Other Abstract | Urban soils were strongly influenced by anthropogenic activities with the increasing intensification of urbanization and industriation and were subject to continuous input of various pollutants. However, little has been reported on the ecological risk of their combined pollution, especially in field due to lack of systematic methodology. The interaction between the co-existed pollutants is rather complex and there is a huge uncertain about the ecotoxic effect of the combined pollutants, especially for the endpoints at high ecosystem level, and there is lack of quantative evaluation method. In this study, we selected heavy metals and typical herbicide siduon as the target combined pollutants. Four field soils with similar properties but contrasting heavy metal contaminated level (C
(1) The results revealed a low sorption of siduron to all the tested soils. The organic carbon normalized distribution coefficient (Koc) of siduron in the studied soils ranged from 117 to 137 L kg-1 and was not significantly correlated to heavy metal levels. No apparent desorption hysteresis was observed with the hysteresis index (HI) ranging from 0.921 to 1.11. More than 50 % of the sorbed siduron was readily released into soil solution. The half-life time of siduron in the studied soils ranging from 37.6 to 70.6 days. Results suggested that siduron was highly mobile and bioavailable in the studied soils. Soil M with the highest content of soil organic matter (SOM), showed the strongest adsorption ability but lowest desorption and degradation ability to siduron among the studied soils. While the difference in the adsorption/desorption and degradation of siduron between the severest heavy metal contaminated soil H and reference clean soil C is slight.
(2) HYDRUS1D simulation showed that siduron accumulated primarily at topsoil 0--20 cm, the average concentration ranging from 1.13 to 107 mg kg20 cm, the average concentration ranging from 1.13 to 107 mg kg--11 under application under application pattern of park & residential turf and golf course greens. Siduron was also transported pattern of park & residential turf and golf course greens. Siduron was also transported downward by the drainage water but slight amount, less than the EU standownward by the drainage water but slight amount, less than the EU standard of 0.1 μg dard of 0.1 μg LL--11 in a 10in a 10--years simulation. Also, soil M showed the strongest tendency to retain siduron years simulation. Also, soil M showed the strongest tendency to retain siduron among the studied soils. The results suggested that SOM rather than the coamong the studied soils. The results suggested that SOM rather than the co--existed heavy existed heavy metals was the dominant factor affecting the fate and the accmetals was the dominant factor affecting the fate and the accumulation of siduron in soils. umulation of siduron in soils. It further means that the adsorption and desorption parameters of organic pollutants in It further means that the adsorption and desorption parameters of organic pollutants in uncontaminated soils after normalizing by the fraction of soil organic matter could be uncontaminated soils after normalizing by the fraction of soil organic matter could be used to estimate the exposure concentrations of thused to estimate the exposure concentrations of the compounds in combined polluted e compounds in combined polluted soils, which could increase the realism and efficiency of the exposure assessment in the soils, which could increase the realism and efficiency of the exposure assessment in the early stages of the risk assessment of combined pollution.early stages of the risk assessment of combined pollution.
(3) Microbial biomass carbon, activity of denitrification enzyme and nitrogenase enase were indicative of MSI and MRS, and the same three parameters plus soil basal were indicative of MSI and MRS, and the same three parameters plus soil basal respiration were indicative of MRL. Significant doserespiration were indicative of MRL. Significant dose--effect relationships between effect relationships between siduron residues in soils and MSI, MRS and MRL undercombined pollution were siduron residues in soils and MSI, MRS and MRL under combined pollution were observed. Heaobserved. Heavy metal polluted soils showed higher sensitivity and lower resistance to vy metal polluted soils showed higher sensitivity and lower resistance to the additional disturbance of herbicide siduron due to the lower microbial biomass, while the additional disturbance of herbicide siduron due to the lower microbial biomass, while the resilience of heavy metal polluted soils was much higher due to the prethe resilience of heavy metal polluted soils was much higher due to the pre--adaption to adaption to the the chemical stresses. The quantifiable indicator microbial functional stability can be chemical stresses. The quantifiable indicator microbial functional stability can be incorporated in the framework of ERA to assess the toxic effect of pollutants at incorporated in the framework of ERA to assess the toxic effect of pollutants at community function level.community function level.
(4) Long--term heavy metal contamination affected overall biodiverterm heavy metal contamination affected overall biodiversity, sity, composition and the network interactions of all microbial speicies due to the strong composition and the network interactions of all microbial speicies due to the strong deterministic selection effect of heavy metals on the microbial assembly process, which deterministic selection effect of heavy metals on the microbial assembly process, which further affected the compositional stability in response to the subsequent stfurther affected the compositional stability in response to the subsequent stress of ress of herbicide siduron. herbicide siduron. Due to Due to the tolerance and adaption of microbes acquired from longthe tolerance and adaption of microbes acquired from long--term pollution of heavy metalsterm pollution of heavy metals, the, the selection selection effect effect of herbicide siduronof herbicide siduron on microbial on microbial community is the lowest, accompanying the community is the lowest, accompanying the shifts in the network characteristishifts in the network characteristic, c, especially the especially the increased keystone speciesincreased keystone species,, which resulted in the highest microbial which resulted in the highest microbial compositional resilience in soil H. Thus, microbial community compositional stability, compositional resilience in soil H. Thus, microbial community compositional stability, especially resilience, results from a combination of microbial interaction and commespecially resilience, results from a combination of microbial interaction and community unity assembly processes triggered by the disturbance history, providing a quantitative assembly processes triggered by the disturbance history, providing a quantitative measure of microbial response to combined pollutants at structure level that can be measure of microbial response to combined pollutants at structure level that can be translated into practice.translated into practice.
(5) Based on single toxicology data of siduron and heavy metals and the availability concentration of heavy metals, the toxicity of siduron to higher plants and soil earthworm in the different studied soils were calculated using independent action (IA) and concentration addition (CA) model. The results showed that the toxicity of siuron to plants and earthworm were the highest in the severest heavy metal contaminated soil H due to the joint toxicity. The accumulation of siduon in the studied soils would exert potential ecologaical risk to the individual plant /earthworm, as well as the intergrated function and structure of microbial community. And the overall ecological risk of siduron in soil H is the highest due to the joint toxicity, although the accumulation level of siduron in soil H is not the highest.
Thus, functional and structural stability of microbial community can be applied as pragmatic endpoints to evaluate the joint toxicity of combined pollution at community level. Joint toxicity was critical in evaluating ecological risks caused by combined pollution of multiple contaminants. On the contrast, environmental behaviors of individual contaminant were affected much more by soil properties rather than co-contaminants.
Key Words: Combined pollution, Environmental exposure, Joint toxicity, Microbial stability, Ecological risk assessment. |
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