Interactive relationship between sorption and decomposition of ¹⁴C-glyphosate: The importance of temperature in influence the availability of glyphosate for decomposition in Australian soils

A laboratory experiment was conducted to investigate the decomposition behaviour of glyphosate in four Australian soils under five temperature regimes using non steady state compartmental analysis (NSSCA). Temperature strongly influenced the partitioning of glyphosate into the soluble and sorbed phases in three soils examined (Hay Alluvium, Walpheup Rendzina, and Walpheup Sandy Soil). At low temperature (5° C and 12° C), only small amounts of glyphosate were partitioned into the soluble phase ranging from 1.4-2.1 % in these soils. As the incubation temperature increased from 12° to 17° and 22° C, the amount of glyphosate partitioned into the soluble phase substantially increased and ranged from (23.2-36.2%). Further increased of the temperature from 22° to 28° C markedly increased the partitioning of glyphosate into the soluble phase in two soils (Walpheup Rendzina and Walpheup Sandy Soil), but the partitioning of glyphosate into the soluble phase slight decreased in the other two soils (Hay Alluvium and Ladysmith Red Podzolic). Temperature has a significant (P <0.05) influence on half life of glyphosate in the soluble and sorbed phases. Half lives of the soluble phase were relatively in the similar order of 1-2 weeks among the 4 soils tested. However, half lives of the sorbed phase varied widely (67-4950 days). Our data suggests that differences in the strengths of binding existed within the sorbed phase in the 4 soils investigated in this study. The rate of glyphosate desorbed from the sorbed form was dependent on soil type and temperature. Glyphosate was shown to be more strongly held in acidic soil than in alkaline soil and its desorption rate (Dsorp) from the sorbed form was correlated with the soil pH, the amount of exchangeable Fe and Al and the incubation temperature. The regression equation for desorption (D-sorp) is: -0.0773 + 0.02314 pH - 0.001707 pH2 + 0.0000717 Exch (Fe+Al) -0.0773 Temperature; R2 = 0.70. This study showed that temperature not only influence partitioning glyphosate into the soluble and sorbed phases which affect the amount glyphosate available for microbial degradation but also influences the amount of residual glyphosate that may be biologically active for non-target organisms.