Factors Influencing Stable Isotope Ratios in CH4 and CO2 Within Subenvironments of Freshwater Wetlands: Implications for δ-Signatures of Emissions

Abstract

Much uncertainty still exists regarding spatial and temporal variability of stable isotope ratios (¹³C/¹²C and D/H) in different CH₄-emission sources. Such variability is especially prevalent in freshwater wetlands where a range of processes can influence stable isotope compositions, resulting in variations of up to ～50‰ for δ¹³C-CH₄ and ～150‰ for δD-CH₄ values. Within a temperate-zone bog and marsh situated in southwestern Ontario, Canada, gas bubbles in pond sediments exhibit only minor seasonal and spatial variation in δ¹³C-CH₄, δD-CH₄ and δ¹³C-CO₂ values. In pond sediments, CO₂ appears to be the main source of carbon during methanogenesis either directly via CO₂ reduction or indirectly through dissimilation of autotrophic acetate. In contrast, CH₄ production occurs primarily via acetate fermentation at shallow depths in peat soils adjacent to ponds at each wetland. At greater depths within soils, σCO₂ and H₂O increasingly exert an influence on δ¹³C- and δD-CH₄ values. Secondary alteration processes (e.g., methanotrophy or diffusive transport) are unlikely to be responsible for depth-related changes in stable isotope values of CH₄. Recent models that attempt to predict δD-CH₄ values in freshwater environments from D/H ratios in local precipitation do not adequately account for such changes with depth. Subenvironments (i.e., soil-forming and open water areas) in wetlands should be considered separately with respect to stable isotope signatures in CH₄ emission models.     

Impact of water table depth on forest soil methane turnover in laboratory soil cores deduced from natural abundance and tracer 13C stable isotope experiments

We investigated turnover of methane (CH4) in soils from a poorly drained UK forest. In situ, this forest exhibited a negligible soil-atmosphere CH4 flux, whereas adjacent grassland plots were sources of CH4. We hypothesised that the forest plots exhibited reduced anaerobic CH4 production through water-table draw down. Consequently, we exposed soil cores from under oak to high and low water-table conditions in the laboratory. Methane fluxes increased significantly in the high water-table (1925+/-1702 mug CH4 m(-2) h(-1)) compared to the low one (-3.5+/-6.8 microg CH4 m(-2) h(-1)). Natural abundance delta13C values of CH4 showed a strong depletion in high water-table cores (-56.7+/-2.9 per thousand) compared to methane in ambient air (-46.0 per thousand) indicative of methanogenic processes. The delta13C values of CH4 from low water-table cores (delta13C-46.8+/-0.2 per thousand) was similar to ambient air and suggested little alteration of headspace CH4 by the soil microbial community. In order to assess the CH4 oxidizing activity of the two treatments conclusively, a 13CH4 spike was added to the cores and 13CO2 production was measured as the by-product of CH4 oxidation. 13CH4 oxidation rates were 57.5 (+/-12.7) and 0.5 (+/-0.1) microg CH4 m(-2) h(-1) for high and low water-tables, respectively. These data show that the lower water-table hydrology treatment impacted methanogenic processes without stimulating methanotrophy.     

Interpreting early land management through compound specific stable isotope analyses of archaeological soils

Compound specific stable isotope analyses of managed soils using isotope ratio mass spectrometry have been undertaken as a means of determining early land use practices. delta (15)N amino acid signals demonstrate differences between manured grassland, unmanured grassland and continuous cereal cultivation under long-term experimental land use control conditions, with delta (15)N in hydrophobic amino acids providing the most distinctive signals. Analysis of early modern/medieval and of Bronze age anthropogenic soils from Orkney demonstrates that such signals are retained in archaeological contexts. delta (13)C analyses of n- alkanoic acid components of the fossil, Bronze Age, anthropogenic soils suggest a major terrestrial input to these soils, with uniform composition of formation materials. Surficial soils demonstrate the assimilation of isotopically lighter carbon, providing a means of assessing the mobility of the n- alkanoic acids within soils and sediments. Copyright 1999 John Wiley & Sons, Ltd.     

