Browsing by Subject "biogeochemistry"
Now showing 1 - 7 of 7
Results Per Page
Sort Options
Item Biogeochemistry of Woody Plant Invasion: Phosphorus Cycling and Microbial Community Composition(2012-07-16) Kantola, Ilsa BethWoody plant encroachment is a globally-prevalent vegetation change phenomenon that has shifted grass-dominated ecosystems to mixed grass and woody plant matrices over the last century. In the Rio Grande Plains of Texas, the introduction of N-fixing woody legumes has increased above- and belowground primary productivity and changed the litter chemistry of the system, accelerating rates of belowground biogeochemical processes. The purpose of this study was to assess the impact of grassland to woodland transition on i) P concentrations in soil physical fractions that differ in their organic matter turnover rates, ii) P availability within the soil over the course of woody encroachment and across the landscape, and iii) microbial community composition and diversity. Soil samples were collected in remnant grasslands and four woody landscape elements (clusters, groves, drainage woodlands, and playas) along a 135-yr chronosequence of woody plant encroachment. P was fractionated by the Hedley method and P concentrations were determined by alkaline oxidation and lithium fusion coupled with ascorbic acid colorimetry. Bacterial and fungal communities were characterized by molecular methods. Whole soil P concentrations were 2-5X greater in woody landscape elements than in grasslands, and nutrient concentrations increased linearly with time following woody plant invasion in all but the slowest-cycling physical fractions. Plant-available P and organic P increased dramatically with time following encroachment. Changes in P availability were more pronounced in drainages and playas than in upland clusters and groves. Analysis of the bacterial and fungal communities demonstrated that microbial communities in grasslands differ at both phylum and genus level from the flora of the wooded landscape elements. This study demonstrates that woody encroachment strongly influences the distribution and availability of soil P and indicates that nutrient cycles in the soil are closely linked and similarly affected by increased woody plant abundance. Microbial communities under woody species differ in composition from those of the grasslands, and are likely contributing to the observed changes in nutrient availability. Since N and P are generally the most limiting nutrients in terrestrial ecosystems, increased stores of P are likely to alter rates of microbial processes, plant-microbe and plant-plant interactions, and successional dynamics in this ecosystem and similar landscapes around the world.Item Diagenesis in seagrass vegetated sediments: biogeochemical processes on diurnal time scales(Texas A&M University, 2005-11-01) Hebert, Andrew BrianSeagrass productivity is largely limited by nutrient and light availability. However, increasing evidence suggests that sedimentary geochemical processes may play an essential role in seagrass productivity/health. Much of this work has been largely phenomenalistic and has not clearly identified the spatio-temporal behavior of the major geochemical parameters involved in diagenesis of seagrass sediments. In this study, a much broader range of both dissolved and solid phase chemical parameters in eelgrass vegetated sediments was investigated. Parallel measurements were made on adjacent unvegetated sediments (<10 m) to more clearly refine the specific influences of seagrass (Zostera marina) on chemical gradients in associated sediments. Previous studies have pointed strongly toward diurnal ??ventilation?? of sediments vegetated with seagrass by the exudation of photosynthetically produced oxygen. However, strong lateral variability of sediment geochemical parameters among and between seagrass vegetated and unvegetated sediments made the observation of diurnal effects sufficiently difficult. Changes resulting from temporal variability were difficult to discern within the spatial variability. A critical question that is often not dealt with in the study of the early diagenesis of sediments is what spatial and temporal sampling intervals are required to account for the dominant source of variability. The auto-covariance function (ACF) was used to determine the optimum scaling length for sample intervals (?x) of ?H2S and Fe2+. Characteristic scale lengths obtained for sediments from seagrass environments are not significantly different from those observed for unvegetated sediments and averaged 13.7?? 2.2 mm. Lateral variations in our scales analyses showed that scale length approximated our sampling interval and that lateral sampling intervals were smaller than the vertical sampling intervals. Our results indicate that macrofauna dwelling in the sediment, the seagrass root/rhizomes, and aggregations of bacteria, microalgae, and meiofauna may be responsible for the vertical and lateral variability. Model calibrations and sensitivity analyses from a sediment-seagrass diagenetic model revealed that changes in physical parameters of the sediments (irrigation, advection, and porosity, for example) had the greatest effect on organic carbon and total dissolved sulfides. This study revealed that sedimentary geochemical parameters that are both vertically and laterally heterogeneous may also affect seagrass productivity.Item Evaluation of kinetic controls on sulfate reduction in a contaminated wetland-aquifer system(2009-05-15) Kneeshaw, Tara AnnOur ability to understand