Endosulfan has no adverse effect on soil respiration

Endosulfan (6,7,8,9,10-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzodioxathiepin-3-oxide) is a chlorinated hydrocarbon used as a broad spectrum contact insect/acaricide. The continuous agricultural production of tomatoes, green beans, and squash make Miami-Dade County, Florida one of the heaviest users of this pesticide in the country. A field study was initiated at the USDA, Subtropical Horticulture Research Station in Miami, FL to examine the effects of endosulfan on soil respiration. Snap bean (Phaseolus vulgaris) and adjacent bare soil plots were treated either with or without endosulfan. Soil respiration was measured weekly as the difference between CO2 in the ambient air and that generated within a 10-cm diameter, 1178-cm3 chamber secured at the soil surface. Treatments were replicated four times and the study continued over three growing seasons. Respiration from bare soil was similar to that from plots planted with snap bean implying soil microbes were the dominant source of CO2 compared to root respiration. Endosulfan applications resulted in inconsistent increases in soil respiration. Increases were short-lived. By the end of the study more CO2 evolved from endosulfan treated than untreated plots. Soil was kept well irrigated and the percent moisture remained near maximum and varied little, soil moisture was not a controlling factor for soil respiration. Endosulfan had a short-lived inhibitory effect on soil fungi but bacteria increased in number in response to endosulfan application. Individual microbial colonies were not identified and it is possible that one or two bacterial species decomposed the endosulfan and rapidly multiplied causing the increased bacterial colony count. There were no adverse effects of endosulfan found on soil respiration.