BIOMINERALIZATION OF CADUSAFOS IN AGRICULTURAL SOILS AS INFLUENCED BY CO-OCCURRING ANTIMICROBIAL AGENTS

Abstract

Cadusafos is a soil-applied pesticide presented as effective against a broad spectrum of nematodes and soil insects on several important crops worldwide. With respect to its toxicity, it is classified as a pesticide of Class I (highly hazardous to human health) and is regarded as ‘dangerous to the environment’ by the regulatory departments. Its presence in the terrestrial environment may pose serious health risks. Triclosan (TCS) and triclocarban (TCC) are the antimicrobial agents that are routinely used in a large number of contemporary consumer and healthcare products. A major fraction of TCS and TCC is washed down the drain after use, and ends up in agricultural soils by the application of biosolids and untreated and partially-treated wastewater. Since TCS and TCC are biocides that are designed to kill microorganisms, their co-occurrence with cadusafos may prolong the persistence of cadusafos in terrestrial environment and cause serious health problems. While living in the soil, the co-existing pesticide and the antimicrobial agents remain in close interaction with the soil constituents, and thus, the soil physicochemical properties may be of vital importance for determining the persistence of the pesticide in the terrestrial environment. Hence, a series of experiments was conducted by using three agricultural soils of different physicochemical properties to study the effect of TCS and TCC on the microbial activity by spiking with biodegradation of 14 14 C-glucose, and on C-cadusafos in adapted (the soil which was previously exposed to cadusafos) and unadapted (the natural soil without previous exposure to cadusafos) agricultural soils. The soils were spiked with 14C-glucose or 14C-cadusafos @ 5 μg/g and TCS @ 30, 90, 270 μg/g or TCC @ 50, 150 and 450 μg/g soil in different experiments. The soils were maintained at 48 and 65% of maximum water holding capacity (MWHC) and were incubated at 28 ± 1 oC up to 28 or 77 days in different experiments. The results revealed a similarity in pattern of biomineralization of 14 C-cadusafos in all the three tested soils. The moisture levels had little effect on biomineralization of well as of 14 14 14 C-glucose as C-cadusafos while soils differed significantly in their potential to degrade C-cadusafos, most likely due to variation in their physicochemical properties. Kinetic analysis revealed that biomineralization of 14 C-cadusafos followed a first order kinetics during the incubation time of the study. Moreover, biomineralization of 14 C-cadusafos was negatively affected by the antimicrobial agents, TCS and TCC, but with different degree of efficacy. The effect of TCS was highly significant and concentration dependent whereas, a very weak and negligible effect of TCC was recorded in the three soils. The TCS application at its highest level (270 μg/g) reduced biomineralization of 14C-glucose in the absence or presence of D-glucose up to 53.6 and 50.1%, respectively. The same dose of TCS caused reduction in 14C-cadusafos biomineralization by 58.4 and 57.4% in the unadapted (fresh) and adapted (conditioned with 14C-cadusafos) soils, respectively. A strong negative correlation was identified between some physicochemical properties (pH and organic matter) and the effect of TCS on biomineralization of 14 C-glucose or 14 C- cadusafos. These findings indicate a strong role of antimicrobial agents, like TCS, in prolonging the persistence of xenobiotics such as cadusafos. This may imply that in addition to investigating the biomineralization of pesticides in soil environment, the continual loading of antimicrobial agents must also be given the due attention. The unrestricted use of the carriers of antimicrobial agents, like biosolids and untreated wastewater, should be avoided for keeping the terrestrial environment safe and healthy. In this connection, if necessary, the laws may also be enacted for the restricted