The ‘Massana soil system’ project: untargeted metabolomics to unravel chemical landscapes in Massane forest soils

Advancements in soil analytical techniques offer new insights into ecosystem structure, functions, and dynamics. Metabolomics has the potential to serve as a unique and efficient way to characterize the soil "chemical landscape." Thirty-three soil samples were collected from the Massane old-growth forest reserve, a UNESCO world heritage site located in the Eastern piedmont of the French Pyrenees. The study sites aimed to investigate three types of forest stands, namely beech forests, beech/oak stands, and mixed forest stands, subsets being also defined within stand types based on forest facies. We hypothesized that soil chemical heterogeneity would reflect forest spatial heterogeneity. Non-targeted metabolomics using liquid chromatography coupled with high-resolution tandem mass spectrometry (UHPLC-MS/MS) and molecular network analyses were employed to map the chemical diversity across the sampled sites. This approach unveiled the presence of various compounds, including lipids (fatty acids and their derivatives, sphingolipids, prenol lipids, steroids), terpenoids (triterpenoids and sesquiterpenes), coumarins, and oligopeptides and lipopeptides. Along with the presence of a rich core metabolome, some heterogeneity was also underscored, suggesting unique chemical structures associated with specific types of sampling sites, notably mature beech stands. These results call for a deeper investigation into these specific compounds, with regards to their biological origin and the diversity and heterogeneity of soil microbial communities.