Continuous field measurements of delta(13)C-CO(2) and trace gases by FTIR spectroscopy

Continuous analysis of the (13)C/(12)C ratio of atmospheric CO(2) (delta(13)C-CO(2)) is a powerful tool to quantify CO(2) flux strengths of the two major ecosystem processes assimilation and respiration. Traditional laboratory techniques such as isotope ratio mass spectrometry (IRMS) in combination with flask sampling are subject to technical limitations that do not allow to fully characterising variations of atmospheric delta(13)C-CO(2) at all relevant timescales. In our study, we demonstrate the strength of Fourier transform infrared (FTIR) spectroscopy in combination with a PLS-based calibration strategy for online analysis of delta(13)C-CO(2) in ambient air. The ability of the instrument to measure delta(13)C-CO(2) was tested on a grassland field-site and compared with standard laboratory-based IRMS measurements made on field-collected flask samples. Both methods were in excellent agreement, with an average difference of 0.4 per thousand (n=81). Simultaneously, other important trace gases such as CO, N(2)O and CH(4) were analysed by FTIR spectroscopy.     

Degradation pathways of dissolved carbon in landfill leachate traced with compound-specific (13)C analysis of DOC

The isotopic compositions of carbon compounds in landfill leachate provide insights into the biodegradation pathways that dominate the different stages of waste decomposition. In this study, the carbon geochemistry of different carbon pools, environmental stable isotopes and compound-specific isotope analysis (CSIA) of leachate dissolved organic carbon (DOC) fractions and gases show distinctions in leachate biogeochemistry and methane production between the young area of active waste emplacement and the old area of historical emplacement at the Trail Road Landfill (TRL). The active area leachate has low DOC concentrations (<200 mg l(-1)) dominated by fulvic acid (FA=160 mg l(-1)), and produces CH(4) dominantly by CO(2) reduction (D- excess=20.6 per thousand). Leachate generated in the area of older waste has high DOC (>4770 mg l(-1)) dominated by FA (4482 mg l(-1)) and simple fatty acids (acetic=1008 mg l(-1) and propionic=608 mg l(-1)), and produces CH(4) by the acetate fermentation pathway (D- excess=9.8 per thousand). CSIA shows an advanced degradation and a progressive accumulation of (13)C of fatty acids in leachate from the older area. The enriched (13)C value of FA (-20 and-26 per thousand for the older and active parts, respectively,) and of low molecular weight DOC (-8 and-27 per thousand) as well as of the bulk DOC (-21 and-25 per thousand) shows more advanced degradation in the older part of the landfill, which is consistent with the shift in the humic/FA ratios (0.05 and 0.18). The (13)C enrichment of acetate (-12 per thousand) above the (13)C of DOC (-21 per thousand) and of propionic acid (-19 per thousand), in older leachate, suggests that this acetate has not evolved from the simple degradation of larger organic molecules, but by homoacetogenesis from the enriched dissolved inorganic carbon (DIC) pool (8 per thousand) and H(2,) which produce a more enriched (13)C of acetate. In contrast, the (13)C of the minor acetate in the active area (-17 per thousand) indicates that CO(2)-reducing bacteria must be the primary consumers of H(2), which has resulted in enriched (13)C(DIC) (10 per thousand) and depleted (13)C(CH4) (-58 per thousand).     

Isotope variations in white-tailed kites from various habitats in California: possible limitations in assessing prey utilization and population dynamics

White-tailed kite (Elanus leucurus) populations in the 1930s were close to extirpation in the United States. But by the 1940s, an upward trend towards recovery was apparent and continued to their current stable population levels. These dramatic fluctuations in kite numbers may have been related to changes in rodent prey populations due to the conversion of native habitats to agriculture. To address this question, we evaluated the use of stable isotope analysis in determining if a shift in diet could be isotopically differentiated in current and historic kite populations. We first compared delta13C, delta15N, and delta34S values from present-day kite flight feathers and prey fur samples from four locations in California. The total ranges of isotope values for kite and their rodent prey were similar within each site. Carbon isotope values ranged from -27.1 to -22.2 per thousand in Arcata, -26.1 to -16.9 per thousand in Davis, -27.0 to -15.0 per thousand in Cosumnes, and -28.2 to -11.6 per thousand in Santa Barbara. Nitrogen isotope values ranged from 3.2 to 15.7 per thousand in Arcata, 2.8 to 12.7 per thousand in Davis, 4.0 to 15.7 per thousand in Cosumnes, and 1.7 to 20.0 per thousand in Santa Barbara. Sulfur isotope values ranged from -7.8 to 12.4 per thousand in Arcata, -1.1 to 9.2 per thousand in Davis, 0.7 to 10.9 per thousand in Cosumnes, and -8.6 to 15.6 per thousand in Santa Barbara. Carbon, nitrogen, and sulfur isotope values at each site reflect typical trophic enrichments due to physiological processes. At each site, delta13C and delta15N values reflected the influence of a predominantly C3 or a mixed C3/C4 plant community. Sulfur isotope values reflect the influence of predominant marine or terrestrial sulfur sources at each site. However, variability in isotope values may limit the usefulness of such analyses for addressing prey utilization and population dynamics.     

