Effects of agricultural land use on fluvial carbon dioxide, methane and nitrous oxide concentrations in a large European river, the Meuse (Belgium).

We report a data-set of CO2, CH4, and N2O concentrations in the surface waters of the Meuse river network in Belgium, obtained during four surveys covering 50 stations (summer 2013 and late winter 2013, 2014 and 2015), from yearly cycles in four rivers of variable size and catchment land cover, and from 111 groundwater samples. Surface waters of the Meuse river network were over-saturated in CO2, CH4, N2O with respect to atmospheric equilibrium, acting as sources of these greenhouse gases to the atmosphere, although the dissolved gases also showed marked seasonal and spatial variations. Seasonal variations were related to changes in freshwater discharge following the hydrological cycle, with highest concentrations of CO2, CH4, N2O during low water owing to a longer water residence time and lower currents (i.e. lower gas transfer velocities), both contributing to the accumulation of gases in the water column, combined with higher temperatures favourable to microbial processes. Inter-annual differences of discharge also led to differences in CH4 and N2O that were higher in years with prolonged low water periods. Spatial variations were mostly due to differences in land cover over the catchments, with systems dominated by agriculture (croplands and pastures) having higher CO2, CH4, N2O levels than forested systems. This seemed to be related to higher levels of dissolved and particulate organic matter, as well as dissolved inorganic nitrogen in agriculture dominated systems compared to forested ones. Groundwater had very low CH4 concentrations in the shallow and unconfined aquifers (mostly fractured limestones) of the Meuse basin, hence, should not contribute significantly to the high CH4 levels in surface riverine waters. Owing to high dissolved concentrations, groundwater could potentially transfer important quantities of CO2 and N2O to surface waters of the Meuse basin, although this hypothesis remains to be tested.

Stable isotope analysis of dissolved organic carbon in soil solutions using a catalytic combustion total organic carbon analyzer-isotope ratio mass spectrometer with a cryofocusing interface.

Stable carbon isotopes are a powerful tool to assess the origin and dynamics of carbon in soils. However, direct analysis of the (13)C/(12)C ratio in the dissolved organic carbon (DOC) pool has proved to be difficult. Recently, several systems have been developed to measure isotope ratios in DOC by coupling a total organic carbon (TOC) analyzer with an isotope ratio mass spectrometer. However these systems were designed for the analysis of fresh and marine water and no results for soil solutions or (13)C-enriched samples have been reported. Because we mainly deal with soil solutions in which the difficult to oxidize humic and fulvic acids are the predominant carbon-containing components, we preferred to use thermal catalytic oxidation to convert DOC into CO(2). We therefore coupled a high-temperature combustion TOC analyzer with an isotope ratio mass spectrometer, by trapping and focusing the CO(2) cryogenically between the instruments. The analytical performance was tested by measuring solutions of compounds varying in the ease with which they can be oxidized. Samples with DOC concentrations between 1 and 100 mg C/L could be analyzed with good precision (standard deviation (SD) < or = 0.6 per thousand), acceptable accuracy, good linearity (overall SD = 1 per thousand) and without significant memory effects. In a (13)C-tracer experiment, we observed that mixing plant residues with soil caused a release of plant-derived DOC, which was degraded or sorbed during incubation. Based on these results, we are confident that this approach can become a relatively simple alternative method for the measurement of the (13)C/(12)C ratio of DOC in soil solutions.

Evalution of sequential extractions on dry and wet sediments.

A five-step sequential extraction procedure was applied on dried and wet Ballastplaat Scheldt estuary sediments. When wet (fresh) sediments were used, all sample handling up to the 3rd extraction step, inclusive, was carried out under inert atmosphere. The repeatability of the procedure was very good on dry samples. For Fe as for Mn, RSD values are lower than 4%, except for Mn in the fifth extraction step where a spread of 10% is observed. The observed RSDs for Pb are of the same order of magnitude as those for Mn. On wet samples the spread of the results is higher than on dried ones. The highest RSDs observed for Fe amount to 20%, for Mn to 15% but for Pb an RSD of up to 44% was found. Better homogenization of the solid sediment part of lyophilized sediments and different porosities of wet sediment sub-samples may be the explanation. These results also indicated that drying/oxidizing of the sediment sample causes a shift from less available/mobile metal fractions to more available/mobile fractions. The Mn and Fe oxyhydroxide spikes added to a wet sediment sample were recovered between 100+/-10%. The results obtained after changing the sequence of the extraction steps (multiple rotations and inversions were tested) corroborated the progressive increase in the aggressive nature of the extraction solutions in our standard scheme. Although there is also no need to change the ratio volume of extractant to amount of sediment, increasing the number of extraction repetitions in steps 1 to 3 resulted, for some of those extraction steps, in a partially modified analyte distribution. Finally the method was applied to sandy and muddy sediment cores of the Scheldt estuary and revealed clear differences between metal distributions in both types of sediment.

