Soil properties mediate the responses of soil microbiota to fungicides and herbicides

The effect of pesticides on non-target soil microbial functions have been widely reported, yet little is known about how soil physicochemical properties mediate these effects. Most studies have examined one or two pesticides in one or few soil types and assessed limited microbial parameters. Here, we evaluated the effects of three herbicides (2,4-D amine, imazamox, propyzamide) and three fungicides (azoxystrobin, flutriafol, tebuconazole) on 16 microbial parameters, including (i) soil microbial enzymatic activities, (ii) potential nitrification, and (iii) the abundance of bacterial and fungal communities and key functional microbial genes (nifH, amoA, cbhI, chiA, and LMCO) across ten agricultural soils during a 28-day laboratory incubation. Fungicides, particularly tebuconazole, exerted a greater number of significant effects on soil microbial parameters than the herbicides. The fungicides stimulated most microbial parameters but consistently inhibited the absolute abundance of amoA genes of ammonia-oxidizing bacteria (AOB) significantly, across most of the ten soils. The herbicides had significant effects on fewer microbial parameters, showing both stimulatory and inhibitory responses. Soil pH emerged as a key driver, with both fungicide-induced stimulation and herbicide-induced inhibition being more pronounced in alkaline soils. The response varied among microbial parameters, with β-glucosidase, cellobiohydrolase activities, and the absolute abundance of amoA (AOB), cbhI, and LMCO genes being particularly responsive to interactions between the effect of pesticides and soil properties. These findings highlight the importance of assessing a broad range of microbial indicators and shows that soil pH is a major driver for the effect of pesticides on soil microbial functions.