Using environmental isotopes 2H and 18O for identification of infiltration processes in floodplain ecosystems of the River Elbe

We examined a floodplain area in the middle section of the river Elbe Valley with regard to hydrogeological and hydrological processes using isotopic methods. Over two years, river water and groundwater have been analysed for temporal and spatial chemical and isotopic (delta2H and delta18O) changes. By these methods we assessed the flow dynamics of the river-groundwater infiltration system. At low and mean river stages there is a general hydraulic gradient from the higher areas at the margin of the valley towards the floodplain. During floods river water infiltrates into the adjacent aquifer not primarily through the river banks but first through surface water inflow from north to south, via depressions and gullies from the back of the floodplain. The early stage of river water infiltration is characterized by a sharp decrease in conductivity and in concentrations of SO4(2-) and Cl- in the hydraulically connected shallow aquifer. delta2H and delta18O values show a similar tendency. We observed a significant minimum in stable isotope ratios during the flood in March 1999. Using a simple mixing equation it was calculated that the groundwater in the upper, shallow aquifer consists of around 70% river water in the transition zone (well 13) during flooding.     

The global methane cycle: isotopes and mixing ratios,sources and sinks

A review of the global cycle of methane is presented with emphasis on its isotopic composition. The history of methane mixing ratios, reconstructed from measurements of air trapped in ice-cores is described. The methane record now extends back to 420 kyr ago in the case of the Vostok ice cores from Antarctica. The trends in mixing ratios and in delta13C values are reported for the two Hemispheres. The increase of the atmospheric methane concentration over the past 200 years, and by 1% per year since 1978, reaching 1.7 ppmv in 1990 is underlined. The various methane sources are presented. Indeed the authors describe the methane emissions by bacterial activity under anaerobic conditions in wet environments (wetlands, bogs, tundra, rice paddies), in ruminant stomachs and termite guts, and that originating from fossil carbon sources, such as biomass burning, coal mining, industrial losses, automobile exhaust, sea floor vent, and volcanic emissions. Furthermore, the main sinks of methane in the troposphere, soils or waters via oxidation are also reported, and the corresponding kinetic isotope effects.     

Diets of nesting laughing gulls (Larus atricilla) at the Virginia Coast Reserve: observations from stable isotope analysis

Food web studies often ignore details of temporal, spatial, and intrapopulation dietary variation in top-level consumers. In this study, intrapopulation dietary variation of a dominant carnivore, the Laughing Gull (Larus atricilla), was examined using carbon, nitrogen, and sulfur isotope analysis of gull tissues as well as their prey (fish, invertebrates, and insects) from the Virginia Coast Reserve estuarine system. As earlier traditional diet studies found evidence of individual dietary specialization within gull populations, this study used stable isotope analysis to assess specialization in a coastal Laughing Gull population. Specifically, blood, muscle, and feather isotope values indicated significant intrapopulation dietary specialization. Some gulls relied more heavily on estuarine prey (mean blood delta13C = -17.5, delta15N = 12.6, and delta34S = 9.3), whereas others appeared to consume more foods of marine origin (mean blood delta13C = -19.4, delta15N = 14.8, and delta34S = 10.4). It is important to account for such dietary variability when assessing trophic linkages in dynamic estuarine systems.     

