Nitrate enhancement of nitrification depth in sediment/water microcosms

Streams draining agricultural catchments in loward England commonly have high nitrate concentrations. Laboratory microcosms were used to study nitrogen transformations in the sediment from such a stream. Solutions of either 0.5 mmol dm⁻³ calcium nitrate or calcium chloride (control) were passed once only over the sediment surface. After 5 days incubation, Eh in the nitrate treatment stabilized at approximately 500 mV at 5 mm below the water/sediment interface, and 285 mV at 10 mmn, whereas in the chloride treatment Eh was −200 mV to −250 mV at all measured depths (5 mm and below). Inorganic N mesurements of the seidment profiles showed that much more ammonium-N was being lost from the nitrate-treated sediment that the control. It was hypothesized that, when nitrate was present in the streamwater, the combined processed of nitrate diffusion and denitrification progressively consumed organic matter, thereby allowing dissolved oxygen to diffuse further into the sediment. A consequence of these processes would be a greater depth of nitrification and hence more ammounium loss. The plausibility of this hypothesis was evaluated by constructing a deterministic model which simulated the nitrate and ammonium profiles from the processes of diffusion, denitrification, nitrification, and ammonification. Kinetic parameters and diffusion impedance factor were determined from independent experiments. This calibrated model provided reasonable simulations of experimental data obtained from an additional experiment, which also showed that peaks in denitrification and nitrification activity were offset from each other by 2–3 mm, and that they migrated down the profile with increasing incubation time.     

High proper motion stars in Kapteyn Selected Area 94

We have applied the astrometric techniques devised by Murray for analysis of 48-inch Schmidt photographic data to SuperCOSMOS scans of UK Schmidt plates centred on Kapteyn's Selected Area 94 ( α =2^h 53^m, δ =0°). In this preliminary study, we combine astrometric data from four short-exposure V -band plates, taken in 1987 August (2 plates) and 1993 August (2 plates), with BVRI photometry from sky-limited plate material, to identify stars with proper motions exceeding 0.1 arcsec yr⁻¹. This paper discusses the completeness of the resulting sample and presents spectroscopy of 30 stars with μ >0.2 arcsec yr⁻¹. Based on the latter observations and the distribution in the [H _V , ( V − I )] reduced proper-motion diagram, we have classified stars in the complete sample as candidate white dwarfs, main-sequence dwarfs and halo subdwarfs, and derived estimates of the disc and halo luminosity functions.     

HISTORICAL SOURCES FOR GEOMORPHOLOGICAL RESEARCH IN THE UNITED STATES*

The use of historical sources in geomorphological analysis has had a long tradition in geography and is now undergoing a resurgence. A substantial body of data is available, and the search for it should extend from the National Archives to state and company records and county and city offices.     

Atomistic simulation of the structure and segregation to the (0001) and surfaces of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>

The segregation of ten isovalent impurities (Al³⁺ Cr³⁺, Eu³⁺, Gd³⁺, Ho³⁺, La³⁺, Lu³⁺, Nd³⁺, Tb³⁺, Y³⁺) to the and the (0001) surfaces of haematite (-Fe₂O₃) have been studied using atomistic simulation where the forces between the atoms are modelled using the Born model of solids. Segregation is found to be energetically favoured in virtually every case. The results for the surface show that the most favourable impurity surface concentration is 33.33%. The (0001) surface has two possible terminations, one terminated by iron atoms and the other by oxygen. No minimum is calculated for the Fe termination of the (0001) surface at low temperatures, but when the effect of raising the temperature is considered, an energy minimum is found, also at 33.33% impurity coverage. In contrast, the O terminated (0001) surface has a minimum in the segregation energy for between 16.67 and 33.33% depending on the cation being considered.     

Photometric solution for the eclipsing binary system AO Camelopardalis

UBV light curves of AO Cam were analyzed with Wilson and Devinney's model. The system was found to be a contact configuration with a degree of overcontact of 1.4%. The mass ratio of the system is about 1.30, and the inclination is 75 _. ⁰ 9. The primary eclipse appears to be an occultation. Thus AO Cam has the characteristics of a W-type W UMa system. Its location in a period-spectral type diagram suggests that the system may be a zero-age contact system.     

Molecular diffusion and the sedimentary environment: results from the in situ determination of whole sediment diffusion coefficients

A dialysis technique has been developed from which whole sediment diffusion coefficients that include the effect of microbially-mediated reactions can be determined in situ. Application of the technique shows that the diffusion of both dissolved sulphate and total dissolved CO₂ (transported primarily as bicarbonate) is related to the changing porosity of a fine-grained marine sediment. Added to this, the transport of dissolved sulphate is significantly affected by the microbial reduction of the pool available for diffusion. In contrast, the diffusion of a far larger pool of total dissolved CO₂ is not noticeably affected by microbial activity.