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TECHNICAL REPORTS

Organic Compounds in the Environment

Reactivity of the Plant Growth Regulator Paclobutrazol (Cultar) with Two Tropical Soils of the Northeast Semiarid Region of Brazil

^a Universidade Federal de Pernambuco, Departamento de Energia Nuclear, Av. Prof. Luiz Freire 1000, Cid. Universitária, CEP 54740-540, Recife, PE, Brazil
^b Laboratoire d'étude de Transferts en Hydrologie et Environnement (LTHE), Univ. Grenoble I, CNRS, BP 53, 38041, Grenoble Cedex 09, França
^c Universidade Federal de Pernambuco, Departamento de Antibióticos, Av. Prof. Arthur de Sá, Cid. Universitária, Recife, PE, Brazil
^d Universidade Federal Rural de Pernambuco, Departamento de Agronomia, Rua Dom Manoel de Medeiros, s/n, Dois Irmãos, CEP 52171-900, Recife, PE, Brazil

^* Corresponding author: (jean.martins{at}hmg.inpg.fr).

Received for publication April 30, 2007. The reactivity of paclobutrazol (PBZ, a plant growth retardant) with a Yellow Ultisol and a Vertisol from the semiarid northeast region of Brazil was evaluated through batch sorption experiments and modeling. Although not instantaneous, the sorption kinetic of PBZ (pure and formulated) was fast (a few hours) in both soils. The sorption kinetics were well described by a second-order (=k₂(S_e2–S_t)²) but not by a first-order model. The sorption isotherms were found to be linear and the calculated K_D values were 8.8 ± 0.11 and 7.4 ± 0.2 L kg^–1 for pure PBZ in the Ultisol and the Vertisol, respectively. The corresponding K_OC values were 1275 ± 34 (logK_OC = 3.11) and 1156 ± 49 (logK_OC = 3.06) L kg^–1, respectively. Considering the very different texture of the two soils and the similar K_OC values determined, these results showed that in both soils, the sorption of PBZ is dominantly controlled by organic matter, although some interactions of PBZ with iron oxides (goethite) were observed in the Ultisol. Based on these sorption parameters a low leachability potential of PBZ in soils is anticipated, as they correspond to a groundwater ubiquity score (GUS) ranging from 2.0 to 2.7, i.e., moderately to not mobile, in contradiction with the actual groundwater situation in Brazil. This work stresses the need to evaluate and predict the risk associated with aquifer contamination by this widely used plant growth regulator.

Abbreviations: PBZ, Paclobutrazol • HPLC, high-performance liquid chromatography • TOC, total organic carbon content • BSSS, Brazilian Society of Soil Science