Changes in activity and structure of the soil microbial community after application of azoxystrobin or pirimicarb and an organic amendment to an agricultural soil

• Spent mushroom substrate (SMS) and pesticides affect soil microbial communities.
• Azoxystrobin was more persistent than pirimicarb in soil over the incubation time.
• Soil dehydrogenase activity was stimulated in SMS-amended soils.
• Soil respiration increased with increasing pesticide concentration in soils.
• Illumina MiSeq analysis shows changes in the eukaryote community composition.

Pesticides and organic amendments are widely used in agricultural soils, the former for pest control and to improve crop yield, and the latter to maintain soil quality and to mitigate pesticide leaching. Both amendments and pesticides can affect soil microbial communities, which may impact crop fitness. Therefore, the objective of the current study was to investigate the changes that occur to soil microbial communities following use of the low-cost organic residue spent mushroom substrate (SMS) (2% and 5% on a dry weight basis) as an amendment to an agricultural soil in combination with the widely used pesticides azoxystrobin or pirimicarb (at 2 and 25 mg kg−1 rates). Dissipation experiments showed that azoxystrobin was more persistent than pirimicarb over the incubation time (90 days). The half-life (DT50) values of azoxystrobin (2 mg kg−1) and pirimicarb (2 and 25 mg kg−1) decreased, while DT50 values of azoxystrobin (25 mg kg−1) increased when SMS was applied to soil. Broad-scale methods such as soil dehydrogenase activity (DHA) and soil respiration showed that SMS rate, pesticide dose and incubation time all affected microbial community metabolism. DHA increased in SMS-amended soils and increased (azoxystrobin) or decreased (pirimicarb) over incubation time. Respiration increased in the soils when the concentration of the pesticide increased and decreased with increasing incubation time. Illumina MiSeq analysis of 18S rRNA genes demonstrated a strong effect of the SMS and incubation time, but not the pesticides, on micro-eukaryote community composition.