Particle-attached and free-living bacterial communities in a contrasting marine environment: Victoria Harbor, Hong Kong

Diversity of particle-attached and free-living marine bacteria in Victoria Harbor, Hong Kong, and its adjacent coastal and estuarial environments was investigated using DNA fingerprinting and clone library analysis. Denaturing gradient gel electrophoresis (DGGE) analysis of 16S rRNA genes showed that bacterial communities in three stations of Victoria Harbor were similar, but differed from those in adjacent coastal and estuarine stations. Particle-attached and free-living bacterial community composition differed in the Victoria Harbor area. DNA sequencing of 28 bands from DGGE gel showed Alphaproteobacteria was the most abundant group, followed by the Bacteroidetes, and other Proteobacteria. Bacterial species richness (number of DGGE bands) differed among stations and populations (particle-attached and free-living; bottom and surface). BIOENV analysis indicated that the concentrations of suspended solids were the major contributing parameter for the spatial variation of total bacterial community structure. Samples from representative stations were selected for clone library (548 clones) construction and their phylogenetic distributions were similar to those of sequences from DGGE. Approximately 80% of clones were affiliated to Proteobacteria, Bacteroidetes and Cyanobacteria. The possible influences of dynamic pollution and hydrological conditions in the Victoria Harbor area on the particle-attached and free-living bacterial community structures were discussed.

Particle-attached and free-living bacterial communities in a contrasting marine environment: Victoria Harbor, Hong Kong

Diversity of particle-attached and free-living marine bacteria in Victoria Harbor, Hong Kong, and its adjacent coastal and estuarial environments was investigated using DNA fingerprinting and clone library analysis. Denaturing gradient gel electrophoresis (DGGE) analysis of 16S rRNA genes showed that bacterial communities in three stations of Victoria Harbor were similar, but differed from those in adjacent coastal and estuarine stations. Particle-attached and free-living bacterial community composition differed in the Victoria Harbor area. DNA sequencing of 28 bands from DGGE gel showed Alphaproteobacteria was the most abundant group, followed by the Bacteroidetes, and other Proteobacteria. Bacterial species richness (number of DGGE bands) differed among stations and populations (particle-attached and free-living; bottom and surface). BIOENV analysis indicated that the concentrations of suspended solids were the major contributing parameter for the spatial variation of total bacterial community structure. Samples from representative stations were selected for clone library (548 clones) construction and their phylogenetic distributions were similar to those of sequences from DGGE. Approximately 80% of clones were affiliated to Proteobacteria, Bacteroidetes and Cyanobacteria. The possible influences of dynamic pollution and hydrological conditions in the Victoria Harbor area on the particle-attached and free-living bacterial community structures were discussed.

Effect of annual weeds on the growth of perennial grass mixtures in the alpine region of the Qinghai-Tibetan Plateau

In the alpine region of the Tibetan Plateau, five perennial grass cultivars, Bromus inermis (B), Elymus nutans (E), Clinelymus nutans (C), Agropyron cristatum (A), and Poa crymophila (P) were combined into nine communities with different compositions and ratios, B+C, E+A, B+E+A, E+B+C,C+E+A,B+E+C+A,B+C+A+P,B+E+A+P and E+C+A+P. Each combination was sown in six 10 X 10 m plots with three hand-weeded plots and three natural-growing plots in a completely randomised design in 1998. A field experiment studied the performance of these perennial grass combinations under the competitive interference of annual weeds in 3 consecutive years from 1998 to 2000. The results showed that annual weeds occupied more space and suppressed the growth of the grasses due to earlier germination and quicker growth in the establishment year, but this pattern changed in the second and third years. Leaf area indexes (LAIs) of grasses were greatly decreased by the competitive interference of weeds, and the negative effect of weeds on LAIs of grasses declined and stabilised in the second and third years. E+B+C, B+E+C+A, and B+E+A+P possessed relatively higher LAIs (P < 0.05) among all grass combinations and their LAIs were close to five when the competitive interference of weeds was removed. Grasses were competitively inferior to weeds in the establishment year, although their competitive ability (aggressivities) increased throughout the growing season. In the second and third years, grasses were competitively superior to weeds, and their competitive ability decreased from May until August and increased in September. Dry matter (DM) yields of grasses were reduced by 29.8-74.1% in the establishment year, 11.0-64.9% in the second year, and 16.0-55.8% in the third year by the competitive interference of weeds. B+E+C+A and B+E+A+P can produce around 14 t/ha of DM yields, significantly higher (P < 0.05) than the production of the other grass combinations in the second and third years after the competitive interference of weeds was removed. It was preliminarily concluded that removal of competitive interference of weeds increased the LAIs of all grass swards and improved the light interception of grasses, thus promoting the production of perennial grass pastures. The germination stage of the grasses in the establishment year was the critical period for weeding and suppression of weeds should occur at an early stage of plant growth. The grass combinations of B+E+C+A and B+E+A+P were productive and can be extensively established in the alpine regions of the Tibetan Plateau. Two or three growing seasons will be needed before determining success of establishment of grass mixtures under the alpine conditions of the Tibetan Plateau.

Productivity and persistence of perennial grass mixtures under competition from annual weeds in the alpine region of the Qinghai-Tibetan Plateau

In the alpine region of the Qinghai-Tibetan Plateau four indigenous perennial grass species Bromus inermis (BI), Elymus sibiricus (ES), Elymus nutans (EN) and Agropyron cristatum (AC) were cultivated as three mixtures with different compositions and seeding rates, BI + EN, BI + ES + AC and BI + ES + EN + AC. From 1998 to 2001 there were three different weeding treatments: never weeded (CK); weeded on three occasions in the first year (1-y) and weeded on three occasions in both the first and second year (2-y) and their effect of grass combination and interactions on sward productivity and persistence was measured. Intense competitive interference by weedy annuals reduced dry matter (DM) yield of the swards. Grass combination significantly affected sward DM yields, leaf area index (LAI) and foliar canopy cover and also species composition DM and LAI, and species plant cover. Interaction between weeding treatments and grass combination was significant for sward DM yield, LAI and canopy cover, but not on species composition for DM, LAI or species plant cover. Grass mixture BI + ES + EN + AC gave the highest sward DM yield and LAI for both weeding and non-weeding treatments. Species ES and EN were competitively superior to the others. Annual weedy forbs must be controlled to obtain productive and stable mixtures of perennial grasses, and germination/emergence is the most important time for removal. Weeding three times (late May, late June and mid-July) in the establishment year is enough to maintain the production and persistence of perennial grass mixtures in the following growing seasons. Extra weeding three times in the second growing year makes only a slight improvement in productivity.