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  • 1
    Publication Date: 2018
    Description: Abstract Receding permafrost may expose the largest store of sequestered carbon to microbes ready to convert it to greenhouse gases. The fluxes of biogenic gases associated with boreal wetlands has historically been difficult to measure, limiting our understanding of what may happen as permafrost thaws. The many difficulties in working in high latitude climates has dramatically limited our understanding on the spatial and temporal distribution of fluxes and what is likely to change with a warming climate. The use of the eddy covariance approach to measure methane emissions has been developing steadily over the past 26 years. Taylor et al ( 2018) is an excellent example of the advancements made in the field. Here, I summarize the history of the eddy covariance methane literature, describe advancements in spatial and temporal coverage, and look at important drivers of methane emissions.
    Print ISSN: 2169-8953
    Electronic ISSN: 2169-8961
    Topics: Biology , Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
    Publication Date: 2018-02-08
    Description: Salt marshes are sinks for atmospheric carbon dioxide that respond to environmental changes related to sea level rise and climate. Here, we assess how climatic variations affect marsh-atmosphere exchange of carbon dioxide in the short-term and compare it to long-term burial rates based on radiometric dating. The five years of atmospheric measurements show a strong interannual variation in atmospheric carbon exchange, varying from -104 to -233gCm −2 a −1 with a mean of -179±32gCm −2 a −1 . Variation in these annual sums was best explained by differences in rainfall early in the growing season. In the two years with below average rainfall in June, both net uptake and Normalized Difference Vegetation Index (NDVI) were less than in the other three years. Measurements in 2016 and 2017 suggest that the mechanism behind this variability may be rainfall decreasing soil salinity which has been shown to strongly control productivity. The net ecosystem carbon balance was determined as burial rate from four sediment cores using radiometric dating and was lower than the net uptake measured by eddy covariance (mean: 110±13gCm −2 a −1 ). The difference between these estimates was significant, and may be because the atmospheric measurements do not capture lateral carbon fluxes due to tidal exchange. Overall, it was smaller than values reported in the literature for lateral fluxes, and highlights the importance of investigating lateral C fluxes in future studies.
    Print ISSN: 0148-0227
    Topics: Biology , Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
    Publication Date: 2018-02-08
    Description: The foundational ecosystem processes of gross primary production (GPP) and ecosystem respiration (ER) cannot be measured directly but can be modeled in aquatic ecosystems from subdaily patterns of oxygen (O 2 ) concentrations. Because rivers and streams constantly exchange O 2 with the atmosphere, models must either use empirical estimates of the gas exchange rate coefficient ( K 600 ) or solve for all 3 parameters ( G P P , E R , K 600 ) simultaneously. Empirical measurements of K 600 require substantial field work and can still be inaccurate. Three-parameter models have suffered from equifinality, where good fits to O 2 data are achieved by many different parameter values, some unrealistic. We developed a new 3-parameter, multi-day model that ensures similar values for K 600 among days with similar physical conditions (e.g., discharge). Our new model overcomes the equifinality problem by (1) flexibly relating K 600 to discharge while permitting moderate daily deviations, and (2) avoiding the oft-violated assumption that residuals in O 2 predictions are uncorrelated. We implemented this hierarchical state-space model and several competitor models in an open-source R package, streamMetabolizer . We then tested the models against both simulated and field data. Our new model reduces error by as much as 70% in daily estimates of K 600 , G P P , and E R . Further, accuracy benefits of multi-day datasets require as few as 3 days of data. This approach facilitates more accurate metabolism estimates for more streams and days, enabling researchers to better quantify carbon fluxes, compare streams by their metabolic regimes, and investigate controls on aquatic activity.
    Print ISSN: 0148-0227
    Topics: Biology , Geosciences
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  • 4
    Publication Date: 2018-02-13
    Description: In 2014, samples from 37 monitoring wells at 17 locations, within or near oil fields, and 1 site 〉5 km from oil fields, in the Los Angeles Basin (LAB), California were analyzed for dissolved hydrocarbon gas isotopes and abundances. The wells sample a variety of depths of an aquifer system comprised of unconsolidated and semi-consolidated sediments under various conditions of confinement. Concentrations of methane in groundwater samples ranged from 0.002 to 150 mg/L – some of the highest concentrations reported in a densely populated urban area. The δ 13 C and δ 2 H of the methane ranged from -80.8 to -45.5 per mil (‰), and -249.8 to -134.9‰, respectively, and along with oxidation-reduction processes, helped to identify the origin of methane as microbial methanogenesis, and CO 2 reduction as its main formation pathway. The distribution of methane concentrations and isotopes are consistent with the high concentrations of methane in LAB groundwater originating from relatively shallow microbial production in anoxic or suboxic conditions. Source of the methane is the aquifer sediments rather than the upward migration or leakage of thermogenic methane associated with oil fields in the basin.
