[14C]Glyphosate transport in undisturbed topsoil columns

Abstract

Although glyphosate (N-(phosphonomethyl)glycine) is one of the most frequently used herbicides, few controlled transport experiments in undisturbed soils have been carried out to date. The aim of this work was to study the in¯uence of the sorption coef®cient, soil-glyphosate contact time, pH, phosphorus concentration and colloid-facilitated transport on the transport of [ 14 C]glyphosate in undisturbed top-soil columns (20 cm height  20 cm diameter) of a sandy loam soil and a sandy soil. Batch sorption experiments showed strong Freundlich-type sorption to both soil materials. The mobility of glyphosate in the soil columns was strongly governed by macropore¯ow. Consequently, amounts of glyphosate leached from the macroporous sandy loam soil were 50±150 times larger than from the sandy soil. Leaching rates from the sandy soil were not affected by soil-glyphosate contact time, whereas a contact time of 96 h strongly reduced the leaching rates from the sandy loam soil. The role of pH and phosphorus concentration in solution was relatively unimportant with respect to total glyphosate leaching. The contribution of colloid-facilitated transport was`1 to 27% for the sandy loam and`1 to 52% for the sandy soil, depending on soil treatment. The risk for glyphosate leaching from the top-soils seems to be limited to conditions where pronounced macropore¯ow occurs shortly after application.

Particle‐facilitated Pesticide Leaching from Differently Structured Soil Monoliths

“…Several soil factors may have contributed to this diff erence with respect to signifi cance of particle-facilitated transport, for instance number of fl ow active continuous macropores, degree of preferential fl ow, the soil surface structure (see next paragraph), etc. For instance, the columns used in this setup were considerably larger (diameter 30 cm, length 50 cm) than columns used in experiments conducted by de Jonge et al (2000) (diameter 20 cm, length 20 cm). Th e larger column size reduces eff ects of lower boundary (saturation at 40-50 cm depth instead of at 15-20 cm depth) and takes better into account eff ects of structural heterogeneity in arable soils.…”

“…Except for Hardy et al (2000), who found that the pesticide difl ufenican was entirely associated with sediment in drain water, other reported quantifi cations of facilitated transport of strongly sorbing pesticides range from < 1 to 11% for leached glyphosate from columns (de Jonge et al, 2000), 13 to 16% and 16 to 31% of leached glyphosate and pendimethalin, respectively, in fi eld drain (Kjaer et al, 2007), to 4.9 to 30% for atrazine sampled in lysimeter (Sprague et al, 2000), and 6% of leached prochloraz sampled in drainage (Villholth et al, 2000). Th us, particle-facilitated transport accounting for 68% of the total leaching of glyphosate in conventionally tilled soil is a very high estimate compared to most other fi ndings.…”

A review of non‐equilibrium water flow and solute transport in soil macropores: principles, controlling factors and consequences for water quality

“…Particulate matter is efficiently filtered out in matrix flow, but significant particle-bound transport in macropores has been demonstrated for strongly sorbing solutes, including some pesticides (e.g. Worrall et al, 1999;Villholth et al, 2000;De Jonge et al, 2000;Zehe & Flu¨hler, 2001a;Petersen et al, 2002) and phosphorus (Ule´n & Persson, 1999;Stamm et al, 1998;Laubel et al, 1999;Uusitalo et al, 2001;De Jonge et al, 2004). An absence of significant sorption retardation during flow in larger macropores has also been indirectly demonstrated in several field experiments that show an equally fast initial breakthrough of solutes irrespective of their sorption characteristics (e.g.…”

“…'rainsplash') with the resident soil water in the distribution zone at or close to the soil surface. Conversely, leaching can be considerably reduced, either if dry weather follows application so that sufficient time is allowed for the solute to diffuse away from the soil surface, or if the solute is 'washed' into the soil matrix by one or more light rain showers which do not generate macropore flow (Isensee et al, 1990;Shipitalo et al, 1990;Edwards et al, 1993;De Jonge et al, 2000). Once the bulk of the chemical has penetrated into the soil matrix below the distribution zone, it is no longer so readily exposed to macropore flow (Gish et al, 1991a), since the micropore volume is much larger than the volume of macropores and very slow diffusion towards macropores becomes the rate-limiting factor.…”

Glyphosate and phosphorus leaching and residues in boreal sandy soil

“…For instance, when the soil water content is high and heavy rainfall occurs shortly after application, macropore transport of glyphosate may take place (de Jonge et al 2000;Kjaer et al 2003). On the other hand, in a lysimeter study Fomsgaard et al (2003) found residues of glyphosate in subsoil and subsurface leachate water even two years after application, when heavy rainfall occurred after a long dry period.…”