Julian J. C. Dawson

Sources of organic and inorganic carbon in a headwater stream: evidence from carbon isotope studies

A combination of stable isotope studies and 14Cdating were used to identify the main sources andprocesses controlling streamwater DOC and TIC in atemperate non-forested watershed. δ13Cvalues for terrestrial (−24.9 to −29.1‰) and aquatic(−30.5 to −33.5‰) plants were similar to valuesreported in the literature for similar ecosystems.δ13C values for DOC in soil solution andstreamwater were consistent with soil and terrestrialvegetation, indicating that the terrestrial ecosystemis the dominant source of aquatic DOC in thiswatershed. δ13C values of soil atmosphereCO2 (−17.2 to −25.2‰) were slightly lessnegative than would be expected for production viaaerobic soil microbial decomposition and rootrespiration. There was a close correspondence betweenδ13C values (−15.5 to −21.5‰) forstreamwater TIC and soil atmospheric CO2 in thecentral part of the catchment where the stream drainsCO2-rich peats. 14C dating showed thatalthough peat has been accumulating in the watershedfor at least 2700 years, DOC in soil pore water andstreamwater contains carbon of predominantly recentorigin (post-AD 1955).

Impact of Global Warming on Soil Organic Carbon

Soils contain a stock of carbon that is about twice as large as that in the atmosphere and about three times that in vegetation. Small losses from this large pool could have significant impacts on future atmospheric carbon dioxide concentrations, so the response of soils to global warming is of critical importance when assessing climate carbon cycle feedbacks. Models that have coupled climate and carbon cycles show a large divergence in the size of the predicted biospheric feedback to the atmosphere. Central questions that still remain when attempting to reduce this uncertainty in the response of soils to global warming are (1) the temperature sensitivity of soil organic matter, especially the more recalcitrant pools; (2) the balance between increased carbon inputs to the soil from increased production and increased losses due to increased rates of decomposition; and (3) interactions between global warming and other aspects of global change, including other climatic effects (e.g., changes in water balance), changes in atmospheric composition (e.g., increasing atmospheric carbon dioxide concentration) and land‐use change. In this chapter, we review trends in warming, factors affecting the response of soil carbon to global warming, evidence on the balance between changes in production and soil organic matter decomposition, recent research on the temperature sensitivity of soil organic carbon pools, methods for measuring soil responses to global warming, approaches to modeling soil responses to global warming, regions/ecosystems likely to be most vulnerable to future warming, and available technologies to reduce vulnerability of soil carbon to the impacts of future global warming.

Is in-stream processing an important control on spatial changes in carbon fluxes in headwater catchments?

Data on small-scale spatial variations in instantaneous fluxes and concentrations of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and free carbon dioxide (CO2) are presented for a small acidic headwater stream in NE Scotland. Chloride is used as a conservative element to estimate additional, diffuse inputs of water into the main stem of the stream, other than those from tributaries. Downstream changes in instantaneous carbon fluxes were calculated and then used to estimate losses and gains of carbon within the stream system. Dissolved organic carbon concentrations in the stream ranged from 1.19-6.06 mg l(-1) at its source to a maximum of 10.0-25.3 mg l(-1) as the stream passed through deep peats; DOC concentrations then declined in the lower part of the catchment. DIC concentrations were initially low, increased to 1.5-3.0 mg l(-1) and then decreased to 0.1-1.65 mg l(-1) at the lowest site. Free CO2 concentrations increased from 0.35 mg l(-1) at the stream source to 3.30 mg l(-1) as the stream passed through the peat dominated area. Continually high inputs of CO2-rich water (> 6.0 mg l(-1)) from tributaries maintained these high concentrations in the main stem, until approximately 1.74 km downstream, when there was a rapid decline in concentration. Significant changes in DOC, DIC and CO2 fluxes occur over a distance of 2.7 km downstream from the stream source to the catchment outlet. Between 5.64-41.5 mg C s(-1) as DOC and 2.52-16.2 mg C s(-1) as DIC are removed from the water column. Between 6.81 and 19.0 mg C s(-1) as CO2 is lost along the stream length as progressive equilibration with the atmosphere occurs. We estimate that 11.6-17.6% of the total DOC flux is removed from streamwater by in-stream processes. Dissolved inorganic carbon (HCO3- and free CO2) losses are in excess of nine times its measured flux at the outlet of the catchment. These results suggest that in-stream processing of DOC and DIC and outgassing of CO2 are important controls on the spatial variability of carbon fluxes within headwater streams in upland catchments dominated by organic-rich soils.

