不同养殖模式池塘沉降颗粒营养物质组成特征

2012年4~10月采用自制沉降颗粒收集装置收集主养草鱼和黄颡鱼池塘生态系统中沉降颗粒物,开展了颗粒物质的垂直沉降量以及沉降颗粒中碳氮磷等营养物质的组成及沉降通量的季节变化特征的研究。结果表明:养殖可以显著提高池塘颗粒物质的垂直沉降量,主养草鱼池塘中颗粒物质垂直沉降量显著高于主养黄颡鱼池塘(p0.01),且两种养殖模式池塘颗粒物质沉降量随着养殖时间推进有显著增加的趋势。主养草鱼的池塘中颗粒物质垂直沉降量在100.39~414.66g/(m2·d)之间变化,平均为224.46g/(m2·d),主养黄颡鱼池塘中沉降颗粒物质垂直沉降量在34.14~272.91g/(m2·d)之间变化,平均为155.18g/(m2·d)。两种养殖模式沉降颗粒的碳氮磷成分在养殖周期内的变化规律不明显但具有相似的变化趋势,主养黄颡鱼池塘沉降颗粒中TN、TC、TOC和TON的含量均比同时期主养草鱼高,且均随着养殖时间的推进,沉降颗粒中碳氮磷的总量呈增加趋势。两种养殖模式池塘沉降颗粒中的C/N比值与沉积物中的C/N比值较为接近,表明沉降颗粒与沉积物营养物质来源具有一定的相似性,同时养殖系统内养殖对象与其所处环境的相互依存和相互影响对颗粒物质的产生以及沉降具有重要作用。     

Field studies of ethoprop movement and degradation in two Florida soils

Unsaturated- and saturated-zone field studies were conducted under two different agricultural conditions in Florida, U.S.A., to measure the movement and degradation of ethoprop.When ethoprop was applied to a sand soil in an orange grove located on the Florida central ridge, soil residues declined with a half-life of 13 days to near the limit of detection (0.01 μg g⁻¹ within 2 months. Ethoprop residues were generally contained in the upper 2 m of soil. Although low levels of ethoprop were found in four groundwater samples within the first 3 months after application, the cause of these isolated and transient residues could not be determined. The data show that there was no widespread plume of ethoprop residues beneath or downgradient from the treated area.On a flatwood soil on the west coast of Florida, ethoprop was incorporated into raised beds and protected by a plastic mulch in which tomatoes were grown. When the plastic mulch was present, ethoprop soil residues declined with a half-life of 40 days and were confined to the upper 0.3 m of soil. After the mulch was removed at crop harvest, the remaining ethoprop dissipated with a half-life of 12 days. Groundwater residues in excess of 1 μg L⁻¹ were confined to a single sample collected after the well had been damaged during a rototilling operation. Resides of < 1 μg L⁻¹ were present in three later samples from this well. At the sampling interval where the highest residues were detected in the damaged well, ethoprop residues of < 1 μg L⁻¹ were also found in three other wells but no residues were detected in these wells in later samples.     

The prebiotic synthesis of pyrimidines in frozen solution

Most prebiotic syntheses depend on the reaction of concentrated precursor compounds to produce bio-organic molecules. It is now believed that the early Earth’s atmosphere was not reducing enough to have permitted copious synthesis of precursor molecules. Freezing allows reaction to occur even from dilute solution. This reaction has been demonstrated for the purines but not for the pyrimidines. It is shown here that dilute solutions of simple prebiotic molecules produce the biological pyrimidines cytosine and uracil upon freezing. Cold environments may have allowed synthesis of all of the RNA bases even from low organic yielding atmospheres, such as those of the early Earth, Mars, Titan and Europa.     

