Soil community structure across native and exotic forests is shaped by edaphoclimatic conditions and eukaryotic biomarkers

The expansion of exotic trees and particularly Eucalyptus globulus in the north of the Iberian Peninsula forests is known to greatly affect ecosystem properties. For instance, it promotes negative effects such as soil erosion, acidification or decreases in soil humidity, which reflect significant functional contrasts with their native counterparts. However, their impact on the communities of millions of organisms living below-ground is much less understood, even though soil biodiversity, abundance and community structure are crucial for forest soil health. We evaluated the effects of native and exotic forests in the north of Portugal on the structure of soil communities through co-occurrence networks and linear discriminant analysis effect size (LEfSe) tools. We aimed to understand the effects of forest type and location and identify community biomarkers or taxa that consistently define the differences between these forests. Although community composition was similar in exotic and native forests, we found that the network structure of these soil communities differed significantly depending on the edaphoclimatic region. Soil communities in native forests exhibited more similar interactions and stability compared to those in exotic forests in interior and coastal regions. Connectance and robustness were higher in native forests of the interior region. On the contrary, exotic forests from the central region showed more homogeneity, stability, connectance and robustness than their native counterparts. A small group of 52 biomarkers dominated by fungi consistently explained the community differences between native and exotic forests, with 30 and 22 taxa in native and exotic forests, respectively. They were also found to be keystone taxa of these soils, as their extraction resulted in significant changes in network structure. Policy implications: Our results point to the importance of taking management decisions for these forests considering regional conditions as well as forest type. The provided list of soil keystone taxa should be taken into account in future research and soil monitoring, which is essential for sustainable management of land.