Interactive relationship between sorption and decomposition: The Importance of temperature in influencing the availability of glyphosate for decomposition in Australian soils

A laboratory experiment was conducted to investigate the sorption-decomposition behavior of glyphosate in four Australian soils. 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 increase 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 slightly decreased in the other two soils (Hay Alluvium and Ladysmith Red Podzolic). Half lives of the soluble phase were relatively in a similar order of 1-2 weeks among the four soils tested. However, half lives of the sorbed phase varied widely (67- 4950 days). Differences in the strengths of binding existed within the sorbed phase in the four soils investigated in this study. Glyphosate was shown to be more strongly held in acidic soil than in alkaline soil and its desorption rate from the sorbed form was correlated with the soil pH, the amount of exchangeable Fe and Al, and the incubation temperature. This study showed that temperature not only influences partitioning of glyphosate into the soluble and sorbed phases which affect the amount glyphosate availability for microbial degradation but also influences the amount of residual glyphosate that may be biologically active for non-target organisms.