Investigation And Transformation Rule Of Microcystins In Agricultural Soils

Microcystins(MCs) is generally detected in the water environment and has the serious harm to aquatic ecosystems and human health, which has attracted widespread attention and a lot of researches on water environment of MCs have been conducted. However, water contaminated with algae blooms may also be transferred to agricultural soil via irrigation, fertilization and so on, leading to soil contamination by MCs released, which endanger the safety of agricultural products and groundwater. In this study, concentration distribution of three commonly found microcystins(MC-LR, MC-RR and MC-YR) in agricultural soil irrigated by frequent algae water was investigated and its risk was assessed. Indoor simulation experiments were carried out to study the degradation behavior of three MCs compounds in the soil and migration patterns and effect mechanism, to explore the environmental risk of MCs in soil. The results would provide theoretical basis for the pollution treatment, a human health risk assessment and safety of agricultural production. The main conclusions were as follows:1) Concentration distribution of three commonly found MCs in 35 soil samples from vegetable fields around a seriously eutrophic lake(Dianchi Lake) was studied using solid phase extraction and liquid chromatography-tandem mass spectrometry(HPLC-MS/MS). Results indicated that MCs were detected in 85.7% of the samples and the total concentrations of three MCs ranged from ND(below LOQ) to 7.8 μg·kg-1 with the average concentration of 1.6 μg·kg-1. MC-RR got the highest detection rate(82.9%) and concentrations(ND~5.3 μg·kg-1) in soil samples, followed by MC-LR and MC-YR. Both the health and ecological risks of three MCs were acceptable. MC-YR displayed the highest health ecological risk and MC-LR showed the highest ecological risk. Intake and dermal contact were primary exposure pathway of MCs to children and adults, respectively. The health risk of MCs exposure to children was higher than that to adults.2) The results of indoor simulation experiment of MCs degradation in the soil showed that degradation of three MCs in the soil become gradually slow over time. In five soils with different physical and chemical properties, degradation of three MCs was most likely to occur in the paddy soil, and the slowest degradation occurred in saline soil and calcareous soil. The degradation speed of three MCs was: MC-YR > MC-LR > MC-RR. Within the concentration range of the experiment, initial concentration of MCs had no obvious effect on the degradation. Sterilization treatment significantly extended the half-life of MC degradation, particularly for MC-RR and MC-YR, indicating that biodegradation was the main way of MC degradation in the soil. Under different temperature and moisture contents, 25 ℃ and moisture contents at 30~60% were the most favorable for the biodegradation of MCs, partial low temperature and low moisture contents(10%) significantly prolonged the degradation half-life. The soil without organic matter reduced significantly the degradation rate of the MCs. Addition of humus(10 mg·kg-1 soil) in the soil had no obvious effect on the degradation of MCs. Under the condition of different illumination, ultraviolet light could promote the degradation of MCs, and MC-RR was easier photodegradation than MC-YR and MC-LR, demostrating that light degradation was also one of the main ways of MCs degradation in the soil. Refer to test guidelines of the chemical pesticide environmental safety evaluation, degradation half-life of MCs in the soil was short, belonging to more easily degradable organic pollutants.3) Using column leaching to study the migration behavior of MCs in the soil profile, the results showed that with the increase of the initial concentrations of the MCs, migration ability of MC-LR and MC-YR was stronger, and the initial concentration had no significant effect of MC-RR leaching. The soil with low organic matter content increased migration ability of MC-RR and MC-YR, but had no significant effect on MC-LR migration. With the increase of leaching solution p H, MC-LR migration ability became stronger, at the same time, MC-YR migration ability became weak, and the p H had no effect on MC-RR. Among three MCs, MC-LR migration ability was the strongest, followed by MC-YR, MC-RR was the weakest, mean for the MC-LR was most likely to occur migration in the soil for pollution of groundwater. Using GUS parameter to evaluate the leaching potential, the results showed that MCs belonged to low mobility pollutants in soil. But deep in the soil microbial activity would be restrained, the migration risk of MCs should be concerned closely yet.