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Abstract:

Organic farming is a key approach to reconcile food production, biodiversity conservation and environmental sustainability. Due to reduced inputs of agrochemicals, the success of organic farming is heavily dependent on the ecosystem services provided by the soil microbial community, and in particular by arbuscular mycorrhizal fungi (AMF). This is especially the case in grapevine (Vitis vinifera) agriculture, were AMF may also contribute to the microbial terroir of the vines, providing distinct characteristics to the grapes and the wine produced. To what extent organic agriculture benefits the AMF communities on vines at regional scales, however, is still poorly understood. Here, we first quantified the relative importance of organic management, soil chemical characteristics, and geography on vineyard AMF diversity and community composition. Second, we tested whether soil nutrients fundamentally change the host-AMF community dynamics through changing universality of dissimilarity overlap curves. To identify AMF communities, we used high-throughput pyrosequencing on 170 root samples from grapevines originating from 18 conventionally and 16 organically managed Belgian and Dutch vineyards. Organic management did not benefit AMF diversity, rather we found strong negative effects of the soil phosphorus content and soil acidity. Together with management type (organic vs. conventional), these two soil variables did explain most of the variation in AMF community composition. The observed accumulation of soil copper, used to control fungal diseases, especially in organically managed vineyards, did not affect AMF communities. AMF communities showed a regularity in interactions among taxa and their host. Under high soil P availability, however, interactions became more irregular. The potential benefits of organic vineyard management in terms of a high diversity of AMF are highly compromised by elevated soil phosphorus levels which may jeopardize the role of these symbionts in improving plant health and soil fertility. In the specific case of grapevine, homogenized AMF communities may also jeopardize the development of a specific microbial terroir. Decreasing nutrient inputs, even organic, is a key step in developing diverse AMF communities in vineyards.