Impacts of Ipomoea cairica and Wedelia trilobata invasion on soil chemical and microbiological characteristics

【Objectives】Ipomoea cairica and Wedelia trilobata, the two general exotic plants, have invaded successfully in southern China and caused lots of serious damages to native ecosystem. In order to explore impacts of the two weeds invasion on soil ecosystem and provide scientific basis for their invasion mechanism in fields, the changes of soil chemical and microbiological properties, including soil nutrient, soil microbial biomass, soil enzyme and metabolic activity, carbon substrate utilization and function diversity of soil microbial community in I. cairica invaded community, W. trilobata invaded community and Neyraudia reynaudiana native community were investigated by using field plot methods. 【Methods】Three sampling sites were selected, including I. cairica invaded site, W. trilobata invaded site and N. reynaudiana native site in the Huolushan Forest Park of Guangzhou city in January 2010. There were four replicates per sampling site (each replicate plot was 4 m2). Soil samples were collected, and soil nutrient, soil enzyme activity, soil microbial biomass and function diversity of soil microbial community were determined. 【Results】 1) Compared with the native plant plot, the contents of soil organic carbon, total nitrogen, total phosphorus, available nitrogen, available phosphorus and available potassium in the I. cairica plot (increased range is 60.38%-230.01%) and the contents of soil organic carbon, total nitrogen, available nitrogen and available phosphorus in the W. trilobata plot (increased range is 50.54%-145.52%) are significantly increased, while the soil C/N ratios are significantly decreased, and there are no significant differences in the contents of total potassium among the three plots. 2) The two exotic plants could significantly increase soil microbial biomass of the invaded plots. The contents of soil microbial biomass carbon, nitrogen and phosphorus of the I. cairica plot and the W. trilobata plot are 105.00%-152.15% and 61.51%-138.27% higher than those in the native plant plot respectively, while there are no significant differences in the contents of soil microbial biomass between the two invaded plots. The ratio of soil microbial biomass carbon to soil organic carbon (Cmic/Corg) in the W. trilobata plot is significantly higher than that in the native plant plot, but the ratios of soil microbial biomass nitrogen to total soil nitrogen (Nmic/Nt) and soil microbial biomass phosphorus to total soil phosphorus (Pmic/Pt) among the three plots are in the same levels. 3) The two exotic plants invasion could significantly increase the activities of soil urase, protease, invertase and cellulose enzymes. The activities of soil urase, protease and cellulose enzymes of the W. trilobata plot are the highest in all the sampling plots, and are 70.35%, 21.51% and 227.86% higher than those of the native plant plot respectively. The activities of soil invertase of the I. cairica plot and the W. trilobata plot are 322.58% and 157.14% higher than that of the native plant plot respectively, while the activities of hydrogen peroxidase are almost the same among the three plots and no significant differences are found. 4) The two exotic plants could significantly increase the metabolic activities of soil microbial community of the invaded plots. The average well color development (AWCD) of all treatments exhibits an order of I. cairica plot ＞ W. trilobata plot ＞ native plant plot during the whole incubating period. The AWCD values of three sampling plots incubated at 72 hours are 1.18, 0.88 and 0.56 respectively, and the differences are significant. Compared to the native plant plot, I. cairica invasion could significantly improve six carbon substrate utilization of soil microbial community, with the increased ranges reach up to 75.00%-162.86%. W. trilobata invasion also significantly increases the utilizations of carbohydrates, carboxylic acids and polymers, with the increased rates of 87.72%, 41.18% and 83.72% respectively. There are some differences in the utilizations of different carbon substrates between the two invasive plants, and the carboxylic acids and amines/amides utilizations of soil from the I. cairica plot are significantly greater than those in the W. trilobata plot. The principal components analysis (PCI) reveals that 71.89% of the variation of carbon substrate utilization data of soil microbial community in different invasion plots could be explained by the first and second components (PC1 and PC2). Carbohydrates, amino acids and polymers are highly correlated with PC1, while PC2 is mainly affected by carbohydrates and carboxylic acids. The two exotic plants could significantly influence the function diversity of soil microbial community. The indexes of Shannon-Wiener (H), Mc Intosh (U), substrate richness (S) and Simpsons dominance (Ds) of soil microbial community in the two invaded plots are significantly higher than those in the native plant plot, but the differences between the two invaded plots are not significant. The Pielou indexes (E) also have not significant differences among all the three treatments. 【Conclusions】 The two exotic plants, I. cairica and W. trilobata, can ameliorate soil nutrition environment in the invaded areas, enhance supply level of soil nutrients, and hence create a favorable microhabitat which can benefit their growth and competitiveness and facilitate their invasion in fields.