How are nematode communities affected during a conversion from conventional to organic farming in southern French vineyards?

The rate of conversion from conventional vineyards to organic farming practices is increasing. Organic farming improves some soil properties, although some organic practices have negative effects on soils. The objective of this work was to study the long-term effects of organic farming through the use of soil nematodes as bio-indicators of soil processes. Our experimentation was conducted in a commercial vineyard where plots belonged to two types of viticulture: conventional viticulture and organic viticulture (for 7, 11 and 17 years). The nematode community structure and nematode indices were determined. The main result was that organic practices increased soil nematode density. An increase in the available resources, as measured by a higher enrichment index (EI), led to an increase in the microbial feeder density and mainly opportunistic fungal-feeding nematodes. A greater density of plant-feeding nematodes was attributed to the presence of a grass cover. The functioning of the soil was shifted with the decomposition channel of the soil organic matter becoming more fungal than bacterial. Even though changes were observed in the nematode community structure following the conversion, the maturity index (MI), the plant-parasitic index (PPI) and the structure index (SI) remained constant. Consequently, the organic practices did not improve the soil food web length or complexity even though the biological activity, as measured by microbial biomass and total nematode density, increased.