| Mikania micrantha H. B. K. (Asteraceae), one of the worst invasive weeds seriously invading South China, has been received considerable attention of ecologists and environmental scientists. The vine Cuscuta campestris Yuncker which is able to parasitize M. micrantha is proposed to be a biological control agent for the invader. But how does C. campestris control M. micrantha is not clear. Can the soil properties of M. micrantha communities be altered after C. campestris parasitizing the invader? Is the growth of the native plant Coixlacryma jobi L. different on the soil invaded by M. micrantha with or without C. campestris parasitizing the invader? Is the growth of C. jobi changed after C. campestris parasitizing M. micrantha? This study is aim at assessing the effects of M. micrantha invasion on soil microbes after C. campestris parasitizing the invader and exploring the role of soil microbes in the growth of native plant C. jobi. The investigated soil properties were pH value, total organic carbon, total nitrogen, organic nitrogen, total phosphorus, available phosphorus, nitrate nitrogen, ammonium nitrogen, inorganic nitrogen, soil microbial biomass carbon (MBC), soil microbial biomass nitrogen (MBN) and soil microbial biomass phosphorus (MBP), soil acid phosphorase activity, soil urease activity, soilβ-D-glucosidase activity, average well color development (AWCD), positive well number and soil carbon utilization pattern. The investigated growth characteristics of C. jobi was the biomass of different parts, biomass allocation pattern, photosynthesis properties, relative content of chlorophyll and chlorophyll fluorescence parameters. We investigated the soil properties in four natural communities and in green house experiments. The growth characteristics and the photosynthesis properties of C. jobi on soil treated with the microbial inhibitors and activated carbon to verify the role of the soil microbes and allelopathy in the growth of C. jobi. The possible ecophysiological, soil nutrient cycle, soil microbial, soil allelopathic mechanisms were provided and discussed.1. We investigated the soil properties through sampling in four plots located in Neilingding island, Guangdong Province, China, including those (1) not invaded by M. micrantha and without C. campestris (WU), (2) invaded by M. micrantha and without C. campestris (W), (3) invaded by M. micrantha and with C. campestris just parasitizing the invader (TW1), and (4) invaded by M. micrantha a and with C. campestris parasitizing the invader for three years (TW3). In plot W, pH value, total organic carbon, total nitrogen, organic nitrogen and ammonium nitrogen of the soil all were significantly greater than those in plot WU. However, soil nitrate nitrogen and inorganic nitrogen was significantly lower than those in plot WU. There was no difference in soil total phosphorus and soil available phosphorus between plot W and WU. MBC, MBN and MBP as well as the activities of soil acid phorsphorase, ureases andβ-D-glucosidase in plot W were significantly higher than those in plot WU. In plot TW1, pH value, total organic carbon, total nitrogen, organic nitrogen, ammonium nitrogen and inorganic nitrogen of the soil were significantly lower than those in plot W, while there was no significant difference in soil total phosphorus, available phosphorus and nitrate nitrogen between plot TW1 and W. MBC, MBN and MBP as well as the activities of soil acid phorsphorase, ureases andβ-D-glucosidase in plot TW1 were significantly lower than those in plot W. But there was certain difference of them between plot TW1 and WU. Total organic carbon, total nitrogen, organic nitrogen and ammonium nitrogen of the soil in plot TW3 were significantly higher than those in plot TW1, which were close to those in plot W but different from those in plot WU. MBC and soil urease activity in plot TW3 were significantly higher than those in plot TW1, while there was no difference in MBN, MBP, soil acid phosphorase activity and soilβ-D-glucosidase between plot TW3 and TW1. AWCD and positive well number of soil in four plots ranked in the decreased order: W>TW1>TW3> WU, indicating the invasion of M. micrantha could increase soil microbial activity and the parasitization could decrease the soil microbial activity with the parasitization time. The carbon utilization pattern of soil in four plots differed significantly. 