Microbial communities and isotopic signatures improve discrimination of pedogenetic trajectories of soil acidification in the Northern Apennines (Italy)

The pedological perspective in examining the influence of soil properties on bacterial communities and stable isotope is often neglected. This study investigated pedogenic horizons of 15 soil profiles in the Northern Apennines (Italy) to: (i) compare bacterial community variation across soil types; (ii) examine δ 13 C and δ 15 N signatures in relation to ongoing pedogenetic processes; and (iii) assess whether microbial and isotopic indicators discriminate soil types. The soil profiles were classified as Fluviwassents, Eutrudepts, and Dystrudepts. Within the Eutrudepts, multivariate analyses identified two clusters. One cluster included soil profiles with pH and exchangeable Ca content of 7.7 and 23.0 cmol(+) kg −1 , respectively. The other cluster included soil profiles, hereafter called Eutric/Dystric transitional soils, with pH and exchangeable Ca content of 5.7 and 6.3 cmol(+) kg −1 , respectively, due to the occurrence of acidification processes likely promoted by forest cover and higher rainfall. In addition, the more alkaline Eutrudepts were characterized by the dominance of Actinobacteriota, whereas Acidobacteriota and Chloroflexota predominated in Dystrudepts and Eutric/Dystric transitional soils. Eutric/Dystric transitional soils showed higher δ 15 N values than Eutrudepts featured by higher pH value (1.60 ‰ and − 0.61 ‰, respectively), suggesting enhanced organic matter turnover associated with lower exchangeable Ca content. Discriminant analysis identified Actinobacteriota, Verrucomicrobiota, and pH as tthe main predictors of soil type. Overall, this study highlights the value of integrating microbial and isotopic indicators into the interpretation of soil development, demonstrating that integrated microbial and isotopic indicators enhance the diagnostic resolution of soil classification and provides novel insights into ongoing pedogenetic processes in mountain environments.