Primary producers sustaining macro-invertebrate communities in intertidal mangrove forests.

In contrast to the large number of studies on the trophic significance of mangrove primary production to the aquatic foodweb, there have been few attempts to provide an overview of the relative importance of different primary carbon sources to invertebrates in the intertidal mangrove habitats. We determined carbon and nitrogen stable isotope ratios (δ13C, δ15N) in sediments, primary producers, and 22 invertebrate species from an intertidal mangrove forest located along the southeast coast of India in order to determine the contribution of mangrove leaf litter and other carbon sources to the invertebrate community. Organic matter in sediments under the mangrove vegetation was characterized by relatively high δ13C values and low C:N ratios, indicating that mangrove-derived organic matter was not the principal source and that imported phytodetritus from the mangrove creeks and adjacent bay significantly contributed to the sediment organic matter pool. Invertebrates were found to display a wide range of δ13C values, most being 3-11‰ enriched relative to the average mangrove leaf signal. The pulmonate gastropod Onchidium sp. showed unusually low δ15N values (-5.6±0.9‰), but further work is needed to adequately explain these data. A compilation of stable isotope data from various sources indicates that significant assimilation of mangrove-derived carbon is only detectable in a limited number of species, and suggests that local and imported algal sources are a major source of carbon for benthic invertebrate communities in intertidal mangrove forests. These results provide new insights into carbon utilization patterns in vegetated tropical intertidal habitats and show a striking similarity with results from temperate salt marsh ecosystems where local plant production has often been found to contribute little to intertidal foodwebs.

Glucocorticoid-induced osteoporosis: is the bone density decrease the only explanation?

BACKGROUND: Glucocorticoids may increase bone fragility via mechanisms independent from their bone mass reducing effect. OBJECTIVE: To study relationships between osteoporotic fractures and bone mineral density in patients on long-term glucocorticoid therapy. PATIENTS AND METHODS: We studied 121 women with a mean age of 60.4 +/- 14.3 years on long-term glucocorticoid therapy (cumulative dose > or = 1 g of prednisone equivalent, duration > or = 6 months) for rheumatoid arthritis (n = 38), polymyalgia rheumatica or giant cell arteritis (n = 26), connective tissue disease (n = 15), asthma (n = 14), another inflammatory joint disease (n = 14), or another condition (n = 14). The control group was composed of 125 subjects who had the same mean age and met the same exclusion criteria as the case group. Bone mineral density was measured at the lumbar spine and femoral neck using a Hologic QDR 4500 unit. In subjects with back pain, radiographs of the thoracic and lumbar spine were obtained to look for fractures. RESULTS: The odds ratio for a bone mineral density decrease of one standard deviation at the femoral neck was 1.68 (1.20-2.35) in patients with a cumulative glucocorticoid dose of 10 g of prednisone equivalent and 1.67 (1.22-2.29) in those with a glucocorticoid therapy duration of 2 years. Sixty-eight fractures were recorded in 56 patients (46% of the overall patient group). Even after adjustment on age, glucocorticoid therapy duration, and dose, mean bone mineral density values at the lumbar spine and femoral neck were significantly lower in the subgroup of patients with fractures than in the subgroup without fractures. Sensitivity and specificity of bone mineral density at the femoral neck and/or lumbar spine for the diagnosis of vertebral fracture and/or peripheral fracture were 73% and 51%, respectively. In the stepwise logistic regression model, factors explaining the presence of fractures were as follows, in hierarchical order: age; absence of calcium/vitamin D supplementation, femoral neck T-score, and glucocorticoid dose. CONCLUSION: Our data are compelling evidence that bone mineral density is a major determinant of the fracture risk in patients with glucocorticoid-induced osteoporosis.

Estimating spatial and seasonal phytoplankton delta13C variations in an estuarine mangrove ecosystem.

In estuarine ecosystems, large spatial and seasonal variations in delta13C values of primary producers can occur, and knowledge of these variations may be crucial when interpreting stable isotope data of higher trophic levels. Obtaining clean phytoplankton samples for isotope analysis is usually impossible in such systems, and analysis of total suspended matter is not a simple proxy for phytoplankton delta13C variations. Based on a few simple assumptions regarding the C and N content of the two end-members (terrestrial detritus and phytoplankton) and the delta13C of the terrestrial component, we here present a simple model to estimate the phytoplankton delta13C variations using an existing dataset on the delta13C and elemental (C:N) composition of suspended organic matter from an estuarine mangrove ecosystem in southeast India. These variations are related to the monthly rainfall pattern during the sampling period. It is stressed that this method estimates approximate phytoplankton delta13C values, which should not be used in e.g., mixing models. However, we propose that in cases where sufficiently large datasets are available, the described procedure could provide a valuable method to semi-quantitatively estimate the seasonal or spatial variations of the phytoplankton delta13C signal.