Carbon flows in the rhizosphere of ryegrass (Lolium perenne)

This study addresses the issue of carbon (C) fluxes through below ground pools within the rhizosphere of Lolium perenne using the ¹⁴C pulse labeling. Lolium perenne was grown in plexiglas chambers on topsoil of a Haplic Luvisol under controled laboratory conditions. ¹⁴C‐CO₂ efflux from soil, as well as ¹⁴C content in shoots, roots, soil, dissolved organic C (DOC), and microbial biomass were monitored for 11 days after the pulsing. Lolium allocates about 48 % of the total assimilated ¹⁴C below the soil surface, and roots were the primary sink for this C. Maximum ¹⁴C content in the roots was observed 12 hours after the labeling and it amounts to 42 % of the assimilated C. Only half of the ¹⁴C amount was found in the roots at the end of the monitoring period. The remainder was lost through root respiration, root decomposition, and rhizodeposition. Six hours after the ¹⁴C pulse labeling soil accounted for 11 %, DOC for 1.1 %, and microbial biomass for 4.9 % of assimilated C. ¹⁴C in CO₂ efflux from soil was detected as early as 30 minutes after labeling. The maximum ¹⁴C‐CO₂ emission rate (0.34 % of assimilated ¹⁴C h^—1) from the soil occurred between four and twelve hours after labeling. From the 5^th day onwards, only insignificant changes in carbon partitioning occurred. The partitioning of assimilated C was completed after 5 days after assimilation. Based on the ¹⁴C partitioning pattern, we calculated the amount of assimilated C during 47 days of growth at 256 g C m^—2. Of this amount 122 g C m^—2 were allocated to below ground, shoots retained 64 g C m^—2, and 70 g C m^—2 were lost from the shoots due to respiration. Roots were the main sink for below ground C and they accounted for 74 g C m^—2, while 28 g C m^—2 were respired and 19 g C m^—2 were found as residual ¹⁴C in soil and microorganisms.     

Food preferences of earthworms for soil fungi

Soil fungi are considered to be an important food source for earthworms. Selection experiments were carried out in order to study the preferences of earthworm species for a variety of soil fungi. Nine fungal species (Cladosporium cladosporioides, Rhizoctonia solani, Mucor sp., Trichoderma viride, Fusarium nivale, Phlebia radiata, Glaeophyllum trabeum, Coniophora puteana, Coriolus versicolor) were grown separately in centrifuge tubes on sterilized sand with potato dextrose. Tubes containing different fungal species, 8–9 per experiment, were arranged in a food choice arena. The preference for the fungi of 5 different earthworm species (Lumbricus terrestris, Lumbricus castaneus, Aporrectodea caliginosa, Aporrectodea rosea, Octolasion cyaneum) was tested by adding one specimen per chamber. Removal of sand from the tubes within 6 days was used as the indicator of preference by earthworms. The food preference of earthworms irrespective of ecological group followed a general pattern. F. nivale and C. cladosporioides were the preferred fungal species, followed by fast-growing species such as Mucor sp. and R. solani. In contrast, basidiomycetes were generally refused. The epigeic species L. rubellus had the strongest preference for a single fungal species, in contrast the endogeic species A. rosea fed more evenly on different fungal species. We conclude that early successional fungal species are used as cues by earthworms to detect fresh organic resources in soil.     

Universality of a surface tension—contact‐angle relation for hydrophobic soils of different texture

Wettability of soil affects a wide variety of processes in soils like infiltration, percolation, preferential flow, and surface runoff. Even though efforts have been made to determine contact angles and surface tension or energy of smooth surfaces, the determination on granular materials like soil particles remains unsolved. One objective of this study was to test the consistency of contact angles (CA) measured with the newly modified and easy‐to‐apply Wilhelmy plate method by using solid particles and liquids with defined variations of surface tension. A second objective was to derive basic physical surface properties for the irregularly shaped and chemically heterogeneous soil particles. Advancing contact angles were determined by using model soils varying in texture from clay to coarse sand to check the impact of grain size on the CA measurements. Varying the solid‐surface tension with dichlorodimethlylsilane treatments provided for pure‐water wetting stages ranging from wettable to extremely hydrophobic. The surface tensions of the liquids were varied from 72 mN m^–1 to 25 mN m^–1 by using water or water‐ethanol mixtures. The surface tensions of the model soils were determined with the zero degree–contact angle method following Zisman's critical–surface tension concept. Results show that the measured CA varied continuously with the variations of the surface tension of the liquid and the solid phase, respectively. A general interpretation of the results is possible by using the concept of the Equation of State Approach.     

