The mineralisation and fate of nitrogen following the incorporation of grass and grass-clover swards

Field and laboratory experiments were carried out between 1992 and 1994 to study the release and fate of nitrogen (N) following the incorporation of grass and grass- clover swards. The effects of sward type, quality of sward residue and previous grazing management on the N cycle were studied. Techniques for the measurement of mineralisation and nitrate (NO3") leaching on medium textured soils were also assessed.Initial work on the measurement of mineralisation using 1 p00j dilution techniques revealed major problems with practical application in the field. Mineral N release was therefore estimated by combining soil mineral N, NO3' leaching, plant uptake and gaseous N loss data.Evaluation of estimated leaching loads and specific tests comparing porous cups with three other estimates of soil water N0 3--N concentrations cast serious doubts on the quantitative accuracy of porous cups on medium textured soils. Results suggest that cups were sampling relatively immobile water and, depending on the source of the solute, this led to over- and under-estimates of leaching loads.The ploughing out of previously grazed swards released a substantial amount of N in the first eighteen months following incorporation (ca. 370 kg N ha-*), particularly during the first two months. Swards subject to an unfertilised cutting regime released much less N following ploughing. This was attributed to a reduction of soil organic matter (SOM) "degradability" and, to a lesser extent, plant residue N supply. Clover residues were found to have a notable effect on mineral N release only when sward clover contents were over 20% of total dry matter (DM) and grass residues were of much lower N content.The ploughing out of grassland produces a considerable short term increase in nitrous oxide (N2O) emissions thought to be initiated by the intimate mixing of readily available carbon sources into the soil during rotavation. The influence of clover residues on N2O emissions was greater than their influence on mineral N release patterns.Manipulation of the post-ploughing N cycle may be possible by altering sward management prior to ploughing. The cessation of grazing for a short period before sward incorporation could reduce mineral N release and N2O emissions. However, the capacity to reduce N2O emissions is constrained by weather conditions at critical times following incorporation

Mixing in numerical models of mantle convection incorporating plate kinematics

The process by which subducted lithosphere is mixed by mantle convection is investigated in numerical calculations. The results show that the observed isotopic heterogeneity of mantle sources and their ancient (1-2 b.y.) apparent ages are consistent with convective mixing. Passive tracers, which are introduced below "trenches," are efficiently dispersed, but nonetheless, heterogeneities in tracer density with a large range of length scales are observed to persist for 40 or more transit times (one transit time is the time to travel the fluid depth with the boundary velocity). In particular, there is a strong tendency to form high-density folds of the tracer strings, which persist much longer than simple shearing indicates. The folds persist because there is a strong tendency for material that enters the flow at the margins of cells to be transferred to adjacent cells, where it is "unmixed." When the simulations are scaled to the whole mantle, the tight clumps (folds) of tracers are shown to persist for up to 1-2 b .y. There is also a tendency for large-scale convection cells to remain isolated from recycled material for 1-2 b.y. These results are consistent with the significant chemical heterogeneity of the mantle as revealed by isotopic studies of oceanic basalts. Despite the spatial heterogeneity in tracer density, the average time tracers remain in the box from subduction at trenches to sampling at ridges (i.e., the residence time) is well constrained and within 20% of the mean residence time expected from an analytic model in which tracers are assumed to be sampled randomly. Model ages of the mantle that explicitly incorporate increased convection rates in the past and assume random sampling of heterogeneities bracket the - 2 b.y. apparent Pb-Pb and Rb-Sr isochrons of midocean ridge basalts and oceanic island basalts. The conclusion of persistent spatial heterogeneity is different from the conclusions drawn from other studies. The different conclusions result, primarily, from our emphasis on the details of spatial variations as opposed to some average of the mixing, from a difference in flow unsteadiness, and from the different ways tracers have been introduced into the flow

Numerical study of high Rayleigh number convection in a medium with depth-dependent viscosity

