## 4 Conclusions  
Most of the scientific reports reviewed in literature seem to indicate that humic materials from different sources have different aggregate morphologies (though the aggregation mechanisms seem to be similar), indirectly underscoring the importance of specific chemistry to humic material geometry. So far, the vast physicochemical heterogeneity of HS, mainly due to the extensive variability of the sources from which they are extracted, justifies the current absence of a unified model capable of describing and predicting their dynamic properties and fate both in the aqueous phase and in the solid state. Table 1 is an attempt to gather most of the scientific reports cited in this review and classified according to the various colloidal or fractal models that inspired them. \begin{table}
\begin{tabular}{c c c} \hline
**Models** & **Methods** & **References** \\ \hline HS are flexible linear colloids at low & & \\ concentrations while at high & & \\ concentrations spherical & & \\ colloids are stable & & \\ \hline “two-phase” model & Vapor-pressure osmometry & [33] \\ \hline biopolymer gels assembly from & & \\ free polymeric DOM & DLS, flow cytometry & [36] \\ \hline HS form ‘micelle-like’ aggregates & & \\ or pseudo-micelles & & \\ through non-covalent interactions & & \\ (H-bonding, van der Waals and & & \\ hydrophobic interactions). & & \\ \hline In aqueous solutions HS exist as soluble & & \\ macroligands or high molecular weight & & \\ macromolecules that assume & & \\ random coil conformations & & \\ \hline HS are non-covalently bonded aggregates & DSC, DOSY, DLS, LDE, UF, & \\ of small molecules in aqu. solutions & CPMAS-NMR & [60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100  
Several questions still remain open on the level of comprehension of HS aggregation: (i) How this aspect may impact on the capacity of soil humic aggregates to be dissolved and then transported to the surface freshwater or as groundwater colloidal dispersions? (ii) If the water carrying the humic material percolates through the soil, would the interaction with metal cations on the soil interfaces help to change the shape and degree of aggregation? Still debated seems to be also the influence of the concentration of the suspension on the pH dependence of the HS configuration. Indeed, several studies demonstrate that at least some types of HS are strongly pH sensitive, in contrast with studies of other humic material, where far less pH sensitivity has been observed. Another very real possibility is that the pH configuration dependence is a function of the chemistry of the HS involved. Last, but not least, (iii) do the aggregates and dissolved molecules of HS influence the H-bonds and 3D structure of surrounding water? Consequently, do the HS aggregates influence also other fundamental water properties similarly as shown for water structural hysteresis? And vice versa, how the water anomalies (i.e., hysteresis) influence HS properties? Is the fate, stability and biological activity of HS aggregates influenced by fluctuation of the environmental conditions, e.g., temperature, pH and ionic strength? Answering these questions may help to create a concept enabling to model the role of HS in biogeochemical cycles. Such a concept is missing and for this reason there are not carbon sequestration strategies that include also HS and their dynamics. Currently, the fundamental strategy is application of HS from various sources as soil amendments or fertilizers to support the yield of agricultural plants. This strategy, however, leads only to C redistribution as HS are relatively fast metabolized or mineralized. Apparently, this is not a meaningful strategy leading to a long-term fixation of C in soils and its protection thereby supporting fundamental soil ecosystem functions.