B21C-0896 0800h
DOC Export from a Small Permafrost Watershed
Approximately 13 percent of the Earth's land surface is currently underlain by permafrost, which is rapidly melting in some northern areas due to the effects of a warming climate. It is hypothesized that the resulting melt will release a large pool of sequestered organic carbon, nutrients, and metals within and from currently frozon soils. As part of a larger USGS water quality study of the 854,700 km$^{2}$ Yukon River Basin, we are conducting a focused "small-scale" study to measure and chemically characterize DOC export from the American Creek sub-watershed of the Yukon River Basin. The 160 km$^{2}$ watershed, located at $64\deg$ North in a remote region of the extreme eastern interior of Alaska, was chosen because its characteristics are generally representative of many watersheds in the Yukon River Basin. Permafrost underlying the American Creek watershed confines the hydrologic flow to interact with the shallow surficial materials. Snowmelt infiltrates into the unfrozen, organic-rich soil surface horizon and then flows laterally, down-gradient along the permafrost table in contact with the organic-rich material directly above the permafrost. DOC and discharge were measured at the watershed outlet during peak snowmelt for two consecutive years to understand how permafrost affects these variables. Maximum DOC concentrations and Specific ultraviolet absorbance (SUVA) were correlated to peak discharge, suggesting that the DOC exiting the watershed during snowmelt is highly aromatic (reactive). Our results suggest that the permafrost confines snowmelt and rainfall to shallow, near-surface flow paths. Increased contact time of the snowmelt with organic-rich material, lack of contact with mineral soil particles, and decreased biodegradation result in enhanced transport of DOC to the stream and increased export of DOC from the watershed. It is expected that with progressive melting of the permafrost, DOC export from the American Creek watershed will initially increase as new carbon from the melted permafrost becomes available. Over the long-term, however, it is possible that DOC export will decrease in response to two processes; 1) the new carbon pool will eventually be depleted due to increased microbial degradation, and 2) flow paths will be deeper with the absence of the permafrost table, moving through the soil mineral horizons to the stream. DOC in ground water along these deeper flow paths will likely react with the mineral horizon, effectively filtering out the DOC before it enters the stream.
B21C-0897 0800h
Hydrologic and Climatic Controls on the Export of Dissolved Organic Carbon from Beverly Swamp
Utilizing a seven-year record this study examines how dissolved organic matter (DOM) dynamics relate to hydrological processes and climatological variability in Beverly Swamp - a forested cedar and hardwood peatland located in southern Ontario. The swamp contains two sub-catchments drained by streams that exhibit different hydrological regimes, one an unconfined stream with a high degree of interaction with the peatland and the other a confined stream with little interaction. These hydrological characteristics, combined with inter-annual and seasonal climate variability, provide the variety of conditions that allow the examination of DOM dynamics in the swamp environment. A mass balance approach is used to characterize the retention and export of dissolved organic carbon (DOC) from the sub-catchments and the entire swamp. The DOC budget results from combining stream discharge and precipitation measurements with their DOC concentrations. Comparison of the DOC budget for the ice-free periods from 1998 through 2004 reveals differences in export of DOC from the sub-catchments tied to the stream's hydrologic regime. Inter-annual and seasonal variations in the characteristics of DOC export relate to differences in seasonal distribution of precipitation and to drought-flood cycles.
