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Allelopathy has been identified as an underlying mechanism of detrimental environmental impacts within commercial plantations. Eucalyptus spp. are known to generate huge amounts of volatile organic compounds (VOCs) that can function as phytotoxins and thus inhibit other plants. In the present study, biochemical markers, including activities of acetylcholinesterase (AChE) and oxidative stress enzymes, such as superoxide dismutase (SOD) and glutathione S-transferase (GST), were assayed to assess changes in Eisenia fetida at the physiological level induced by different doses of VOCs as part of an acute toxicity test over 7 and 14-day exposures. In addition, the toxicities of VOCs were investigated using a soil avoidance test and comet assay. The results revealed that E. fetida exhibited significant avoidance behavior towards the highest concentrations of undecane, decane, 2,4-dimethyl heptane, and 2,2,4,6,6-pentametyl heptane. The tail DNA percentages were significantly increased for all experimental treatments relative to control. However, under the treatments of VOCs, Olive tail moment content and comet tail length also display an obvious increase compared to control, except for that of octane, undecane and decane treatments. As VOC concentrations and durations increased in the soil, activities of AChE, SOD, and GST were either stimulated or inhibited. Among the VOCs, decane, 2,4-dimethyl heptane, 2,2,4,6,6-pentamethyl heptane, and 2,4-di tert buyl phenol exerted stronger effects on enzymatic activities. In summary, VOCs in rhizosphere soils of E. grandis might exert a toxic impact on E. fetida, among which 2,4-dimethyl heptane, 2,2,4,6,6-pentamethyl heptane, and 2,4-di tert buyl phenol have the strongest effects.

Issues around the loss of diversity caused by fast-growing tree plantations such as Eucalyptus have aroused controversy for many years.It is generally believed that Eucaluptus plantations bring about a decrease of plant′s biodiversity by influencing resource competitions,allelopathy,or the level of soil fertility.Above and below-ground of forest ecosystems interact implicitly.Complex interactions between above-belowground biodiversity may provide important feedbacks regulating ecosystem.However,seldom information was available on the soil organism′s diversity in Eucalyptus plantations.Furthermore,most of the previous studies were being conducted in the short-term rotation Eucalyptus plantations with a certain plantation age,which might limit our understanding of the actual plantation ecosystem process.Therefore,the abundance and diversity of plants and soil biodiversity were simultaneously measured across a range of eucalypt plantation ages(1—10 years) in southwest China to determine how Eucalyptus grandis afforestation of agricultural lands affected the native biodiversity.The present results showed that afforestation of E.grandis caused changes in plant and soil biodiversity with soil depth and was dependent on the stand age.The species richness,abundance and the diversity of understory plants and soil organisms showed a parallel development during the process of the succession,viz.both of them increased significantly with time but with a 'dip' around 4 years old.CCA analysis showed that the plant′s life forms transited from liana,hemicryptophytes and therophytes into perennial herbaceous-phanerophytes and phanerophytes.Bacteria quantitatively dominated the soil microbe in E.grandis regardless of plantation age.The individual proportion of saprozoic group quantitatively dominated in the soils of the all the study sites,followed by omnivorous group.The proportion of saprozoic groups decreased in 1—7 year old stands and then increased significantly with time.The proportion of omnivorous group increased as S shape with the plantation age.The proportion of predators and phytophage groups decreased significantly with time.The individual percentage of nematode decreased with the plantation age,but that of acarina and the number ratio of the acarina and collembola present the S shape increasing trend over time.The multiple comparisons test showed that plant diversity of the understory vegetation in 1,2 and 4—7 years-old plantations were significantly lower than that in the control forests(Pinus massoniana).Plant diversity in 1—10 year-old E.grandis plantations were significantly higher than that of the control arable land.Soil biodiversity in the plantations before the rotation period has no significant differences compared with agricultural lands,but were significantly higher in the plantations on or after the rotation period than the crop land.There were significant correlations between the plant′ s and soil organism′s diversity index and the correlation coefficient decreased with soil depth.