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Soil Chemistry

2,4-D Residues in Aggregates of Tropical Soils as a Function of Water Content

^a Faculdades Integradas de Rondonópolis, União de Escolas Superiores de Rondonópolis, Rua Floriano Peixoto, 597, CEP 78700-040, Rondonópolis, Mato Grosso, Brazil
^b Lab. de Ecotoxicologia, Centro de Energia Nuclear na Agricultura, Univ. de São Paulo, Caixa Postal 97, CEP 13400-970, Piracicaba, São Paulo, Brazil
^c Dep. de Solos e Nutrição de Plantas, Escola Superior de Agricultura "Luiz de Queiroz", Univ. de São Paulo, Caixa Postal 09, CEP 13418-900, Piracicaba, São Paulo, Brazil

^* Corresponding author (regitano{at}cena.usp.br)

The effects of soil moisture on reactions between pesticides and soil aggregates are little understood, particularly in soils from the humid tropics. Thus, the influence of soil water content on retention and mineralization of the herbicide 2,4-D [(2,4-dichlorophenoxy)acetic acid], as well as its distribution on the aggregate fractions (diameter >150, 53 to 150, 20 to 53, 2 to 20, and <2 µm) were evaluated. Soil samples from a Xanthic Dystrudox, a Xanthic Acrudox, and two Rhodic Acrudoxes were collected and moisture was adjusted to 25, 50, and 75% maximum water holding capacity. ¹⁴C 2,4-D was then applied and the flasks incubated in a semi-dark, climatized room at 22 ± 2°C, for 42 d. 2,4-D mineralization was determined weekly. After three extractions with a 0.01 mol L^–1 CaCl₂ solution, soil aggregates were sonicated using water as a dispersing agent. 2,4-D retention increased with soil moisture. Higher soil moisture favored 2,4-D retention in aggregates < 20 µm, ratifying the hypothesis of 2,4-D diffusion to less accessible soil sites. 2,4-D mineralization was also enhanced with soil moisture. For the Acrudoxes, in which the clay fraction was less dispersed in water, 2,4-D retention was higher in larger aggregates (>53 µm), having higher total organic C (OC) content and higher degree of humification of its organic matter (OM).

Abbreviations: LSS, liquid scintillation spectrometry • MWHC, maximum water holding capacity • OC, organic C • OM, organic matter • SSA, specific surface area • WDC, water dispersible clay