Stable Isotope Fractionation And Bioavailability Of Pyrethroids During Microbial Biodegradation In Soil

Synthetic pyrethroids, a class of bionic insecticides with high activity for killing insecticides, low toxicity for mammals and low persistence in the environment, have gradually taken over the market of organochlorine pesticides. However, heavy use of pyrethroids also brought some environmental problems, such as residues in environmental media and agricultural products as well as toxic effects on non-target organisms. Since pyrethroids are mainly degraded by microorganisms in the environment under natural conditions, it is very important to assess the microbial degradation of pyrethroids. Compound-specific stable isotope analysis (CSIA) has been developed and applied for the qualitative detection and quantitative evaluation of biodegradation of organic pollutants. Compared to traditional approaches based on concentration index, CSIA is better to indicate the occurrence of biodegradation. In recent years, scientistes have carried out a number of researches on biodegradation of pesticides with application of CSIA. However, the method has never been reported for assessment of pyrethroids biodegradation.In this study, the carbon isotope fractionation of six pyrethroids (bifenthrin, fenpropathrin, permethrin, alpha-cypermethrin, fenvalerate and deltamethrin) was investigated during their microbial degradation in soil. The influence of soil properties was also studied on the microbial degradation. Furthermore, a new approach was preliminarily developed to dynamically assess bioavailability of pyrethroids. The main results are as follows:(1) The degradation of pyrethroids in soil accorded with the first-order reaction kinetics model. The degradation rates of pyrethroids in unsterilized soil were obviously faster than those in sterilized soil, indicating that microbial degradation was the main way of the degradation of pyrethroids. The microbial degradation of bifenthrin, permethrin and fenvalerate showed no obvious carbon isotope fractionation during microbial degradation. On the other hand, the microbial degradation of fenpropathrin, alpha-cypermethrin and deltamethrin induced significant carbon isotope fractionation.(2) The carbon isotope enrichment factors of fenpropathrin, alpha-cypermethrin and deltamethrin were determined as-1.88‰,-1.82‰ and -2.00‰, respectively. Accordingly, CSIA can be applied to qualitatively detect and quantitatively evaluate the microbial degradation of these pyrethroids in soil.(3) The physicochemical properties of soil can affect the microbial degradation rate of alpha-cypermethrin. Correlation analysis showed a significantly positive correlation (p<0.01) between the microbial degradation rate of alpha-cypermethrin and the content of organic carbon in soil. Moreover, the correlation was stronger with longer incubation time, indicating that the content of organic carbon was one of the main factors affecting the microbial degradation rate of alpha-cypermethrin. Furthermore, the microbial degradation rate of alpha-cypermethrin showed a weak negative correlation with the content of NHU+-N, NO3--N and inorganic N, indicating that the nitrogen in soil may lead to a certain inhibitory effect on the microbial metabolism of alpha-cypermethrin.(4) During the degradation of pyrethroids in soil, the bioavailability of pyrethroids gradually decreased with incubation time. And the decreasing rates of bioavailability accelerated over time. For different pyrethroids with the same initial concentration, the pyrethroid with a larger degradation rate showed a faster decreasing rate of bioavailability.