Influence of Dry Density on Soil-Water Retention Curve of Unsaturated Soils and Its Mechanism Based on Mercury Intrusion Porosimetry

The soil-water retention curve(SWRC)can be used to evaluate the ability of unsaturated soils to attract water at various water contents and suctions. In this study, drying SWRCs for a kind of sandy soil were obtained in the laboratory by using self-modified SWRC apparatus. In addition, the porosity and the pore size distribution of the samples were investigated by a mercury porosimetry test in order to analyze the effect of dry density. Results showed that the soil-water retention of the soil specimens was strongly dependent on the dry density. Under zero suction, soil specimens with a higher dry density exhibited lower initial volumetric water content. The higher the dry density of soil, the more slowly the volumetric water content decreased with the increase of suction. There was a general and consistent trend for a soil specimen to possess a larger air-entry value and residual suction, while smaller slope of SWRC when it had a higher density. This was probably attributed to the presence of smaller interconnected pores in the soil specimen with a higher dry density. The proportion of large diameter pores decreased in comparison to pores with small diameters in the soil tested. The measured total pore volume of the soil specimen, which had a larger dry density, was lower than that of the relatively loose specimens.

New macroalgal fossils of the Kaili Biota in Guizhou Province, China

The Kaili Biota of Jianhe County, Guizhou Province is found in a Burgess Shale type of deposits, and as such it provides a key fossil record for studying biotic differentiation following the Cambrian explosion. Macroalgae represent several important groups of primary producers and yet have not been well studied. Here, we report two new forms of macroalgae from the Kaili Biota, including a new genus and species and an indeterminate genus and species. The new genus and species, Parallelphyton tipica, is a heterotrichous alga with thalli composed of a prostrate cardinal axis, and parallel bundles of erect trichomes on the axis. The bifurcation of the trichomes is somewhat similar to the living heterotrichous Fritschiella (Chlorophyta), representing "architectonic differentiation" only known in higher plants as an adaptation to terrestrial conditions. The indeterminate genus and species consists of an arched, thick, cylindrical major shaft and irregularly spaced, curved lateral branches attached to the major shaft and subdivided into subbranches. The characteristics of the thick and strong major shaft and the lateral branch subdivided branches are analogous to the dichotomous branching of Psilophyton in Protopteridophyta. Thus, the differentiation of the thalli in these two algal forms represents the expression of properties of some higher metaphytes, indicating that the ecological differentiation of the macroalgae was well underway at the time of the Kaili Biota.