Geochemical characterization and redox properties of humic substances in lagoon environments

The humification model and consequently the existence of humic substances (HS), was recently harshly questioned in favor of a new vision of soil organic matter, the Soil Continuum Model, proposed by Lehmann and Kleber. For this reason, the first part of this thesis examines the integrity of the alkaline extraction of HS. The aim is to put to scrutiny the criticism regarding the so called ‘humic substances paradigm’, discussing and examining the argumentations of the Soil Continuum Model theory in the light of recent existing literature. A vast volume of interdisciplinary scientific evidence supports the formation of relevant non-pre-existing complex molecules exhibiting various types of structures. These molecules form during degradation and decay of biological cell components. The chemically active role of pedofauna is also highlighted. To support such literature analysis, an experimental investigation was carried out to verify the possible formation of artefacts during alkaline extraction of HS. Sphagnum moss and peats at different stages of decomposition were extracted by both alkaline (sodium hydroxide and sodium pyrophosphate) and neutral (neutral sodium pyrophosphate and water) solutions and extracts were fractionated according to the classic solubility scheme. Results show that extraction yields vary with the extractant pH: alkaline extractants extract more organic matter from the different substrates. Spectroscopic properties are conserved when different extractants are used. Moreover, substances extracted from sphagnum differ both in their solubility properties and in their spectroscopic characteristics from HS extracted from peat. This allowed to observe structural differences among HS extracted from substrates at different stages of humification. After having ascertained the reliability of the humic substances approach, electrochemical and structural changes which peat humic acids (HA) undergo when exposed to either aerobic or anaerobic incubation are examined. Under anaerobic conditions, HA may act as terminal electron acceptors, allowing facultative anaerobic bacteria to obtain energy from anaerobic respiration. In peatlands, extended drought periods induced by climate change could alter redox properties of HA and affect ratios of greenhouse gases emissions. Cyclic voltammetry experiments showed that microbial reduction increases the number of electrons that can be directly transferred from HA and a wide potential distribution of redox-active moieties are present in HA molecules. Pseudo-first order kinetic constants indicated that, after reduction, HA can donate electrons faster. The kinetics of the oxidation of humic substances are investigated throughout using the redox mediator ABTS. The co-existence of fast and slow reaction steps is highlighted using electron paramagnetic resonance (EPR) spectroscopy. Once the interaction between humic substances and ABTS was widely investigated, the terrestrial and marine contribution of humic acids in sediments along a river-lagoon transect (Marano and Grado Lagoon, Northern Adriatic Sea) are studied. Results highlight the existence of a complex but continuous pattern of terrestrial and marine contributions to C sequestration and humification, even in transitional environments where allochthonous humic C inputs are restricted due to insolubilization of humic substances by Ca2+. Geochemical characteristics of humic acids extracted from sediments were then compared to those extracted from saltmarshes of the same lagoon, and finally correlated with the electron donating capacity. The results obtained in this work confirm the importance of contributions of aromatic structures of terrestrial origin for the EDC capacity of HA in transitional environments. Moreover, the geochemical characteristics of soils and humic acids are strongly related to the electron donating capacity of HA and should be taken into account when redox processes are studied in transitional environments