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

Abstract

This study aimed to evaluate the role of different plant covers on brown soil profile development changing SOM stability in the epipedons. In particular, two brown soils (Eutric Cambisols), located in Monghidoro at the Italian Apennines, were analysed in order to test the hypothesis that plant cover affects the brown soil profile development under eutric qualifier changing SOM stability and soil pH. Also we verified if soil biochemical activities and microbial biomass size could be promoted by factors supporting Eutric Cambisols development. The two soil profiles were formed under coniferous (M con) and beech cover (M beech), respectively; both of them had similar lithology, morphology, exposition, slope and elevation. The development of soil profiles was evaluated by the determination of their physical, chemical and biochemical properties, while SOM stability of the epipedons was assessed through the determination of some properties of humic substances by thermal (TG-DSC) and spectroscopic (FTIR) analysis, humification indexes and eco-physiological indicators such as microbial quotient (C mic:C org) and metabolic quotient (qCO 2). The C org content decreased along both soil profiles. The epipedon of M beech soil was richer in SOM that in turn showed a higher level of humification than M con. The pH value, CEC and the concentrations of all earth-alkaline cations were also higher in M beech than in M con soil profile. Differences in soil reaction and SOM quality seemed to affect weathering process, since the content of all Al forms (amorphous—Alo, crystalline—Ald and linked to SOM—Alp) increased as the soil pH decreased, and the Fe forms increased as the humification level of SOM increased. Nevertheless, the active iron ratio (Feo/Fed) calculated in the two soil profiles did not highlight significant differences between them in terms of podzolitation intensity. The activity of hydrolytic enzymes (β-glucosidase, cellulase and chitinase) was higher in M beech soil than in M con 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 only in the endopedons of M beech. Although biochemical properties were not correlated with physical and chemical properties of soil profile, significant correlations were observed between enzyme activity and the soil organic carbon or nitrogen content. As far as the epipedons SOM quality is concerned, the content of humified C, the microbial biomass carbon (C mic), as well as the C mic:C org were higher in M beech than in M con, whereas the qCO 2 was lower in M beech than in M con. Also the FTIR and TG-DSC analysis showed differences on HAs and FAs hydrophilic character and thermal stability between epipedons of M beech and M con. The SOM quality of epipendon under coniferous resulted in a more recalcitrant and hydrophobic composition than that of epipedon under beech. In conclusion the type of plant cover (coniferous vs. beech-tree) affected the weathering of the pedogenetic substrate changing the SOM properties and soil pH. Even if the beech-tree cover promoted SOM accumulation in soil profile, which represented the organic substrates for microbial biochemical activities, the weathering of soil under coniferous appeared more marked than beech-trees covered soil. Consequently, soil biochemical activity of microbial biomass along soil profile was not promoted by factors supporting weathering processes.