Invasive Chromolaena odorata species specifically affects growth of its co-occurring weeds.

Plant-plant interaction is essential to weed invasion success and is related to impacts on the environment. To understand interactions of the well-known invasive plant siamweed (Chromolaena odorata) and its neighbors (exotic Praxelis clematidea and native cadillo) in South China, and their competitive mechanisms above- and belowground, a multicultivation experiment was conducted. Competitive indices, plant morphological traits, soil nutrient contents, enzyme activities, and microbial biomass were measured. Competitive balance index and morphological traits revealed balanced competition between P. clematidea and siamweed, and suppressive effect of siamweed on cadillo. In coculture of siamweed and P. clematidea, the branch length of siamweed slightly lengthened, while the branch number of P. clematidea increased compared with their respective monocultures accordingly. Overall impacts of the two invaders on soil properties were near averages of their single impacts. In coculture of siamweed and cadillo, siamweed was more competitive in both light and nutrient capture; soil urease activity and acid phosphatase activity were magnified and mitigated compared with the averages of those in their respective monocultures, respectively. The species-specific results of siamweed competing with its co-occurring weeds would contribute to a better understanding of mechanism in synergistic effect of siamweed with the other invasive plants.

Effects of simulated acid rain on microbial characteristics in a lateritic red soil.

A laboratory experiment was performed to examine the impact of simulated acid rain (SAR) on nutrient leaching, microbial biomass, and microbial activities in a lateritic red soil in South China. The soil column leaching experiment was conducted over a 60-day period with the following six SAR pH treatments (levels): 2.5, 3.0, 3.5, 4.0, 4.5, and 5.0 and one control treatment (pH = 7). Compared with the control treatment, the concentrations of soil organic matter, total nitrogen, total phosphorus, total potassium, soil microbial biomass carbon (MBC), soil microbial biomass nitrogen (MBN), and average well color density (AWCD) in the Ecoplates were all significantly decreased by leaching with SAR at different pH levels. The decrease in MBC and MBN indicated that acid rain reduced the soil microbial population, while the decrease in AWCD revealed that acid rain had a negative effect on soil bacterial metabolic function. Soil basal respiration increased gradually from pH 4.0 to 7.0 but decreased dramatically from pH 2.5 to 3.0. The decrease in soil nutrient was the major reason for the change of soil microbial functions. A principal component analysis showed that the major carbon sources used by the bacteria were carbohydrates and carboxylic acids.

[Impact of Mikania micrantha invasion on soil meso- and micro-invertebrate community structure].

Mikania micrantha, a notorious exotic weed of Asteraceae family, has invaded successfully in southern China, and caused serious damages to native ecosystems. In this paper, a field survey was conducted in the Huolushan Forest Park of Guangzhou, China, aimed to understand the impact of M. micrantha invasion on the soil meso- and micro-invertebrate community. Three sampling sites were installed, including M. micrantha-invaded site, ecotone, and native vegetation site. Through four samplings in 2009, a total of 5206 soil meso- and micro-invertebrate individuals were collected, belonging to 4 phyla, 10 classes, and 19 orders, among which, Nematoda was the dominant group, and Acarina, Collembolan, and Rotifera were the common groups. M. micrantha invasion altered the characteristics of soil meso- and micro-invertebrate community structure. Compared with those at the other two sampling sites, the numbers of total individuals, Nematoda, and Acarina at M. micrantha-invaded site increased significantly, but the groups of soil meso- and micro-invertebrates had less change. At M. micrantha-invaded site, the density-group index (DG) of soil meso- and micro-invertebrates was significantly higher, Margalef richness index (D) and Simpson dominance index (C) tended to ascend, but Pielou evenness index (E) and Shannon index (H') tended to descend. The similarity coefficient of soil meso- and micro-invertebrate community between M. micrantha-invaded site and ecotone was higher than that between M. micrantha-invaded site and native vegetation site. The changes of local climate conditions, plant litters, root secretions, and soil physical-chemical properties caused by M. micrantha invasion could be the major contributing factors that altered the community structure of soil meso- and micro-invertebrates at M. micrantha-invaded site.

[Effects of rice-duck farming on paddy field water environment].

Field experiment was conducted to investigate the effects of rice-duck farming on the water environment in paddy field. The results showed that under rice-duck farming, the temperature and pH value of the surface water in paddy field decreased, and the electrical conductivity, oxidation-reduction potential, turbidity, and the contents of nitrogen (N), phosphorus (P) and potassium (K) increased. The total N, P, and K increased by 1.85-5.06 times, 2.01-8.70 times, and 42.79%-109.18%, respectively, as compared to those in conventional rice farming. All of these illustrated that rice-duck farming improved the paddy field water environment and nutrient supply, optimized the ecological environment of paddy field, and promoted the growth and development of rice.

[Effects of rice-duck farming system on physiological characters of rice].

A field experiment with four treatments of rice-duck farming was conducted in the Zengcheng Teaching and Research Base of South China Agricultural University to further understand the effects of this farming system on the physiological characters of rice. The results showed that rice-duck farming could decrease the plasma membrane permeability and the malondialdehyde and proline contents of rice leaves, but increase their soluble sugar content and superoxide dismutase, peroxidase and catalase activities, indicating that this farming system had positive effects to the improvement of rice plant resistance and the postponement of leaf aging, being of significance on the rice growth and development.