and predict the fate and transport of contaminants in natural systems is vital if we are to be successful in protecting our water resources. One important aspect of understanding chemical fate and transport in natural systems is identifying key kinetic controls on important redox reactions such as sulfate reduction. Anaerobic microbial activities like sulfate reduction are of particular interest because of the important role they play in the degradation of contaminants in the subsurface. However, current rate estimates for sulfate reduction have a wide range in the literature making it difficult to determine representative rates for a given system. These differences in rate data may be explained by varying kinetic controls on reactions. Push-pull tests were used to evaluate sulfate reduction rates at the wetland-aquifer interface. Anaerobic aquifer water containing abundant sulfate was injected into sulfate-depleted wetland porewater. The injected water was subsequently withdrawn and analyzed for geochemical indicators of sulfate reduction. Complexities in rate data, such as presence of a lag phase, changing rate order and spatial variability, were observed and are hypothesized to be linked to activities of the native microbial population. Subsequent experiments explored the response of native microorganisms to geochemical perturbations using a novel approach to measure directly the effects of a geochemical perturbation on an in situ microbial population and measure rates of resulting reactions. In situ experiments involved colonization of a substrate by microorganisms native to the wetland sediments followed by introductions of native water amended with sulfate and tracer. Experimental results showed that higher sulfate concentrations and warmer seasonal temperatures result in faster sulfate reduction rates and corresponding increases in sulfate reducing bacteria. Findings from this research provide quantitative evidence of how geochemical and microbiological processes are linked in a system not at equilibrium.Item Linking Molecular Microbiology and Geochemistry to Better Understand Microbial Ecology in Coastal Marine Sediments(2012-02-14) Reese, Brandi KielThe overall objective of the research presented here was to combine multiple geochemical parameters and molecular characterizations to provide a novel view of active microbial community ecology of sediments in a large-river deltaic estuary. In coastal and estuarine environments, a large portion of benthic respiration has been attributed to sulfate reduction and implicated as an important mechanism in hypoxia formation. The use of high-resolution sampling of individual sediment cores and high throughput nucleic acid extraction techniques combined with 454 FLX sequencing provided a robust understanding of the metabolically active benthic microbial community within coastal sediments. This was used to provide further understanding and show the importance of simultaneously analyzing the connectivity of sulfur and iron cycling to the structure and function of the microbial population. Although aqueous sulfide did not accumulate in the sediments of the northern Gulf of Mexico, active sulfate reduction was observed in all locations sampled. Microbial recycling and sequestration as iron sulfides prevented the release of sulfide from the sediment. Prominent differences were observed between the sample locations and with depth into the sediment column. This study emphasized the importance of combining novel molecular techniques with simultaneous traditional geochemical measurements to show the interdependence of microbiology and geochemistry. In addition, this study highlights the need to consider microbial community biogeography along with small-scale variations in geochemistry and biology that impact the overall cycling of redox elements when constructing biogeochemical models in marine sediments.Item Particle flux transformation in the mesopelagic water column: process analysis and global balance(Texas A&M University, 2008-10-10) Guidi, LionelMarine aggregates are an important means of carbon transfers downwards to the deep ocean as well as an important nutritional source for benthic organism communities that are the ultimate recipients of the flux. During these last 10 years, data on size distribution of particulate matter have been collected in different oceanic provinces using an Underwater Video Profiler. The cruise data include simultaneous analyses of particle size distributions as well as additional physical and biological measurements of water properties through the water column. First, size distributions of large aggregates have been compared to simultaneous measurements of particle flux observed in sediment traps. We related sediment trap compositional data to particle size (d) distributions to estimate their vertical fluxes (F) using simple power relationships (F=Ad^b). The spatial resolution of sedimentation processes allowed by the use of in situ particle sizing instruments lead to a more detailed study of the role of physical processes in vertical flux. Second, evolution of the aggregate size distributions with depth was related to overlying primary production and phytoplankton size-distributions on a global scale. A new clustering technique was developed to partition the profiles of aggregate size distributions. Six clusters were isolated. Profiles with a high proportion of large aggregates were found in high-productivity waters while profiles with a high proportion of small aggregates were located in low-productivity