The equations of motion are solved numerically for a Boussinesq fluid with infinite Prandtl number in a square 2-D box where the viscosity increases with depth. Three heating modes are employed: bottom heating, internal heating, and half bottom and half internal heating. In all cases the boundaries are free slip. The range of Rayleigh numbers employed is 10^4-10^7. The viscosity increases as 10^(β(1-y)), where y is distance measured from the bottom upwards and β is a free parameter. In the bottom heated cases, the convective velocities slow near the bottom and result in a large temperature drop between the bottom boundary and interior compared with the top boundary and the interior. This results in increased buoyancy in the ascending limb. In the internally heated case, the flow in the top half of the box resembles Rayleigh-Bènard convection and in the bottom half it approaches a conductive thermal regime for β greater than about 2. In this case the top surface heat flux decays from ascending to descending limb and the ascending and descending limbs become more equal in their buoyancy. Increasing β decreases the efficiency of heat transport, but has little effect on the exponents of Nu-Ra and Pe-Ra relations. There is a larger decrease in heat transport efficiency for a given β in the bottom heated case compared to the internally heated case

Bank resolution and safeguarding the creditors left behind

Not for the first time, the global banking crisis illustrated the vulnerability of banks to a loss of confidence by their depositors, other creditors and counterparties. The experience highlighted the need to have special arrangements for dealing with failing banks - ‘special resolution regime’ — that provides the authorities with the tools necessary to reduce the systemic risks arising from a bank’s failure while at the same time limiting the taxpayers’ exposure to the costs. The United Kingdom’s own Special Resolution Regime for dealing with failing banks and building societies was born out of the difficulties in dealing with the failure of Northern Rock in the autumn of 2007.

Measurement of elastic velocities of MgO under shock compression to 500 kilobars

The velocities of rarefaction waves in shock-compressed MgO have been measured by observing the reduction of the shock front velocity near the sample edges due to the rarefaction waves propagating from the edges. The extent of this ‘edge effect’ is difficult to determine accurately because of its emergent nature. Arrangements sensitive to differences in shock front velocity yielded rarefaction wave velocities close to predicted longitudinal velocities in the high-pressure shock state. Velocities closer to the hydrodynamic sound speed in the shock state were obtained from less sensitive arrangements. These results can be interpreted in terms of a two-stage elastoplastic model of the decompression. The longitudinal velocities measured in shock states up to 528 kb imply second pressure derivatives of the elastic moduli c_(ij)″, given by K_0c_(ij)″ = −1 ± 15, where K is the bulk modulus

A proposed equation of state of stishovite

The available shock-wave data for solid α quartz in the stishovite pressure regime are reduced to a 25°C isotherm and an adiabat, centered at standard conditions, using recent standard density, enthalpy, and coefficient of thermal expansion data. The calculated isothermal bulk modulus, 3 Mb, as determined from the Birch-Murnaghan equation, depends critically on the value of (dK/dP)T at zero pressure and to a yet unknown extent on the form of the equation of state. The high-temperature value of Grüneisen's ratio (0.8 to 0.9) along the a quartz (stishovite regime) Hugoniot was obtained from the pressure offsets of the fused quartz and porous quartz Hugoniot. The high value for γ obtained from thermochemical data at standard conditions (1.5±0.3) suggests that a marked decrease in the value of γ to 0.8 occurs with increasing temperature

Math and Science Education

Resnick provides an excellent brief account of current work in cognitive psychology and its important implications for math and science education. As she indicates, most cognitive psychologists view knowledge as consisting of highly organized schemata into which new experiences are assimilated and view the learner as actively constructing new knowledge. This view is consistent with the ideas that Piagetian theorists and educators have been propounding for many years, although Resnick’s discussion is rooted in the more detailed analysis of specific knowledge and learning in specific content areas that typifies the information-processing paradigm of modern cognitive science

Past major tsunamis and the level of tsunami risk on the Aitape coast of Papua New Guinea

