Characterization of Pesticide Sorption and Degradation in Macropore Linings and Soil Horizons of Thatuna Silt Loam

  • Mallawatantri A
  • McConkey B
  • Mulla D
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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 (Kf- 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 Kf 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 (81.8%) 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 (

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  • A P Mallawatantri

  • B G McConkey

  • D J Mulla

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