Influence of climatic and edaphic factors on persistence of glyphosate and 2,4-D in forest soils

Persistence in soil of the two herbicides glyphosate and 2,4-D was investigated after application for brush control in conifer reforestation areas. Field experiments were carried out at five sites in southern Sweden and six in northern Sweden. Initially, glyphosate disappeared faster in northern soils than in southern soils. This was probably a result of the higher biological activity in the northern soils. However, small amounts of glyphosate were detected in the northern area long after all traces had disappeared in the south, presumably because of the long period during which the soil remained frozen in the northern area and because of the slow release of vegetation-bound herbicide. One metabolite of glyphosate, aminomethylphosphonic acid (AMPA), persisted longer than glyphosate itself. After 2 years, 8% of the theoretical amount was found in the northern area and, after 1 year, 1% in the southern area. 2,4-D disappeared rapidly from all sites, although minor amounts persisted for several years, probably because of slow release from vegetation-bound residues.