Anecic earthworms benefit from organic management practices in apple orchards under the Mediterranean climate

Abstract

Earthworms, due to the important roles they play in the soil, are considered a cornerstone in agroecology. Despite this, the effects of different management practices (organic versus conventional) on their abundance, biomass, and diversity are not yet fully understood, especially in perennial crops. We took apple orchards in Southeastern France, with up to 30 pesticides applied annually, as a case study. We conducted a comparative analysis of earthworm communities, soil properties, and pesticide use among a total of 68 selected orchards. This dataset comprises 31 under Integrated Pest Management (IPM), 27 organic and 10 abandoned orchards. The soil properties were rather similar, and no significant difference was observed between organic and IPM orchards. Earthworm community composition differed between abandoned and commercial orchards mainly due to the presence of Prosellodrilus fragilis. Interestingly, total earthworm biomass and abundance were similar in all orchards, commercial or abandoned, with an average of 197 individuals m−2. However, the diversity and the equitability of these communities were significantly lower in IPM orchards compared to both other types of orchards. The most striking difference was the almost 3-fold increase in the mean abundance of Lumbricus terrestris in the organic orchards compared to IPM ones and, in turn, the 2-fold increase in the anecic character of the communities. As the anecic ecological category is associated with typical behavioral facets (creation of deep vertical burrows and intense forging at the soil surface), it is assumed that organic and IPM orchards will have different soil functioning. When evaluating the risks associated with pesticide use, based on their potential effect on earthworms in standardized toxicity tests, IPM orchards are characterized by a much higher (+ 280 %) risk mainly related to insecticide use (including broad-spectrum compounds). However, pesticides are not the only possible explanation for the modification of the earthworm communities since we did not observe a clear link between mean risk per orchard and L. terrestris abundance. Other practices such as the use of organic fertilizers as in the case of organic farming could also play a role. Our study suggests that an accurate characterization of the different practices and farming systems is required to better understand which practices should be applied by farmers to reinforce the services provided by earthworms in their soil.