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961.
Rebecca L. Stepien DVM MS Keith G. Benson DVM Lisa J. Forrest VMD 《Veterinary radiology & ultrasound》1999,40(6):606-610
Right lateral, left lateral and ventrodorsal radiographs were obtained in 20 normal ferrets (11 male). Three independent observers recorded measurements of the cardiac silhouette and results for each parameter were averaged. Long axis (length), short axis (width) and total of length plus width (L+W) were recorded in each view. Comparative measurements were calculated, including the ratio of L+W/length of thoracic vertebrae 5-8, and a modified vertebral heart score (VHS) method, measuring the heart in vertebral units. Measurements made in ventrodorsal views were usually larger than corresponding measurements in lateral views. Weight and most absolute measurements differed significantly between male and female ferret (P<0.05), but the differences in absolute measurements were not apparent when related to measures of body size (Ratio and modified VHS methods). Given the variability in body weight and size in ferrets, measurements of the cardiac silhouette normalized for body size may be more universally applicable than absolute measurements. 相似文献
962.
963.
Studies aiming at quantification of roots growing in soil are often constrained by the lack of suitable methods for continuous, non‐destructive measurements. A system is presented in which maize (Zea mays L.) seedlings were grown in acrylic containers — rhizotrons — in a soil layer 6‐mm thick. These thin‐layer soil rhizotrons facilitate homogeneous soil preparation and non‐destructive observation of root growth. Rhizotrons with plants were placed in a growth chamber on a rack slanted to a 45° angle to promote growth of roots along the transparent acrylic sheet. At 2‐ to 3‐day intervals, rhizotrons were placed on a flatbed scanner to collect digital images from which root length and root diameters were measured using RMS software. Images taken during the course of the experiment were also analyzed with QUACOS software that measures average pixel color values. Color readings obtained were converted to soil water content using images of reference soils of known soil water contents. To verify that roots observed at the surface of the rhizotrons were representative of the total root system in the rhizotrons, they were compared with destructive samples of roots that were carefully washed from soil and analyzed for total root length and root diameter. A significant positive relation was found between visible and washed out roots. However, the influence of soil water content and soil bulk density was reflected on seminal roots rather than first order laterals that are responsible for more than 80 % of the total root length. Changes in soil water content during plant growth could be quantitifed in the range of 0.04 to 0.26 cm3 cm—3 if image areas of 500 x 500 pixel were analyzed and averaged. With spatial resolution of 12 x 12 pixel, however, soil water contents could only be discriminated below 0.09 cm3 cm—3 due to the spatial variation of color readings. Results show that this thin‐layer soil rhizotron system allows researchers to observe and quantify simultaneously the time courses of seedling root development and soil water content without disturbance to the soil or roots. 相似文献
964.
965.
Crop yields are an important determinant of C input into the soil and ultimately of soil organic carbon (SOC). Models to predict changes in SOC require crop yield and future yield growth information. In this review, yield trends for 11 major crops in the US for the period 1939–1994 are analyzed. Historical data are analyzed to detect evidence of a yield plateau. Yields are extrapolated to the year 2020 based on linear and exponential yield trends estimated in the paper. Forward-looking information on yield gaps, on the biological potential for future yield growth, and socio-economic determinants of yield growth is examined for evidence that yield growth may plateau in the near future. A linear model of yield growth indicates a compound annual rate of growth of between 0.7% and 1.3% per year for major US crops through 2020. Under an exponential model, growth rates could range up to 3% per year. This could lead to substantial increases in SOC if crop residues are retained. Biophysical limits, including those imposed by agronomic and tillage practices, do not appear to prevent rates of growth consistent with the linear model. How socioeconomic forces will affect future growth are subject to greater uncertainty. 相似文献
966.
967.
Purpose
Roadside ditches line more than 6.3 million km of roadways in the USA, dissecting the natural topography and altering the flow of runoff from the catchments that drain into them. In agricultural regions, more than 30% of a watershed may directly drain into the roadside ditch system. Quantifying soil erosion and sediment export from agricultural watersheds is a crucial component when considering long-term soil sustainability.
Materials and methodsOur study evaluated the relation of catchment soil erosion and ditch sedimentation at six representative roadside ditches in Lime Creek watershed (eastern Iowa) and quantified the effectiveness of possible catchment conservation practices to reduce soil erosion and ditch sedimentation.
Results and discussionStudy results provide clear evidence linking roadside ditches to the agricultural catchments that drain into them. Among the six ditch sites, catchment erosion was found to be inversely related to sediment storage within the ditch due to erosive power of water entering the ditches from their basins. Of four catchment scenarios to reduce soil erosion, no-till with cover and graded terrace did not require land to be taken out of production and provided the most significant reductions in catchment erosion rates.
