Plant cover and epipedon SOM stability as factors affecting brown soil profile development and microbial activity

The type of plant cover is one of the factors affecting soil organic matter (SOM) properties, microbial biomass and its activity within the soil, in particular for natural forest ecosystems. This study aimed to evaluate the role of different plant cover (coniferous vs. beech) on SOM stability in the epipedons (A1 and A2 horizons) of two brown soils Eutric Cambisols with similar lithology, morphology, exposition, slope and elevation - named Mcon and Mbeech and located in Monghidoro, Italian Apennines, through the determination of some properties of humic substances by thermal and spectroscopical analytical techniques, humification indexes and eco-physiological indicators such as microbial and metabolic quotient. Moreover the influence of the epipedon SOM quality on the evolution of soil profiles was evaluated by the determination of physical, chemical and biochemical properties in the whole profile, namely A1 and A2 horizons of epipedon and AB and Bw horizons of endopedon. The Corg content decreased along both soil profiles; in the epipedon, Mbeech soil was richer in SOM that showed a higher levels of humification indexes than Mcon. The pH, CEC and the concentrations of all earth-alkaline cations were lower in Mcon soil profile than Mbeech. The different form of Fe, amorphous (Feo),crystalline (Fed) and Fep linked at SOM, were not correlated with Al forms (Alo, Ald and Alp) while these latest were inversely correlated with pH. The active iron ratio (Feo/Fed) calculated in the two soil profiles doesn t propose significant differences between them in terms of podzolitazion intensity. The activity of hydrolytic enzymes (-glucosidase, cellulase and chitinase) was higher in Mbeech soil than Mcon soil profile. The activity of all enzymes, on mass basis, decreased along both soil profiles, while the specific activity (per unit of organic carbon), increased in the epipedons of Mbeech. The specific activity of -glucosidase and chitinase decreased along the profile of Mcon. The physical and chemical properties were not correlated with soil biochemical properties. We studied epipedons SOM proprties in detail. The content of humified C (C-HAs and C-FAs), the microbial biomass (Cmic) as well as the microbial quotient (Cmic:Corg) were significantly higher in the epipedons of Mbeech than in Mcon. The metabolic quotient (qCO2) was significantly lower in the epipedons of Mbeech than in Mcon. Spectroscopic analysis (FTIR) highlighted a hydrophilic peculiarity of Mbeech HAs and FAs (exclusively A1 horizon) as indicated by the Band B absorbance (1720-1640 cm-1). The thermal pattern of HAs (TG-DSC) showed two main organic pools with different stability. This oxidation kinetics was observed in HAs extracted from A1 and A2 horizons of two soil profiles. DSC traces of FAs showed different thermal behaviour with respect to Has; the total amount of heat involved in the oxidation reaction was lower in FAs than in HAs The type of plant cover (coniferous vs. beech-tree) affect the weathering of the pedogenetic substrate changing the SOM properties and soil pH. The SOM quality of epipendon under coniferous results in a more recalcitrant and hydrophobic composition than that of epipedon under beech. Moreover, the soil under coniferous showed lower pH, higher SOM humification and higher mobile-acid organic compounds in the endopedons than soil under beech. Since the content of all Al forms increased as the soil pH decreased, and the Fe forms increased as the humification level of SOM increased, the weathering of soil under coniferous appeared more marked than beech-trees covered soil. Nevertheless, the beech-tree cover promoted SOM accumulation in soil profile which represented the organic substrates for microbial physiological demand, favouring biochemical activities. Consequently, soil biochemical activity of microbial biomass along soil profile are not positively related to factors supporting weathering processes.