A method for measuring free CO2 in upland streamwater using headspace analysis

Abstract Existing titration-based methods for the measurement of dissolved free CO 2 are indirect and require the measurement of a number of other determinands (e.g. pH); they may underestimate free CO 2 concentrations, because analysis is carried out frequently in an open vessel from which some free CO 2 may be lost prior to measurement. Here, a method of headspace analysis is described; this minimises CO 2 loss and provides a more direct technique for determining free CO 2 in low ionic strength, organic-rich upland streamwaters. A sample of streamwater is collected in a sealed flask and a headspace is created by pumping out a known volume of sample, replacing it with CO 2 -scrubbed air. After equilibration of CO 2 between the remaining water and the headspace, the concentration of CO 2 in the headspace is measured using an Infra Red Gas Analyser. The concentration of free CO 2 in the original sample is then calculated using Henrys law. This method measured free CO 2 in standard solutions containing 1–10 mg l −1 free CO 2 to within 0.1 mg l −1 . The method was used to measure free CO 2 in streamwater from 19 sites on the River Dee in north-east Scotland and the results were compared with those reported for streams elsewhere. Free CO 2 concentrations measured by headspace analysis were significantly higher than those found using acidimetric titration.

Is the composition of dissolved organic carbon changing in upland acidic streams

The quantity and composition of dissolved organic carbon (DOC) exported from upland soils to surface waters is a key link in the global carbon cycle and economically important for treating potable waters. The relationship between ultraviolet (UV) absorbance and DOC concentrations can be used to infer changes in the proportion of hydrophobic (aromatic, recalcitrant) carbon and hence biodegradability of DOC. This study describes a significant change in the relationship between UV absorbance and DOC over 22 years at two upland moorland catchments in Scotland, UK. Despite increases in long-term DOC concentrations, analysis suggests that the proportion of hydrophobic material has declined. A statistical mixed-effect modeling approach was used to examine the likely mechanisms that could explain these observations. Annual nonmarine sulfate load was the only significant forcing factor that could explain the observed long-term trend in the UV absorbance-DOC relationship at both sites. It is hypothesized that enhanced heterotrophic decomposition of organic matter and increased solubility of carbon compounds in soils where sulfate driven acidification is being reversed are the dominant mechanisms behind this change in DOC composition. These trends will impact on carbon substrate dynamics by potentially increasing biodegradability of exported organic matter, influencing carbon cycling in terrestrial and aquatic ecosystems.

Application of luminescent biosensors for monitoring the degradation and toxicity of BTEX compounds in soils.

Aims:  To assess the changes in acute toxicity and biodegradation of benzene, toluene, ethylbenzene and xylene (collectively referred to as BTEX) compounds in soil over time and compare the performances of biological and chemical techniques.

Characterizing Pb mobilization from upland soils to streams using 206Pb/207Pb isotopic ratios.