12 samples from WU, W, TW1 and TW3 plots could be divided into four groups using principal components analysis (PCA) method. The invasion of M. micrantha, the parasitization of C. campestris and the parasitization time could significantly change the carbon utilization ability of soil microbes and the most important factors might be soil total nitrogen, total carbon and pH value according to canonical correspondence analysis (CCA) method. Base on the data of carbon utilization ability, Shannon diversity index, evenness index, Simposon diversity index and richness index ranked in the decreased order: W>TW1>TW3> WU, suggesting the invasion of M. micranhta, the parasitization of C. campestris and the parasitization time could change the soil microbial functional diversity.2. In 7 weeks green house experiments, the soil chemical properties, soil enzyme activity, soil microbial biomass and the carbon utilization pattern of soil microbes in soil of C. jobi monoculture (Y soil), C. jobi invaded by M. micrantha (YW soil), C. jobi invaded by M. micrantha and with C. campestris lightly parasitizing the invader (LYWT soil), C. jobi invaded by M. micrantha and with C. campestris moderately parasitizing the invader (MYWT soil), C. jobi invaded by M. micrantha and with C. campestris heavily parasitizing the invader (HYWT soil) were investigated and compared. There was no significant difference in soil chemical properties between YW and Y soil. Total organic carbon in LYWT soil was significantly higher than that in YW soil, pH in MYWT soil was significantly lower than that in LYWT soil. Soil chemical properties in HYWT soil recovered and were close to those in Y soil. Soilβ-D-glucosidase acivity in YW soil was significantly higher than that in Y soil. Soil acid phosphorase activity in HYWT soil was significantly lower than that of the other soil. Soil urease activity in MYWT soil was significantly higher than that of the other soil. MBC in YW soil was significantly higher than that in Y soil. MBC, MBN and MBP in LYWT, MYW and HYWT soil were all significantly lower than those in YW soil. MBC in MYWT and HYWT soil were significantly lower than that in LYWT soil. AWCD and positive well number in YW soil were significantly higher than that in Y soil. AWCD and positive well number of LYWT and HYWT soil were significantly lower than those in YW soil, while those in MYWT soil was the highest. This indicated that the invasion of M. micrantha could increase soil microbial activity and the parasitization of C. campestris could decrease the soil microbial activity, except for MYWT soil. The invasion and parasitization could change the carbon utilization pattern of soil microbes. 20 soil samples could be divided into five groups using PCA method. YW and LYWT soil were far away from each other, and far from Y soil. MYWT soil was nearer to Y soil, while HYWT was nearest to Y soil. Base on the data of carbon utilization ability, Shannon diversity index, Evenness index and Simpson diversity index ranked in the decreased order: YW>MYWT>LYWT> HYWT>Y, but the Richness index showed no significant difference. This indicated that the invasion of M. micrantha could increase soil microbial activity, the carbon utilization pattern and the functional diversity, while the parasitization could also change soil microbial activity, the carbon utilization pattern and the functional diversity. The microbial functional groups recovered after C. campestris parasitizing the invader and those in HYMT soil was nearest to those in Y soil.3. We also investigated the soil properties through sampling in three plots located in the abandoned field in Dengshuiling Villiage, Dongguan City, Guangdong Province, China, including those (1) not invaded by M. micrantha and without C. campestris (WU), (2) invaded by M. micrantha and without C. campestris (W), and (3) invaded by M. micrantha and with C. campestris parasitizing the invader (TW). Except for soil total phosphorus and the activity of soil phosphorase, the rule of change of soil chemical properties, soil enzyme activity, soil microbial biomass were similar to those in Neilingding Island. Effects of these three types of soil on the growth of C. jobi before and after treatment with different microbial inhibitors and activated carbon were analyzed and the possible ecophysiological, soil nutrient cycle, soil microbial, soil allelopathic mechanisms were provided and discussed. The leaves, stems, above-ground and total biomass of C. jobi ranked in the decreased order: W>TW>WU, suggesting the invasion could promote the growth of C. jobi and the parasitization could weaken the promotion effect. Root biomass ratio and root top ratio of C. jobi ranked in the decreased order: TW>W>WU. The invasion could decrease net photosynthesis (Pn), intercellular CO2 concentration (Ci), and light utilization efficiency (WUE), and the parasitization could increase them again. The photosynthesis efficiency could be influenced by stomata factors. The relative content of chlorophyll, F0 and Fm of C. jobi on W soil was significantly higher, while Fv/Fm and Fv/F0 were lower than those on WU soil. Those on TW soil recovered, but there was no significant