Pesticide soil microbial toxicity: setting the scene for a new pesticide risk assessment for soil microorganisms (IUPAC Technical Report)

Abstract

Pesticides constitute an integral part of modern agriculture. However, there are still concerns about their effects on non-target organisms. To address this the European Commission has imposed a stringent regulatory scheme for new pesticide compounds. Assessment of the aquatic toxicity of pesticides is based on a range of advanced tests. This does not apply to terrestrial ecosystems, where the toxicity of pesticides on soil microorganisms, is based on an outdated and crude test (N mineralization). This regulatory gap is reinforced by the recent methodological and standardization advances in soil microbial ecology. The inclusion of such standardized tools in a revised risk assessment scheme will enable the accurate estimation of the toxicity of pesticides on soil microorganisms and on associated ecosystem services. In this review we (i) summarize recent work in the assessment of the soil microbial toxicity of pesticides and point to ammonia-oxidizing microorganisms (AOM) and arbuscular mycorrhizal fungi (AMF) as most relevant bioindicator groups (ii) identify limitations in the experimental approaches used and propose mitigation solutions, (iii) identify scientific gaps and (iv) propose a new risk assessment procedure to assess the effects of pesticides on soil microorganisms.

The effect of natural products used as pesticides on the soil microbiota: OECD 216 nitrogen transformation test fails to identify effects that were detected viaq‐PCR microbial abundance measurement

“…As for 3,5-DCA, it is primarily known for its inhibitory impact on soil enzymatic activities and the soil microbiota, particularly at concentration levels surpassing 6-10 mg kg −1 . 46 We further extended our measurements to AOMs, which have been identified as potential bioindicators for assessing the toxicity of pesticides on soil microorganisms 32,34 and have a key role in nitrification, a process considered also in the N transformation OECD 216 test. Dihydrochalcone, aliphatic phenol, spinosad, and 3,5-DCA significantly reduced the abundance of AOA, with the former showing the most consistent inhibitory effect across soils.…”

“…This has been illustrated in earlies studies of Papadopoulou et al 78 where significant, still transient, effects of the antioxidant ethoxyquin on soil AOA and AOB were evident only via RNA-based q-PCR approaches. However, as suggested by Karpouzas et al, 32 the labile nature of RNA molecules in soil has precluded the extensive use of such approaches in soil microbial ecotoxicology. Still, we consider them particularly useful if a refinement of the assessment of the toxicity of pesticides on key functional microbial groups like AOMs is requested based on non-conclusive evidence by DNAbased measurements.…”

“…31 In the same report, they highlighted the need for novel testing approaches and identified arbuscular mycorrhizal fungi (AMF) as promising bioindicators for assessing the toxicity of pesticides on soil microbiota. Reflecting on this, Karpouzas et al 32 stressed the need to abandon the OECD 216 test and instead incorporate novel, well standardized molecular tools (e.g. ISO 17601 Estimation of abundance of selected microbial gene sequences by quantitative PCR from DNA directly extracted from soil 33 ) that could provide a highresolution analysis of pesticide toxicity on soil microorganisms.…”

“…In the same report, they highlighted the need for novel testing approaches and identified arbuscular mycorrhizal fungi (AMF) as promising bioindicators for assessing the toxicity of pesticides on soil microbiota. Reflecting on this, Karpouzas et al 32 . stressed the need to abandon the OECD 216 test and instead incorporate novel, well standardized molecular tools (e.g.…”

Considerations for the inclusion of metabarcoding data in the plant protection product risk assessment of the soil microbiome

“…This, therefore, indicates the possibility that changes in microbial community composition and diversity may become an endpoint in a future European PPP risk assessment. Outside of EFSA publications, there have also been calls from the academic community to consider including microbial community composition, as measured via amplicon sequencing approaches, within a risk assessment framework (Karpouzas et al, 2022; Walder et al, 2022; Webster et al, 2018), demonstrating the significant wider interest in this area.…”