ConclusionsResults indicated that reducing nutrient and sediment loads to ditches by incorporating in-field conservation practices in ditch catchments may be more economical and environmentally sustainable than current management practices for both farmers and roadway managers because they trap detached soil sediments before they enter the ditch.
相似文献968.
Keith A. Kelling Douglas I. Rouse Phillip E. Speth 《American Journal of Potato Research》2017,94(5):481-489
Research has shown that while fumigation and use of ammonium N can both reduce the severity of verticillium wilt (Verticillium dahliae Kleb.) of potatoes (Solanum tuberosum L.), the use of the two practices together raises concerns over feeding the crop only ammonium N under reduced nitrification conditions. To assess the validity of this concern, we conducted two 3-year field split-plot experiments with both using metam sodium fumigant (none, fall or spring applied) as the main plot. For the first experiment, N source (134 kg N ha?1 as ammonium sulfate, urea, or ammonium nitrate) was the split, whereas for the second trial in-season N rate (0, 67, 134, or 202 kg N ha?1 all as ammonium sulfate) was the split. For both trials, in 2 of the 3 years, fumigation significantly increased tuber yield by an average of 9.9 Mg ha?1 and decreased late-season verticillium severity ratings from 77 to 45%. In some years, fumigation also increased the proportion of U.S. No. 1 tubers and tubers >170 g. No differences in crop yield or quality were observed between the various N sources applied. This was true even on spring-fumigated areas with the highest rate of ammonium N applied. These experiments confirm that the choice between in-season potato N fertilizer should be based on factors such as potential for benefits or N losses, cost, and convenience of use rather than concern over an interaction between fumigation and ammonical N. While both fumigation and N rate reduced verticillium severity ratings in some years, the lack of interaction suggests these factors are functioning independently. 相似文献
969.
Tom Barker Keith Hatton Michael O'Connor Les Connor Lee Bagnell Brian Moss 《水产资源保护:海洋与淡水生态系统》2008,18(3):221-240
- 1. The alternative stable states hypothesis for the behaviour of shallow lake communities requires switches to transform clear-water macrophyte-dominated communities to turbid algal-dominated ones. Such switches have rarely been demonstrated experimentally. This study shows the role of rising salinity as such a switch while contributing a solution to the conservation problems of an important nature reserve.
- 2. Hickling Broad changed from a clear-water, charophyte-dominated lake to a turbid, phytoplankton-dominated lake in the early 1970s, probably owing to guanotrophication by gulls and to increased salinity from more intensive pumping of the agricultural land that separates its main inflow from the nearby North Sea. Following a decline in nutrient loading as the gull flock moved away, the plants began to return during the 1980s and 1990s. In 1998/99, the water cleared and charophytes, including some very rare species, were abundant.
- 3. This was welcome to conservation bodies, but the vigorous growth precluded competitive sailing and there were conflicts with the local sailing club. The plants, however, began an irregular decline in 2000, though nutrient loadings and other conventional chemical drivers have remained steady.
- 4. Our hypothesis was that the unstable nature of the plant community was linked to high salinity, and that if salinity were lowered there would be vigorous and reliable growth, enabling annual cutting of plants to allow sailing races. In an experiment using mesocosms, salinities straddling the current values in the Broad led to declines in plant biomass, macrophyte species richness and macrophyte Shannon–Weaver diversity through increased release of phosphorus from the sediments, increased algal turbidity and reduction of zooplankton grazer activity.
- 5. Stabilization of the plant community of Hickling Broad would be achieved by a reduction of present salinities by about 20%. This would be possible by use of existing Environmentally Sensitive Area (High Level Environmental Stewardship) arrangements or diversion of some pumped drainage water to the sea. There remain some uncertainties about the future of the area because of rising sea levels.
970.
Donald B. Bechtel Jeff D. Wilson W. D. Eustace Keith C. Behnke Thomas Whitaker Gary L. Peterson David B. Sauer 《Cereal Chemistry》1999,76(2):270-275
Wheat contaminated with teliospores of Tilletia controversa Kühn (TCK) was mixed with uncontaminated wheat and processed through the Kansas State University pilot mill. Two 50-bu lots of the contaminated mixture were cleaned, tempered, and milled. Approximately 500 samples of wheat, cleanings, and mill fractions were collected and examined for the presence of intact and broken TCK teliospores. Whole wheat samples (50 g) were washed, sieved through a 60-μm nylon sieve, and pelleted by centrifugation. Contents of the pellet were examined microscopically for the presence of TCK spores. The procedure was modified as needed to accommodate cleanings and mill fractions. Levels of spore contamination in whole wheat samples decreased at each step during the handling process, and large numbers of spores were found in materials that were sieved or aspirated from the grain. Very few spores were found in bran, germ, and shorts; none were detected in red dog or straight-grade flour. The results showed that a high percentage of spores can be removed from wheat by mechanical cleaning but that it is not feasible to remove all of them. 相似文献