Sorption, desorption and mineralisation of the herbicides glyphosate and MCPA in samples from two Danish soil and subsurface profiles

The vertical distribution of the sorption, desorption and mineralisation of glyphosate and MCPA was examined in samples from two contrasting soil and subsurface profiles, obtained from a sandy agricultural site and a non-agricultural clay rich site. The highest mineralisation of 14C-methylen]glyphosate, with 9.3-14.7% degraded to 14CO2 within 3 months was found in the deepest sample from the clay site. In the deeper parts of the sandy profile high sorption and low desorption of glyphosate coincided with no or minor mineralisation indicating a limited glyphosate bioavailability. MCPA was readily mineralised except in the deepest samples from both sites. The highest MCPA mineralisation was detected just below the surface layers with 72% or 44% degraded to 14CO2 at the sandy or the clay sites, respectively. MCPA sorped to a minor extent in all samples and no indications of sorption-controlled mineralisation was revealed. None of the herbicides were mineralised under anoxic conditions.