Role of soil organic matter characteristics in a pedological survey: “Bosco Frattona” natural reserve (Site of Community Importance, Italy) case study

Changes in land use over time impact on soil organic carbon content and turnover. In the present study a pedological survey was carried out in order to determine the evolution of epipedon of forest soils by studying soil organic matter turnover in relation to certain pedogenesis factors. The study area was the “Bosco Frattona” natural reserve, harbouring a relict of the mixed broadleaves woodland that once covered the Apennine hills of the Po Valley in north Italy, and a significant diversification of pedogenetic factors could be found therein despite its limited extent. The aim of the survey was to identify potentially vulnerable sites and to investigate how some parameters used to characterise soil organic matter in the epipedon – such as the living fraction, humification and thermal stability indices – were correlated to pedogenetic factors such as lithology, geomorphology and plant cover, which have contributed most to the differentiation of soils belonging to the Ustepts suborder. Total organic carbon (Corg), soil biomass carbon (Cmic), extractable and humified carbon together with corresponding humification parameters were determined to quantify carbon distribution in different organic matter pools. Stable carbon isotope measurements (δ13C) have been used to assess previous vegetation, land use changes and the organic matter turnover rate. Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC) analyses were applied on soil humified substances in order to highlight differences in their thermal stability and composition. The results showed a correspondence between the diversification of the pedogenetic characteristics of soil and quali-quantitative characteristics of the organic matter of the epipedons. The abundance estimation of the different organic carbon fractions consistently indicated two pedons (both Typic Dyistrustepts) as being potentially vulnerable due to their very low content of Corg, the considerable presence of labile fractions versus humified ones and a high Cbiom/Corg ratio, which suggested that these soils had a poor humification capacity and an energetically non-conservative system insofar as the organic resource is concerned. Statistical treatment of data through Principal Component Analysis (PCA) highlighted the role of pedogenetic substrate and type of plant cover as discriminant pedological factors. Among the TG data relating to the extracted humic substances, weight loss at low temperature, generally attributed to adsorbed water and thermo-labile functional groups, proved to be discriminant vis-à-vis the type of vegetation. This result seemed to confirm the influence of plant cover on the turnover of organic carbon in soil and on the process of humification. In the group of chemical parameters, PCA revealed a cluster made up of the soil samples that had formed upon the stony silty sand alluvial deposits and also referred to the same pedotype (Typic Calciustepts). They showed higher microbial and humified carbon contents and higher pH values, whereas no differentiation was found in connection with vegetation changes arising from past land use (δ13C). From a methodological viewpoint, the study confirmed that the data set of parameters used was able to identify potentially vulnerable sites and thus to act as soil quality indicators.