Sources and Transformations of Organic Matter in Surface Soils and Sediments from a Tidal Estuary (North Inlet, South Carolina, USA) (original) (raw)
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Aquatic Geochemistry, 2011
High sedimentation rates along river-dominated margins make these systems important repositories for organic carbon derived from both allochthonous and autochthonous sources. Using elemental carbon/nitrogen ratios, molecular biomarker (lignin phenol), and stable carbon isotopic (bulk and compound-specific) analyses, this study examined the sources of organic carbon to the Louisiana shelf within one of the primary dispersive pathways of the Mississippi River. Surface sediment samples were collected from stations across the inner, mid, and outer Louisiana shelf, within the Mississippi River plume region, during two cruises in the spring and fall of 2000. Lignin biomarker data showed spatial patterns in terrestrial source plant materials within the river plume, such that sediments near the mouth of the Mississippi River were comparatively less degraded and richer in C 4 plant carbon than those found at mid-depth regions of the shelf. A molecular and stable isotope-based mixing model defining riverine, marsh, and marine organic carbon suggested that the highest organic carbon inputs to the shelf in spring were from marine sources (55-61% marine organic carbon), while riverine organic carbon was the highest (63%) in fall, likely due to lower inputs of marine organic carbon at this time compared with the spring season. This model also indicated that marsh inputs, ranging from 19 to 34% and 3-15% of the organic carbon in spring and fall, respectively, were significantly more important sources of organic carbon on the inner Louisiana shelf than previously suggested. Finally, we propose that the decomposition of terrestrial-derived organic carbon (from the river and local wetlands sources) in mobile muds may serve as a largely unexplored additional source of oxygen-consuming organic carbon in hypoxic bottom waters of the Louisiana shelf.
The cycling and oxidation pathways of organic carbon in a shallow estuary along the Texas Gulf Coast
Estuarine, Coastal and Shelf Science, 2008
The cycling and oxidation pathways of organic carbon were investigated at a single shallow water estuarine site in Trinity Bay, Texas, the uppermost lobe of Galveston Bay, during November 2000. Radio-isotopes were used to estimate sediment mixing and accumulation rates, and benthic chamber and pore water measurements were used to determine sediment-water exchange fluxes of oxygen, nutrients and metals, and infer carbon oxidation rates. Using 7 Be and 234 Th XS , the sediment-mixing coefficient (D b ) was 4.3 AE 1.8 cm 2 y À1 , a value that lies at the lower limit for marine environments, indicating that mixing was not important in these sediments at this time. Sediment accumulation rates (S a ), estimated using 137 Cs and 210 Pb XS , were 0.16 AE 0.02 g cm À2 y À1 . The supply rate of organic carbon to the sediment-water interface was 30 AE 3.9 mmol C m À2 d À1 , of which w10% or 2.9 AE 0.44 mmol C m À2 d À1 was lost from the system through burial below the 1-cm thick surface mixed layer. Measured fluxes of O 2 were 26 AE 3.8 mmol m À2 d À1 and equated to a carbon oxidation rate of 20 AE 3.3 mmol C m À2 d À1 , which is an upper limit due to the potential for oxidation of additional reduced species. Using organic carbon gradients in the surface mixed layer, carbon oxidation was estimated at 2.6 AE 1.1 mmol C m À2 d À1 . Independent estimates made using pore water concentration gradients of ammonium and C:N stoichiometry, equaled 2.8 AE 0.46 mmol C m À2 d À1 . The flux of DOC out of the sediments (DOC efflux ) was 5.6 AE 1.3 mmol C m À2 d À1 . In general, while mass balance was achieved indicating the sediments were at steady state during this time, changes in environmental conditions within the bay and the surrounding area, mean this conclusion might not always hold. These results show that the majority of carbon oxidation occurred at the sediment-water interface, via O 2 reduction. This likely results from the high frequency of sediment resuspension events combined with the shallow sediment mixing zone, leaving anaerobic oxidants responsible for only w10e15% of the carbon oxidized in these sediments.