Avian and Mammalian Translocations: Update and Reanalysis of 1987 Survey Data

In 1993 we conducted a follow-up study of the 1987 survey by Griffith et al. (1989) of 421 avian and mammalian translocation programs in North America, Australia, and New Zealand to reassess the programs' status and the biological and methodological factors associated with success. Our survey response rate was 81%. Approximately 38% of usable programs in 1993 reported a change in outcome from 1987 (e.g., a translocated population was "declining" but now is "self-sustaining"), but the difference between the overall success rates was not statistically significant (66% in 1987 and 67% in 1993). Since 1987, an increase was observed in the median number of animals translocated per program (31.5 to 50.5), median duration of releases (2 to 3 years), and proportion of programs releasing more than 30 animals (46% to 68%). Multiple logistic regression analyses indicated that release into the core of the historical range, good-to-excellent habitat quality, native game species, greater numbers of released animals, and an omnivorous diet were positively associated with translocation success. Moreover, our results indicate that translocated birds were less successful at establishing self-sustaining populations than translocated mammals. Our findings, using comparable logistic analyses, generally corroborate the results of Grifftth et al. (1989). Variables not found to be significantly correlated with translocation success include species' reproductive potential (number of offspring and first age of reproduction), number and duration of the releases, and source of the translocated animals (wild-caught versus captive-reared).     

Effects of restoration and reflooding on soil denitrification in a leveed Midwestern floodplain.

River floodplains have the potential to remove nitrate from water through denitrification, the anaerobic microbial conversion of nitrate to nitrogen gas. An important factor in this process is the interaction of river water with floodplain soil; however, many rivers have been disconnected from their historic floodplains by levees. To test the effect of reflooding a degraded floodplain on nitrate removal, we studied changes in soil denitrification rates on the Baraboo River floodplain in Wisconsin, USA, as it underwent restoration. Prior to this study, the site had been leveed, drained, and farmed for more than 50 years. In late fall 2002, the field drainage system was removed, and a gate structure was installed to allow controlled flooding of this site with river water. Soil moisture was extremely variable among zones and months and reflected local weather. Soil organic matter was stable over the study period with differences occurring along the elevation gradient. High soil nitrate concentrations occurred in dry, relatively organic-poor soil samples and, conversely, all samples with high moisture soils characterized by low nitrate. We measured denitrification in static cores and potential denitrification in bulk samples amended with carbon and nitrogen, one year before and two years following the manipulation. Denitrification rates showed high temporal and spatial variability. Static core rates of individual sites ranged widely (from 0.00 to 16.7 microg N2O-N x [kg soil](-1) x h(-1), mean +/- SD = 1.10 +/- 3.02), and denitrification enzyme activity (DEA) rates were similar with a slightly higher mean (from 0.00 to 15.0 microg N2O-N x [kg soil](-1) x h(-1), 1.41 +/- 1.98). Denitrification was not well-correlated with soil nitrate, organic matter content, or moisture levels, the three parameters typically thought to control denitrification. Static core denitrification rates were not significantly different across years, and DEA rates decreased slightly the second year after restoration. These results demonstrate that restored agricultural soil has the potential for denitrification, but that floodplain restoration did not immediately improve this potential. Future floodplain restorations should be designed to test alternative methods of increasing denitrification.     

Inorganic nitrogen metabolism in marine bacteria: The intracellular free amino acid pools of a marine pseudomonad

The marine pseudomonad bacterium PL₁ contains an intracellular pool of free amino acids which consist mainly of glutamate with small amounts of glutamine and aspartate when grown in a nutrient medium containing 0.2 M NaCl. When the NaCl concentration of the growth medium is increased to 0.8 M, proline becomes a major component of the intracellular pool together with glutamate—at this molarity and under suitable nutrient conditions these amino acids comprise 20% of total bacterial amino acid nitrogen. When grown in a nutrient growth medium containing a constant level of NaCl, the intracellular pool size can vary by a factor of 4 depending on the concentration of carbon and nitrogen in the medium. Experiments show that the amino acid pool can act as a nitrogen reserve but has little function as a carbon reserve. At high NaCl concentrations there is a marked dependence for growth on the presence of sufficient potassium in the medium. However, no correlation between K⁺ and glutamate concentration in either nitrogen or K⁺-limited cultures has been found. None of the enzymes associated with glutamate biosynthesis was influenced by NaCl levels between 0.2 and 0.5 M. Neither Na⁺ or K⁺ stimulated the activity of these enzymes when tested in vitro.