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61.
Soil moisture strongly controls the uptake of atmospheric methane by limiting the diffusion of methane into the soil, resulting in a negative correlation between soil moisture and methane uptake rates under most non-drought conditions. However, little is known about the effect of water stress on methane uptake in temperate forests during severe droughts. We simulated extreme summer droughts by exclusion of 168 mm (2001) and 344 mm (2002) throughfall using three translucent roofs in a mixed deciduous forest at the Harvard Forest, Massachusetts, USA. The treatment significantly increased CH4 uptake during the first weeks of throughfall exclusion in 2001 and during most of the 2002 treatment period. Low summertime CH4 uptake rates were found only briefly in both control and exclusion plots during a natural late summer drought, when water contents below 0.15 g cm−3 may have caused water stress of methanotrophs in the A horizon. Because these soils are well drained, the exclusion treatment had little effect on A horizon water content between wetting events, and the effect of water stress was smaller and more brief than was the overall treatment effect on methane diffusion. Methane consumption rates were highest in the A horizon and showed a parabolic relationship between gravimetric water content and CH4 consumption, with maximum rate at 0.23 g H2O g−1 soil. On average, about 74% of atmospheric CH4 was consumed in the top 4-5 cm of the mineral soil. By contrast, little or no CH4 consumption occurred in the O horizon. Snow cover significantly reduced the uptake rate from December to March. Removal of snow enhanced CH4 uptake by about 700-1000%, resulting in uptake rates similar to those measured during the growing season. Soil temperatures had little effect on CH4 uptake as long as the mineral soil was not frozen, indicating strong substrate limitation of methanotrophs throughout the year. Our results suggest that the extension of snow periods may affect the annual rate of CH4 oxidation and that summer droughts may increase the soil CH4 sink of temperate forest soils. 相似文献
62.
Ian C. Grieve Donald A. Davidson Patricia M.C. Bruneau 《Soil biology & biochemistry》2006,38(2):229-234
A pulse of 13CO2 was added to the above ground vegetation in an upland grassland to determine the effects of faunal diversity on the flux of carbon to the surface horizons of the soil. Faunal diversity was manipulated by liming and biocide treatments for three years prior to the pulse addition. The relocation of 13C within roots and rhizosphere soil was determined by analysis of samples of bulk soil and of specific features identified on soil thin sections on four dates after the addition of the 13CO2 pulse. Analysis of bulk soils showed only a small enrichment in 13C and no significant effects of the treatments. Analysis by isotope ratio mass spectrometry of the products of in situ laser combustion of root material and aggregates formed from faunal excrement showed that the distribution of the newly photosynthesised 13C is very localised, with large spatial variability in soil and root δ13C at scales of less than 1 mm. δ13C values ranged from the natural abundance level of around −28‰ to −4.9‰ in roots and to −8.4‰ in aggregates. The small pulse and large spatial variability masked any effects of the liming and biocide treatments in these soils. However, the variability in the relocation of newly photosynthesised carbon may help to explain the large spatial variability found in bacterial numbers at the sub-mm scale within soils and emphasises the importance of the accessibility of substrates to decomposers in undisturbed structured soils. 相似文献
63.
Anterior uveal melanoma, with secondary keratitis, cataract, and glaucoma, in a horse. 总被引:1,自引:0,他引:1
H J Davidson G L Blanchard C A Wheeler J A Render 《Journal of the American Veterinary Medical Association》1991,199(8):1049-1050
Intraocular melanoma was diagnosed in a 13-year-old horse. Secondary clinical findings included keratitis, cataract, and glaucoma. The eye was enucleated. Follow-up information did not give an indication of metastatic disease. 相似文献
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Effects of tylosin feeding on the antibiotic resistance of selected gram-positive bacteria in pigs 总被引:2,自引:0,他引:2
P J Christie J N Davidson R P Novick G M Dunny 《American journal of veterinary research》1983,44(1):126-128
The effect of tylosin on macrolide resistance of gram-positive bacteria of pigs was determined. After an initial base-line period during which the pigs were given antibiotic-free feed, 1 group of 8 pigs was given tylosin feed (100 g/US ton of feed), and a 2nd group of 7 was given antibiotic-free feed. Samples were taken at 2- to 3-week intervals. For each pig, rectal, skin, and nasal swab samples were collected for enumeration of fecal streptococci and skin and nasal staphylococci. Percentages of macrolide resistant organisms of each group were tabulated on the basis of colony counts from antibiotic free and erythromycin-containing plates. After the introduction of tylosin into the feed of 1 group, a clear difference between the 2 groups with respect to the macrolide resistance of their gram-positive microflora was observed. The data indicate that tylosin feeding results in an increase in macrolide resistance of the bacterial flora of pigs. 相似文献
67.
A van der Woerdt M P Nasisse M G Davidson 《Journal of the American Veterinary Medical Association》1992,201(6):921-926
During a 5-year period, phacolytic uveitis was diagnosed in 202 eyes of 151 dogs admitted to the veterinary teaching hospital. The diagnosis of phacolytic uveitis was based on the finding of a cataractous lens and anterior uveitis, unassociated with other identifiable causes of uveal inflammation. The most commonly affected breeds were the Toy and Miniature Poodle (35%) and the American Cocker Spaniel (19%). The mean age was 7.0 years for all breeds, 5.1 years for the Cocker Spaniel, and 9.0 years for the Poodle breeds. Evidence of cataract resorption was visible in 72% of the eyes. Fifty-one dogs were affected bilaterally and 100 dogs unilaterally. The mean interval between recognition of the cataract and the onset of lens-induced uveitis (LIU) was 17 months; mean times of 25 and 11 months were seen in the Poodle breeds and American Cocker Spaniel, respectively. The mean age of dogs requiring greater than 1 revisit before the inflammation had subsided was 5.5 years. Complications, referable to the uveitis, were seen in 14% of eyes, the most important of which were glaucoma (16 eyes) and phthisis bulbi (9 eyes). Lens extraction surgery was done on 50 LIU-affected eyes, and on 35 normal eyes in LIU-affected animals. The 2- and 6-month success rates for LIU-affected eyes were 78 and 39%, respectively, and for normal eyes in LIU-affected animals were 85 and 71%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
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