TILLAGE AND COVER CROP EFFECTS ON CYANAZINE ADSORPTION AND DESORPTION KINETICS

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Abstract
Accumulation of partially decomposed plant residues under no-tillage (NT) and cover crop management systems can affect herbicide fate in the soil. This study evaluated adsorption and desorption of cyanazine {2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methylpropanenitrile} in soils and herbicide-killed Italian ryegrass (Lolium multiflorum Lam.) residues collected from a long-term conventional tillage (CT) and NT cotton field. The four cotton production systems included were CT and NT, each with and without ryegrass as a cover crop. Adsorption was determined by reacting 0.5 g of soil or ryegrass residue with 8 mL of 14C-cyanazine solution (five concentrations: 0.13 to 15.68 μmol L−1) for 48 h. The Freundlich Kf values were higher in NT than in CT soils and higher in soils from ryegrass cover crop than in soils from no cover crop. The Kf was higher in ryegrass residue (13.33) than in soils (1.77 to 2.94). The N values for soils (>0.90) and ryegrass residue (>0.95) indicated nearly linear adsorption. Time-course adsorption data analyzed by an equilibrium/kinetic model indicated that adsorption was rapid initially (within 1 h), followed by a slow increase in CT and NT soils from ryegrass plots. In contrast, adsorption achieved equilibrium within 48 h of reaction time in ryegrass residue. Cyanazine adsorption increased with increased decomposition of plant residues. The Kf for ryegrass residues sampled at 5 weeks after cotton planting was 17% higher than the residues sampled at 3 weeks before planting. The CaCl2-desorbable cyanazine in two consecutive 24-h cycles ranged from 77 to 88% in soils and from 46 to 47% of that adsorbed in ryegrass residues. Two additional 24-h desorptions with methanol removed most of the remaining cyanazine. Under field conditions, the plant residues on the soil surface in NT and cover crop systems can apparently intercept and temporarily retain cyanazine.