    Print ISSN: 0148-0227
    Topics: Biology , Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
    Publication Date: 2018-02-13
    Description: Timing and accumulation of snow are among the most important phenomena influencing land surface phenology in mountainous ecosystems. However, our knowledge on their influence on alpine land surface phenology is still limited, and much remains unclear as to which snow metrics are most relevant for studying this interaction. In this study, we analyzed 5 snow and phenology metrics, namely timing (snow cover duration, SCD); last snow day, LSD), accumulation of snow (mean snow water equivalent, SWE m ), and mountain land surface phenology (start of season, SOS; length of season, LOS) in the Swiss Alps during the period 2003–2014. We examined elevational and regional variations in the relationships between snow and alpine land surface phenology metrics using multiple linear regression and relative weight analyses, and subsequently identified the snow metrics that showed strongest associations with variations in alpine land surface phenology of natural vegetation (NV) types. We found that the relationships between snow and phenology metrics were pronounced in high elevational regions and alpine natural grassland and sparsely vegetated areas. SOS was influenced primarily by SCD, secondarily by SWE m , while LOS was equally affected by SCD and SWE m across different elevational bands. We conclude that SCD plays the most significant role compared to other snow metrics. Future variations of snow cover and accumulation are likely to influence alpine ecosystems, for instance their species composition due to changes in the potential growing season. Also, their spatial distribution may change as a response to the new environmental conditions if these prove persistent.
    Print ISSN: 0148-0227
    Topics: Biology , Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
    Publication Date: 2018-02-13
    Description: Large storm events possess significant erosive energy capable of mobilizing large amounts of sediment and particulate organic matter (POM) into fluvial systems. This study investigated how stream POM composition varied as a function of the watershed POM source, particle size, storm event magnitude, and seasonal timing. POM composition was characterized for multiple watershed sources and for stream POM following storms in a second order forested stream. Carbon (C) and nitrogen (N) amount, C:N ratio and isotopic content ( 13 C and 15 N) were determined for solid phase POM, whereas dissolved organic C (DOC), total N concentrations and fluorescence characteristics were determined for solution/extracted POM. Key findings from this study were: (1) Composition of POM varied greatly with watershed sources with forest floor litter being C and N rich and labile while stream banks and bed were C and N poor and recalcitrant. (2) Summer storms mobilized more carbon and nitrogen rich labile sources, while winter events mobilized more carbon and nitrogen poor refractory material from near stream sources. (3) POM composition varied by size class, with the coarse POM (CPOM) showing more C and N rich and labile properties while the fine POM (FPOM) displayed more degraded and refractory properties. If climate variability increases the magnitude and intensity of large storm events, our observations suggest that this will not only increase the inputs of POM to aquatic systems but also result in the delivery of coarser, C and N rich, and more bioavailable POM to the stream drainage network.
    Print ISSN: 0148-0227
    Topics: Biology , Geosciences
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  • 7
    Publication Date: 2018-02-14
    Description: Hurricane-induced rainfall over Puerto Rico has characteristic δ 18 O values which are more negative than local rainfall events. Thus, hurricanes may be recorded in speleothems from Larga cave, Puerto Rico, as characteristic oxygen isotope excursions. Samples of 84 local rainfall events between 2012 and 2013 ranged from -6.2 to +0.3 ‰ , whereas nine rainfall samples belonging to a rain band of hurricane Isaac (23-24 August 2012) ranged from -11.8 to -7.1 ‰. Cave monitoring covered the hurricane season of 2014 and investigated the impact of hurricane rainfall on drip water chemistry. δ 18 O values were measured in cumulative monthly rainwater samples above the cave. Inside the cave, δ 18 O values of instantaneous drip water samples were analyzed and drip rates were recorded at six drip sites. Most effective recharge appears to occur during the wet months (April-May and August-November). δ 18 O values of instantaneous drip water samples ranged from -3.5 to -2.4 ‰ . In April 2014 and April 2015 some drip sites showed more negative δ 18 O values than the effective rainfall (-2.9 ‰), implying an influence of hurricane rainfall reaching the cave via stratified seepage flow months to years after the event. Speleothems from these drip sites in Larga cave have a high potential for paleotempestology studies.
