Second hyperpolarizability of carbon tetrachloride

Although present theories of nonlinear optics agree with observed behavior in simple atoms such as helium, more complex molecules containing many electrons, such as carbon tetrachloride (CCI4), cannot consistently be described by theory. Through experimental analysis of nonlinear materials, a new, more sophisticated model for describing their properties could be realized. The purpose of our experiment was to measure the nonlinear behavior of the second harmonic signal generated from CCI4 and to compare the results with the prediction by the CCSD(T) molecular model

Uneven distribution of cobamide biosynthesis and dependence in bacteria predicted by comparative genomics.

The vitamin B12 family of cofactors known as cobamides are essential for a variety of microbial metabolisms. We used comparative genomics of 11,000 bacterial species to analyze the extent and distribution of cobamide production and use across bacteria. We find that 86% of bacteria in this data set have at least one of 15 cobamide-dependent enzyme families, but only 37% are predicted to synthesize cobamides de novo. The distribution of cobamide biosynthesis and use vary at the phylum level. While 57% of Actinobacteria are predicted to biosynthesize cobamides, only 0.6% of Bacteroidetes have the complete pathway, yet 96% of species in this phylum have cobamide-dependent enzymes. The form of cobamide produced by the bacteria could be predicted for 58% of cobamide-producing species, based on the presence of signature lower ligand biosynthesis and attachment genes. Our predictions also revealed that 17% of bacteria have partial biosynthetic pathways, yet have the potential to salvage cobamide precursors. Bacteria with a partial cobamide biosynthesis pathway include those in a newly defined, experimentally verified category of bacteria lacking the first step in the biosynthesis pathway. These predictions highlight the importance of cobamide and cobamide precursor salvaging as examples of nutritional dependencies in bacteria

The Tropical Grassland Society of Australia Incorporated

The Tropical Grassland Society of Australia was formed in 1962 and became incorporated in 1987 and has the following aims: To publicise information of interest to primary producers and scientists To improve the relevance of research and adoption of technology through the flow of ideas between scientists and producers To publicise the findings of Australian pasture research and development to overseas workers, and to draw on their experience for application in Australi

RESIDUE MANAGEMENT TO CONTROL SOIL EROSION BY WATER

The Erosion Process Erosion of topsoil begins when water detaches individual soil particles from clod and other soil aggregates. A single raindrop may seem insignificant, yet collectively, raindrops strike the ground with surprising force. During an intense storm, rainfall can loosen and detach up to 100 tons of soil per acre and can be especially erosive when residue mulch or vegetation are not present to absorb their impact. Two problems often occur during rainstorms. The rate of rainfall can exceed the rate at which water can enter the soil and raindrop impact forces can partially seal the soil surface. In the first in distance, the excess water either collects on or runs off the soil surface and in the second, less water can infiltrate into the soil, causing more runoff. This runoff will travel downhill, carrying soil particles with it. Runoff from steeper areas flows at greater velocities and may transport considerable amounts of soil. Further, longer slopes have greater flows because water is concentrated from a larger area. As runoff flow across unprotected soil surfaces, additional soil particles are dislodged, thus creating even more soil erosion. Residue Reduces Erosion Crop residue helps protect the soil surface from raindrop impact. It. also reduces surface crusting, sealing and rainfall-induced soil compaction, all of which increase water runoff by reducing infiltration. In addition, runoff is reduced because pieces of residue form a complex series of small dams and obstructions that slow the runoff. Years of research show that no-till planting systems, which leave the greatest amount of residue cover, can reduce soil erosion by 90 to 95 percent of that occurring from cleanly tilled systems. As little as a 30 percent residue cover can reduce erosion by 65 percent as shown in the illustration. Prior land use, crop canopy and surface roughness also influence erosion from different tillage and planting systems, but residue cover is the single most important factor

Duality Symmetric Strings, Dilatons and O(d,d) Effective Actions

We calculate the background field equations for the T-duality symmetric string building on previous work by including the effect of the Dilaton up to two-loops. Inclusion of the Dilaton allows us to obtain the full beta functionals of the duality symmetric sigma model. We are able to interpret the result in terms of a dimensionally reduced O(d,d) invariant target space effective action.Comment: 15 pages, latex; v2 reference added, typos fixe

Background Field Equations for the Duality Symmetric String

This paper describes the background field equations for strings in T-duality symmetric formalisms in which the dimension of target space is doubled and the sigma model supplemented with constraints. These are calculated by demanding the vanishing of the beta-functional of the sigma model couplings in the doubled target space. We demonstrate the equivalence with the background field equations of the standard string sigma model.Comment: 26 pages, latex, v2 typos correcte

Tillage Systems for Row Crop Production

Selecting the tillage system best suited to a particular farming situation is an important management decision. Formerly, the traditional system was a moldboard plow operation followed by several secondary tillage operations before planting. This system can be appropriate for poorly drained soils having little or no slope and low erosion potential. However, plowing has several disadvantages . The potential for soil erosion is high on sloping lands, and labor and fuel requirements can be substantially higher than with other tillage and planting systems. Today, conservation tillage systems are used to reduce preplant tillage operations, thus reducing soil erosion and moisture loss while saving labor and fuel. The label conservation tillage represents a broad spectrum of farming methods, and is most often defined by the amount of residue cover remaining on the soil surface. The minimum amount recommended is 20 to 30 percent after planting. Research in Nebraska and other Midwestern states has shown that leaving at least this much residue will reduce erosion by more than 50 percent of that occurring from a cleanly tilled field. To achieve effective erosion control, this minimum residue cover should be maintained during the critical soil erosion period between spring seedbed preparation and crop canopy establishment. Conservation tillage does not necessarily require new equipment. Most conventional farm implements can be used. For corn, grain sorghum, or wheat residue, one or two passes with a field cultivator, disk, or chisel plow will usually leave more than the 20 percent minimum cover. Additional operations reduce the amount of residue, and thus reduce erosion control. Other tillage and planting systems such as ridge-plant (till-plant) and no till leave even more residue, and thus offer greater erosion control. However, no-till planting is the only method that consistently leaves the minimum surface cover in the more fragile and less abundant soybean residue. No single tillage system is best for all situations at all times. Selecting the best tillage system for a particular soil and cropping situation requires matching the operation to the crop sequence, topography, and soil type. Rotating systems to coincide with crop rotations often provides an excellent combination. For example, a no till system could follow soybeans while a chisel or disk system might follow corn. This tillage rotation provides the best erosion control following soybeans, and provides an opportunity for some tillage in the less fragile and more abundant corn residue

G91-1046 Conservation Tillage and Planting Systems

Tillage system descriptions and comparisons are included here. Moldboard plowing, followed by such secondary tillage operations as disking and harrowing, was once the most common, or traditional, tillage system before planting. Soil erosion potential from rainfall on sloping lands was great and requirements for labor and fuel were high compared to other tillage and planting systems. One of the most commonly used tillage systems in Nebraska today is two diskings followed by field cultivation. Unfortunately, the potential for soil erosion may be great because the number of tillage operations involved may not leave adequate residue cover for erosion control. Today conservation tillage systems reduce soil erosion and moisture losses while saving labor and fuel. Conservation tillage can represent a broad spectrum of farming methods, provided at least 30 percent of the soil surface remains covered with crop residue following planting. Research in Nebraska and other midwestern states has shown that leaving at least 30 percent residue cover reduces erosion from water by more than 50 percent, as compared to a cleanly tilled field