Measurements of agricultural N2O: a comparative study of static chamber and eddy covariance fluxes
Nitrous oxide (N2O) emitted from soils is a strong greenhouse gas and catalyst of ozone destruction by virtue of its long persistence in the atmosphere. Our understanding of both agricultural and natural N2O emissions has significantly improved over the past decade, but difficulties with precise soil N2O emission quantification still exist, related mostly to the large flux variability (both temporally and spatially) and the diversity of factors affecting N2O formation. The most commonly used methodologies for N2O measurements are conventional chambers and more recently-developed (and more expensive) micrometeorological methods. However, the differences in the flux-footprint between those two methods result in a large uncertainty in the integrated flux estimates, especially for areas with non-uniform land use and vegetation. In this study, comparative N2O flux measurements were performed on dairy manure fertilized cropland (on a field split between corn (Zea mays) and alfalfa (Medicago sativa)) in New York State. The field area was monitored simultaneously with (1) micrometeorological eddy covariance technique and (2) arrays of conventional static chambers. For eddy covariance measurements, a tunable diode laser absorption spectroscopy (TDLAS) trace gas analyzer (TGA100a) and 3D sonic anemometer were used, with the high- frequency data averaged over 30 min periods. The chamber design included a set of 28 chambers (14 on the alfalfa, and 14 on the corn site), and the sampling was time-synchronized with TDLAS/TGA100 measurements. The comparative analysis of the two N2O emission data sets helped to estimate the agreement between the methodologies and the spatial distribution of integrated N2O flux formation.
Effects of Reservoirs on O2 and N Dynamics in the Grand River
Macrophyte communities as an important seasonal sink for P and N in an urban and agricultural watershed in Southern Ontario.
Riverine macrophytes and macroalgae are important to riverine nutrient cycling processes, and can often reach nuisance levels of biomass in agriculturally and urban impacted streams. Elevated biomass can result in lowered dissolved oxygen conditions and enhanced anaerobic processes which create problems for riverine biota and for human uses of the water resource, as well may allow for enhancement of N2O and CH4 production. Controlling and removing macrophyte and macroalgal biomass has been an endeavor of river managers for several decades, however stands may represent a large seasonal sink for inorganic forms of N and P. Rivers are heterogeneous environments, which complicate studies of primary productivity, thus spatial variations within riverine systems are important to consider for developing an understanding of benthic vegetation and it's impact on oxygen, phosphorus and nitrogen cycling. Here we present multi-year spatial data on submersed macrophyte communties in the Grand River watershed (Southern Ontario), a heavily populated urban and agricultural watershed, and demonstrate that macrophyte communities represent an important seasonal sink for nutrients P and N. We also give a watershed wide estimate for the temporary nutrient storage capacity of nutrients by macrophyte communities in this system.
Separating Agricultural and Urban Inputs to N Cycling in a Large Eutrophic River, Southern Ontario, Canada
Effective management of eutrophic rivers requires an understanding of the sources of N pollution (NO3-, NH4+, N2O) and determination of in-stream cycling processes. The Grand River, southern Ontario, Canada, is a eutrophic river receiving agricultural runoff and effluents from 26 wastewater treatment plants (WWTPs). Population in the catchment (currently 925 000) is rapidly growing and hypoxia events (dissolved oxygen < 2 mg/L) are common at some locations in summer. We used variety of techniques (whole-river surveys, diel sampling, WWTP effluent sampling, isotopic modelling) to examine (1) coupled oxygen and nitrogen cycling; (2) the relative importance of agricultural and urban NO3- and NH4+ for river health; (3) the significance of dilution versus uptake of dissolved NO3-; (4) location, timing and processes responsible for N2O production; and (5) if stable isotopes of NO3- , NH4+, and N2O can trace N sources and processes. Extreme shifts in N cycling processes on the diel scale were observed downstream of WWTPs, tightly coupled with large diel dissolved oxygen cycles. Whole-river surveys indicated that loads of N from urban sources were larger than agricultural inputs and some urban NO3- was removed at the river mouth, over 100 km downstream. N2O production peaked in summer during hypoxia events below WWTPs. Stable isotopic ratios of NO3- and N2O often changed on a diel scale, and were often distinct at sites above and below WWTPs, as well as distinct from WWTP effluents. Isotopic modelling and laboratory sediment incubation studies are necessary to distinguish N2O production pathways. As the magnitude and relative importance of different N-cycling processes and sources vary diurnally, seasonally and spatially along the Grand River, and are closely tied to agricultural and urban inputs, a variety of sampling techniques is necessary to understand river function and optimize management.
