Exploring the soil microbiota of maritime pine (Pinus pinaster) groves, nearby woody crops (vineyards) and rainfed cereal crops, and its relation to tree decline

Maritime pine (Pinus pinaster) is a conifer widely distributed in the western European Mediterranean Basin, which provides important resources, such as resin, pulpwood, wood, pellets and bark. These forests are seriously threatened by decline, which reduces the area of maritime pines in the Mediterranean Basin every year. Different crops, such as vineyards and rainfed cereals, are often established adjacent to these pine forests. The objective of this work is to explore for the first time the bacterial and fungal diversity of the soils of pine forests and surrounding crops, in order to establish a relationship with the presence or absence of tree decline. Soil samples were taken from three different areas in the south of the province of Burgos (Spain), where pine forests, vineyards and rainfed cereals were grown in the surrounding area, and the microbial diversity present was analyzed by metabarcoding (bacterial 16S and fungal ITS). The main bacterial phyla reported in pine forests were Proteobacteria and Actinobacteria, and the fungal phylum was Ascomycota. With regard to bacterial families and the different plant cover where they were found, the greatest co-occurrence was found in areas where there were diseased pine forests, unlike what occurred with fungal genera. The highest alpha diversity of bacterial families was reported in the soils of diseased pine forests and surrounding vineyards. While the highest alpha diversity for fungal genera was found in rainfed cereals associated with diseased pine forests. The bacterial families Hyphomonadaceae and Koribacteraceae and the fungal genus Volutella, found in the soils of diseased pine forests, include plant pathogens that could explain the presence of decline. In the soils of healthy pine forests was reported the presence of bacterial families such as Azospirillaceae and Bacillaceae, as well as the fungal genera Amphinema, Emmonsiellopsis and Harmoniella, possibly related to the absence of diseases in the trees. In the soils of diseased pine forests, ectomycorrhizal fungi were reported as the main functional niche, while in the surrounding crops, plant pathogenic and saprotrophic fungi were found. Therefore, the soil microbial diversity present in pine forests and surrounding crops could contribute to explain the presence or absence of tree decline, along with the study of other factors, such as abiotic conditions or pest infestations. The study of microbial diversity, along with other analyzes, could help prevent the onset of disease.