Does Phosphate Affect Soil Sorption and Degradation of Glyphosate? – A Review

Glyphosate is worldwide the most used herbicide because it is an efficient weed killer with favorable environmental and toxicological properties such as very low human and animal toxicity. Chemically, glyphosate is a phosphonate, i.e. it resembles phosphate. Consequently, glyphosate is strongly sorbed by the same soil minerals as phosphate, especially aluminium and iron oxides, allophane/imogolite and other variable charge minerals, whereas sorption by 2:1-layer silicates is modest. However, this similarity between the two compounds indicates that phosphate may affect glyphosate sorption and degradation. Therefore, phosphate may affect the leachability of glyphosate and, in turn, the risk of groundwater contamination with this xenobioticum. Although the effect of phosphate on glyphosate sorption was pointed out shortly after the introduction of the herbicide in 1974, much is still unclear about interactions between glyphosate and phosphate. Therefore, it has been attempted to summarize available knowledge on influence of phosphate on sorption and degradation of glyphosate and its main metabolite, aminomethylphosphonic acid (AMPA) at different scales from the isolated mineral to the field level. The summary has shown that the interaction between glyphosate and phosphate can exhibit great variation from mineral to mineral and from soil to soil according to investigations in several laboratories. Phosphate has been shown to suppress glyphosate sorption by some minerals and soils but for most minerals and soils the effect is limited or even absent. This discrepancy has been interpreted as existence of two kinds of sorption sites including common sites subject to glyphosate-phosphate competition and to sites specific for either glyphosate or phosphate, where the two sorbates are sorbed independently of each other (additive sorption). Phosphate seems to have a positive effect on glyphosate degradation in some soils but no or little effect in other soils. Transport of presumably colloid-bonded glyphosate and AMPA in structured clayey soils with macropores has been demonstrated both in several lysimeter studies and few field trials; the leaching is augmented by heavy rainfall shortly after glyphosate application. However, the role of phosphorus in this transport has not been investigated. In sandy soils without macropores, glyphosate and AMPA do not seem to leach. These results are in agreement with those of a Danish field trial, where concentrations of glyphosate and AMPA in soil samples and leachates were monitored during the year following glyphosate application to plots on a flat sandy soil that have received different amounts of phosphorus (and lime) since 1944. This trial showed that less than 0.1 g/ha corresponding to 0.013 % of applied glyphosate was translocated during the monitoring period and the leachate concentrations of glyphosate and AMPA were less than the EU drinking water threshold, 0.1 μg/L and often below the detection limit (0.01 μg/L). The contents of glyphosate and AMPA in the soil samples did not change significantly during the monitoring period, except for decreasing glyphosate content in the samples from one of the four plots investigated, but this decrease was ascribed to favorable pH conditions for the microbial degraders because of liming. The very limited translocation of glyphosate and AMPA found in the leachate and subsoil during the monitoring was attributed to transport by the plants as also seen in other investigations. Therefore, a main conclusion of this trial was that the soil phosphorus status did not influence glyphosate and AMPA dissipation at that site.