B21C-0898 0800h
Dissolved Organic Matter Controls on Terrestrial Carbon Sequestration and Export in Contrasting California Ecosystems
Here we report results from the first year of a study characterizing dissolved organic carbon (DOC) fluxes within two first-order watersheds: (1) a humid coastal forest and (2) a semi-arid coastal prairie. Our goals are to quantify (1) the importance of DOC in redistributing C within the soil profile; (2) the magnitude of DOC losses relative to respiratory losses; and (3) seasonal changes in DOC export (both in magnitude and composition) from these upland soils to the local stream network. We are continuously monitoring hydrologic conditions and periodically collecting water samples for chemical analysis in order to construct detailed DOC budgets for these two watersheds. We are utilizing $^{13}$C, $^{14}$C and $^{13}$C NMR analyses on DOC and relevant soil C fractions to reveal numerous insights into information on the internal transformations of organic matter as it moves through the soil in and out of the aqueous phase. Analyzing both the solid and dissolved phase of C allows us to discern the sources and sinks of DOC as rainfall moves down through the soil and ultimately out into the stream network. Results thus far (for the 03/04 water year) show that, in the redwood stand, rainfall mobilized 450 kgC ha$^{-1}$ yr$^{-1}$ as DOC from the O horizons, but while only 80 kgC ha$^{-1}$ of that was leached below the A horizon and less than 10 kgC ha$^{-1}$ was exported to the stream. Laboratory leaching and sorption experiments support these data. DOC concentrations and stable C isotope composition both in soil and stream water samples exhibited pronounced seasonal trends, reflecting changes in DOC sources. Radiocarbon results for DOC and soil density fractions will also be presented. While the redwood site appears to retain DOC, the coastal prairie site is a large net exporter of DOC (stream flux = 24 kgC ha$^{-1}$ yr$^{-1}$), with concentrations and isotopic composition varying in a more complex matter with time and space that seems to correlate with the hydrologic flow path.
B21C-0899 0800h
Distribution and Turnover of DOC in Natural and Constructed Wetlands in the Florida Everglades
Stable and radiocarbon isotopic contents of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and particulate organic carbon (POC) were used to examine the source and turnover rate of carbon in natural and constructed wetlands in the Florida Everglades. DOC concentrations decreased with decreasing phosphorus (P) concentrations along a water quality gradient from the agriculturally impacted areas in the northern Everglades to the more pristine Everglades National Park. $\delta$$^{13}$C values of DOC in the area reflect contributions of both wetland vegetation and sugarcane from agriculture. Radiocarbon ages of DOC, POC and DIC in the Everglades ranged from 2010 years BP to "$<$modern". The old $^{14}$C ages of DOC and POC were found in impacted areas near the Everglade Agricultural Area (EAA) in the northern Everglades. In contrast, DOC and POC in pristine marsh areas had near modern or "$<$modern" $^{14}$C ages. These data indicate that a major source of POC and DOC in impacted areas is the degradation of historic peat deposits in the EAA. In the pristine areas of the marsh, DOC represents a mix of modern and historic carbon sources, whereas POC comes from modern primary production as indicated by positive $\Delta$$^{14}$C values. High $\Delta$$^{14}$C values of DIC indicate that dissolution of limestone is not a significant source of DIC in the Everglades wetlands. A 68.5% turnover of DOC from the northern to the southern Everglades was estimated using radiocarbon data. Our study suggests that $\Delta$$^{14}$C measurements can be a useful indicator of the progress of ecosystem restoration in the Everglades. As a restored wetland moves towards its "original" or "natural" state, the $^{14}$C signatures of DOC should approach that of modern atmosphere. In addition, measurements of concentration and carbon isotopic composition of DOC in two small constructed wetland test cells indicate that these freshwater wetland systems contain a labile DOC pool with rapid turnover times and that the test cells are overall net sinks of DOC.
