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Two complementary studies were performed to examine (1) the effect of 18 years of nitrogen (N) fertilization, and (2) the effects of N fertilization during one growing season on soil microbial community composition and soil resource availability in a grassland ecosystem. N was added at three different rates: 0, 5.44, and 27.2 g N m−2 y−1. In both studies, Schizachyrium scoparium was the dominant plant species before N treatments were applied. Soil microbial communities from each experiment were characterized using fatty acid methyl ester (FAME) analysis. Discriminant analysis of the FAMEs separated the three N fertilizer treatments in both experiments, indicating shifts in the composition of the microbial communities. In general, plots that received N fertilizer at low or high application rates for 18 years showed increased proportions of bacterial FAMEs and decreased fungal FAMEs. In particular, control plots contained a significantly higher proportion of fungal FAMEs C18:1(cis9) and C18:2(cis9,12) and of the arbuscular mycorrhizal fungal (AMF) FAME, C16:1(cis11), than both of the N addition treatment plots. A significant negative effect of N fertilization on the AMF FAME, C16:1(cis11), was measured in the short-term experiment. Our results indicate that high rates of anthropogenic N deposition can lead to significant changes in the composition of soil microbial communities over short periods and can even disrupt the relationship between AMF and plants.  相似文献   
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A simple HPLC method is presented to quantify the low concentration of hydroxylysyl pyridinoline (PYD) cross-links in Atlantic salmon (Salmo salar L.) muscle. The method involved the extraction of tissue with NaOH prior to hydrolysis, which greatly reduced the amount of protein to be hydrolyzed and made downstream operations easier and more reproducible. The concentration of PYD was 426 pmol g(-)(1) dry mass muscle in post-rigor muscle stored at 0 degrees C and sampled 3 d after death. Hydroxproline (HYP) concentration was determined following NaOH extraction as a measure of collagen content. In post-rigor samples, the alkaline-insoluble HYP fraction comprised 18.3% of the total HYP. Scanning electron microscopy revealed shrinkage of muscle fibers and a retraction of the connective tissue matrix in smoked salmon. PYD concentration was relatively resistant to processing to the smoked product, decreasing by around 11.7%, as compared to a 22.2% decrease in HYP. There was a positive correlation between PYD concentration and the firmness of post-rigor muscle samples as measured by an instrumental texture analyzer, explaining 25% of the total variation. A weaker but still significant correlation was found between PYD concentration and firmness in the smoked product. There was no relationship between fillet firmness and total collagen concentration, although the correlation with HYP in the alkaline-insoluble fraction was significant at the 6% level (P = 0.057). Our results indicate that only 1-3% of collagen molecules are linked by nonreducible mature cross-links in harvest size farmed Atlantic salmon and that PYD concentration is an important raw material characteristic for flesh quality.  相似文献   
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Microbial activity is known to continue during the winter months in cold alpine and Arctic soils often resulting in high microbial biomass. Complex soil nutrient dynamics characterize the transition when soil temperatures approach and exceed 0 °C in spring. At the time of this transition in alphine soils microbial biomass declines dramatically together with soil pools of available nutrients. This pattern of change characterizes alpine soils at the winter-spring transition but whether a similar pattern occurs in Arctic soils, which are colder, is unclear. In this study amounts of microbial biomass and the availability of carbon (C), nitrogen (N) and phosphorus (P) for microbial and plant growth in wet peaty soils of an Arctic sedge meadow have been determined across the winter-spring boundary. The objective was to determine the likely causes of the decline in microbial biomass in relation to temperature change and nutrient availability. The pattern of soil temperature at depths of 5-15 cm can be divided into three phases: below −10 °C in late winter, from −7 to 0 °C for 7 weeks during a period of freeze-thaw cycles and above 0 °C in early spring. Peak microbial biomass and nutrient availability occurred early in the freeze-thaw phase. Subsequently, a steady decrease in inorganic N occurred, so that when soil temperatures rose above 0 °C, pools of inorganic nutrients in soils were very low. In contrast, amounts of microbial C and soluble organic C and N remained high until the end of the period of freeze-thaw cycles, when a sudden collapse occurred in soluble organic C and N and in phosphatase activity, followed by a crash in microbial biomass just prior to soil temperatures rising consistently above 0 °C. Following this, there was no large pulse of available nutrients, implying that competition for nutrients from roots results in the collapse of the microbial pool.  相似文献   
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The records of 3,952 equine patients presenting to the Veterinary Teaching Hospital at North Carolina State University College of Veterinary Medicine were evaluated to determine risk factors associated with thrombocytopenia. Of 2,346 horses from which a CBC was obtained, 35 (1.49%) were thrombocytopenic (platelet count < 75,000/μL). A reference population of 189 horses with normal platelet counts (75,000 to 300,000/μL) was also studied. Standardbred horses were at increased risk for thrombocytopenia. but age and gender were not identified as significant risk factors. Horses with infectious or inflammatory diseases were at increased risk for thrombocytopenia. The potential association of clinical and clinicopathologic factors with thrombocytopenia were assessed by reviewing a series of multiple logistic regression models. Clinical and clinicopathologic variables significantly associated with thrombocytopenia in the final model included increased PCV, increased band neutrophil count, increased total WBC, and decreased plasma protein concentration. Increased mature neutrophil count was associated with normal platelet counts. Thrombocytopenic horses were significantly more likely to die or be euthanized than were horses with normal platelet counts. J Vet Intern Med 1996;10:127–132. Copyright © 1996 by the American College of Veterinary Internal Medicine .  相似文献   
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