Exploring the interactions of glyphosate in soil: the sorption scenario upon soil depletion and effect on waterleaf (Talinum triangulare) growth

Abstract

The pesticide glyphosate has contributed immensely to the ease of farming and high yields. However, the ever-increasing environmental input of pesticides is of particular interest due to several unintended effects on non-target organisms. In soil, the sorption, transport, possible uptake, and effect on plant growth are still not well understood, and much so for the sub-Sahara. Sorption processes are contingent on the soil composition, characteristics, and ambient conditions, and these are becoming increasingly affected by climate change in a way that may alter pesticide fate. Hence, representative sub-Saharan whole soil (WS) treated to eliminate organic matter (OMR) and iron oxides (IOR) was employed to ascertain the contributions of these major constituents to glyphosate sorption processes, as well as ascertain the effect of glyphosate in soil on the growth of Talinum triangulare–waterleaf. Glyphosate sorption for all treatments was rapid with equilibrium at around 720 min. The sorption decreased as pH increased, and was concentration-dependent, gradually increasing with glyphosate concentration. The process was endothermic, and sorption data were better described by the fractal pseudo-second-order and Freundlich adsorption isotherm models, suggesting a complex interplay of interactive sorption forces. The IOR sample (with iron oxide depleted but organic matter intact) exhibited higher sorption than the OMR and WS, highlighting the contribution of organic matter in glyphosate sorption. Hysteresis was high for all samples and increased with temperature. Considering the unregulated usage of glyphosate in the sub-Sahara, the poor sorption, especially in treated soils, observed in this study suggests a high possibility of glyphosate leaching into the aquifer and poisoning of this water source, while the high hysteresis implied the bio-availability of glyphosate in surface soil for plant absorption, hence affecting growth; as confirmed in the waterleaf growth study where growth in the organic-matter/iron-oxide-depleted soils was substantially stunted. Hence, glyphosate affects waterleaf growth, especially in organic-matter/iron-oxide-depleted soils.