance estimation revealed higher N mineralization rates in invaded sites than non-invaded sites in summer, and in high density invaded sites than non-invaded sites overall (p < 0.05). There was not a corresponding increase in extractable N. No differences between invaded and noninvaded sites were observed for other variables. Conclusions Nitrogen-rich F. alnus leaf litter (3.2 % of dry mass) may contribute to elevated N mineralization at these sites, though pre-existing conditions may be responsible. Results suggest that F. alnus alters N cycling but has little impact on soil carbon pools and microbial communities.