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DIVISION S-2 - SOIL CHEMISTRY

Effect of Exogenous Carbon on Movement of Simazine and 2,4-D in Soils

^a Instituto de Recursos Naturales y Agrobiología de Sevilla, CSIC, P.O. Box 1052, 41080 Sevilla, Spain
^b Dep. Soil, Water & Climate, Univ. Minnesota, St. Paul, MN 55108
^c USDA-ARS, Soil and Water Management Research Unit, 1991 Upper Buford Cir. Rm 439, St. Paul, MN 55108

* Corresponding Author (cornejo{at}irnase.csic.es)

Organic amendment (OA) addition is an agricultural practice that can greatly affect pesticide behavior in soil. The influence of three OAs, consisting of a liquid amendment (LOA), solid (SOA) humic amendment, and a solid urban waste (SUW), on sorption, persistence, and leaching potential of simazine (2-chloro-4,6-bis (ethylamino)-s-triazine) and 2,4-D (2,4-dichlorophenoxyacetic acid) in a sandy soil (P2) has been investigated. The OAs were added to P2 at a rate of 5% (w/w). Sorption was studied using the batch equilibration technique, degradation by incubations at 20°C and -0.33 kPa moisture content, and leaching using handpacked soil columns. Simazine sorption increased in the order of P2 P2 + LOA < P2 + SUW < P2 +S OA. The herbicide, 2,4-D, did not sorb on P2 soil, but in amended soils sorption increased in the same order as simazine. The lower sorption of both herbicides in P2 + LOA as compared with P2 + SOA and P2 + SUW is attributed to higher dissolved organic matter (DOM) content of LOA. Simazine's half-life was reduced upon amendment. Degradation of 2,4-D followed a sigmoidal rate in P2, P2 + SOA, and P2 + SUW, but not in P2 + LOA, in which 2,4-D persistence was much higher. The higher stability of 2,4-D in P2 + LOA has been attributed to 2,4-D-dissoved organic C interactions. Leaching studies indicate that degradation affects movement to a higher extent than sorption, specially in the case of 2,4-D in P2 + LOA soil, in which the amounts leached are much higher than those in unamended soil. In the case of simazine and P2 + SUW soil, greater reduction in large-size conducting pores upon amendment, as revealed in Hg porosimetry studies, resulted in a greater reduction of leaching than that suggested from the small differences in sorption and degradation between P2 and P2 + SUW soil.

Abbreviations: BTC, breakthrough curve • Ce, equilibrium concentration • Ci, initial concentrations • Cs, solid concentration • DOC, dissolved organic C • DOM, dissolved organic matter • HPLC, high performance chromatography • LOA, liquid organic amendment, OA, organic amendment • P2, sandy soil • SOA, solid organic amendment • SUW, solid urban waste

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