LAUSR

Abstract Biodiesel presents a higher degradability than petroleum hydrocarbons in soils. Since microorganisms dominate biodegradation processes, impacts on soil microbiota provide information regarding the course of fuel attenuation. We aimed to evaluate the effects of biodiesel (B100) and a diesel–biodiesel blend (B8) on soil microbial indicators. In microcosms, an oxisol was spiked with B100 (5% and 10%, w w-1 dry soil) or B8 (5% and 10%, w w-1 dry soil). In addition to microbial parameters, phytotoxicity, pH, and fuel attenuation were assessed during 90 days. Spiked soils exhibited increased cumulative respiration than non-contaminated controls; however, it was lower with B100 (140–204%) than with B8 (347–420%). Initial pH (6.0) slightly diminished with B8 after 90 days (~5.7), but decreased to ~3.9 after 30 days with B100. Heterotrophic and B8-degrading bacteria were 1.6–2.4 and 1.5–2.5 Log higher with B8 as compared to controls; conversely, heterotrophic and B100-degrading populations were reduced by B100 (3.5–4.5 and 4.7–6.8 Log lower than controls). B100 resulted in more prominent negative impacts on microbial biomass, metabolic quotient and soil enzyme activities. Phytotoxicity towards cucumber seeds tended to be mitigated in B8-5% soil, whereas the opposite has been observed with B100. Biodiesel fatty acid methyl esters (FAME) were completely (or highly) attenuated in B8 microcosms; nonetheless, in B100 soils, partial attenuation occurred only for unsaturated FAME. These findings are particularly discussed considering the prominent acidification in B100-spiked soils. In addition to B8, the results indicate that heavy soil contaminations with biodiesel pose significant environmental hazards.