Modelling the biogeochemical cycle of silicon in soils: Application to a temperate forest ecosystem

We investigated the biogeochemical cycling of silicon (Si) in an acidic brown soil covered by a coniferous forest (Douglas fir). Based on published and original data, we constructed a conceptual model and used a modified version of the reactive transport code MIN3P for model testing and quantification purposes. The model was first calibrated and further validated with respect to biomass data and Si-concentrations in capillary solutions, which were collected monthly over several years by means of suction-cup lysimeters placed at different soil depths.Following sensitivity tests, the model was calibrated quite accurately (limited to a 10% concentration error) by the adjustment of kinetic constants, longitudinal dispersion, and apparent activation energy for K-feldspar dissolution. Calibrated parameter values were constrained by ranges reported in the literature, when available. Mass balance calculations indicate that an average of 60% of the biogeochemical cycle of Si was controlled by biological processes (i.e. Si-uptake and dissolution of phytoliths). Sensitivity analyses suggest that no more than 55% of the Si-cycle is controlled by weathering of primary silicates. Such a large contribution of biological turnover to Si-cycling may be explained by the combined effects of a relatively large Si-content in the litter fall (i.e. specifically in the needles) and high biomass productivity of the coniferous species considered. In addition to potential implications for the global Si cycle, this investigation raises several fundamental questions concerning the nature of Si-uptake mechanisms and physiological use of Si by trees in natural systems.     

Input of Nutritionally Rich Organic Matter from the Mississippi River to the Louisiana Coastal Zone

Isotopes have often been used to discern riverine subsidies to coastal food chains, but there are few direct measurements of nutritional quality of riverine particulates. We tested for nutritionally enriched organic matter in the Mississippi River suspended sediment and evidence for its delivery to Louisiana coastal sediments by measuring enzymatically hydrolysable amino acids (EHAA). Riverine suspended sediments contained EHAA concentrations of up to 5 mg g^?1, higher than reported in any coastal sediment. Pigment concentrations indicated that EHAA in some river samples were dominated by phytoplankton, but many samples contained significant non-algal EHAA. Coastal sediments showed EHAA concentrations lower than riverine sediments but still higher than most reported shelf values. Incubation of riverine sediment showed losses of 28–34% of their EHAA over 6 days, similar to differences found between riverine and coastal sediments. EHAA concentrations decreased more rapidly than total nitrogen, indicating the relative lability of this pool of material in the studied region. These EHAA-enriched materials provide fuel for various coastal biota whose composition likely depends on factors such as disturbance regimes.     

Evaluating nitrate uptake in a Rocky Mountain stream using labelled 15N and ambient nitrate chemistry

Background aqueous chemistry and ¹⁵N_nitrate tracer injection methods were used to calculate in‐stream nitrate uptake metrics at Red Canyon Creek, a third‐order stream in the Rocky Mountains in the state of Wyoming, United States. ‘Net’ nitrate uptake lengths, which reflect both nitrate uptake and regeneration, and ‘gross’ nitrate uptake lengths, which exclude re‐mineralization, were quantified separately from background nitrate chemistry and ¹⁵N labelling tracer data, respectively. Gross nitrate uptake lengths, from tracer injections of ¹⁵N labelled nitrate, ranged from 502 to 3140 m. Net nitrate uptake lengths, from background nitrate chemistry downstream of a point source, ranged from 1170 to 4330 m. Diurnal changes in uptake lengths suggest the importance of nitrate utilization by autotrophs in the stream and benthic zone. The differences between net and gross nitrate uptake lengths along lower reaches of Red Canyon Creek allowed us to estimate the nitrate regeneration rate, which was 0·056–0·080 µmol m^?2 s^?1 during the day and 0·0062–0·0083 µmol m^?2 s^?1 at night. Spatial patterns of streambed pore water chemistry indicate those areas of the hyporheic zone where denitrification was likely occurring. Permanent log dams generated stronger redox gradients in the hyporheic zone than areas with transient beaver dams. By combining isotopically labelled nitrate additions, estimates of uptake from background aqueous nitrate chemistry and characterization of redox conditions in the hyporheic zone, we were able to determine the nitrate regeneration rate and the redox processes responsible for nitrogen cycling in the hyporheic zone. Copyright © 2010 John Wiley & Sons, Ltd.     

Evaluation of Seasonal Factors on Petroleum Hydrocarbon Vapor Biodegradation and Intrusion Potential in a Cold Climate

A detailed seasonal study of soil vapor intrusion at a cold climate site with average yearly temperature of 1.9 °C was conducted at a house with a crawlspace that overlay a shallow dissolved‐phase petroleum hydrocarbon (gasoline) plume in North Battleford, Saskatchewan, Canada. This research was conducted primarily to assess if winter conditions, including snow/frost cover, and cold soil temperatures, influence aerobic biodegradation of petroleum vapors in soil and the potential for vapor intrusion. Continuous time‐series data for oxygen, pressure differentials, soil temperature, soil moisture, and weather conditions were collected from a high‐resolution monitoring network. Seasonal monitoring of groundwater, soil vapor, crawlspace air, and indoor air was also undertaken. Petroleum hydrocarbon vapor attenuation and biodegradation rates were not significantly reduced during low temperature winter months and there was no evidence for a significant capping effect of snow or frost cover that would limit oxygen ingress from the atmosphere. In the residual light nonaqueous phase liquid (LNAPL) source area adjacent to the house, evidence for biodegradation included rapid attenuation of hydrocarbon vapor concentrations over a vertical interval of approximately 0.9 m, and a corresponding decrease in oxygen to less than 1.5% v/v. In comparison, hydrocarbon vapor concentrations above the dissolved plume and below the house were much lower and decreased sharply within a few tens of centimeters above the groundwater source. Corresponding oxygen concentrations in soil gas were at least 10% v/v. A reactive transport model (MIN3P‐DUSTY) was initially calibrated to data from vertical profiles at the site to obtain biodegradation rates, and then used to simulate the observed soil vapor distribution. The calibrated model indicated that soil vapor transport was dominated by diffusion and aerobic biodegradation, and that crawlspace pressures and soil gas advection had little influence on soil vapor concentrations.     

