Effect of Temperature and Moisture on the Degradation and Sorption of Florasulam and 5-Hydroxyflorasulam in Soil

The degradation rate and sorption characteristics of the triazolopyrimidine sulfonanilide herbicide florasulam and its principal degradation product 5-hydroxyflorasulam (5-OH-florasulam) were determined as a function of temperature and moisture in three different soils. The half-life for degradation of florasulam ranged from 1.0 to 8.5 days at 20−25 °C and from 6.4 to 85 days at 5 °C. The half-life for degradation of 5-OH-florasulam ranged from 8 to 36 days at 20−25 °C and from 43 to 78 days at 5 °C. The degradation rate of both compounds was strongly influenced by temperature, with activation energies ranging from 57 to 95 kJ/mol for florasulam and from 27 to 74 kJ/mol for 5-OH florasulam. Soil moisture content had negligible impact on the degradation rate. Apparent (nonequilibrium) sorption coefficients for florasulam and 5-OH-florasulam at 0 days after treatment (DAT) were 0.1−0.6 L/kg and increased linearly with time for both florasulam and 5-OH-florasulam (r 2 > 0.90) to levels as high as 12−23 L/kg. Heats of adsorption were calculated on one soil as a function of time. Heat of adsorption values for both florasulam and 5-OH-florasulam increased as incubation time increased and the amount of each compound decreased; values were near 0 kJ/mol initially and increased to a maximum of 91 and 66 kJ/mol for florasulam and 5-OH-florasulam, respectively. Keywords: Florasulam; 5-hydroxyflorasulam; 5-OH-florasulam; aerobic soil metabolism; sorption; aerobic soil degradation; apparent K d