Environmental Microbiology, 2007
This biogeochemical, molecular genetic and lipid biomarker study of sediments (∼4 m cores) from the Skagerrak (Denmark) investigated methane cycling in a sediment with a clear sulfate-methane-transition zone (SMTZ) and where CH4 supply was by diffusion, rather than by advection, as in more commonly studied seep sites. Sulfate reduction removed sulfate by 0.7 m and CH4 accumulated below. 14C-radiotracer measurements demonstrated active H2/CO2 and acetate methanogenesis and anaerobic oxidation of CH4 (AOM). Maximum AOM rates occurred near the SMTZ (∼3 nmol cm−3 day−1 at 0.75 m) but also continued deeper, overall, at much lower rates. Maximum rates of H2/CO2 and acetate methanogenesis occurred below the SMTZ but H2/CO2 methanogenesis rates were × 10 those of acetate methanogenesis, and this was consistent with initial values of 13C-depleted CH4 (δ13C c.−80‰). Areal AOM and methanogenic rates were similar (∼1.7 mmol m−2 day−1), hence, CH4 flux is finely balanced. A 16S rRNA gene library from 1.39 m combined with methanogen (T-RFLP), bacterial (16S rRNA DGGE) and lipid biomarker depth profiles showed the presence of populations similar to some seep sites: ANME-2a (dominant), ANME-3, Methanomicrobiales, Methanosaeta Archaea, with abundance changes with depth corresponding to changes in activities and sulfate-reducing bacteria (SRB). Below the SMTZ to ∼1.7 m CH4 became progressively more 13C depleted (δ13C −82‰) indicating a zone of CH4 recycling which was consistent with the presence of 13C-depleted archaeol (δ13C −55‰). Pore water acetate concentrations decreased in this zone (to ∼5 μM), suggesting that H2, not acetate, was an important CH4 cycling intermediate. The potential biomarkers for AOM-associated SRB, non-isoprenoidal ether lipids, increased below the SMTZ but this distribution reflected 16S rRNA gene sequences for JS1 and OP8 bacteria rather than those of SRB. At this site peak rates of methane production and consumption are spatially separated and seem to be conducted by different archaeal groups. Also AOM is predominantly coupled to sulfate reduction, unlike recent reports from some seep and gassy sediment sites.
Interactions between Methane Oxidation and Nitrification in Coastal Sediments
Geomicrobiology Journal, 2003
Galveston Bay sediments exhibit substantial spatial and seasonal variability in rates of nitrification and aerobic methane oxidation. We examined the biogeochemical and microbiological controls on these processes using aerobic enrichment slurries. Potential aerobic methane and ammonia oxidation rates from unamended control slurries were compared to rates in slurries amended with methane, ammonium, or methane + ammonium. Bacterial community composition was monitored using denaturing gradient gel electrophoresis (DGGE) analysis of PCR amplified ribosomal and functional gene DNA. Potential methane and ammonia oxidation rates increased over time in sediments amended with methane and ammonium, respectively. The highest potential methane oxidation rates occurred in treatments receiving both ammonium and methane suggesting that methanotrophs in the enrichment cultures were nitrogen limited. The highest ammonia oxidation rates occurred in treatments amended with ammonium only. Treatments receiving both ammonium and methane exhibited ammonia oxidation rates and porewater ammonium concentrations similar to those measured in the unamended control suggesting that methanotrophs may have inhibited ammonia oxidation by sequestering available ammonia. Sequence analysis revealed a decrease in general bacterial community diversity over time and a shift in ammonia-oxidizing bacterial composition corresponding with methane availability. However, methanotroph community composition similarities between treatments with different relative methane oxidation rates suggest that changes in physiological activity, as well as shifts in community composition, contributed to the observed patterns in potential rates. from TAMU-Galveston are acknowledged for arranging laboratory space and access to small boats and instrumentation. We thank Tim Hollibaugh for use of laboratory equipment, Nasreen Bano for assistance with DGGE, and Alison Buchan, Nasreen Bano, and three anonymous reviewers for providing valuable comments that improved this manuscript. This work was supported by the National Science Foundation and the Texas Water Development Board .
Estuarine, Coastal and Shelf Science, 2010
Keywords: carbon isotopes saturated hydrocarbons salt marshes sea level carbon cycle a b s t r a c t Sources of sedimentary organic matter to a Morse River, Maine (USA) salt marsh over the last 3390 AE 60 RCYBP (Radiocarbon Years Before Present) are determined using distribution patterns of nalkanes, bulk carbon isotopic analysis, and compound-specific carbon isotopic analysis. Marsh foraminiferal counts suggest a ubiquitous presence of high marsh and higher-high marsh deposits (dominated by Trochammina macrescens forma macrescens, Trochammina comprimata, and Trochammina inflata), implying deposition from w0.2 m to 0.5 m above mean high water. Distributions of n-alkanes show a primary contribution from higher plants, confirmed by an average chain length value of 27.5 for the core sediments, and carbon preference index values all >3. Many sample depths are dominated by the C 25 alkane. Salicornia depressa and Ruppia maritima have similar n-alkane distributions to many of the salt marsh sediments, and we suggest that one or both of these plants is either an important source to the biomass of the marsh through time, or that another unidentified higher plant source is contributing heavily to the sediment pool. Bacterial degradation or algal inputs to the marsh sediments appear to be minor. Compound-specific carbon isotopic analyses of the C 27 alkane are on average 7.2& depleted relative to bulk values, but the two records are strongly correlated (R 2 ¼ 0.89), suggesting that marsh plants dominate the bulk carbon isotopic signal. Our study underscores the importance of using caution when applying mixing models of plant species to salt marsh sediments, especially when relatively few plants are included in the model.
