TILLAGE AND COVER CROP EFFECTS ON CYANAZINE ADSORPTION AND DESORPTION-KINETICS

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-methylpropanen itrile) in soils and herbicide-killed Italian ryegrass (Lolium multifl orum Lam.) residues collected from a long-term conventional tillage (C T) and NT cotton field, The four cotton production systems included we re CT and NT, each with and without ryegrass as a cover crop, Adsorpti on was determined by reacting 0.5 g of soil or ryegrass residue with 8 mL of C-14- cyanazine solution (five concentrations: 0.1.3 to 15.68 m u mol L-1) for 48 h. The Freundlich K-f 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 K, was higher in ryegrass residue (13.33) tha n 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 adsor ption data analyzed by an equilibrium/kinetic model indicated that ads orption was rapid initially (within 1 h), followed by a slow increase in CT and NT soils from ryegrass plots, In contrast, adsorption achiev ed equilibrium within 48 h of reaction time in ryegrass residue, Cyana zine adsorption increased with increased decomposition of plant residu es, The K-f for ryegrass residues sampled at 5 weeks after cotton plan ting was 17% higher than the residues sampled at 3 weeks before planti ng, The CaCl2-desorbable cyanazine in two consecutive 24-h cycles rang ed from 77 to 88% in soils and from 46 to 47% of that adsorbed in ryeg rass residues, Two additional 24-h desorptions with methanol removed m ost of the remaining cyanazine, Under field conditions, the plant resi dues on the soil surface in NT and cover crop systems can apparently i ntercept and temporarily retain cyanazine.