203 research outputs found
Response of microbial activity to labile C addition in sandy soil from semi-arid woodland is influenced by vegetation patch and wildfire
Nutrient cycling in semi-arid woodlands is likely to be influenced by patchy vegetation, wildfire and the supply of easily available organic C, e.g. root exudates. The study assessed the effect of wildfire and vegetation patch on response of microbial activity to labile C addition in soil from a semi-arid Eucalyptus woodland. Two sites were studied: one unburnt and the other exposed to wildfire four-month before sampling. Top soil (0 – 30 cm) from under trees, under shrubs or in open areas from each site was air-dried and sieved to < 2 mm. The soils were incubated at 80% of maximum water holding capacity for 24 days without or with addition of 5 g C kg-1 as glucose. Soil organic carbon (TOC), microbial biomass C, N and P availability and cumulative respiration were greater under trees than in open areas. Fire decreased TOC and cumulative respiration only under trees and had little effect on available N, microbial biomass C and P concentrations. The greater increase in cumulative respiration by glucose addition under shrubs and in open areas compared to under trees and, in a given patch, greater in burnt than unburnt soils, indicate lower availability of native organic carbon.Qiaoqi Sun, Wayne S. Meyer, Georgia R. Koerber, Petra Marschne
On moduli and effective theory of N=1 warped flux compactifications
The moduli space of N=1 type II warped compactions to flat space with generic
internal fluxes is studied. Using the underlying integrable generalized complex
structure that characterizes these vacua, the different deformations are
classified by H-twisted generalized cohomologies and identified with chiral and
linear multiplets of the effective four-dimensional theory. The Kaehler
potential for chiral fields corresponding to classically flat moduli is
discussed. As an application of the general results, type IIB warped Calabi-Yau
compactifications and other SU(3)-structure subcases are considered in more
detail.Comment: 54 pages; v3: comments and references added, version published in
JHE
Universal de Sitter solutions at tree-level
Type IIA string theory compactified on SU(3)-structure manifolds with
orientifolds allows for classical de Sitter solutions in four dimensions. In
this paper we investigate these solutions from a ten-dimensional point of view.
In particular, we demonstrate that there exists an attractive class of de
Sitter solutions, whose geometry, fluxes and source terms can be entirely
written in terms of the universal forms that are defined on all SU(3)-structure
manifolds. These are the forms J and Omega, defining the SU(3)-structure
itself, and the torsion classes. The existence of such universal de Sitter
solutions is governed by easy-to-verify conditions on the SU(3)-structure,
rendering the problem of finding dS solutions purely geometrical. We point out
that the known (unstable) solution coming from the compactification on SU(2)x
SU(2) is of this kind.Comment: 20 pages, 3 figures, v2: added reference
Under a new light: validation of eddy covariance flux with light response functions of assimilation and estimates of heterotrophic soil respiration
Estimation of the basal or heterotrophic soil respiration is crucial for determination of whether an ecosystem is emitting or sequestering carbon. A severe bushfire in January 2014 at the Calperum flux tower, operational since August 2010, provided variation in ecosystem respiration and leaf area index as the ecosystem recovered. We propose ecosystem respiration is a function of leaf area index and the y-intercept is an estimate of heterotrophic soil respiration. We calculated an assimilation rate from eddy covariance data for light response functions to calculate ecosystem respiration incorporating suppression of the daytime autotrophic respiration. Ecosystem respiration from light response functions correlated with data processing calculations of ecosystem respiration by OzFluxQC (y0 = 0.161x + 0.0085; Adj. r2 = 0.698). The relationship between ecosystem respiration and leaf area index (y0 = 1.43x +0.398; Adj. r2 = 0.395) was also apparent. When this approach was compared to field measurements of soil respiration and mass balance calculations from destructive leaf area, leaf area index calculations and litter fall, the year of data corresponding to the year of soil respiration measurements, the y-intercept was 0.432 µmol m−2 s−1 or 163.44 gC m−2 year−1 (y0 = 1.37x + 0.432, Adj. r2 = 0.325). The mass balance approach for the net primary productivity when subtracted from the tower NEE estimated heterotrophic soil respiration of 134.59 gC m−2 year−1. This is only 28.9 gC different, therefore the y-intercept approach indeed provides an estimate of heterotrophic soil respiration.Georgia R. Koerber, Wayne S. Meyer, Qiaoqi SUN, Peter Cale, and Cacilia M. Ewen