Stable isotope ratios and uric acid preservation in termites belonging to three feeding habits in Thailand

Nitrogen and carbon stable isotope ratios and uric acid concentrations in termites sampled from a dry evergreen forest in Thailand, were determined across three kinds of feeding habits. Feeding habits of Microcerotermes crassus, which is an abundant wood-feeder, and Dicuspiditermes makhamensis, a common soil-feeding termite, were confirmed by isotopic signatures. Lichen feeding termites (Hospitalitermes birmanicus, H. bicolor and H. ataramensis) were characterized by low delta15N values, suggesting that they assimilated nitrogen deposited from the atmosphere. There was also a significant difference in uric acid concentrations between termites representing different feeding habits. No significant relationships were found between uric acid concentrations and delta15N or delta13C in Hospitalitermes. However, delta15N values were correlated with C/N ratios in H. birmanicus, except in one colony of H. ataramensis. delta13C values in both species were negatively correlated with C/N ratios.     

A dynamic study of earthworm feeding ecology -using stable isotopes

Changes in the specific diet of earthworms with time in relation to landuse changes and two different climates were studied by analysing (13)C and (15)N natural abundance in soils and animals. Soil samples from three depths (0-10, 10-20 and 20-30 cm) and earthworms were collected from two sites: Santiago (Northwest Spain) and North Wyke (Southwest England) both consisting of replicated long-term grasslands and recently converted to maize plots. Earthworms were hand-sorted in the field at the peak of the maize growth and after harvesting at both sites. In the Spanish plots, nine and eight earthworm species, all belonging to the three ecological categories (epigeic, anecic and endogeic), were found under maize and permanent pasture, whereas at the English site five and seven different species were, respectively, identified. At both sites (13)C isotopic values of the earthworm tissues reflected changes in diet from C(3) to C(4) with epigeic and epi/anecic worms in the maize plots showing one delta unit difference in relation to the ones found in the grassland plots. Anecic worms seemed to be less responsive to landuse changes. The higher (13)C values of the Spanish soils were also reflected in the earthworm tissues when compared with the English samples. (15)N values showed no clear relationship with the cropping treatments but were clearly related to the ecological grouping, with endogeic worms reaching the highest values whereas for the epigeic and epi/anecic species the lowest values were obtained. This finding was also previously recorded by other authors1 and suggests that, in the future, stable isotope techniques could also be a useful tool in taxonomic studies. Copyright 1999 John Wiley & Sons, Ltd.     

The fate of organic matter in a papyrus (Cyperus papyrus L.) dominated tropical wetland ecosystem in Nyanza Gulf (Lake Victoria, Kenya) inferred from delta13C and delta15N analysis

Papyrus swamps usually form at the interface between river inlet and open lake. From one such wetland ecosystem (the Kibos system located in the Nyanza Gulf, Lake Victoria, Kenya), three sediment cores were recovered using piston corer in order to determine the fate of organic matter derived from papyrus and possible nutrient pathways in this system. The coring represented a transect from the river through the floating papyrus mat to the lake. Two short cores were retrieved from the lake and river. One long core (2 m) was recovered on a floating papyrus mat. The C:N ratio showed similar trends down core from the three locations. This may possibly be due to diagenic processes such as autolysis, dissolution and microbial mineralisation occurring in the sediments. Statistical analysis through one-way ANOVA revealed no significant differences in the C:N ratios between stations. Results of the stable carbon isotope ratios revealed that the delta(13)C of the river and lake samples were persistently more negative than -20 per thousand over the whole profile indicating possible contribution from terrestrial derived carbon. Regarding the floating mat core, the delta(13)C values ranged from -18.99 per thousand on the top of the floating mat but gradually increased to -16.82 per thousand towards the bottom of the core indicating possible contribution of carbon from Cyperus papyrus that has a delta(13)C value of -13.45+/-0.62 per thousand. Statistical analysis through one-way ANOVA revealed significant differences in the delta(13)C values between stations. The stable nitrogen isotope values were highly positive both in the river and in the lake station (delta(15)N > 10 per thousand), indicating possible contamination from sewage wastes. Values in the swamp were less positive suggesting first, the formation of ammonium depleted in (15)N from intense organic matter mineralisation, secondly indicating the delta(15)N signal of papyrus and, finally that nitrogen fixation processes were possibly occurring in the swamp. Statistical analysis through one-way ANOVA revealed significant differences in the delta(15)N values between stations. The stable isotope findings suggested that carbon derived from papyrus is retained in the swamp. Impoverished oxygen concentration in the swamp suggests high mineralisation of organic matter in the swamp indicating that the retained papyrus-derived carbon is largely respired. We conclude that further studies should be undertaken to determine the respiration rates in the wetland.     

