Toxicity of carboxin on the crustacean Thamnocephalus platyurus : Influence of two entisols with different texture and Pseudomonas aeruginosa strain.

Little is know about the toxic effects on soils of the pesticide Carboxin, a systemic fungicide used to control plant diseases caused by Basidiomycetes. Studies on abiotic transformation of carboxin report its rapid oxidation to sulfoxide, that is idrolized to other degradation products. This chemical process is favoured by natural sunlight in water, presence of humic substances and soil, UV light in organic solvent. This research was designed to assess the influence of soils with different texture, but similar pedogenetic features, and the possible activity of Pseudomonas aeruginosa on the toxicity reduction of this fungicide. Toxicity tests were performed on the microcrustacean Thamnocephalus platyurus in presence and absence of soils and the bacterium. The results show that the degradation of carboxin in less toxic by-products is significantly increased also in the dark. The phenomenon is influenced by concentration level of the clay in the soil, that causes a rise in the relative adsorbing surface. As a consequence, the availability of carboxin sulfoxide, the first by-oxidation product, is reduced. The results show that a similar consequence is also partially verified for soils that are relatively poor in clay (MND soil), but the reduction of carboxin sulfoxide is in that case due to the presence of organic and inorganic colloids naturally present in soils. It is also proved that the degradation’s kinetic is faster and more complete when P. aeruginosa is present. In this case, along the twenty days of observation, the biotic degradation of carboxin gives inorganic nitrogen products, such as ammonia and nitrite with a final toxicity level lower than the sole use of soils.