Characterization of pesticide sorption and degradation in macropore linings and soil horizons of Thatuna silt loam

The effect of changes in soil characteristics with depth because of soil horizonation and macroporosity on fate of pesticides is poorly understood. Soil from the surface A horizons (0-0.65 m), albic E horizons (1.05-1.35 m), argillic Bt horizons (1.4-3.5 m), and surface linings of macropores in the argillic horizons (Bt-M, 1.4-3.5 m) of a Thatuna silt loam (fine-silty, mixed, mesic Xeric Argialbolls) were characterized for their physical, chemical, and microbiological effects on adsorption, desorption, and mineralization of 2,4-D, carbofuran, and metribuzin. Organic carbon (OC) contents decreased for soil materials in the order A (1.22%) > Bt-M (0.36%) > Bt (0.25%) = E (0.20%), and were correlated with the number of soil microbial colony forming units (log CFU) that decreased in the order A (7.4) > Bt-M (6.1) > E (5.6) = Bt (5.4). Percent macroporosity (pores 2-5 mm diam.) decreased in the order Bt (1.1%) > A (0.7%) > E (0.6%), which is the same order for decreases in saturated hydraulic conductivity (log m s^-1), namely; Bt (-6) > A (-6.2) > E (-7.0). Freundlich adsorption partition coefficients (K(t) - L kg^-1) for A horizon soil with 2,4-D (1.1), carbofuran (1.0), and metribuzin (1.6) were significantly greater than in subsurface E or Bt matrix material. Sorption K(t) values in macropore linings with carbofuran (1.1) and metribuzin (2.0) were comparable to or greater than their respective values of 1.0 and 1.6 in A horizon material. Percent mineralization in the A horizon after 139 d for [U-¹⁴C] 2,4-D (81.0%) and carbofuran (14.7%) was significantly greater than in all subsurface soil materials. Mineralization in Bt macropore linings for 2,4-D (17.0%) and carbofuran (8.4%) was significantly greater than in other subsurface soil materials. Mineralization of [U-¹⁴C] metribuzin was negligible (<3%) in all soil materials after 139 d.