    Print ISSN: 0148-0227
    Topics: Biology , Geosciences
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  • 8
    Publication Date: 2018-02-08
    Description: The Mid-Atlantic coastal forests in Virginia are stressed by episodic disturbance from hurricanes and nor'easters. Using annual tree-ring data, we adopt a dendroclimatic and statistical modeling approach to understand the response and resilience of a coastal pine forest to extreme storm events, over the past few decades. Results indicate that radial growth of trees in the study area is influenced by age, regional climate trends, and individual tree effects, but dominated periodically by growth disturbance due to storms. We evaluated seven local extreme storm events to understand the effect of nor'easters and hurricanes on radial growth. A general decline in radial growth was observed in the year of the extreme storm and three years following it, after which the radial growth started recovering. The decline in radial growth showed a statistically significant correlation with the magnitude of the extreme storm (storm surge height and wind speed). This study contributes to understanding declining tree growth response and resilience of coastal forests to past disturbances. Given the potential increase in hurricanes and storm surge severity in the region, this can help predict vegetation response patterns to similar disturbances in the future.
    Print ISSN: 0148-0227
    Topics: Biology , Geosciences
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  • 9
    Publication Date: 2018-02-09
    Description: A paired-watershed approach was used to compare the quality and fluxes of dissolved organic matter (DOM) during stormflow and baseflow in two lowland tropical rainforest streams located in northeastern Costa Rica. The Arboleda stream received regional groundwater (RGW) flow, whereas the Taconazo stream did not. DOM quality was assessed with absorbance and fluorescence and stable carbon isotope (δ 13 C-DOC) values. RGW DOM lacked detectable fluorescence and had specific ultraviolet absorption (SUVA 254 ) and absorbance slope ratio ( S R ) values consistent with low aromaticity and low molecular weight material, respectively. We attributed these properties to microbial degradation and sorption of humic DOM to mineral surfaces during transport through bedrock. SUVA 254 values were lower and S R values higher in the Arboleda stream during baseflow compared to the Taconazo stream, presumably due to dilution by RGW. However, no significant difference in SUVA 254 or S R occurred between the streams during stormflow. SUVA 254 was negatively correlated to δ 13 C-DOC ( r 2 =0.61, p〈0.001), demonstrating a strong linkage between stream DOM characteristics and the relative amounts of RGW flow and local watershed runoff containing soil and throughfall C sources. Mean DOC export from the Taconazo stream during the study period was 2.62±0.39 g C m -2 yr -1 , consistent with other tropical streams, yet mean DOC export from the Arboleda stream was 13.79±2.07 g C m -2 yr -1 , one of the highest exports reported and demonstrating a substantial impact of old regional groundwater from outside the watershed boundary can have on surface water carbon cycling.
    Print ISSN: 0148-0227
    Topics: Biology , Geosciences
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  • 10
    Publication Date: 2018-02-09
    Description: Peatlands can accumulate organic matter into long-term carbon (C) storage within the soil profile. This study used solid state 13 C nuclear magnetic resonance ( 13 C-NMR) to investigate the transit of organic C through a peatland ecosystem to understand the molecular budget that accompanies the long term accumulation of C. Samples of biomass, litter, peat soil profile, particulate organic matter (POM), and dissolved organic matter (DOM) were taken from the Moor House National Nature Reserve, a peat-covered catchment in northern England where both the dry matter and C budget for the ecosystem were known. The results showed that: The interpretation of the 13 C-NMR spectra shows that polysaccharides are preferentially removed through the ecosystem while lignin components are preferentially retained and come to dominate the organic matter accumulated at depth in the profile. The DOM is derived from the oxidation of both biomass and the degradation of lignin while the POM is derived from erosion of the peat profile. The DOM is differentiated by its proportion of oxidised functional groups and not by its aromatic content. The changes in functionality leading to DOM production suggest side-chain oxidation resulting in C–C cleavage/depolymerisation of lignin; a common reaction within white-rot fungi. The 13 C-NMR budget shows that O-alkyl functional groups are disproportionately lost between primary production and accumulation in the deep peat while C-alkyl functional groups are disproportionately preserved. The carbon lost as gases (CO 2 and CH 4 ) was estimated to be comprised of 93% polysaccharide-derived carbon and 7% lignin-derived carbon.
    Print ISSN: 0148-0227
    Topics: Biology , Geosciences
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