Crystallography of the hexagonal ferrites

The hexagonal ferrites form an unusual group of complex, ferrimagnetic oxides embodying some 60 known crystal structures. These include phases for which the structural unit cell is larger than that in any known inorganic materials. The various hexagonal ferrite modifications fall into two distinct structural series, each formed by the ordered interlayering (stacking) of two discrete building blocks; these blocks stack along the c crystallographic axis in varying ratios and varying permutations to form strictly coherent, reproducible crystal structures. This mixed-layering aspect of the hexagonal ferrites permits direct, visual observation of the sequence of their subunit-cell stacking elements, after etching, by means of electron microscopy. The sequence of stacked blocks in such structures constitutes the only information lacking for a complete, three-dimensional structure determination. Direct access to this information provides an immediate, unique solution of the crystal structure problem in each case and thereby avoids the dilemmas of a classical diffraction approach to such large unit cells. Ferrite structures with hexagonal c dimensions of 1455 and 1577 angstroms have been uniquely solved by direct electron microscopic readout of surface etch features. One must exercise caution, however, in generalizing these findings to other materials. The method is successful in the case of the hexagonal ferrites because these are mixed-layer structures, wherein the building blocks react at different rates to a specific etchant. Mixed-layer systems are not uncommon in crystallography, and it is likely that similar techniques can be developed for other such materials. Regardless of the validity of this prognosis, however, it is quite evident that high-resolution replica electron microscopy is a most promising tool for the direct observation of surface structure on an ultramicro scale. During the studies reported here replica resolution capability was improved to about 10 angstroms; final resolution is limited by the particle size of the platinum shadowing material. Careful control of experimental conditions during replica preparation or an alternate choice of shadowing material, or both, might reasonably improve the resolution by a factor of 2. This resolution is within the range of most unit cell dimensions and approaches interatomic distances in solid-state materials. The potential of such an experimental capability needs no elaboration.     

Effects of mustard residues on the carbon and nitrogen turnover in undisturbed soil microcosms

The C- and N-turnover of undisturbed, irrigated 25 cm soil microcosms of a Typic Agrudalf from loess were followed during a laboratory incubation of about 200 days. Besides the untreated control columns two treatments were installed to test the effects of the application of white mustard (Sinapis alba L.) on both nutrient cycles (either mixing the dried plant residues to 0–8 cm or surface application). Each of the treatments is represented by 4 soil columns to account for soil variability. The C-budget was mainly altered for soil respiration, which increased for the microcosms treated with mustard rapidly after application. After 4 weeks no differences could be proved however. The loss of soluble C-compounds, which was unimportant for the budget, reflected no one of the treatments with mustard. Net N mineralization, as calculated from the NO₃-leaching, could be well fitted by zero-order kinetics. Although the high variability of the net N mineralization of the replicates mustard application significantly rises the N-release. No difference could be proved for the different techniques of application however. In the mixed treatment 38% of the nitrogen added with the mustard was decomposed during the incubation period, in the surface treatment 34%.     

An architectural framework that may lie at the core of the postsynaptic density

The postsynaptic density (PSD) is a complex assembly of proteins associated with the postsynaptic membrane that organizes neurotransmitter receptors, signaling pathways, and regulatory elements within a cytoskeletal matrix. Here we show that the sterile alpha motif domain of rat Shank3/ProSAP2, a master scaffolding protein located deep within the PSD, can form large sheets composed of helical fibers stacked side by side. Zn2+, which is found in high concentrations in the PSD, binds tightly to Shank3 and may regulate assembly. Sheets of the Shank protein could form a platform for the construction of the PSD complex.     

Variation of serum inorganic phosphorus and association with haemoglobinuria and osteomalacia in female water buffaloes in Pakistan

Data from an animal health service in the Punjab of Pakistan showed that 39 adult female buffaloes with haemoglobinuria were 21 times more likely to have serum inorganic phosphorus (serum P) levels < 0.97 mmol/1 than 24 controls sampled during the period of case occurrence (December 1984–March 1985). Age > 7 years or early lactation (1–60 days post partum) were unrelated to the disease. Similarly, symptoms of osteomalacia in 19 multiparous buffaloes were associated with low P (OR = 14.3) but not with age. Subsequently, a serum survey was carried out from February 1985 to July 1987 to investigate serum P variations with season and host factors. Data from 139 farms (426 adult female buffaloes, 468 lactations) indicated strong farm and seasonal effects on serum P. Serum P declined during the study period and was lowest during December–March 1985/1986 and again 1986/1987. Calving season, parity > 1, high pregnancy > 6 months, or daily milk production were not related to serum P in the final model. Seasonal effects were interpreted as soil borne and related to feed changes from maize to berseem in December.     

Effect of fungicides on the physiology of plants. Part IV: Protection of cellular organelles of senescent wheat leaves by carbendazim

Wheat leaves (cv. Kormoran) floated on water and kept in darkness turned yellow. This induced senescence was accompanied by a loss of electrolytes and amino acids, and by disorganisation of all cellular organelles except cell walls. Treatment with a solution of carbendazim (20 μg ml^?1) prevented the leakage of electrolytes and disorganisation of cell organelles. At 100 μg ml^?1, the fungicide did not prevent the damage caused by senescence but stimulated the loss of electrolytes from the leaves. It is proposed that one of the major mechanisms of the antisenescent activity of carbendazim is its protective effect on membranes.     

Application of the DNDC model to predict N2O emissions from sandy arable soils with differing fertilization in a long‐term experiment

Modeling crop growth and soil N dynamics is difficult due to the complex nature of soil–plant systems. In several studies, the DNDC model has been claimed to be well‐suited for this purpose whereas in other studies applications of the model were less successful. Objectives of this study were to test a calibration and validation scheme for DNDC‐model applications to describe a field experiment with spring wheat on a sandy soil near Darmstadt (SW Germany) using different fertilizer types (either application of mineral fertilizer and straw—MSI; or application of farmyard manure—FYM) and rates (low—MSI_L, FYM_L; and medium—MSI_M, FYM_M). The model test is based on a model parameterization to best describe the case MSI_L and applies this parameterization for a retrospective simulation of the other cases (MSI_M, FYM_L, FYM_M) including crop growth and N₂O emissions. Soil water contents were not accurately simulated using either the DNDC default values for a loamy sand or for the next finer texture class or using results from the pedotransfer function provided by ROSETTA. After successful calibration of the soil water flow model using a soil texture class that led to the best fit of the measured water content data, grain yield of spring wheat and cumulative N₂O emission were slightly underestimated by DNDC and were between 91% and 86% of the measured data. A subsequent calibration of the yields and cumulative N₂O emissions from soils of the MSI_L treatment gave a good prediction of crop growth and N₂O emissions in the MSI_M treatment, but a marked underestimation of yields of the FYM treatments. Cumulative N₂O emissions were predicted well for all MSI and FYM treatments, but seasonal dynamics were not. Overall, our results indicated that for the sandy soil in Germany, site‐specific calibration was essentially required for the soil hydrology and that a calibration was useful for a subsequent prediction where greater amounts of the same fertilizer were used, but not useful for a prediction with a different fertilizer type.