Soil fauna in agroforestry contributes to the suppressiveness to plant-parasitic nematodes: A case study in a Mediterranean area

Abstract

Plant-parasitic nematodes are known to impair plant development and can cause severe crop loss. Agroforestry is a promising land use management system for the preservation of soil fertility and biodiversity conservation, but very few studies have focused on the regulation of plant-parasitic nematodes with the inclusion of tree alleys in cropping systems. In this study, the capacity of the soil to suppress plant-parasitic nematodes was assessed in Southern France. Fresh and heated soils from different combinations of land use (monospecific crop, agroforestry or tree plantation) and plant cover (crop or tree) were tested for their capacity to suppress Meloidogyne javanica in a laboratory assay. In the samples taken under tree cover, the suppressive capacity of fresh soils was improved compared to that of the monospecific crop samples. As the suppressive capacity of the heated soils remained low, we considered that soil fauna was responsible for part of the soil suppressiveness. The characterization of nematode communities revealed no Meloidogyne spp. on the experimental site, but other plant-parasitic nematodes were found. The total density of Pratylenchus spp. was lower while the relative density of Helicotylenchus spp. was higher under tree cover, compared with the monospecific crop soils. In agroforestry, the relative density of herbivores was ca 200 % higher under tree cover compared to under crop cover but the parasitic pressure (assessed by the Plant-Parasitic Index/Maturity Index ratio) was ca 30 % lower. Moreover, the crop soils had the highest Enrichment Index while the tree row soils in agroforestry had the highest Structural Index. The suppressive capacity in agroforestry was associated with a specific nematofauna, including more predatory taxa than in the monospecific crop. This study showed that including trees in a cropping system in a Mediterranean area created a favorable niche for potential herbivore regulators among the soil fauna. Further investigations are required to validate the regulation of plant-parasitic nematodes, and to disentangle the complex interactions explaining it in agroforestry.