difference between TW and W soil. The photosystem II of C. jobi on W soil was destroyed and the photosynthetic efficiency decreased, but the relatively content of chlorophyll increased, which promote the photosynthesis intensity and the growth of C. jobi eventually. The destroy of the photosystem II of C. jobi on TW soil was recovered, but the relative content of chlorophyll decreased at the same time, which could decrease the photosynthesis intensity and inhibit the growth of C. jobi eventually. There was significantly positive correlation between the leaves, above-ground biomass and the soil pH, total phosphorus, total nitrogen, total organic carbon. There was significantly positive correlation between the stem, total biomass and the soil pH, total phosphorus, total nitrogen. This suggested that the invasion and the parasitization could indirectly influence the growth of C. jobi through the change of soil chemical properties. There was significantly positive correlation between the biomass of the different parts of C. jobi and the soil microbial biomass, indicating soil microbes might play important role in the effects of invasion and parasitization on the growth of C. jobi. The biomass of different parts of C. jobi decreased significantly after WU soil was treated with fungal inhibitor or bacteria and fungal inhibitors, but there was no significant effect of bacterial inhibitor on the biomass of C. jobi. This suggested that soil fungal might play important role in the growth of C. jobi. The biomass of the different parts of C. jobi increased after W soil was treated with different microbial inhibitors. The total biomass of C. jobi on W soil treated with fungal inhibitor, bacterial inhibitor, fungal and bacterial inhibitors were 3.93, 1.44, 2.39 times higher that those on WU soil treated with the corresponding microbial inhibitors, respectively. But the total biomass on W soil without treatment was 1.65 times higher that that on WU soil without treatment. This suggested that soil fungal in W soil might inhibit the growth of C. jobi and the bacteria might slightly promote the growth of C. jobi. The biomass of C. jobi on TW soil treated with bacterial and fungal inhibitors was significantly higher that that without treatment, while there was no significant difference in treatment of fungal inhibitor and bacteria inhibitor. But the biomass of C. jobi on MYWT soil treated with fungal inhibitor increased significantly. The total biomass of C. jobi on TW soil treated with fungal inhibitor, bacterial inhibitor, fungal and bacterial inhibitors were 3.50,1.34, 2.84 times higher than those on WU soil treated with the corresponding microbial inhibitors, respectively. But the total biomass on TW soil without treatment was 1.47 times higher than that on WU soil without treatment. This suggested that soil fungal in TW soil could still inhibit the growth of C. joib and soil bacteria could still promote the growth of C. jobi. The total biomass of C. jobi on W and TW soil treated with activated carbon was 2.05 and 1.61 times higher than those without treatment, respectively. This indicated that the allelopathy in W and TW soil might significantly inhibit the growth of C. jobi and the inhibition effect of W soil was stronger than that of TW soil. The total biomass of C. jobi on WU, W and TW soil treated with activated carbon were 1.12, 1.40, 1.24 times higher than those without treatment, respectively. And this indicating that the effect of allelopathy was little. The relative content of chlorophyll, Fv/Fm and Fv/F0 of C. jobi on three types of soil treated with activated carbon were higher than those without treatment, while F0 and Fm decreased. This suggested that the inhibition effect of the allelopathy in soil on the photosynthesis could be weakened after it was absorbed by activated carbon.4. In 7 weeks green house experiments, the biomass of different parts of C. jobi on soil of C. jobi monoculture (Y), C. jobi and M. micrantha (YW), C. jobi and M. micrantha parasitized by C. campestris at light degree (LYWT), C. jobi and M. micrantha parasitized by C. campestris at middle degree (MYWT), C. jobi and M. micrantha parasitized by C. campestris at high degree (HYWT) treated with different microbial inhibitors and activated carbon were analyzed and the possible ecophysiological, soil nutrient cycle, soil microbial, soil allelopathic mechanisms were provided and discussed. The leavs, above-ground and total biomass of C. jobi increased after M. micrantha was parasitized by C. campestris and those of C. jobi on HYWT increased significantly than those on YW soil. The root biomass ratio and root top ratio in YW was significantly higher than that in Y. There was no significant difference among LYWT, MYWT and HYWT. There was no significant