Concurrent patterns of sorption-degradation for oryzalin and degradates

Sorption and degradation are linked, dynamic processes which largely govern the potential for herbicides and their degradation products to reach groundwater. Patterns of concurrent degradation and sorption of the dinitroaniline herbicide oryzalin (3,5-dinitro-N-4,N-4-dipropylsulfanilamide and its degradates were determined on four soils representative of typical use areas. Radiolabeled oryzalin was applied to soil at either 1.64 or 3.14 mu g/g and incubated at 25 degrees C under aerobic conditions for up to 6 months. The time required for 50% and 90% degradation of oryzalin ranged from 24 to 42 and 79 to 140 days, respectively. The radiocarbon was rapidly incorporated into the soil organic matter; up to 57% of the applied radiocarbon was recalcitrant to extraction after 6 months. The degradation pathway deduced is similar to that for the related dinitroaniline herbicide trifluralin. Apparent distribution coefficients were measured separately for the pool of material in the soil solution (K-ss, determined from a 5-min desorption) and the total desorbable pool of material (K-d, determined from a 24-h desorption). Sorption coefficients increased by a factor of 3 over 6 months, indicating that the total pool of extractable oryzalin became less available for desorption and degradation. Values of K-ss remained constant over time, indicating that material in the soil solution and the desorbable pool were in equilibrium. The amount of oryzalin and degradates in soil solution was a function of the desorption energy from the surface and was not limited by solubility. Similar, though less consistent, trends were observed for oryzalin degradation products.