The aim of this work was to estimate the dissipation of mesotrione applied at three doses (2, 10 and 50 mg kg−1
dw) in an unamended agricultural soil, and this same soil amended with two organic residues (green compost
(C) and sewage sludge (SS)). The effects of herbicide and organic residue on the abundance and activity of soil
microbial communities were also assessed by determining soil microbial parameters such as biomass, dehydrogenase
activity (DHA), and respiration. Lower dissipation rates were observed for a higher herbicide dose. The
highest half-life (DT50) values were observed in the SS-amended soil for the three herbicide doses applied.
Biomass values increased in the amended soils compared to the unamended one in all the cases studied, and
increased over the incubation period in the SS-amended soil. DHA mean values significantly decreased in the
SS-amended soil, and increased in the C-amended soil compared to the unamended ones, under all conditions.
At time 0 days, respiration values were significantly higher in SS-amended soils (untreated and treated with
mesotrione) than in the unamended and C-amended soils. The effect of mesotrione on soil biomass, DHA and respiration
was different depending on incubation time and soil amendment and herbicide dose applied. The results
support the need to consider the possible non-target effects of pesticides and organic amendments simultaneously
applied on soil microbial communities to prevent negative impacts on soil quality.