THE DISAPPEARANCE OF 2,4-D, DIALLATE AND MALATHION FROM SOIL AND SOIL COMPONENTS

Abstract

This paper contrasts the routes of loss of three pesticides from a silt loam soil. The biological and nonbiological breakdown of 2,4-D, diallate, and malathion were assessed in both whole soil and its separated inorganic and organic fractions in an attempt to identify the major influences affecting the decay of each pesticide. The breakdown of 2,4-D in soil followed the well-established pattern of a brief lag phase (8 days), during which microbial enrichment occurred, succeeded by a rapid first-order decline in herbicide concentration. In the sand, organo-mineral and silt fraction breakdown was somewhat slower following a 10-day lag, whilst no loss was observed from the clay. Virtually no degradation at all occurred in the microbially sterile irradiated soil, nor in the autoclaved soil and soil components. In contrast, the disappearance of diallate appeared to be a mostly nonbiological event due to volatilisation. Rates of loss increased in the coarser soil fractions (sand and silt) and neither irradiation nor autoclaving reduced these rates. The insecticide, malathion, was degraded rapidly in both nonsterile and irradiated soils but not in autoclaved soil. Breakdown was also rapid in separated organic and organo-mineral fractions, somewhat slower in sand and silt and nonexistent in all autoclaved fractions. From comparisons of the breakdown of these three pesticides in a silt loam soil it was clear that different mechanisms were involved in each instance. 2,4-D decay was predominantly microbial, diallate decay due to volatilisation, and malathion decay a combination of microbial and exoenzyme metabolism. This type of approach to the measurement of pesticide loss from an individual soil – by separating and identifying the contribution of each component to the total decay pattern – may prove a rewarding avenue of research, and lead to quantitative and even predictive assessments of pesticide behaviour in soil.