This paper reports the results of an investigation into past major tsunamis on the Aitape coast of Papua New Guinea. The investigation was mounted to gather information to help assess the level of ongoing tsunami risk, in the aftermath of a catastrophic tsunami that struck this coast in 1998. We found that local residents have a strong oral tradition of a great tsunami at some time in the past, date unknown. A possible geological record of past major tsunamis was found in a submerged rock face that comprised clay-rich mudstone with three centimetric interbeds of peat, two of which contained coarse detrital sediment of marine origin. The topmost peat contained much marine detrital sediment, some of it very coarse (pebbles to 4 cm), and was dated at around AD 1440–1600. The second peat contained a much smaller proportion of detrital sediment, finer sediment than was in the topmost, and was dated at around AD 1150–1240. The lowermost peat was dated at around AD 980–1050. The two occurrences of coarse detrital sediments are presumed to be a record of past marine incursions into coastal swamps, probably as tsunamis or possibly as storm waves. The more recent, and more energetic, incursion, at around AD 1440–1600, was very likely the great tsunami of legend. In the thousand years recorded in the submerged rock face, there have been, at most, three major tsunamis, at approximate intervals of 300–500 years.We thank the Australian aid program for a grant which covered the cost of drilling

Elasticity of Solids at High Pressures and Temperatures : Theory, Measurement, and Geophysical Application

A theory for describing the elasticity of solids at simultaneous high pressures and high temperatures is developed by incorporating the fourth-order ahnarmonic theory of lattice dynamics into finite strain theory. The theory is applied to the analysis of a variety of data for MgO, SiO2 and NaCl, and the results for MgO and SiO2 used as the basis of a discussion of the constitution of the lower mantle. New results are reported of measurements of elastic properties of MgO shock-compressed to over 500 Kb. The condition that finite strain equations be frame- indifferent is shown to require that only strain tensors belonging to a class of frame-indifferent strain tensors be used in finite strain expansion. It is shown that the generality of finite strain theory is not impaired by the inclusion of an explicit theory of thermal effects. Explicit equations for isotherms, isentropes and Hugoniots and for the effective elastic moduli of materials of cubic symmetry under hydrostatic stress are derived. The primary parameters of these equations are related to the elastic moduli and their pressure and temperature derivatives in an arbitrary reference state using thermodynamic identities, some of which are derived here. Hugoniot data corresponding to different initial sample densities of MgO, SiO2 and NaCl and original ultrasonic data of NaCl are used to test both the compressional and thermal parts of the theory, and to refine the equations of state of these materials. The frame-indifferent analogue, E, of the usual "Eulerian" strain tensor, ε, is found to usually give faster convergence of finite strain expansions than the "Lagrangian" strain tensor, η. The effect of usinq differ­ent strain measures on the values of parameters derived from data is demonstrated, and the adverse effects of using inappropriately derived parameters in extrapolation equations is demonstrated. Thermal effects in Hugoniot data are reasonably well described, but higher-order anharmonic effects appear to be required in the theory in order to describe the high temperature ultrasonic and thermal expansion data. Measured velocities of rarefaction waves propagating into shocked MgO are in accord with a two-stage longitudinal (elastic)-hydrodynamic (plastic) decompression model, and constrain the high-pressure elastic moduli of MgO. The effects on the determination of the lower mantle constitution of temperature, varying composition, the presence of phases denser than oxides mixtures, and the presence of iron in the "low-spin" electronic state are estimated, and a trade-off between many of these factors demonstrated. Iron content could range between 6% and 15% by weight of FeO. Silica content could range from 33% to 50% or more by weight. Phases a few percent denser than oxides mixtures seem to be likely. The temperature is very indeterminable.</p

Interaction of mantle dregs with convection: Lateral heterogeneity at the core-mantle boundary

Preliminary numerical models indicate that chemically denser material (dregs) at the base of the mantle would have substantial lateral variations in thickness induced by convection of the overlying mantle, and might well form discontinuous aggregations below mantle upwellings. A model with a density contrast of about 2 per cent and an initial uniform thickness of the denser layer of 100 km yields a discontinuous distribution with maximum thickness 230 km and bottom topography of several kilometers amplitude, in reasonable accord with recent seismological observations of vertical and lateral structure. Heat flux out of the core is probably strongly modulated laterally by mantle convection, while mantle dregs will complicate and possibly amplify this effect. Such modulation may be relevant to long-term (10^7 - 10^8 year) variations in the magnetic field