Intact soil food webs are pivotal to maintain essential soil functions such as carbon recycling, sequestering and biomass production. While the functional role of micro- (e.g. bacteria and fungi) and macrofauna (e.g. earthworms) is comparatively well established, the importance of the mesofauna community (e.g. abundance and diversity of Acari and Collembola) in maintaining soil functionality is less clear. We investigated this question in a six-month field experiment in arable soil by actively manipulating mesofauna abundance and biodiversity through the application of two legacy insecticides (lindane and methamidophos) at sufficiently high doses to reduce mesofauna abundance (well above previously registered application rates; 2.5 and 7.5 kg a.s./ha for lindane, and 0.6 and 3 kg a.s./ha for methamidophos) and measure the impact, on organic matter degradation. Our results show that both insecticides had reduced Collembola and Acari abundances by up to 80% over the 6 months study duration. In addition, we observed less pronounced and more complex changes in mesofauna biodiversity over time. These included insecticide-dependent temporal fluctuations (both reduction and increase) for different estimates (indices) of local (alpha)-diversity over time and no lasting impact for most estimates after six months. Even at these exceptionally high field rates Collembola and Acari diversity was observed to generally recover by six months. In contrast, considering organic matter breakdown, we found no evidence for a treatment related effect. These results suggest that organic matter breakdown in arable soils is likely driven by other trophic levels (e.g. microorganisms or earthworms) with only a limited influence of the mesofauna community. We discuss these findings with regard to their implications for our current understanding of soil food web function and future European soil risk assessments.

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