The effective theory of type IIA AdS4 compactifications on nilmanifolds and cosets
We consider string theory compactifications of the form AdS4 x M6 with
orientifold six-planes, where M6 is a six-dimensional compact space that is
either a nilmanifold or a coset. For all known solutions of this type we obtain
the four-dimensional N=1 low energy effective theory by computing the
superpotential, the Kaehler potential and the mass spectrum for the light
moduli. For the nilmanifold examples we perform a cross-check on the result for
the mass spectrum by calculating it alternatively from a direct Kaluza-Klein
reduction and find perfect agreement. We show that in all but one of the coset
models all moduli are stabilized at the classical level. As an application we
show that all but one of the coset models can potentially be used to bypass a
recent no-go theorem against inflation in type IIA theory.Comment: 47 pages main text, 28 pages appendix, 3 tables, 7 figures, v2: added
references, corrected typo
Nonabelian Phenomena on D-branes
A remarkable feature of D-branes is the appearance of a nonabelian gauge
theory in the description of several (nearly) coincident branes. This
nonabelian structure plays an important role in realizing various geometric
effects with D-branes. In particular, the branes' transverse displacements are
described by matrix-valued scalar fields and so noncommutative geometry
naturally appears in this framework. I review the action governing this
nonabelian theory, as well as various related physical phenomena such as the
dielectric effect, giant gravitons and fuzzy funnels.Comment: Lecture at Leuven workshop on ``The quantum structure of spacetime
and the geometrical nature of fundamental interactions'' (September 13-19,
2002); ref.'s adde
T-duality and Generalized Kahler Geometry
We use newly discovered N = (2, 2) vector multiplets to clarify T-dualities
for generalized Kahler geometries. Following the usual procedure, we gauge
isometries of nonlinear sigma-models and introduce Lagrange multipliers that
constrain the field-strengths of the gauge fields to vanish. Integrating out
the Lagrange multipliers leads to the original action, whereas integrating out
the vector multiplets gives the dual action. The description is given both in N
= (2, 2) and N = (1, 1) superspace.Comment: 14 pages; published version: some conventions improved, minor
clarification
Standard Model tests with trapped radioactive atoms
We review the use of laser cooling and trapping for Standard Model tests,
focusing on trapping of radioactive isotopes. Experiments with neutral atoms
trapped with modern laser cooling techniques are testing several basic
predictions of electroweak unification. For nuclear decay, demonstrated
trap techniques include neutrino momentum measurements from beta-recoil
coincidences, along with methods to produce highly polarized samples. These
techniques have set the best general constraints on non-Standard Model scalar
interactions in the first generation of particles. They also have the promise
to test whether parity symmetry is maximally violated, to search for tensor
interactions, and to search for new sources of time reversal violation. There
are also possibilites for exotic particle searches. Measurements of the
strength of the weak neutral current can be assisted by precision atomic
experiments using traps of small numbers of radioactive atoms, and sensitivity
to possible time-reversal violating electric dipole moments can be improved.Comment: 45 pages, 17 figures, v3 includes clarifying referee comments,
especially in beta decay section, and updated figure
Flux compactification on smooth, compact three-dimensional toric varieties
Three-dimensional smooth, compact toric varieties (SCTV), when viewed as real
six-dimensional manifolds, can admit G-structures rendering them suitable for
internal manifolds in supersymmetric flux compactifications. We develop
techniques which allow us to systematically construct G-structures on SCTV and
read off their torsion classes. We illustrate our methods with explicit
examples, one of which consists of an infinite class of toric CP^1 bundles. We
give a self-contained review of the relevant concepts from toric geometry, in
particular the subject of the classification of SCTV in dimensions less or
equal to 3. Our results open up the possibility for a systematic construction
and study of supersymmetric flux vacua based on SCTV.Comment: 27 pages, 10 figures; v2: references, minor typos & improvement
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