土壤环境科学实验室知识库

      随着城市化以及工业化进程的加快，城市土壤受到人为活动的强烈影响，城市土壤中复合污染现象普遍存在，多种污染物质并存。但是目前关于复合污染生态风险评估还存在很大不确定性，很难揭示复合污染的真实风险。复合污染物在环境行为上的交互作用较为复杂，而且关于复合污染生态效应评估的方法也存在很大不确定性，尤其是在生态系统高水平的评价终点，缺乏定量的评估方法。本研究针对城市绿地中的典型复合污染物重金属和除草剂环草隆，选取了4个金属污染程度不同的土壤（污染程度C       （1）所研究的土壤对环草隆的吸附能力较低，有机碳分配系数Koc值在117至137之间。超过50 ％的被吸附的环草隆易于释放到土壤溶液中，没有观察到明显的吸附-解吸滞后现象。环草隆在土壤中的平均降解半衰期在37.6-70.6天。因此，在研究的土壤中，环草隆具有很高的移动性和生物利用率。环草隆在不同土壤中的吸附，解吸及降解行为特征不同。由于有机质的强吸附作用，环草隆在有机质含量最高的土壤M中吸附强度最高，解吸及降解能力最弱。但是与清洁土壤相比，环草隆在重金属污染最严重的土壤H中的吸附/解吸及降解行为差异不显著。
      （2）HYDRUS1D模拟结果表明，在公园/居民区草坪和高尔夫球场草坪的施用模式下，环草隆主要积累在0-20 cm表层土壤上，平均浓度在1.13至107 mg kg-1之间。在十年模拟中，环草隆还通过排水向下运输，但量很小，低于欧盟标准的0.1 μg L-1。环草隆在不同土壤中的累积和迁移趋势也不同，在有机质含量最高的土壤M中累积量最高，淋溶浓度最大。这也进一步表明土壤有机质是影响环草隆在土壤中累积和迁移的决定性因子，相比之下，重金属没有产生明显影响。因此，在暴露评估的早期阶段，为了提高评估的效率，可以不考虑复合污染物之间在环境行为上的交互作用。
      （3）微生物生物量碳，反硝化酶和固氮酶的活性，可以用于指示在环草隆胁迫下土壤微生物群落功能的综合敏感性（MSI）和抵抗力（MRS），而相同的三个参数加上土壤基础呼吸可以指示综合恢复力（MRL）。在复合污染下，土壤中的环草隆残留浓度与MSI，MRS和MRL之间存在显著的剂量效应关系。由于微生物生物量较低，重金属污染土壤对环草隆表现出较高的敏感性和较低的抵抗力，而由于对化学胁迫的预先适应，重金属污染土壤的恢复力则更高。微生物群落功能稳定性作为可量化的指标，可以被纳入生态风险评估（ERA）框架，从更高的群落水平进行生态效应评估。
      （4）由于重金属对微生物群落聚集过程的强烈的确定性选择作用，长期的重金属污染影响了整个微生物群落物种的总体物种多样性，组成和物种间相互作用，这进一步影响了对后续环草隆胁迫的响应，从而影响了其结构稳定性。与清洁土壤相比，在重金属污染土壤中，由于预先适应了污染物的胁迫，附加的环草隆对微生物群落的选择效应较低，并且伴随着关键物种数量的增加和网络相互作用加强，使微生物群落结构表现出更高的恢复力。微生物群落结构稳定性与环草隆残留浓度之间存在显著的剂量-效应关系。因此，微生物结构稳定性可以在群落结构水平上，定量评估复合污染物对微生物群落的生态效应。
      （5）基于环草隆和重金属的单一毒性数据和土壤中重金属的有效性浓度，利用独立作用（IA）和浓度增加（CA）模型，计算了环草隆在重金属污染土壤中的毒性效应。结果表明，由于联合毒性效应，在重金属污染最严重的土壤H中，环草隆对植物根伸长和蚯蚓致死的毒性最强。利用风险商和证据-权重法表征了环草隆在土壤中累积的对土壤生物的生态风险。结果表明环草隆不仅对植物和动物个体，而且对微生物群落的整体结构和功能也存在潜在的生态风险。在土壤H中，虽然环草隆的累积量不是最高的，但由于环草隆与重金属的联合毒性效应，环草隆的累积对土壤生物的综合生态风险是最高的。
      因此，污染物在复合污染土壤中的环境行为特征，更多的是受土壤理化性质影响。土壤微生物群落结构和功能稳定性，作为可定量的评估终点，可以在群落水平，评估复合污染的生态效应。联合毒性对于评估多种污染物联合污染所引起的生态风险至关重要。

      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.