20,359 research outputs found
Effects of long-term farmyard manure applications on soil organic matter, nitrogen mineralization and crop yield – a modeling study –
To develop sustainable cropping systems we need to predict both short-term and long-term effects of management practices on soil fertility. For this purpose agro-ecosystem simulation models are valuable tools. We used the Daisy model to simulate a three-year crop rotation (beetroot, onion, white clover, potato) over a period of 40 years. With this rotation, three rates of farmyard manure were tested (0, 15, 28 t ha-1 year-1). After 40 years without manure soil organic matter carbon (SOM-C) decreased by approximately 40%, and increased by 27% with the highest application rate. SOM turnover did not reach equilibrium at the end of the experiment. Nitrogen mineralization from SOM followed in the long-term (40 years) the slowly changing time courses of SOM. However, manure applications affected mineralization and hence crop yield and nitrogen losses much more in the short-term (1 to 2 years) than in the long-term
Time-Reversal of Nonlinear Waves - Applicability and Limitations
Time-reversal (TR) refocusing of waves is one of fundamental principles in
wave physics. Using the TR approach, "Time-reversal mirrors" can physically
create a time-reversed wave that exactly refocus back, in space and time, to
its original source regardless of the complexity of the medium as if time were
going backwards. Lately, laboratory experiments proved that this approach can
be applied not only in acoustics and electromagnetism but also in the field of
linear and nonlinear water waves. Studying the range of validity and
limitations of the TR approach may determine and quantify its range of
applicability in hydrodynamics. In this context, we report a numerical study of
hydrodynamic TR using a uni-directional numerical wave tank, implemented by the
nonlinear high-order spectral method, known to accurately model the physical
processes at play, beyond physical laboratory restrictions. The applicability
of the TR approach is assessed over a variety of hydrodynamic localized and
pulsating structures' configurations, pointing out the importance of high-order
dispersive and particularly nonlinear effects in the refocusing of hydrodynamic
stationary envelope solitons and breathers. We expect that the results may
motivate similar experiments in other nonlinear dispersive media and encourage
several applications with particular emphasis on the field of ocean
engineering.Comment: 14 pages, 17 figures ; accepted for publication in Phys. Rev. Fluid
In vivo laser Doppler holography of the human retina
The eye offers a unique opportunity for non-invasive exploration of
cardiovascular diseases. Optical angiography in the retina requires sensitive
measurements, which hinders conventional full-field laser Doppler imaging
schemes. To overcome this limitation, we used digital holography to perform
laser Doppler perfusion imaging of the human retina in vivo with near-infrared
light. Wideband measurements of the beat frequency spectrum of optical
interferograms recorded with a 39 kHz CMOS camera are analyzed by short-time
Fourier transformation. Power Doppler images and movies drawn from the zeroth
moment of the power spectrum density reveal blood flows in retinal and
choroidal vessels over 512 512 pixels covering 2.4 2.4 mm
on the retina with a 13 ms temporal resolution.Comment: 5 pages, 5 figure
On Properties of Update Sequences Based on Causal Rejection
We consider an approach to update nonmonotonic knowledge bases represented as
extended logic programs under answer set semantics. New information is
incorporated into the current knowledge base subject to a causal rejection
principle enforcing that, in case of conflicts, more recent rules are preferred
and older rules are overridden. Such a rejection principle is also exploited in
other approaches to update logic programs, e.g., in dynamic logic programming
by Alferes et al. We give a thorough analysis of properties of our approach, to
get a better understanding of the causal rejection principle. We review
postulates for update and revision operators from the area of theory change and
nonmonotonic reasoning, and some new properties are considered as well. We then
consider refinements of our semantics which incorporate a notion of minimality
of change. As well, we investigate the relationship to other approaches,
showing that our approach is semantically equivalent to inheritance programs by
Buccafurri et al. and that it coincides with certain classes of dynamic logic
programs, for which we provide characterizations in terms of graph conditions.
Therefore, most of our results about properties of causal rejection principle
apply to these approaches as well. Finally, we deal with computational
complexity of our approach, and outline how the update semantics and its
refinements can be implemented on top of existing logic programming engines.Comment: 59 pages, 2 figures, 3 tables, to be published in "Theory and
Practice of Logic Programming
Rural-urban food, nutrient and virtual water flows in selected West African cities
Food consumption / Water quality / Nutrients / Urban agricuture / Food production
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