221 research outputs found
Nitrogen-fixation technology for increased agricultural sustainability
Non-Peer ReviewedThe advent of the world energy crisis in 1973 aroused interest in the potential of nitrogen-fixation for increasing agricultural production. Since then, sustainable agriculture has become the focus of attention. Sustainable agriculture involves design and management of procedures which work with natural processes to conserve resources, and minimize waste and environmental damage, while maintaining farm profitability. As agriculture moves towards greater sustainability, the potential role of nitrogen-fixation processes increases. As a result, the agronomic significance of nitrogen-fixation has generated renewed scientific excitement in this area of microbial-plant interaction. A number of significant advances in applied nitrogen-fixation technology have been made: improved rhizobial delivery systems, increased nitrogen-fixation capacities, greater yield and improved quality of legumes, and better definition of the role of nitrogen-fixation in N cycling. Shifts in agricultural sustainability that involve these improved nitrogen-fixation technologies require continued research efforts to assess the levels of resource conservation, impacts on the environment and the economic consequences
How much phosphorus do crop residues release under conventional and zero tillage systems?
Non-Peer ReviewedIn a field experiment conducted at Fort Vermilion in north-western Alberta, we used the litter bag method to quantify phosphorus (P) release from red clover green manure, field pea, canola and wheat residues under conventional tillage and zero tillage. Wheat residues added significantly less P (1.7 kg ha-1) to the soil than the other residues (5.6-8.5 kg P ha-1). Tillage had no significant effect on residue P applied, but the trend was for slightly greater amounts under ZT than under CT. Clover released the most P (3.8 kg ha-1 under CT and 2.8 kg ha-1 under ZT, compared with 1.4 kg ha-1 or less from the other residues). There were no significant tillage effects on the amounts released by clover, pea and canola residues, but wheat immobilized 0.2 kg P ha-1 under ZT compared with 0.4 kg P ha-1 released under CT. Soil phosphate contents were not significantly different between tillage systems. Phosphate contents decreased with soil
depth, especially under ZT. However, there was no tillage by soil depth interaction with canola residues, where soil phosphate was greater (although not significantly) under ZT than under CT at all depths. Uptake of P by wheat was significantly greater where pea and, to a lesser extent, canola residues had been applied than where clover and wheat residues had been applied, and tillage had no significant effects on P uptake
Anomalies of ac driven solitary waves with internal modes: Nonparametric resonances induced by parametric forces
We study the dynamics of kinks in the model subjected to a
parametric ac force, both with and without damping, as a paradigm of solitary
waves with internal modes. By using a collective coordinate approach, we find
that the parametric force has a non-parametric effect on the kink motion.
Specifically, we find that the internal mode leads to a resonance for
frequencies of the parametric driving close to its own frequency, in which case
the energy of the system grows as well as the width of the kink. These
predictions of the collective coordinate theory are verified by numerical
simulations of the full partial differential equation. We finally compare this
kind of resonance with that obtained for non-parametric ac forces and conclude
that the effect of ac drivings on solitary waves with internal modes is exactly
the opposite of their character in the partial differential equation.Comment: To appear in Phys Rev
Optical investigation of the charge-density-wave phase transitions in
We have measured the optical reflectivity of the quasi
one-dimensional conductor from the far infrared up to the
ultraviolet between 10 and 300 using light polarized along and normal to
the chain axis. We find a depletion of the optical conductivity with decreasing
temperature for both polarizations in the mid to far-infrared region. This
leads to a redistribution of spectral weight from low to high energies due to
partial gapping of the Fermi surface below the charge-density-wave transitions
at 145 K and 59 K. We deduce the bulk magnitudes of the CDW gaps and discuss
the scattering of ungapped free charge carriers and the role of fluctuations
effects
Energy landscape, two-level systems and entropy barriers in Lennard-Jones clusters
We develop an efficient numerical algorithm for the identification of a large
number of saddle points of the potential energy function of Lennard- Jones
clusters. Knowledge of the saddle points allows us to find many thousand
adjacent minima of clusters containing up to 80 argon atoms and to locate many
pairs of minima with the right characteristics to form two-level systems (TLS).
The true TLS are singled out by calculating the ground-state tunneling
splitting. The entropic contribution to all barriers is evaluated and
discussed.Comment: 4 pages, RevTex, 2 PostScript figure
Variable-range hopping in quasi-one-dimensional electron crystals
We study the effect of impurities on the ground state and the low-temperature
dc transport in a 1D chain and quasi-1D systems of many parallel chains. We
assume that strong interactions impose a short-range periodicicity of the
electron positions. The long-range order of such an electron crystal (or
equivalently, a charge-density wave) is destroyed by impurities. The 3D
array of chains behaves differently at large and at small impurity
concentrations . At large , impurities divide the chains into metallic
rods. The low-temperature conductivity is due to the variable-range hopping of
electrons between the rods. It obeys the Efros-Shklovskii (ES) law and
increases exponentially as decreases. When is small, the metallic-rod
picture of the ground state survives only in the form of rare clusters of
atypically short rods. They are the source of low-energy charge excitations. In
the bulk the charge excitations are gapped and the electron crystal is pinned
collectively. A strongly anisotropic screening of the Coulomb potential
produces an unconventional linear in energy Coulomb gap and a new law of the
variable-range hopping . remains
constant over a finite range of impurity concentrations. At smaller the
2/5-law is replaced by the Mott law, where the conductivity gets suppressed as
goes down. Thus, the overall dependence of on is nonmonotonic.
In 1D, the granular-rod picture and the ES apply at all . The conductivity
decreases exponentially with . Our theory provides a qualitative explanation
for the transport in organic charge-density wave compounds.Comment: 20 pages, 7 figures. (v1) The abstract is abridged to 24 lines. For
the full abstract, see the manuscript (v2) several changes in presentation
per referee's comments. No change in result
Jahn-Teller polarons and their superconductivity in a molecular conductor
We present a theoretical study of a possibility of superconductivity in a
three dimensional molecular conductor in which the interaction between
electrons in doubly degenerate molecular orbitals and an {\em intra}molecular
vibration mode is large enough to lead to the formation of
Jahn-Teller small polarons. We argue that the effective polaron-polaron
interaction can be attractive for material parameters realizable in molecular
conductors. This interaction is the source of superconductivity in our model.
On analyzing superconducting instability in the weak and strong coupling
regimes of this attractive interaction, we find that superconducting transition
temperatures up to 100 K are achievable in molecular conductors within this
mechanism. We also find, for two particles per molecular site, a novel Mott
insulating state in which a polaron singlet occupies one of the doubly
degenerate orbitals on each site. Relevance of this study in the search for new
molecular superconductors is pointed out.Comment: Submitted to Phys. Rev.
The analytic solution of near-tip stress fields for perfectly plastic pressure-sensitive material under plane stress condition
Different from dense metals, many engineering materials exhibit pressure-sensitive yielding and plastic volumetric deformation. Adopting a yield criterion that contains a linear combination of the Mises stress and the hydrostatic stress, the analytic solutions of plane-stress mode I perfectly-plastic near-tip stress fields for pressuresensitive materials are derived. Also, the relevant characteristic fields are presented. This perfectly plastic solution, containing a pressure sensitivity parameter μ, is shown to correspond to the limit of low-hardening solutions, and when μ=0 it reduces to the perfectly plastic solution of near-tip fields for the Mises material given by Hutchinson [1]. The effects of material pressure sensitivity on the near-tip fields are discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42771/1/10704_2004_Article_BF00034180.pd
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