MULCHING AND SOIL ORGANIC AMENDMENTS: EFFECT ON THE ADSORPTION-DESORPTION OF HERBICIDES

1. ABSTRACT Conservative tillage practices and application of organic amendments to soil improves its fertility and its present and future agronomic value. These practices increase the organic matter content in soil and can modify its physical, chemical, biological and hydrological properties (OM, bulk density, soil structure, porosity, etc.). Consequently, they can modify the behaviour and environmental fate of herbicides in soil. In this thesis project we studied the adsorption-desorption of three herbicides (S-metolachlor, foramsulfuron, and thiencarbazone-methyl) on wheat straw mulch at three different decomposition rate (M1, M2 and M3) and size particle (M1 crop residue milled (MM1)), and in two soils non-amended (S1 and S2) and amended with MM1 (S1+MM1 and S2+MM1) as well as the properties of herbicides, mulches and soils that effected the adsorption-desorption of these compounds. Adsorption-desorption isotherms were obtained by the batch equilibrium technique and fitted to Freundlich model. The Kf values for S-metolachlor (1.3 < Kf < 65.8) increased in the order: S1 < S2 < S1+MM1 < S2+MM1 << MM1 < M1 < M2 < M3 according to the increase in OC and/or DOC content of the adsorbents. This order was the same as observed for foramsulfuron (0 < Kf < 34.3) except on MM1. The adsorption of thiencarbazone-methyl (0.01 < Kf < 1.10) increased in the order: S1 ~ S1+MM1 < S2 < S2 + MM1 = M3 < M2 ~ M1 < MM1. The desorption Kfd values varied between 3.39 and 65.1 for S-metolachlor, 0 – 26.3 for foramsulfuron, and 0.19 - 7.65 for thiencarbazone-methyl. In general, Kfd values were higher for mulches than for soils (unamended and amended). CO and COD play an important role in adsorption-desorption of the 3 compounds; however, CO was the most important variable controlling the Kf, Kfd, of S-metolachlor, Kd of foramsulfuron and Kf of thiencarbazone-methyl. DOC was the one that controlled the desorption Kfd of the thiencarbazone-methyl.