Conservation agriculture improves the balance between beneficial free-living and plant-parasitic nematodes for low-input rainfed rice crop

Conservation agriculture systems leaning on living mulch show particular promise thanks to their benefits on soil biological activity, but weed pressure in these cropping systems strongly depends on the amount of mulch. To assess the ability of these cropping systems to sustain soil health considering pest regulation, we investigated the combined influence of tillage and crop management (conventional, CONV and no-tillage with living mulch, NTLM) and weeding regimes (weekly hand-weeding and none) on soil free-living and plant-parasitic nematodes. To do so, we leant on a split-plot field experiment in Madagascar highlands 7 years after crop establishment. Overall, the abundance of soil free-living nematodes was 3.9 times higher in NTLM than CONV, primarily due to a preferential increase in fungal-feeders (+585 %) and in omnivores and predators (+633 %). Conversely, plantparasitic nematodes had the same abundance in both systems, but not the same taxonomic composition, with a dominance of endoparasitic taxa in CONV, and of ectoparasitic taxa in NTLM. Weeding management affected only populations in NTLM, leading to increased abundance of fungal-feeders (+191 %) and lower abundance of semi-endoparasites (-89 %) in the unweeded systems, which were associated with changes in plant community diversity. In this context, conservation agriculture and no-weeding proved beneficial for promoting free-living nematode communities but also to decrease the overall plant parasitic pressure through plant diversification. As no weeding may nonetheless affect crop yield, a trade-off has therefore to be found to promote soil ecosystem services while maintaining crop production.