Influence of soil property variability and terrain attributes on the spatial distribution in the field dissipation of the herbicides: Acetochlor and isoxaflutole

Soil and landscape characteristics that influence herbicide behavior through soil/sorption and transport processes vary spatially at the scales of most agricultural fields, so it is possible that herbicide dissipation will also vary spatially. Knowledge of the spatial variability in herbicide dissipation could lead to site-specific management practices to reduce off-site transport while maintaining viable weed control. Experiments on a 6 ha watershed were conducted in order to characterize the spatial variability of acetochlor and isoxaflutole behavior. Spatial distribution of surface soil properties were determined for the watershed. Terrain attributes were derived from a DEM in order to ascertain if they could enhance the prediction of acetochlor and isoxaflutole behavior. Acetochlor and isoxaflutole were applied at uniform recommended rates in 2000 and 2001. At various times, soil samples were taken to a depth of 70 cm at numerous georeferenced locations having a range in soil properties representative of the site. It was found that acetochlor dissipation was rapid and varied across the field. Isoxafluole degradation to DKN was also rapid and variable across the field; however DKN did not rapidly degrade between sampling times. Geostatistical analysis revealed that acetochlor and DKN dissipation rates exhibit spatial structure. The temporal spacing in the sampling events was not adequate to capture the spatial dependency of the rapid isoxaflutole dissipation. Complete spatial variability could not be accounted for in the pedotransfer functions (PTF's) generated for acetochlor and isoxaflutole dissipation rates from soil properties and terrain attributes. The relationships between acetochlor dissipation and soil/terrain attributes were found to be complex and varied between sampling times and years. Elevation and % sand explain up to 40% of the variability in DKN residues across the field at two days, 1 and two weeks after application in both years. The inclusion of terrain attributes along with soil properties in PTF's improve the overall ability to predict the dissipation of acetochlor and isoxaflutole that could lead to specific site management. Ultimately, however, information describing the distribution of microbial populations may be more useful in predicting the behavior of acetochlor and isoxaflutole.