Effect of an alternative viticulture on native entomopathogenic nematodes and other organisms linked to their soil food web in DOCa Rioja vineyards stars

Abstract

Entomopathogenic nematodes (EPNs) are biological control agents that often occur naturally in crop soils. The conventional agricultural practices of regular tillage and agrochemical applications predispose to soil biodiversity losses, compromising soil health and disrupting the natural balance of abiotic and biotic factors that might modulate EPN abundance and activity. The vineyard, which supports a relevant socio-economic sector worldwide, is one of the most intensively managed cropping systems. Therefore, approaches rather than reliance on mechanization and agrochemicals are needed to achieve more sustainable viticulture. We hypothesized that alternative strategies to tillage for soil management and the release of agrochemicals for pests, diseases, and weed management, such as cover cropping, mulching, and organic farming, would favor the native EPN community in vineyard soils. Therefore, our objective was to evaluate the impact of differentiated viticulture practices on native EPNs and other targeted organisms associated with their soil food web and how their assemblage might signal soil health in vineyards. We implemented traditional and innovative methodologies to isolate and identify mesofauna to achieve this aim. Firstly, we estimated different soil activities, including those associated with EPNs, by baiting the soil samples with Galleria mellonella larvae. Besides, we used speciesspecific primers/probe qPCR sets to screen and quantify the occurrence and abundance of 10 EPN species and 12 organisms linked to their soil food web: four free-living nematodes (FLNs), six nematophagous fungi (NF), and two ectoparasitic bacteria (EcPB). Lastly, a third soil subsample set was employed to estimate the soil properties. Following this procedure, we performed three independent studies to evaluate the impact of different management practices on the EPN community and associated soils organisms in The Appellation of Origin (DOCa) Rioja vineyards (Northern Spain): (i) diverse cover crops (seeded with Bromus catharticus, flower-driven, and spontaneous) compared to regular tillage in an experimental vineyard, (ii) cover cropping and organic viticulture compared to regular tillage and Integrated Pest Management (IPM) in a survey comprising 80 vineyards, and (iii) various organic mulches (based on grape pruning debris, straw, and spent mushroom compost) compared to regular tillage and herbicide applications in an organic and IPM experimental vineyards. We found seven EPN species and all the other screened species except the NF Arthrobotrys musiformis and the EcPB Paenibacillus nematophilus. The only EPNs reported in the three studies were Heterorhabditis bacteriophora, Steinernema feltiae, and the new EPN species S. riojaense, identified and isolated during the progress of this Thesis. Overall, EPN abundance and activity were higher for cover cropping and mulching than conventional soil management practices in both studies performed in experimental vineyards. However, the results obtained in the DOCa Rioja survey did not support this trend. It is possible that differential effects of diverse alternative strategies to regular tillage also affected the soil properties and, therefore, the EPN soil food web differentially. Indeed, we found lower numbers of potential enemies of EPNs, particularly NF, for spontaneous cover cropping and mulching based on spent mushroom compost, the treatments for which higher EPN activity rates and abundance were recorded. On the other hand, in agreement with our hypothesis, organic viticulture enhanced the activity of native EPNs and the abundance and activity of the predominant EPN species, S. feltiae, in the DOCa Rioja survey. In addition, we obtained similar results for the organic vineyard in the mulching study. Organic viticulture also supported a higher FLN abundance and richness of the overall nematode species screened since the EPN species Steinernema affine, S. carpocapsae, and S. kraussei, as well as the FLN species Oscheius onirici, only occurred in organic vineyards. Our results showed that organic viticulture and specific soil management practices that restrict or avoid regular tillage might support native EPNs in the vineyard, contributing to the maintenance of the ecosystem service these soil organisms offer as biological control agents. Moreover, these studies have illustrated how evaluating the EPN soil food web can signal soil health and the suitability of some viticulture practices over others. Applying innovative molecular tools and statistical analyses will improve understanding of the factors that determine the occurrence and distribution of EPNs in crop soils.