title: Degradation of soil fumigants as affected by initial concentration and temperature.
abstract: Soil fumigation using shank injection creates high fumigant concentration gradients in soil from the injection point to the soil surface. A temperature gradient also exists along the soil profile. We studied the degradation of methyl isothiocyanate (MITC) and 1,3-dichloropropene (1,3-D) in an Arlington sandy loam (coarse-loamy, mixed, thermic Haplic Durixeralf) at four temperatures and four initial concentrations. We then tested the applicability of first-order, half-order, and second-order kinetics, and the Michaelis-Menten model for describing fumigant degradation as affected by temperature and initial concentration. Overall, none of the models adequately described the degradation of MITC and 1,3-D isomers over the range of the initial concentrations. First-order and half-order kinetics adequately described the degradation of MITC and 1,3-D isomers at each initial concentration, with the correlation coefficients greater than 0.78 (r2> 0.78). However, the derived rate constant was dependent on the initial concentration. The first-order rate constants varied between 6 and 10x for MITC for the concentration range of 3 to 140 mg kg(-1), and between 1.5 and 4x for 1,3-D isomers for the concentration range of 0.6 to 60 mg kg(-1), depending on temperature. For the same initial concentration range, the variation in the half-order rate constants was between 1.4 and 1.7x for MITC and between 3.1 and 6.1x for 1,3-D isomers, depending on temperature. Second-order kinetics and the Michaelis-Menten model did not satisfactorily describe the degradation at all initial concentrations. The degradation of MITC and 1,3-D was primarily biodegradation, which was affected by temperature between 20 and 40 degrees C, following the Arrhenius equation (r2 > 0.74).
authors.affiliation: USDA-ARS, Soil Physics and Pesticides Research Unit, George E Brown Jr Salinity Lab, Riverside, CA 92507, USA.
authors.forename: Q L
authors.initials: QL
authors.lastname: Ma
authors.forename: J
authors.initials: J
authors.lastname: Gan
authors.forename: S K
authors.initials: SK
authors.lastname: Papiernik
authors.forename: J O
authors.initials: JO
authors.lastname: Becker
authors.forename: S R
authors.initials: SR
authors.lastname: Yates
mesh_terms: D000498:Allyl Compounds; D005651:Fumigation; D006540:Herbicides / Q000378:metabolism*; D006843:Hydrocarbons, Chlorinated; D007306:Insecticides / Q000378:metabolism*; D017879:Isothiocyanates / Q000378:metabolism*; D007700:Kinetics; D008962:Models, Theoretical; D013696:Temperature
chemical_list: D000498:Allyl Compounds; D006540:Herbicides; D006843:Hydrocarbons, Chlorinated; D007306:Insecticides; D017879:Isothiocyanates; C023891:1,3-dichloro-1-propene; C005227:methyl isothiocyanate