Dissipation of herbicides within a heterogeneous landscape

The herbicides 2,4-D (2,4-dichlorophenoxy acetic acid) and dicamba (2-methoxy-3,6-dichlorobenzoic acid) are among the most widely used and mobile herbicides in use today. This study was conducted to characterize dissipation of these herbicides in different components of the landscape, to evaluate the role of microbial biomass as a controller of herbicide degradation capacity in soil and to compare dissipation rates measured under field and laboratory conditions. Five different environments were represented in an 80-day laboratory microcosm study including a home lawn, a cornfield, an upland hardwood forest, a wetland forest and aquifer material. An 80-day field plot experiment was established using each of these sites except the aquifer material. A modified method was developed to extract and quantify 2,4-D and dicamba residues in soils using high-performance liquid chromatography. Herbicide, microbial biomass and mineral nitrogen (NH{dollar}sb4sp+{dollar} and NO{dollar}sb3sp-{dollar}) analyses were performed on soil samples on days 5, 10, 20, 40, and 80 following herbicide application in each study. 2,4-D was completely dissipated by day 80 of the laboratory study in the home lawn, hardwood forest and wetland soils and less than 5% of the amount applied was present in the cornfield soil and aquifer material on day 80. 2,4-D was completely dissipated by day 20 in each of the soils during the field plot study. Dicamba was dissipated totally or nearly so by day 80 of the laboratory study in the hardwood forest and wetland soils, while the cornfield, home lawn and aquifer material soils had significantly greater amounts of dicamba present. Dicamba was completely eliminated from the home lawn and cornfield soils by day 20 of the field study and measurable quantities were present in the hardwood forest and wetland soils on day 80. Microbial biomass carbon was negatively correlated with the amount of dicamba and 2,4-D remaining on day 20 of the laboratory study. Microbial biomass carbon and nitrogen were positively correlated with the amount of dicamba remaining on day 20 of the field study. The microbial biomass appears to control the dissipation of 2,4-D and dicamba from soil though environmental factors, especially sorption, may modify or override this effect. Laboratory studies should be validated by field research whenever possible.