## 1 Introduction  
The organic matter in terrestrial and aquatic environments is being classified as particulate organic matter (POM) and dissolved organic matter (DOM). POM includes particles of natural organic matter (NOM) characterized by average sizes ranging from 50 \(\upmu\)m to 2 mm. [1]. DOM is the corresponding fraction of dissolved POM that can pass through a 0.45 mm mesh filter [2]. In the marine environment this rather artificial distinction between POM and DOM is sometimes questioned and replaced by a broader definition where organic matter is considered as a continuum of gel-like polymers, replete with colloids cross-linked by polymer strings, sheets, and bundles while the size can reach hundreds of micrometers [3]. An important part of DOM comes from the carbon mobility processes occurring in the environment. However, much of it is lost through various mechanisms such as substrate to soil microorganisms or adsorbed on soil sediments and eventually diffuses into the ocean. Much of the carbon DOM present in terrestrial and aquatic compartments is oxidized to CO\({}_{2}\) through microbial metabolism and then accumulates in the atmosphere [4]. DOM is a parameter that responds to changes in ecological processes and is capable to promote the dissolution and transport of metals such as micronutrients and organic compounds, including pollutants [5; 6; 7; 8]. Organic compounds in DOM have been shown to affect the chemical reactivity of soil minerals [9] and the complex process of podzolization of soil [10]. The production of greenhouse gases such as methane, and soil denitrification is connected to the bacterial degradation of DOM [11]. Furthermore, the localization of the DOM in the soil, for example, located in pores of various sizes, influences its mobility, ecological fate and availability [2]. The major pools of DOM are molecules solubilized and leached from soil organic matter (SOM). Those can be biomolecules originating from both fresh materials, such as leaf and grass litter, root exudates, decaying fine roots, soil microorganisms etc. and micro-biologically processed, humidified, fractions such as humic substances (HS) [12]. Due to the environmental conditions, the dissolved organic carbon cycle is relatively fast. The controlling factors for DOM biodegradability, and thus carbon dynamics, are influenced by several factors. Higher content of non-humified hydrophilic compounds such as carbohydrates, organic acids and proteins increase DOM degradability. On the contrary, the presence of aromatic and hydrophobic residues leads to a reduction of its biodegradability, due to the high resistance shown by the aromatic compounds towards degradation phenomena and to the inhibitory action of the enzymatic activity. Effects of heavy metal concentrations on DOM biodegradability is not clear yet since various studies reported conflicting results. As degradation is dependent on microbial activity, the type and concentration of microbial population must be taken into account as well [13]. HS are the most abundant part of DOM and their chemical structure and even their concept is still a matter of discussion [14]. For decades, it had been accepted that HS are macromolecules formed through a two-step process. In the first step, molecular building blocks are produced from the degradation of plant-, microbial-, and animal-derived organic systems. In the second step, secondary reactions are believed to occur, including condensation of resulting fragments into larger products, each hardly referable to the parent feedstock [15]. This view has been revised and new models have been developed to explain the chemical-physical properties of HS related with their long-term persistence and their molecular architecture [16]. From a chemical-physical point of view, HS are described as dark-coloured, chemically, and presumably biologically refractory, heterogeneous organic compounds playing the main role in soil fertility [17].