Anthropogenically deposited lead (Pb) binds efficiently to soil organic matter, which can be mobilized through hydrologically mediated mechanisms, with implications for ecological and potable quality of receiving waters. Lead isotopic ((206)Pb/(207)Pb) ratios change down peat profiles as a consequence of long-term temporal variation in depositional sources, each with distinctive isotopic signatures. This study characterizes differential Pb transport mechanisms from deposition to streams at two small catchments with contrasting soil types in upland Wales, U.K., by determining Pb concentrations and (206)Pb/(207)Pb ratios from soil core profiles, interstitial pore waters, and stream water. Hydrological characteristics of soils are instrumental in determining the location in soil profiles of exported Pb and hence concentration and (206)Pb/(207)Pb ratios in surface waters. The highest Pb concentrations from near-surface soils are mobilized, concomitant with high dissolved organic carbon (DOC) exports, from hydrologically responsive peat soils with preferential shallow subsurface flows, leading to increased Pb concentrations in stream water and isotopic signatures more closely resembling recently deposited Pb. In more minerogenic soils, percolation of water allows Pb, bound to DOC, to be retained in mineral horizons and combined with other groundwater sources, resulting in Pb being transported from throughout the profile with a more geogenic isotopic signature. This study shows that (206)Pb/(207)Pb ratios can enhance our understanding of the provenances and transport mechanisms of Pb and potentially organic matter within upland soils.

Comparative evaluation of a bioluminescent bacterial assay in terrestrial ecotoxicity testing

Despite the widespread and successful use of luminescence-based bioassays in water testing, their applications to soils and sediments is less proven. In part this is because such bioassays have mainly been carried out in an aqueous-based medium and, as such, favour contaminants that are readily water-soluble. In this study, aqueous solutions and soils contaminated with heavy metals (HM), polar organic contaminants and hydrophobic organic contaminants (HOCs) were tested using a range of luminescence-based bioassays (Vibrio fischeri, Escherichia coli HB101 pUCD607 and Pseudomonas fluorescens 10586r pUCD607). For the first two chemical groups, the assays were highly reproducible when optimised extraction procedures were employed but for HOCs the bioassay response was poor. Quantitative structure-activity relationships (QSARs) obtained from aqueous solutions had a linear response although correlation for the chemicals tested using bacterial bioassays was significantly less sensitive than that of sublethal tests for Tetrahymena pyriformis. Bacterial and Dendrobaena veneta bioassay responses to extracts from HM amended soils showed that a clear relationship between trophic levels could be obtained. There is no doubt that the wide range of bioluminescent-based bioassays offers complementary applications to traditional testing techniques but there is a significant need to justify and optimise the extraction protocol prior to application.

The biogeochemical reactivity of suspended particulate matter at nested sites in the Dee basin, NE Scotland

Variation in the organic matter content associated with suspended particulate matter (SPM) is an often overlooked component of carbon cycling within freshwater riverine systems. The potential biogeochemical reactivity of particulate organic carbon (POC) that affect its interactions and fate, i.e. respired and lost to the atmosphere along river continua or ultimately exported to estuarine and oceanic pools was assessed. Eleven contrasting sites draining nested catchments (5-1837 km(2)) in the River Dee basin, NE Scotland were sampled during summer 2008 to evaluate spatio-temporal variations in quantity and quality (biogeochemical reactivity) of SPM during relatively low flow conditions. Mean SPM concentrations increased from 0.21 to 1.22 mg L(-1) between the uppermost and lowest mainstem sites. Individually, POC concentrations ranged from 0.08 to 0.55 mg L(-1) and accounted for ca. 3-15% of total aqueous organic carbon transported. The POC content was partitioned into autotrophic (2.78-73.0 mg C g(-1) SPM) and detrital (119-388 mg C g(-1) SPM) biomass carbon content. The particulate respired CO(2)-C as a % of the total carbon associated with SPM, measured by MicroResp™ over 18 h, varied in recalcitrance from 0.49% at peat-dominated sites to 3.20% at the lowermost mainstem site. Significant (p<0.05) relationships were observed between SPM biogeochemical reactivity measures (% respired CO(2)-C; chlorophyll α; bioavailable-phosphorus) and arable and improved grassland area, associated with increasing biological productivity downstream. Compositional characteristics and in-stream processing of SPM appear to be related to contributory land use pressures, that influence SPM characteristics and biogeochemistry (C:N:P stoichiometry) of its surrounding aqueous environment. As moorland influences declined, nutrient inputs from arable and improved grasslands increasingly affected the biogeochemical content and reactivity of both dissolved and particulate matter. This increases the potential for recycling of the organic matter that is either transported from upstream or entering further along the riverine continuum.