Does altered rainfall regime change pesticide effects in soil? A terrestrial model ecosystem study from Mediterranean Portugal on the effects of pyrimethanil to soil microbial communities under extremes in rainfall

• We examined interaction effect of fungicide (PYR) and altered rainfall.
• PYR effects on microbial community structure differed with rainfall regime.
• The interaction effect on structure persisted for eight weeks.
• PYR effects on soil functions differed with rainfall regime at two weeks.
• But this interaction effect on soil function was short-lived.

In this century, agroecosystems are subjected to multiple global change stressors acting in concert such as alterations in rainfall regimes and pesticide use. Alterations in rainfall regimes, characterised by more extreme intra-annual rainfall regimes, have been forecasted for the Mediterranean region. At the same time, the use of pesticides continues to rise. Here, we report the responses of soil microbial community to a model pesticide, i.e., fungicide pyrimethanil (PYR) under altered rainfall regimes (i.e., drought and heavy rainfall) two and eight weeks after PYR application. We measured the functional responses as enzyme activities, potential nitrification and BIOLOG carbon substrate utilisation. We also characterised the soil bacterial communities using polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE) method. After two weeks, enzyme activities were mainly responsive to PYR and kinetic parameters, calculated from BIOLOG carbon substrate utilisation, indicated interaction effects from PYR and rain treatments. Bacterial band richness increased with PYR treatment under normal rain and drought regimes, but bacterial band richness was higher at 1X than 5X PYR under heavy rainfall. Bacterial community structure was also different with the PYR and rainfall treatments. By week eight, PYR treated soils remained functionally different from untreated soils. Bacterial band richness was consistent across PYR treatment regardless of rain regime. However, the bacterial community structure remained significantly different among the PYR treatments under different rain regimes. We conclude that rainfall extremes can alter the effect of PYR on the soil microbial community structure without altering PYR effects on soil functions (measured as enzyme activities, potential nitrification and BIOLOG carbon substrate utilisation).