**1.5 Soil properties and glyphosate mobility**

*Sorption in 2020s*

**Figure 1.**

cantly enhance glyphosate degradation.

ronments compared with oxic environments [6].

times larger than that from the sandy soil [10].

streams and groundwater systems [8].

**1.3 Mobility of glyphosate**

*Chemical structure of glyphosate [1].*

glyphosate application [9].

**1.4 Glyphosate residues**

converting glyphosate to AMPA (amino-methyl phosphonic acid) which is further decomposed and finally excreted to the environment. Glyphosate degradation can also occur via C-P lyase pathway to sarcosine, rather than AMPA. A bacterial strain *Bacillus subtilis* Bs-15 degraded 18% (12 h) to 67% (96 h) of glyphosate in sterile soil and 19% (12 h) to 72% (96 h) in unsterilized soil. It indicates that Bs-15 can signifi-

The binding mechanisms of clay minerals and organic colloids result in nonoccurrence of free glyphosate, but leaching of glyphosate complexes via preferential flow paths through the soil and transfer to waterways can occur, which could be a concern from the environmental pollution point of view [4]. In another study related to the desorption rate of glyphosate from goethite mineral surfaces, the rate of glyphosate desorption is mainly controlled by the breaking of the Fe-glyphosate bond through a dissociative or a dissociative interchange mechanism [5]. Soil redox condition is also an important factor controlling the mobility of glyphosate. Microbial degradation and mineralization of glyphosate were slow in anoxic envi-

In US soils, glyphosate and AMPA have been detected together and found widely

In a study related to 14C glyphosate transport in undisturbed topsoil columns, the amounts of glyphosate leached from the macroporous sandy loam were 50–150

Glyphosate and its decomposition product AMPA have been reported in stream water samples in areas of Zurich, Switzerland, with median concentrations of 0.11 and 0.20 μg/l; however, these compounds were not detected in groundwater [11]. In a Canadian study, glyphosate residues were observed in both upland and wetland settings; however, the concentrations were well below the Canadian guidelines for drinking water quality. Many other studies have reported glyphosate residues in

in the environment. The occurrence was more frequent in soils and sediments, ditches and drains, and rivers and streams and less in lakes, ponds, wetlands, soil water, and groundwater [7]. In western Switzerland, the surface runoff has been suggested as the major reason for the occurrence of glyphosate and AMPA in surface waters [8]; however, in a study related to Danish soils, limited leaching of glyphosate was reported in non-structured sandy soils, while subsurface leaching to drainage systems was observed in a structured soil when high rainfall followed

**106**

Data from sorption studies indicated that sorption coefficients are the most sensitive parameters for environmental risk assessment and soil properties like pH and clay content govern the glyphosate adsorption in Argentinian soils. In a related study in Argentina, high glyphosate sorption with low desorption in mollisols and ultisols indicated a low risk of groundwater contamination [14].

In another study on glyphosate mineralization in different agricultural soils, exchangeable acidity (H+ and Al3+), exchangeable Ca2+ ions, and ammonium lactate extractable K were the key soil parameters governing mineralization [15]. In a study related to glyphosate sorption with high soil phosphate levels, glyphosate sorption distribution constant Kd in soils ranged from 173 to 939 l Kg <sup>−</sup><sup>1</sup> under very strong to strongly acidic conditions, but the Kd was always <100 l Kg <sup>−</sup><sup>1</sup> under moderately acidic to slightly alkaline conditions suggesting that glyphosate may become mobile by water in soils with high phosphate levels [16]. This is important concerning the application of phosphatic fertilizers, as the phosphate ion would desorb glyphosate from adsorption sites resulting in the mobility of glyphosate towards aquatic environments [17].

Generally, iron and aluminum oxides adsorb a greater amount of glyphosate and phosphates in comparison to layer silicates [18] supporting the role of soil mineralogy concerning glyphosate sorption. As high phosphorus application can desorb glyphosate from sorption sites, application of char can be effective in these scenarios concerning sorption of glyphosate. The rapid degradation of glyphosate in surface waters and its practically irreversible sorption indicated a low potential environmental risk [19].

An investigation on adsorption of the herbicide glyphosate and its main metabolite AMPA found that pH(CaCl2) values, available phosphate, and amorphous iron and aluminum contents were the major parameters to predict the adsorption constants for these molecules [20]. In a similar study, while examining the effect of humic acid (HA) on the adsorption/desorption behaviour of glyphosate on goethite minerals, the herbicide was desorbed by two parallel processes: (i) a direct detachment from the surface, which is first order in adsorbed glyphosate, and (ii) a ligand exchange with HA molecules, which is first order in adsorbed glyphosate and first order in dissolved humic acid [21]. Glyphosate is adsorbed by humic acids via hydrogen bonding [22].

A laboratory study related to the fate of glyphosate and degradation in cover crop residues and underlying soil indicated that the differences in sorption and degradation levels were due to differences in the composition of the crop residues and availability to microorganisms [23]. In a related study of adsorption and mobility of glyphosate in different soils under no-till and conventional tillage, adsorption of glyphosate was influenced by the soil clay content and cation exchange capacity (CEC) and negatively related to pH and phosphorus. High Freundlich parameter (KF) values obtained in isotherm studies were the dominant factor influencing glyphosate mobility. KF values indicate the adsorption capacity of the soil [24].