Sensitivity of subglacial Lake Vostok's flow regime on environmental parameters

Subglacial lakes provide unique habitats, but the exact nature of physical and geochemical conditions are still a matter of debate and await direct sampling of water. Due to its isolation from external atmospheric forcing other environmental parameters influence the flow characteristics within the lake. In this study we use an improved treatment of the physical processes at the ice–water boundary interface to identify and quantify the impact of (1) the geothermal heat flux, (2) the heat flux from the lake into the ice, (3) the influence of the salinity of the lake water, and (4) the ice thickness on the size of the freezing area and the freeze/melt rates. We show that the modelled basal mass imbalance (that is the produced melt water minus the re-frozen water) depends on the geothermal heating as well as the heat flux into the ice. The circulation and the temperature distribution within subglacial Lake Vostok are rather stable against variations of geothermal heat flux, heat flux into the ice sheet, salinity of the lake, and small changes of the ice thickness above the lake. However, the flow regime for any subglacial lake with less than 2000 m ice thickness above, will be substantially different from those that experience higher pressures. This is because the buoyancy–temperature relationship reverses at this depth.     

Response of bed morphology and bed material texture to hydrological conditions at an asymmetrical stream confluence

Changes in bed morphology and sedimentology at stream confluences in relation to specific hydrological events have not been documented extensively. Moreover, few, if any, studies have investigated the effect of net hydrological conditions on channel change at a confluence over a period of many years. Repeat cross section surveys and sampling of bed material at a small, asymmetrical stream confluence reveal the short-term effects of hydrological variability on bed morphology and spatial patterns of bed material texture. Survey data over a 15-year period illustrate the long-term response of channel morphology at the confluence to net hydrological conditions. Results confirm that this confluence is a dynamic fluvial environment, involving shifts in the zone of scour and repeated deposition and erosion of a bar complex at the downstream junction corner as the discharge ratio changes in response to hydrological variability in the tributary streams. Persistent faster runoff-response times of the lateral tributary compared to the main stem have resulted in long-term net erosion of the channel bank opposite this tributary. The documented morphological and sedimentological changes are consistent with and can be explained by results of experimental studies and by past investigations at this confluence that have documented changes in patterns of mean flow structure and turbulence in response to variation in the discharge ratio of contributing streams.     

Spatio-temporal variability of moisture conditions within the Urban Canopy Layer

Summary ¶Re-analysed data from an urban climate research project in Munich, Germany, were used to investigate the spatio-temporal variability of moisture conditions (expressed here in vapour pressure VP) within the Urban Canopy Layer UCL. The results, which apply to three main sites and additional subsidiary ones, cover both summer and winter months. The summer month variation of VP is characterised by higher monthly mean values of VP for all three sites, howbeit with considerable inter-site differences. The temporal variability of mean VP values at diurnal time scales is also examined. With respect to the UCL, they reveal different amplitudes and times of occurrence of their extreme values. In addition, results of car traverses performed during clear sky conditions in downtown Munich show a remarkable small-scale spatio-temporal variability of VP.In relation to a sealed downtown site within a courtyard in Munich, a time-dependent urban moisture excess (UME) was formed. A positive correlation between UME and the urban heat island (UHI) could be verified in general. However, it was slightly negative with a very low coefficient of determination in the summer month when the maximum UME preceded the maximum UHI up to 5hrs. As example for the effects of air moisture on the urban climate within the UCL, the role of VP on a thermal index (physiologically equivalent temperature PET) was investigated. Based on one-year data from another urban climate project in Munich, a positive correlation between PET and VP was found, although the coefficient of determination was somewhat low. However, during a human-biometeorological case study on a typical summer day in the northern downtown of Freiburg, a medium-sized city in southwest Germany, PET and VP showed a negative correlation (possibly because the specific temporal course of VP at the measuring points was mainly influenced by thermally induced turbulence).     

Experiment on Obtaining Mercury's Images by the Short Exposure Method

December 1–3, 1999, observations of the planet Mercury were carried out at the Abastumani Astrophysical Observatory of the Republic of Georgia by the short exposure method with the aid of a charge-coupled device (CCD) camera. The materials of these observations are presented in this paper. It is shown that the reduction of the exposure down to 10 ms eliminates the image blurring caused by the atmospheric instability and considerably improves resolution. As regards the image distortions, they can be eliminated only by selecting acceptable images from a sufficiently large number of pictures obtained. The short exposure method allows one to obtain new results from the ground-based observations of Mercury.