waters. The aggregate size and mass flux in the mesopelagic layer were correlated to the nature of primary producers (micro-, nano-, picophytoplankton fractions) and to the amount of integrated chlorophyll a in the euphotic layer using a multiple regression technique on principal components. Finally, a mesoscale area in the North Atlantic Ocean was studied to emphasize the importance of the physical structure of the water column on the horizontal and vertical distribution of particulate matter. The seasonal change in the abundance of aggregates in the upper 1000 m was consistent with changes in the composition and intensity of the particulate flux recorded in sediment traps. In an area dominated by eddies, surface accumulation of aggregates and export down to 1000 m occured at mesoscale distances (<100 km).Item Relating the Expression of Soil Redoximorphic Features to Soil Texture, pH, and Cation Exchange Capacity(2010-01-14) Mersmann, Ryan S.Three laboratory studies were performed to elucidate the influence of soil texture, pH, and cation exchange capacity (CEC) on the concentration of ferrous Fe in soil solution and the resulting expression of soil redoximorphic features. The objectives were: 1) assess the buffering effects of CEC on ferrous Fe concentration in soil solution, 2) evaluate the effects of pH on the concentration of ferrous Fe in soil solution, and 3) observe the expression of redoximorphic features in soils with varying texture and CEC. The studies concentrated on seasonally wet soils from the Texas Gulf Coast Prairie. Selected soils included Alfisols and Vertisols with characteristics ranging from coarse-loamy to very-fine in texture, strongly acidic to neutral in soil reaction, and siliceous, mixed, and smectitic in mineralogy. The soils included the Pledger clay microlow (acidic, fine-textured), Pledger clay microhigh (neutral, fine-textured), China clay (acidic, fine-textured), Cieno loam (acidic, fine-loamy), Orelia sandy clay loam (neutral, fine-loamy), Gessner fine sandy loam (acidic, coarse-loamy), and Orelia fine sandy loam (neutral, coarse-loamy). The studies provided the following information: 1) fine-textured soils with higher CEC contained more ferrous Fe in solution, 2) ferrous Fe concentrations in the acidic fine-loamy and coarse-loamy soils were higher than the neutral soils for the same textural class, 3) acidic and neutral fine-textured soils contained more ferrous Fe in solution than the remaining soils, 4) the highest percentage of redox concentrations was observed in the acidic, fine-textured soil, 5) the acidic fine-loamy and coarse-loamy soils exhibited a greater percentage of Fe depletions, and 6) a higher percentage of redox features were observed by micromorphic analysis (i.e., point counts under a binocular stereoscopic microscope) than by macromorphic descriptions. This research showed that differing soil characteristics affect the reductive dissolution and translocation of Fe, and subsequent formation of redox features.Item Temporal influences of seasonal hypoxia on sediment biogeochemistry in coastal sediments(Texas A&M University, 2004-11-15) Sell, Karen S.Bottom water hypoxia and its influence on the environment have been topics of increasing concern for many coastal regions. This research addresses both spatial and temporal variability in sediment biogeochemistry at the southeastern region of Corpus Christi Bay, TX, where seasonal (summer) hypoxia occurs. Traditional techniques for determination of a variety of dissolved and solid components, benthic oxygen demand, and sulfate reduction rates were augmented by measurements using solid state microelectrodes to simultaneously determine concentrations of dissolved O2, Mn2+, Fe2+, and [sigma]H2S in multiple small - interval (1 mm) depth profiles of sediment microcosms. Oxygen concentrations in the overlying water were manipulated in the sediment microcosms and electrode depth profile measurements were made over ~ 500 hours of experimentation. Laboratory and field microelectrode results were in good agreement for both norm - oxic and anoxic time periods. Results indicated that iron (Fe2+) and sulfide ([sigma]H2S) were the redox reactive species in these sediments. During hypoxic conditions an upward migration of dissolved Fe2+and [sigma]H2S through the sediment column and, at times, into the overlying water was observed as the dissolved oxygen concentrations decreased. A corresponding decline in the vertical extent of these redox species occurred when the overlying water was re-oxidized. When both dissolved iron and sulfide coexisted, FeS minerals were formed in the sediment, preventing sulfide diffusion into the overlying water. However, after a long duration of hypoxia (> 200 hours) this buffering capacity was exceeded and both iron and sulfide penetrated into the overlying waters. Results indicated that iron may have a greater influence on hypoxia than sulfide because its concentration in the overlying waters during induced hypoxia was an order of magnitude greater than those of sulfide. Moreover, in the southeastern region of the Bay, where mixing was minimal and the water column was shallow, the sediments alone may have caused the onset of the hypoxic event in a relatively short time period (< 5.5 days). These results demonstrated that in shallow marine environments where seasonal hypoxia occurs, such as Corpus Christi Bay, the associated major changes that take place in the sediment biogeochemistry must be included in benthic - pelagic models for overlying water hypoxia.