Minor effects of herbicides on microbial activity in agricultural soils are detected by N-transformation but not enzyme activity assays

The potential effect of herbicides on soil functions, including nitrogen (N) transformations, is of interest to farmers, advisers, regulators and the community, yet comparisons of herbicides with different modes of action in a range of soil types are scarce. We applied seven commonly used herbicides (glyphosate acid, 2,4-dichlorophenoxyacetic acid [2,4-D], metsulfuron-methyl, trifluralin, diuron, atrazine and diflufenican) and one fungicide (tebuconazole) to five different Australian cropping soils at a recommended and five times recommended rate. Mineral N levels and the activities of eight enzymes involved organic matter cycling, including β-N-acetylglucosaminidase and leucine aminopeptidase contributing to organic N transformation, were monitored over a 28-d period. Only one herbicide (atrazine) at a recommended rate statistically (P < 0.05) and agronomically (>25% inhibition cf. control treatment) reduced NO3− formation in one soil type at the end of the incubation. The remaining herbicides at recommended rates had inconsistent, transitory or negligible effects on enzyme activities and mineral N levels. However, five times the recommended rate of metsulfuron-methyl or 2,4-D caused alterations to mineral N levels, either as reduced NO3− or increased NH4+, across multiple soil types. This study suggests that N-transformation assays were at least, if not, more sensitive to herbicides than enzyme assays. Overall, our study confirms that single recommended rate applications of herbicides typically have limited effects on soil functions such as N-transformation and soil enzyme activities involved in organic matter and nutrient cycling. The observation of significant effects at higher-than-label rates highlights the need to follow label instructions and prevent herbicide accumulation in soil.