A laser extraction/combustion technique for in situ delta(13)C analysis of organic and inorganic materials

A CO(2) laser extraction system is described for in situ delta(13)C analysis of organic and inorganic materials. Carbonaceous compounds volatilized by the laser are quantitatively converted to CO(2) gas by a combustion furnace mounted after the sample chamber. Gases produced by the laser and combustion processes are swept by helium carrier gas and separated by a packed gas chromatography column prior to their introduction to an isotope ratio monitoring mass spectrometer. A sample of lentil bean was analyzed at a spatial resolution of 200 μm and yielded delta(13)C values with precision of +/- 0.3 per thousand. The accuracy of delta(13)C measurements was better than +/- 0.5 per thousand from NBS 22 (mineral oil), USGS 24 (graphite), and IAEA CO-1 (calcium carbonate). Copyright 1999 John Wiley & Sons, Ltd.     

Sampling soil-derived CO2 for analysis of isotopic composition: a comparison of different techniques

A new system for soil respiration measurement [P. Rochette, L.B. Flanagan, E.G. Gregorich. Separating soil respiration into plant and soil components using analyses of the natural abundance of carbon-13. Soil Sci. Soc. Am. J., 63, 1207-1213 (1999).] was modified in order to collect soil-derived CO2 for stable isotope analysis. The aim of this study was to assess the suitability of this modified soil respiration system to determine the isotopic composition (delta13C) of soil CO2 efflux and to measure, at the same time, the soil CO2 efflux rate, with the further advantage of collecting only one air sample. A comparison between different methods of air collection from the soil was carried out in a laboratory experiment. Our system, as well as the other dynamic chamber approach tested, appeared to sample the soil CO2, which is enriched with respect to the soil CO2 efflux, probably because of a mass dependent fractionation during diffusion and because of the atmospheric contribution in the upper soil layer. On the contrary, the static accumulation of CO2 into the chamber headspace allows sampling of delta13C-CO2 of soil CO2 efflux.     

Determination of delta13 CV-PDB and delta15NAIR values of cocaine from a big seizure in Germany by stable isotope ratio mass spectrometry

In this study, delta(13)C(V- PDB) and delta(15)N(AIR) values of 132 cocaine samples from a big seizure in Germany in 2002 were determined using elemental analyser isotope ratio mass spectrometry. The 1.2 tons of cocaine were packed in 1 kg packages and the cocaine bricks inside these packages showed certain logos. Twenty different logos could be identified. Results show a large variability among some samples, for delta(15)N(AIR) values ranging from-17 to -2 per thousand. Furthermore, the possibility of linking samples with the same logo was checked. The results show that, in general, there is no relationship between the determined isotope ratio and a certain logo.     

Food web implications of delta13C and delta15N variability over 370 km of the regulated Colorado River USA

Dual stable isotope analysis in the regulated Colorado River through Grand Canyon National Park, USA, revealed a food web that varied spatially through this arid biome. Down-river enrichment of delta13C data was detected across three trophic levels resulting in shifted food webs. Humpack chub delta13C and delta15N values from muscle plugs and fin clips did not differ significantly. Humpback chub and rainbow trout trophic position is positively correlated with standard length indicating an increase in piscivory by larger fishes. Recovery of the aquatic community from impoundment by Glen Canyon Dam and collecting refinements for stable isotope analysis within large rivers are discussed.     

Tracing the pathways of neotropical migratory shorebirds using stable isotopes: a pilot study

