Biogeochemical Proxies of Anthropic Impact in Mediterranean Forest Soils: Anthropic impact in Mediterranean forest soils

Quantitative changes in soil organic matter (SOM) from undisturbed Mediterranean forests and neighbouring deforested sites were assessed by analysing soil lipids and humic acids (HAs), in total studying 80 variables. Changes in the composition of free lipids reflected vegetation types, whereas HAs analysed by visible and 13C NMR spectroscopy, sodium perborate degradation and Curie-point pyrolysis indicated the extent of structural alteration of lignin in soil. The molecular fractions released by degradation techniques applied to HAs showed that demethoxylation and oxidation were associated with the removal of forest vegetation, and the aromatic compounds consisted mainly of methoxyphenols and benzenecarboxylic acids. Decreased concentration of alkanes and increased amounts of alcohols were observed after removal of forests. The chain lengths of alkyl compounds also tended to decrease. In forest soil, there was a series of diterpene resin acids, whereas in pasture soil steroids from animal origin and a root-derived triterpenoids with friedelan structure were found. In relation to the total quantity and quality of soil C, the overall chemical descriptors indicated that clearing and cultivation (semiarid cereal fields) lead to intense mineralization of SOM (mainly particulate, free organic matter) but the residual humic substances have enhanced maturity in terms of structural condensation and potential resilience. Conversely, the cleared forest soil under pasture had a comparatively higher potential for C sequestration, but the HA characteristics suggested selective preservation of plant biomacromolecules directly incorporated as underground biomass.