Agricultural Nutrient Cycling at the Strawberry Creek Watershed: Insights Into Processes Using Stable Isotope Analysis
When nitrogen availability exceeds biological demand, excess nitrogen, especially nitrate, may subsequently pollute ground and surface water. Agricultural practices in Southern Ontario typically supplement soils with organic and inorganic nutrients to aid in crop development, and employ various management techniques to limit nutrient loss. Excess nitrogen has several potential fates, which are controlled by the net effects of numerous nitrogen cycling reactions in the soil that are often difficult to measure directly. Nitrogen cycling in soils is controlled in large part by soil moisture, as it affects microbial activity and soil redox conditions. Stable isotope geochemistry is a powerful tool that provides information on nitrogen sources and processes. This study uses crop nitrogen and carbon isotope ratios to provide insights into the net effects of soil nitrogen cycling and nitrogen fate. This research was conducted at the Strawberry Creek Watershed (SCW), an agricultural research watershed located between Kitchener-Waterloo and Guelph, Ontario. The SCW exhibits elevated nitrate concentrations in groundwater, tile discharge, and the stream itself. Previous isotopic work revealed that this nitrate is largely derived from chemical fertilizer and manure applications. Field-scale hydrological processes lead to areas where the fate of applied nitrogen differs, which has an isotopic effect on the residual nitrogen that is available to plants. Results of this study indicate significant patterns in the isotopic signature of plant tissue, in both temporal and spatial scales. At the plot-scale where soil conditions are similar, there is little to no variation in foliar isotope values, but at the field-scale there appears to be a significant amount of variability related to soil moisture and nitrogen loss. This relationship can potentially provide insight into ideal conditions for nitrogen uptake efficiency. Reducing agricultural nitrogen leaching to ground and surface water requires a better understanding of nitrogen fate in the soil zone, and will result in more effective agricultural nutrient management.
Total Mercury and Methylmercury Mass Fluxes From Three Coastal Plain Watersheds With Contrasting Land Use
To examine how coastal plain watersheds might respond to future changes in the atmospheric deposition of mercury, a long-term watershed monitoring program was recently initiated in three small watersheds with contrasting land use (forest, agriculture, and mixed), at the Smithsonian Environmental Research Center on Chesapeake Bay. Weekly flow-weighted composite samples were collected using automated clean sampling and analyzed for both dissolved and particulate concentrations of total mercury (THg) and methylmercury (MeHg). During the first full year of data in 2008, the forested watershed had the largest per-area yields of both THg and MeHg (4.6 and 0.19 g km-2 yr-1, respectively). Despite wetland coverage of <1% in all three of the study watersheds, MeHg concentrations in watershed runoff increased several-fold between winter and late- spring, with the highest concentrations (up to 3 ng/l) observed in the forested watershed. High MeHg concentrations (up to 12 ng/l) in streamside wells in this watershed point to the likelihood of riparian zone methylation. Long-term monitoring projects, such as this one, will be critical to assessing the impact of future trends in atmospheric mercury deposition on watershed transport to surface waters.
Effects of pH on Dissolved Organic Matter From Freshwater Algal Species
Dissolved organic matter (DOM) is ubiquitous in all natural waters. The nature and composition of aquatic DOM depends on its origin (autochthonous vs. allochthonous) and the physical chemical conditions (pH) of the system. It is clear that autochthonous DOM of algal origin is an important contributor to the DOM pool in most aquatic systems. Little is known on its nature and composition. In this study, algal monocultures of S. acutus and F. crotonensis were grown at two different pHs (pH 7 and 5). The production of exudates was monitored over time and characterized by dissolved organic carbon content, absorbance and synchronous fluorescence. Results indicate a significant difference in the concentration of dissolved organic carbon (DOC) formed per species. The ratio of DOC to chlorophyll a is ten times greater in S. acutus than F. crotonensis. In terms of composition, the production of humic-like compounds varies between species with F. crotonensis producing up to four fold more at natural pH. At lower pH, the production of algal DOM is less but there were more proteins and humic materials generated by both species under decreasing pH, with a significant increase in the S. acutus species. Therefore, the concentration and composition of DOM depends not only on algal species but also on the physical chemical condition (pH level) indicating that water acidification would have a major impact on DOM composition.