B21C-0900 0800h
A Comparison of Molecular and Isotopic Chemistry of Overland Flow DOM and yPOM to Soil Sources From a Small Mid-western Agricultural Basin
In agricultural watersheds, the mobilization of terrestrial organic matter into yaquatic environments has been linked to increased primary productivity and ymicrobial activity in the tributaries of large-order streams and rivers. The yincrease in primary productivity and microbial activity results in downstream ynutrient export which can increase decomposition rates, turbidity, release of ycarbon dioxide to the atmosphere, and reduce the dissolved oxygen levels that yaquatic fauna rely upon to survive. The intensity and frequency of storms is a ycritical factor in determining the mass and chemical character of organic matter ymobilized as overland flow from agricultural watersheds. We will present results yfrom biogeochemical characterization of size fractionated aquatic and soil yorganic matter collected during storm events from a 2.5 Km2 drainage area in ycentral Indiana, part of the U.S. Geological Survey National Water-Quality yAssessment. Molecular and isotopic techniques were applied to size fractions of ysource surface soils and to the resultant dissolved, colloidal, and particulate yaquatic fractions isolated by cross-flow ultra-filtration at the overland flow site and ydown stream. Alkaline CuO oxidation of the size fractions was performed to yrelease lignin and aliphatic biopolymer (cutin and suberin) components. yPreliminary results indicate that dissolved organic components released during ythe storm are more degraded than particulate and colloidal materials. Compound yspecific and bulk carbon isotope analyses of the fractions will help us discern if yselective mobilization and decomposition is a factor in controlling the organic ymatter discharge volume from either the added C3 soybean or C4 corn in this ycorn/soybean rotation system.y
B21C-0901 0800h
Carbon Storage in Wetland Soils and Export to Streams in Southeastern Alaska
The majority of terrestrial carbon in southeast Alaska is stored in extensive wetlands dominated by peat. This carbon stock is believed to contribute much dissolved organic carbon (DOC) to streams, though few studies have documented the magnitude of this supply throughout the diverse watersheds in southeast Alaska. Wetland soils in southeast Alaska vary widely in organic matter depth and decomposition that influence both carbon and nutrient cycles within these different soils. We characterized soil type, storage, and export of carbon in a bog (peatland) and a forested wetland to establish reference levels of nutrient export from these wetland types. The bog had a deeper peat accumulation compared to the forested wetland, although DOC concentrations were higher in lysimeters in the forested wetland (70 to 90 mg C/L) than the peatland (20 to 49 mg C/L). DOC concentration in wells (15 to 45 mg C/L) and export from tributaries (5 to 41 mg C/L) draining both the forested wetland and peatland was similar with peak DOC concentrations and export linked to high soil temperatures and storm flushing in mid-summer. This relationship would support a positive feedback of increased DOC export with warmer temperatures and higher rainfall in the hypermaritime ecosystem of coastal North America.
B21C-0902 0800h
Trace metal mobility and microbial community structure in tropical soils: examples from adjacent forest and grassland ecosystems
Many factors determine the quality and sustainability of a soil environment and changes in land use can impact significantly soil geochemistry and the associated soil microbial communities. Native tropical forests and human-constructed grasslands on Barro Colorado Island provide an excellent setting for comparing changes in soil ecosystems in undisturbed and altered landscapes. The goals of this study were to examine biological, chemical, and mineralogical changes in soil properties as a function of land use changes during the wet and dry seasons. Soil pits were excavated at two study sites, a tropical forest and an adjacent plot that has been converted to grassland, during March 2002 and August 2003. The 1 meter deep pits were sampled at 5 cm intervals and characterized for soil organic matter content, soil moisture, community structure and total lipid biomass of the soil microbial community, mineralogy, and trace metal distribution using a sequential extraction method. Results demonstrate that forested soils exhibit higher organic matter content than grassland soils regardless of soil moisture content. Total lipid biomass of the active soil microbial population decreases with depth in both soils, but is elevated in the forested soil, likely correlating with the organic matter content in this system. Diversity of the soil microbial community, determined by PLFA analysis, decreases sharply at the base of the root zone and general trends in community structure are similar in both soils. XRD analysis of the soils reveal that the weathering profile in the forest has extended to a greater depth, but these differences in the mineralogy profile do not exert significant control on trace element mobility. Vanadium, copper, zinc, and aluminum show strong affinities for the organically bound fraction in both soils.
B21C-0903 0800h
Development of a Starch Iodide Method for the Determination of Phosphite in Natural Waters.
