Chemical and microbiological soil characteristics controlling glyphosate mineralisation in Danish surface soils

Adsorption and mineralisation of the herbicide glyphosate in five contrasting Danish surface soils were investigated using ¹⁴C labelled glyphosate. A comparison was made to several chemical and microbiological soil characteristics to identify their role in regulating the fate of glyphosate in these soils. Glyphosate was rapidly adsorbed to iron and aluminium oxides, but were later released from these pools during mineralisation. In soils with high mineralisation rates the metabolite AMPA was formed and adsorbed. Of all the soil factors tested, the rate of mineralisation was best correlated with the population size of Pseudomonas spp. bacteria in the soils. Phosphate addition had a stimulating effect on glyphosate degradation in soils with low mineralisation rates, but no effect or a negative effect on mineralisation in soils with high mineralisation rates. Finally, mineralisation rates were higher in soils from organically managed soils than in soils from conventional farming. The results indicate that the activity of glyphosate mineralising bacteria (e.g. Pseudomonas spp.) was a major factor controlling the fate of glyphosate in the soils.