Spatial Variability of Diuron Sorption In a Hilly Apple Orchard.

Modeling the distribution and transport of pesticides through a landscape is important to minimize its environmental risk. Soil type, topography and within field management practices are among the major factors that affect the behavior of pesticides and likelihood of off-site transport. Few studies however, have been conducted in an intensively managed orchard system. We investigated how the sorption of diuron (3’-(3,4-dichlorophenyl)-1,1-dimenthyl-urea), a moderately hydrophobic herbicide, is affected by the above-mentioned factors. We carried out soil-landscape analysis in a 5.6 ha apple orchard in the Adelaide Hills, South Australia, which has a soil with strong texture contrast between A and B horizons and landform with relief difference of 50 m. One thousand soil samples were collected to a depth of 10 cm and composited to produce 100-pair sampling locations (alleys and tree line). Total organic carbon (TOC), pH_w, EC, and per cent clay were determined by diffuse reflectance infrared Fourier-transform spectroscopy and traditional chemical methods. Five digital terrain models (5 m pixel) were generated from elevation and drainage data sets.  Diuron sorption (K_d) was predicted using mid-infrared – spectroscopy and chemometrics after calibration based on measured K_d for 25% of the samples employing a conventional batch sorption technique. Correlation and stepwise linear regression analyses were used to determine significant soil and terrain variables.

Soil properties explained much of the variance of diuron K_d (R² = 0.64) with TOC and pH_w the key variables. The addition of terrain parameters improved model reliability (R² = 0.72), highlighting their relevance. Lower correlation coefficient values of diuron K_d and the different independent properties for tree line than for alley suggest within-field management practices affect how pesticide is distributed in this kind of production system. Mean differences of diuron K_d were also significantly different among the management zones.