CONCLUSIONS
In all the studied soils the initial dissipation of glyphosate was fast, followed by a decrease in the dissipation rate. Over time, glyphosate becomes less bioavailable, making the remaining fraction more persistent especially in those soils with high adsorption capacity (PAR and PER). In this sense, the partitioning of the herbicide between the aqueous and the solid phase will influence the degradation, as it becomes less available to microorganisms, while it adsorbs to the soil matrix. There were no differences in dissipation between NT and CT, indicating that glyphosate degrading microflora was not modified with the different tillage managements. Also, tillage practices did not alter the general soil properties therefore; glyphosate bio-availability was not affected by NT or CT management.
Glyphosate initial dissipation was fast, whereas the accumulation of its metabolite, AMPA, was scarce. This suggests a fast AMPA degradation or that the preferred degradation pathway in these soils is via sarcosine.
The estimated half-lives for glyphosate in the studied soils under optimal temperature and moisture conditions ranged between 9 and 38 days. However, the less available residues can remain in the soil for almost a year after application. In the field, glyphosate might persist for even longer periods if conditions are temporarily less favorable for degradation (e.g. cold or dry seasons). The implications of this study are that glyphosate residues may accumulate in agricultural soils, especially if it is applied 2 to 3 times per year which is frequently the case. This may lead to negative impacts on the soil biota and furthermore, it increases the risk of polluting surface waters, by soil runoff, and groundwater resources, by vertical transport. It is important that glyphosate applications are kept to the needed minimum, in order to avoid its environmental accumulation and distribution.