Freshwater Biology, 2014
1. Many rivers are oversaturated in methane (CH 4 ) and carbon dioxide (CO 2 ) relative to the atmosphere, but we know little about the biological controls on the balance between these two important greenhouse gases and how they might respond to warming. 2. We characterise the potential response to temperature in the biological production of CO 2 and CH 4 and the subsequent microbial oxidation of that CH 4 , that is the sink and source components of the CH 4 cycle, in contrasting river bed sediments: fine sediments, which are largely anoxic, and oxic, coarse gravels. 3. In the fine sediments, anaerobic production of both CH 4 and CO 2 increased with temperature, with apparent activation energies for each being 0.51 eV and 0.24 eV, respectively. The difference between the two resulted in a 4% increase in the ratio of CH 4 :CO 2 production for a 1°C increase in temperature. 4. In the coarse gravels, microbial CH 4 oxidation showed no response to temperature at CH 4 concentrations characteristic of these gravel beds (30-200 nmol CH 4 L À1 ), due to strong substrate limitation. In contrast, at higher (although still rate limiting) CH 4 concentrations, more characteristic of the fine sediment patches (2-4 lmol CH 4 L À1 ), CH 4 oxidation exhibited an increasingly strong response to temperature, eventually exceeding that for CH 4 production. 5. In the fine sediment, the surface layers had a CH 4 oxidation capacity over 100 times greater than the gravels and the kinetic response to differing pore water CH 4 concentrations meant CH 4 was oxidised some 2000 times faster in the fine sediment patches compared with the coarse gravels. 6. The calculated kinetic and temperature responses showed that with warming, methanogenesis is unlikely to outstrip methanotrophy and the ratio of CO 2 to CH 4 emitted could be conserved. Consequently, any changes in the efflux ratio of CH 4 to CO 2 are unlikely to be due to the incapacity of methanotrophy to respond to CH 4 production, but rather to a physical bypassing of the methanotrophic community (e.g. through ebullition or transport via plant stems) or contraction of the oxic layer.
Does dissolved organic carbon regulate biological methane oxidation in semiarid soils?
Global Change Biology, 2013
In humid ecosystems, the rate of methane (CH 4) oxidation by soil-dwelling methane-oxidizing bacteria (MOB) is controlled by soil texture and soil water holding capacity, both of which limit the diffusion of atmospheric CH 4 into the soil. However, it remains unclear whether these same mechanisms control CH 4 oxidation in more arid soils. This study was designed to measure the proximate controls of potential CH 4 oxidation in semiarid soils during different seasons. Using a unique and well-constrained 3-million-year-old semiarid substrate age gradient, we were able to hold state factors constant while exploring the relationship between seasonal potential CH 4 oxidation rates and soil texture, soil water holding capacity, and dissolved organic carbon (DOC). We measured unexpectedly higher rates of potential CH 4 oxidation in the wet season than the dry season. Although other studies have attributed low CH 4 oxidation rates in dry soils to desiccation of MOB, we present several lines of evidence that this may be inaccurate. We found that soil DOC concentration explained CH 4 oxidation rates better than soil physical factors that regulate the diffusion of CH 4 from the atmosphere into the soil. We show evidence that MOB facultatively incorporated isotopically labeled glucose into their cells, and MOB utilized glucose in a pattern among our study sites that was similar to wet-season CH 4 oxidation rates. This evidence suggests that DOC, which is utilized by MOB in other environments with varying effects on CH 4 oxidation rates, may be an important regulator of CH 4 oxidation rates in semiarid soils. Our collective understanding of the facultative use of DOC by MOB is still in its infancy, but our results suggest it may be an important factor controlling CH 4 oxidation in soils from dry ecosystems.
Inorganic Nitrogen Stimulates Methane Oxidation in Coastal Lagoon Sediments
Oecologia Australis
Methane (CH4) oxidation is a critical process to reduce CH4 emissions from aquatic environments to the atmosphere. Considering the continuous increase in nitrogen in rivers, lakes, and lagoons from human sources, we re-evaluated the still controversial potential effect of inorganic nitrogen on CH4 oxidation. Here, we approached three shallow coastal lagoons that represent great environmental heterogeneity and used slurry sediments as a model system. The addition of ammonium chloride (NH4Cl) and potassium nitrate (KNO3) significantly stimulated CH4 oxidation in the sediments of all studied lagoons, indicating the potential limitation of nitrogen for the growth of CH4 oxidizing bacteria. Our findings contrast to some previous reports, where ammonium and nitrate inhibited CH4 oxidation in sediments. Indeed, our experiment was performed in a more realistic range in relation to natural concentrations of inorganic nitrogen in aquatic systems (0.5 to 1 mM) and was opposed to extreme concen...