Relationship between farming practices, soil macrofauna and litter decomposition in organic versus conventional banana agroecosystems

Many agricultural practices used in conventional farming can have negative impacts on soil biodiversity and decomposition process. Conversion to organic farming could reduce the effect of these disturbances, but in situ consequences of these effects are still poorly investigated. In order to better support the ecological transition towards an organic agriculture in tropical agroecosystems, it is important to better understand the links between agricultural practices, soil macrofauna and the decomposition process. To reach this goal, we conducted a field experiment in banana agroecosystems in Martinique (Lesser Antilles). We selected six organic and six conventional banana fields located in two bioclimatic zones. In order to measure the microbial and macrofaunal contribution to decomposition, 264 litterbags, with either a small or large mesh size, were positioned and left in the field for three months. Glyphosate-contaminated and glyphosate-free banana leaf litters were tested. Biotic and abiotic field characteristics were measured. Our results confirmed the overall beneficial effect of converting to organic agriculture on the decomposition process (+24 %) for bananas leaf litter. Macrofaunal decomposition was increased more (55 %) than microbial decomposition (20 %), indicating that organic farming removes a constraint of conventional farming especially affecting macrofauna. Glyphosate contamination of the litter did not have a significant effect on overall litter decomposition. By using structural equations, we were able to link farming practices to macrofauna diversity and to decomposition process, through a cascading chain of effects. We found that organic farming enhanced macrofaunal contribution to decomposition by enhancing both macro-arthropod and earthworm richness by providing a diversity of microhabitats with dense and species-rich plant cover. Our results suggested a weak effect of the direct toxicity by glyphosate-contaminated litter ingestion, but an indirect effect of herbicides by destroying the soil weed cover providing resources and microhabitats for soil macrofauna. Soil weed cover thus appears to be an important element of the agroecosystem ensuring the sustainability of ecosystem processes.