Assessing the effects of plant protection products on organic matter breakdown in arable fields—litter decomposition test systems

There is a need for plant protection products (PPPs) to be assessed for their effects on the breakdown of organic matter (OM), which is an important functional process in terrestrial ecosystems. Little information is available on to how to assess effects of PPPs on this complex system and formal guidelines for a standardised test method are lacking. We critically reviewed the literature to determine appropriate methods to investigate OM breakdown for the risk assessment of PPPs. Five methods appeared to be potentially suitable: namely the use of mini-containers or litter-bags to enclose OM, cotton-strip and bait-lamina assays which provide an artificial OM substrate, and stable isotopes to track the chemical decomposition of OM. These methods were compared on the basis of 10 suitability criteria, which included ecological relevance, ease of use and relevance to risk assessors. Each test method has limitations but the use of litter-bags, which is the most frequently used method, has distinct advantages over the other approaches. Accordingly, literature describing OM breakdown in litter-bags when applying PPPs are reviewed, gaps in the methodology are highlighted and recommendations for the development of a standardised and validated test method are proposed.