Carbon mineralization of flooded boreal soil and vegetation under different temperature and oxygen conditions
Flooding of terrestrial ecosystems significantly alters carbon (C) mineralization rates, which results in increasing emissions of carbon dioxide (CO2) and methane (CH4). To better understand the changes after water impoundment, C mineralization under flooded conditions needs to be investigated. This study investigates CO2 and CH4 fluxes from flooded boreal soil and vegetation, compares them to the fluxes of non- flooded treatment, and examines how environmental factors affect the fluxes. We conducted short-term in vitro experiments using boreal forest soil (FH layer), peat soil (0 to 5 and 5 to 15 cm) layer, and black spruce needles and small twigs, and shrub, sedge, lichen, and moss tissues. Flooded samples were incubated in 1- L Mason jars without light, under three temperatures (5, 12, and 24degC) and 0 and 50 percent of ambient oxygen (O2) concentration, and non-flooded ones were incubated in 1-L plastic containers under same light and temperature conditions to those of flooded samples and ambient oxygen concentration. We collected gas samples after flushing with nitrogen gas and air, and the fluxes of CO2 and CH4 were determined by gas chromatography. The average CO2 and CH4 fluxes in all materials were 200 and 0.8 microgram C/g organic matter/day, with smaller CO2 fluxes and larger CH4 fluxes than the fluxes of non-flooding (CO2 and CH4: 370 and 0.2 microgram C/g organic matter/day). Among the flooded samples, forest and peatland ground vegetation showed much high CO2 fluxes, and peat soils released more CH4 than other materials. Higher temperatures increased emissions of both CO2 and CH4, and the lower O2 concentration increased CH4 emissions. These results suggest the flooded vegetation and peat soil largely contribute to the total C emission in the flooded ecosystem and that spatial and temporal variability in CO2 and CH4 emissions can be related to substrate type, temperature and O2 concentration.
Dissolved Carbon Flux and Mass Balance From a Wetland-Dominated Karstic Headwater Catchment
The stream-borne dissolved carbon efflux of peatland-draining catchments is dominated by organic carbon, whereas inorganic carbon dominates the flux from calcareous bedrock catchments. The export of dissolved carbon from calcareous bedrock catchments with significant wetland coverage has not previously been determined. This study documents the spatiotemporal variability of dissolved carbon (inorganic + organic) along a headwater stream in southern Ontario, Canada, as it drains three distinct wetland types: a calcareous fen, a riparian cedar swamp, and a cattail marsh. Upon emergence from the groundwater seeps, the spring water contained 28 times more CO2 than in equilibrium with the atmosphere. This supersaturation decreased to just 5 times equilbrium as the stream leaves the catchment through the marsh, representing a decrease in CO2 concentration of 11 mg L-1, lost to the atmosphere as exsolution. The groundwater seeps contained an average of 1.25±0.75 mg L-1 of dissolved organic carbon (DOC) from May to November 2007, one of the driest years on record in the region. At the catchment outlet through the marsh, DOC concentrations were slightly higher and more variable during the same period at 2.27±1.29 mg L-1, as a fall flushing event resulted in concentrations > 7 mg L-1. This DOC concentration is small compared to the 58.72±3.9 mg L-1 of dissolved inorganic carbon (DIC, as bicarbonate ion) contained within the water leaving the catchment. At 0.21 and 0.17 g m-2 d-1 from May-July and August-November 2007, respectively, the DIC dominated the carbon flux out of the watershed, compared with 0.007 and 0.008 g m- 2 d-1 DOC and 0.015 and 0.009 g m-2 d-1 CO2 exsolution during the same period. Results of the 2007 season will be contrasted to the 2008 season, one of the wettest on record. The watershed is underlain by Silurian dolomite that exhibits karst fractures, resulting in a complex subsurface hydrogeology that influences carbon transport and mass balances between the wetland types. Significant stream recharge back into the dolomite occurred through the calcareous fen, resulting in the peatland acting as a net 'sink' of DOC and DIC, which was stronger in the drier summer than the wetter autumn. The stream became unconfined through the cedar swamp, with appreciable depression storage that resulted in the wetland becoming a greater net sink of DOC and DIC in the autumn when increased stream discharge into the swamp resulted in greater depression storage. The marsh received significant groundwater inputs from the karstic dolomite, diluting the stream of DOC, but increasing the flux of DIC, resulting in the wetland acting as a sink of DOC but a source of DIC. These data demonstrate that the presence of calcareous bedrock overwhelms the influence of wetland sediments on the dynamics of terrestrial-stream linkages to carbon transport, and that groundwater systems and wetland type can significantly affect the stream-borne flux of carbon from watersheds.