HSE soil project (PS2404) – final report published

Since SANCO (2002) was implemented more than 20 years ago many new survey techniques, analytical approaches and statistical methods have been developed for the study of soil invertebrates and microorganisms. This has resulted in considerably improved understanding of soil invertebrate ecology in agroecosystems, and offers opportunities for higher tier extension of the risk assessment for soil microorganisms. It is also evident that owing to a lack of clarity in the recommended study guideline for measuring the effects of pesticides on soil microorganisms (OECD TG 216) there is inconsistency in how the results are reported and uncertainty in their interpretation. Assisted by colleagues at Cambridge Environmental Assessments Ltd, during 2024 Enviresearch Ltd led HSE contract HSE/T4325 and reviewed available information to provide HSE CRD with:

1a) clarification of the use and interpretation of OECD TG 216 in relation to the soil microbial processes risk assessment.

1b) a proposal for a tiered testing framework for the soil microorganism risk assessments.

2) a reassessment of current higher tier refinements and evaluation of the suitability of alternatives within a tiered testing framework for the soil meso- and macro-fauna risk assessments.

In accordance with the original intentions of the working groups behind the development of OECD TG 216, we concluded that only endpoints based on time-interval rate calculations for nitrate accumulation data should be used for risk assessment. This is because the rate at which bacteria convert organic nitrogen into into nitrate varies during the study period. Based on decades of laboratory experience on measuring nutrient release in soils we also identified numerous opportunities within OECD TG 216 where with methodological clarification variability between replicates could be reduced. In our proposed alternative, tiered testing framework, we recommend that the non-agrochemical dose-response version of OECD TG 216, with its defined, familiar and well understood EC₅₀ endpoint is used at Tier I. The current agricultural version of OECD TG 216 should be retained as a higher tier test because it enables an assessment of recovery.

Other recommended higher tier options include ‘soil’ sensitivity distributions, which albeit in a slightly different form, are an accepted refinement within the current REACH assessment, and ‘tea bag’ litter bag field studies because the methodology is standardised and both the size of organisms entering the bag and plant material undergoing decomposition are defined. The merits of other approaches such as isotope and nucleic acid methods are also reviewed, but are concluded to be either too expensive, or too difficult to interpret in a regulatory context to be suitable as accessible tests for routine, high-throughput studies.

We identified more than 21 potential options available for refining the soil meso- and macro-organism risk assessment, if low risks cannot be concluded at Tier I. Some of which are established methods and were re-evaluated e.g. litter bag field studies, but others are emerging approaches, currently only used in research studies e.g. soil eco-acoustics. Refinement options were categorised into five tiers: Tier I (single species studies, predictive toxicity), Tier II (additional species testing, natural soils, recovery studies, indoor microcosm studies), Tier III (field studies, litterbag and bait lamina studies), Tier IV (TKTD modelling, population modelling), and Tier V (post-registration monitoring). We reviewed both in silico and in vivo approaches, discussing their strengths, weaknesses and highlighted knowledge gaps and where impact analysis would be needed prior to implementation. We framed our considerations in the context of soil meso- and macro-organism protection goals and emphasised making use of existing data/information and the development of a robust, relevant narrative for refining the risk assessment. Most of the refinement options considered do not feature in the SANCO (2002) soil risk assessment framework, and therefore their implementation in regulatory submissions is currently challenging, requiring change to GB soil risk assessment guidance, or legislative change.

The full, published report can be found here.