Biological Activity of Two Stereoisomers of the N-Thienyl Chloroacetamide Herbicide Dimethenamid

Due to the presence of an asymmetrically substituted C atom, dimethenamid [2-chloro-N-(2,4-dimethyl-3-thienyl)-N-(2-methoxy-1-methylethyl)acetamide], a recently introduced N-thienyl chloroacetamide herbicide, exists as two stereoisomers (S and R) having differing herbicidal activities as demonstrated with a selection of weeds and Lemna minor. The activity of the two isomers was investigated in greater detail with the green alga Scenedesmus acutus and compared to that of alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide]. As with alachlor, the S isomer (5 μM ) strongly inhibited algal growth and fatty acid desaturation while the R isomer had no effect. In short-term experiments (up to 5·5 h), the S isomer and alachlor (100 μM ) inhibited [¹⁴C]acetate uptake and its incorporation into fatty acids in the same manner, while the R isomer did not. Incorporation of [¹⁴C]acetate into a non-lipid fraction of the algae was strongly inhibited by alachlor and the S isomer (100 μM ) and only slightly by the R isomer. A 50% inhibition of incorporation of [¹⁴C]oleic acid into the same non-lipid fraction was attained with less than 10^-7 M of the S isomer while 10^-5 M of the R form of dimethenamid achieved only a 40% inhibition. The same stereospecificity of the compound on growth, fatty acid desaturation, acetate uptake and oleic acid incorporation provides strong evidence that dimethenamid may act upon a primary, specific target in lipid metabolism. Furthermore, the comparable biological activities of dimethenamid and alachlor indicate that this target is common to both N-phenyl and N-thienyl chloroacetamide herbicides. © 1997 SCI.     

Tree shelters and weed control: Effects on protection,survival and growth of cherrybark oak seedlings planted on a cutover site

In the southern USA oaks (Quercusspp.) are often favored by forest owners havingmultiple objectives for forest ownership as oaksprovide mast for wildlife, are consideredaesthetically pleasing, and are valuable for timberproducts. Regeneration and early seedling growth isa concern to those forest owners interested insustaining oaks as a component of their forests. Theeffects of tree shelters and herbaceous weed controlon second-year seedling survival, browse by deer andrabbits, and seedling growth of hand-plantedcherrybark oak (Quercus pagoda Raf.) wereexamined. The study was established on a cutovermixed pine-hardwood forest in Alabama. Fourtreatments were: weed control only, tree shelter only,tree shelter with weed control, and a controlconsisting of a seedling without a tree shelter orweed control. No significant difference in seedlingsurvival was found among the treatments after twoyears. Tree shelters were effective in preventingbrowsing. No seedlings in the tree-shelter-onlytreatment were browsed. There was no significant,difference, however, in the percentage of seedlingsbrowsed between the control treatment and the weedcontrol treatment. The use of tree shelters with weedcontrol was the most effective treatment for promoting2-year ground-line diameter, height, and stem volumegrowth.     

猪卵母细胞电激活参数的研究

以体外成熟的猪卵母细胞为对象,研究不同卵龄、电场强度、脉冲次数和激活液Ca ,Mg 对猪卵母细胞电激活效果的影响.结果表明,1次脉冲就足以激活猪卵母细胞,电场强度以1 250～1500V/cm为优,含Ca ,Mg 的激活液激活效果显著高于非电解质液,卵龄以54h为宜.     

A Preliminary Study of the Spatial Variation in Growth of Raft-Cultured Blue Mussels Mytilus trossulus in Northern Puget Sound, Washington

Abstract.– The effects of culture rope density (high and low), position (inside, middle. and outside), and depth (1, 3, and 5 m) on growth rates, expressed as mean shell length increase (mm) per day, were quantified in raft-cultured blue mussels Mytilus trossulus in northern Puget Sound during the fall of 1993. Growth rates differed by as much as 20% after the 90-d experimental period, and varied significantly according to culture rope position, but not according to culture rope density or depth. Furthermore, a three-way analysis of variance revealed a significant first-order interaction effect, culture rope density × depth. The significant culture rope position effect was due to the higher growth rate (mean ± SEM) of mussels on outside culture ropes (0.199 ± 0.009 mm/d) compared to inside culture ropes (0.168 ± 0.007 mm/d). The interaction effect was due to the significantly higher growth rates of mussels at I and 3 m (0.197 ± 0.009 and 0.204 ± 0.011 mm/d, respectively) compared to 5 m (0.162 ± 0.014 mm/d) under low culture rope density conditions, and the significantly higher growth rate of mussels at 3 m under low culture rope density conditions (0.204 ± 0.011 mm/d) compared to high culture rope density conditions (0.168 ± 0.012 mm/d). Practical raft management is affected by such variation in growth. Commercial mussel growers may find it necessary to alter their culture methods to insure a more uniform product from seed to harvest.     

The interrelationships of stride lengths and stride times to velocities of galloping horses

Side view slow motion movies of galloping horses were taken and analyzed to determine horse velocity, limb velocity, stride length, stride time, and the swing and support timesof the stride. Multivariate statistical methods were used to evaluate the interrelationships of theseparameters. A linear relationship between horse velocity and limb velocity was observed; however, two groups could be distinguished within the trials. In one group, in which the horses were galloping normally, a one-to-one relationship of limb velocity to horse velocity was noted. In the other group, in which the majority of the horses were either lame or restrained by the rider, limb velocities were higher than horse velocities.In general, as horse velocity increased, stride length increased and stride time decreased due to decreases in both the swing and support times of the stride. Increases in stride length contributed more to increases in horse velocity than changes in either swing or support times. Changes in stride length had the greatest effect upon the velocities of those horses that were either lame or restrained by the rider, while the effects of changes in swing and support times contributed relatively more to changes in velocities of the horses which were galloping normally.This study suggests that in lame horses, limb velocity is proportionally greater than horse velocity due to decreases in stride length and stride time.