Effect of glyphosate and a commercial formulation on soil functionality assessed by substrate induced respiration and enzyme activity

Glyphosate is the most widely used herbicide globally. While concerns have been raised that glyphosate may modify soil ecosystems, the specific effects of glyphosate and commercial formulations of glyphosate on soil microbial community function are still not fully understood. This study investigated the effect of increasing doses (0-79 mg kg−1) of glyphosate and the formulation RoundupCT® on substrate induced respiration (SIR) and enzyme activities representing C-, N-, S- and P- cycling in three contrasting agricultural soils, over a 27 d period. Soil characteristics and the form of herbicide were dominant factors controlling potential effects on soil functionality. The light-textured Tenosol was more responsive to herbicide treatments than either the clay Vertosol or loamy Chromosol. In the Tenosol, there was a significant interaction between dose and herbicide form at 3 d after treatment: application of RoundupCT® at the two highest doses (26 and 79 mg glyphosate kg−1 soil) enhanced SIR of a number of C-substrates, while the highest dose of glyphosate inhibited SIR. Roundup CT® also triggered significantly greater consumption of arabinose, glucose, N-acetylglucosamine and proline in the Tenosol 27 d after application compared to glyphosate alone, but application dose was no longer significant. Effects in both the Chromosol and Vertosol were less clear, with glyphosate increasing SIR of glucose and malic acid in the Chromosol at day 3 only cf. Roundup CT, while SIR of arabinose, glucose and malic acid was stimulated by RoundupCT® in the Vertosol cf. glyphosate. In the Vertosol, glyphosate and RoundupCT® application at 79 mg kg−1 significantly increased respiration of arabinose 3 d after application, and oxalic acid at both time points, compared with the untreated control. Although some minor effects on enzyme activities were observed, they were generally less sensitive than measures of SIR. The exception was a significant reduction of cellulase activity in Vertosol 27 d after treatment with glyphosate (but not RoundupCT®) at rates equal or greater than 2.9 mg kg−1. Overall, effects of glyphosate or RoundupCT® at label rates were minor or periodic. This study demonstrated that the soil type and formulation of the herbicide are important factors when assessing potential impacts of herbicides on soil functions.