correlation between the biomass and the soil chemical properties for the short culture period. There was significantly negative correlation between the biomass of different parts and the soil urease activity except for the seed and underground biomass. There was significantly negative correlation between the leaves, total biomass and the soilβ-D-glucosidase. This suggested that the nutrient cycle in soil, especially N and P cycle, might play important role in the growth of C. jobi. The photosynthesis efficiency of C. jobi increased after M. micrantha was parasitized by C. campestris and ranked in the decreased order: HYWT>MYWT>LYWT. The photosynthesis efficiency could also be influenced by stomata factors. The water utilization efficiency (WUE) and LUE of C. jobi ranked in the decreased order: MYWT>HYWT>LYWT>YW. The relatively content of chlorphyll of C. jobi in MYWT and HYWT was higher than that in YW and LYWT, and the photosynthesis was promoted and the growth of C. jobi was increased eventually. There was significantly negative correlation between the biomass and soil MBC, MBN and MBP, indicating that the invasion could increase the number of microbes which could inhibit the growth of C. jobi. The parasitization of C. campestris could decrease the number of these microbes, weaken the inhibition and increase the biomass of C. jobi. The total biomass of C. jobi on five types of soil treated with microbial inhibitors all ranked in the deceased order: bacterial inhibit>bacterial and fungal inhibitors>fungal inhibitor indicating that soil fungal might be necessary for the growth of C. jobi and promote the growth while soil bacteria might inhibit the growth of C. jobi. This also suggested that there might be a certain antagonistic effect between soil fungal and bacteria. Without treatment of microbial inhibitors, the total biomass of C. jobi in MYWT was 1.87 times higher than that in YW. But after treatment of fungal inhibitor, bacterial inhibitor, fungal and bacteria inhibitors, the total biomass of C. jobi in MYWTwas 2.10, 1.61 and 1.72 times higher than those in YW, respectively. This might be explained by two reasons: one was that soil microbes might be altered and the activity of microbes which could inhibit the growth of C. jobi was promoted by the medium degree parasitization of C. campestris; and the other was that soil microbes could be altered and the activity of microbes which could not be inhibited by the fungal inhibitor and might be beneficial for the growth of C. jobi be promoted by the medium degree of parasitization. The total biomass of C. jobi in YW, LYWT, MYWT and HYWT treated with activated carbon were 1.027, 0.581, 0.809 and 1.032 times higher than those without treatment, respectively. This indicated that the allelopathy in YW and HYWT soil could inhibit the growth of C. jobi and the allelopathy in LYWT and MYWT soil could promote the growth of C. jobi. The relative content of chlorophyll, Fv/Fm and Fv/F0 of C. jobi in MYWT and HYWT treated with activated carbon were higher than those without treatment, while F0 and Fm decreased. This suggested that the inhibition effect of the allelopathy in soil on the photosynthesis could be weakened after it was absorbed by activated carbon. This also indicated that the allelopathy produced by M. micrantha could inhibit the growth of C. jobi and the parasitization of C. campestris could change the allelopathy and promote the growth of C. jobi.5. From the results, it could be concluded that M. micrantha invasion could increase soil microbial activity, increase the function of soil microbes, increase soil nutrient cycle resulting in increase soil nutrient fertility and then promote the growth of C. jobi. In addition, the parasitization could disorder the below-ground microbial communities through decreasing the activity and function of soil microbes, decreasing soil nutrient cycle resulting in decrease soil nutrient fertility and then weaken the promotion effect of M. micranhta invasion on the growth of C. jobi. It is soil fungal, but not bacteria and allelopahty, that might play important role in the indirect promotion effect. In green house experiments, the parasitization of C. campestris could promote the growth of C. jobi and the promotion effect increased with the degree of parasitization. This might be due to the competition effect beween M. micrantha and C. jobi and soil fungal also played important role in the indirect effect. The increase of the chlorophyll might be he main ecophyisological mechanism for the promotion of the growth of C. jobi. This helps to enrich the theory and methodology of ecology of parasitic plant and helps to understand the use of C. campestris in biological controlling of M. micrantha.
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