Phosphorus (P) is believed to occur almost exclusively in the environment as fully oxidized phosphate (H$_{3}$PO$_{4}$, oxidation state +V). Recent developments in the field of microbiology and research on the origin of life have suggested a possibly significant role for reduced, inorganic forms of P such as phosphorus acid [H$_{3}$PO$_{3}$, P(+III)], hypophosphorus acid [H$_{3}$PO$_{2}$, P(+I)] and various forms of phosphides [P(-III)] in the biogeochemical cycling of P. In order to confirm the importance of reduced forms of P, we need to develop new and better methods for the detection of these compounds in the environment, often at levels below 1 mM. Traditional methods of P determination are based on the reaction of P with acidified molybdate in aqueous solution to yield phosphomolybdate heteropolyacid, which is then reduced and analyzed spectrophotometrically to quantify the total amount of P in the sample. Limitations in this technique have resulted in our development of a new method using a starch iodide complex for the detection of phosphorus acid or phosphite. Under laboratory conditions, phosphite [P (+III)] is oxidized to phosphate [P (+V)] by tri-iodide (I$_{3}$). Starch reacts with excess tri-iodide present in solution to form a blue colored complex having a $\lambda$max of 580 nm. The I$_{3}$, as well as other species such as I$_{5}$, are responsible for the formation of the complex when they fit inside the coiled amylose structure of starch. Linear determination is possible in water samples containing 1 - 80 mM of P (+III). Measurement of the loss of blue color complex can then be correlated with phosphite concentration in the starting sample. Current efforts to optimize this method in order to reach limits of detection below 1 mM are underway. We believe that with better detection methods, the evidence for reduced P in the environment will increase. Due to chemical kinetics, microbial activity, surface catalyzed reactions and possible storage effects, it is difficult to effectively measure the low concentrations present after removing the samples from their natural environments for laboratory analysis. Therefore, to fully understand the importance of reduced P, ultimately, we hope to develop the starch iodide method into an in situ detection technique for measuring phosphite directly in natural waters where sensitive and reliable field-based methods of analysis are needed.
B21C-0904 0800h
Assessing Spatial and Temporal Variability in Aquatic Photosynthesis and Respiration with Oxygen and Carbon Stable Isotopes
Periodic low dissolved oxygen (DO) conditions in the deep water shipping channel (DWSC) of the San Joaquin River at Stockton, CA are an issue of concern because they inhibit the upstream migration of salmon and other anadromous fish. Improved understanding of chemical and biological reactions that contribute to low DO conditions will enable calibration of water quality models geared toward aiding managerial decisions for ameliorating this environmental problem. We apply novel analytical techniques to obtain the isotopic composition of dissolved oxygen (\delta$^{18}$O-DO) and of dissolved inorganic carbon (\delta$^{13}$C-DIC) in an effort to quantify photosynthesis and respiration taking place diurnally at different depths in the DWSC. We collected samples from five depths (1m, 3m, 5m, 7m, 9m) at a stationary point in the DWSC every two hours across a thirty-hour period in the late summer, when low DO conditions tend to develop; surface samples were also collected at 9 adjacent sites. This sampling strategy was repeated twice: during a spring tide and during a neap tide. Samples for nitrate and seston isotopes were also collected but have not yet been analyzed. Values of \delta$^{18}$O-DO range from +19\permil to +25\permil. Shifts in \delta$^{18}$O-DO on the order of 6\permil are evident in the shallow (1m) samples across a diurnal cycle, but are less pronounced at greater depths where values varied between +22\permil and +25\permil. Values of \delta$^{18}$O-DO strongly decrease at 1m depth during the late afternoon when photosynthetic activity would have been at an expected maximum, while remaining nearly constant at 9m depth. Values of \delta$^{13}$C-DIC ranged from -9\permil to -11\permil, with more negative values generally occurring with increasing depth; however, the greatest variability in \delta$^{13}$C-DIC was also observed in the deepest (9m) samples. \delta$^{13}$C-DIC values were strongly negatively correlated with DIC concentrations at 5m, indicating more pronounced heterotrophic respiration at that depth, particularly during the daylight hours. These preliminary data suggest depth-dependent correlations between the \delta$^{18}$O-DO and \delta$^{13}$C-DIC of these samples. More positive \delta$^{13}$C-DIC values correlate with lower \delta$^{18}$O-DO values at the shallow 1m depth, consistent with a photosynthetic signature. At mid-depth in the channel (5m), a correlation between \delta$^{18}$O-DO and \delta$^{13}$C-DIC is still apparent yet weaker, and at 9m the correlation breaks down. These preliminary data illustrate the potential of a multi-isotope approach for understanding sources and sinks of DO in hypoxic environments.