Legacy effects of herbicides on soil nitrifying guilds exposed to drought

Micro-organisms are essential for the functioning of agricultural soils but face increasing stress due to pollution and climate change. However, direct and legacy effects of agrochemicals on non-target micro-organisms are poorly considered in environmental risk assessment. Here, we set up a two-phase microcosm experiment to assess the effect of the herbicides clopyralid, metribuzin, and tembotrione (phase 1) on the abundance and activity of ammonia and nitrite oxidizing micro-organisms involved in nitrification, a key step in soil N cycling, and how it influences their response to subsequent drying and rewetting stress (phase 2). Pesticide exposure in phase 1 did not affect the nitrifying guilds and nitrification activity. By contrast, drying-rewetting affected the abundance of the different guilds, with ammonia-oxidizing archaea and Nitrospira-type nitrite oxidizers showing low resistance to rewetting, but with minor differences between herbicide-treated and no-herbicide treated soils. Legacy effects of herbicide exposure were instead captured by the soil nitrate pools, where differences between droughted and control soils appeared larger in the no-herbicide than in the herbicide-treated soils, potentially indicating differences in drought-coping strategies depending on prior stress exposure. Our results highlight that multiple stressor scenarios can reveal effects not captured by end-point measurements in risk assessment procedures.