We evaluated the potential use of stable isotopes to establish linkages between the wintering grounds and the breeding grounds of the Pectoral Sandpiper (Calidris melanotos), the White-rumped Sandpiper (Calidris fuscicollis), the Baird's Sandpiper (Calidris bairdii), and other Neotropical migratory shorebird species (e.g., Tringa spp.). These species molt their flight feathers on the wintering grounds and hence their flight feathers carry chemical signatures that are characteristic of their winter habitat. The objective of our pilot study was to assess the feasibility of identifying the winter origin of individual birds by: (1) collecting shorebird flight feathers from several widely separated Argentine sites and analyzing these for a suite of stable isotopes; and 2) analyzing the deuterium and 18O isotope data that were available from precipitation measurement stations in Argentina. Isotopic ratios (delta13C, delta15N and delta34S) of flight feathers were significantly different among three widely separated sites in Argentina during January 2001. In terms of relative importance in separating the sites, delta34S was most important, followed by delta15N, and then delta13C. In the complete discriminant analysis, the classification function correctly predicted group membership in 85% of the cases (jackknifed classification matrix). In a stepwise analysis delta13C was dropped from the solution, and site membership was correctly predicted in 92% of cases (jackknifed matrix). Analysis of precipitation data showed that both deltaD and delta18O were significantly related to both latitude and longitude on a countrywide scale (p < 0.001). Other variables, month, altitude, explained little additional variation in these isotope ratios. Several issues were identified that will likely constrain the degree of accuracy one can expect in predicting the geographic origin of birds from Argentina. There was unexplained variation in isotope ratios within and among the different wing feathers from individual birds. Such variation may indicate that birds are not faithful to a local site during their winter stay in Argentina. There was significant interannual variation in the deltaD and delta18O of precipitation. Hence, specific locations may not have a constant signature for some isotopes. Moreover, the fractionation that occurs in wetlands due to evaporation significantly skews local deltaD and delta18O values, which may undermine the strong large-scale gradients seen in the precipitation data. We are continuing the research with universities in Argentina with a focus on expanding the breadth of feather collection and attempting to resolve the identified issues.     

Isotopic characteristics of meteoric waters in the Belgrade region

The stable isotope composition of hydrogen (delta(2)H) and oxygen (delta(18)O) in monthly precipitation and river water (Sava River and Danube) samples in the Belgrade area gathered between 1992 and 2005 are determined. The local meteoric water line delta(2)H=7.8 (+/-0.2) delta(18)O+7.3(+/-1.6) (r(2)=0.98, n=60, sigma=0.52) for the whole period of observation is close to the global meteoric water line. The amount-weighted mean delta(2)H and delta(18)O values of precipitation were-65+/-27 per thousand and-9.4+/-3.4 per thousand, respectively. Good correlation between delta(18)O values (r approximately >0.67) and ambient temperature and relative humidity was obtained. Stream-water data ranged from-94 to-60 per thousand for delta(2)H and from-11.0 to approximately 5.7 per thousand for delta(18)O with highly statistically significant difference (p>0.01) between the Sava River and the Danube. In addition, the isotopic compositions of local precipitation and adjacent river water at monitoring sites were compared. Obtained data will give an opportunity to improve the knowledge of mixing stream water and local groundwater, and assessment of potential groundwater risks and pressures in the Belgrade basin.     

Consistent predictable patterns in the hydrogen and oxygen stable isotope ratios of animal proteins consumed by modern humans in the USA

Published datasets of proteinaceous animal tissues suggest that co-variation between amino acid hydrogen (δ2H) and oxygen (δ1?O) isotope ratios is a common feature in systems where isotopic variation is driven by geographic or temporal variation in the δ2H and δ1?O values of environmental water. This has led to the development of models relating tissue δ2H and δ1?O values to those of water, with potential application in a number of fields. However, the strength and ubiquity of the influence of environmental water on protein isotope ratios across taxonomic groups, and thus the relevance of predictive models, is an open question. Here we report strong co-variation of δ2H and δ1?O values across a suite of terrestrial and aquatic animal meats purchased in American food markets, including beef, poultry (chicken and turkey), chicken eggs, pork, lamb, freshwater fish, and marine fish. Significant isotope co-variation was not found for small collections of marine bivalves and crustaceans. These results imply that isotopic signals from environmental water were propagated similarly through most of the diverse natural and human-managed foodwebs represented by our samples. Freshwater fish had the largest variation in δ2H and δ1?O values, with ranges of 121‰ and 19.2‰, respectively, reflecting the large isotopic variation in environmental freshwaters. In contrast marine animals had the smallest variation for both δ2H (7‰ range, crustaceans) and δ1?O (3.0‰ range, bivalves) values. Known-origin beef samples demonstrated direct relationships between the variance of environmental water isotope ratios and that of collected meats.