Soil quality depends on the quantity, quality, and dynamics of the soil organic matter (SOM) (Lal 2000). Up to 70-80 % of SOM in mineral soil is composed of Humic Substances (HS) (Piccolo 2002). HS are composed of chemically complex, non-biochemical organic components, which are largely hydrophilic, amorphous, dark colored, and resistant to chemical and biological degradation (Schintzer 1991; Hedges and Oades 1997). The definition of humic substances (HS) is only operational, being based on the properties of solubility in the aqueous solutions used as extractants (Piccolo 2002). The term Humic Acid (HA) is used to indicate the HS soluble in dilute alkali but insoluble in dilute acid (pH < 1) (Piccolo 2002). The HA is negatively-charged colloid recalcitrant to biodegradation so it can be stored in soil for a long time (Qualls 2004). These characteristics mean HA plays an important role in determining soil characteristics by influencing its chemical, physical, and biological properties. As a result, the balance of this fraction in soil is a key factor in maintaining soil fertility and preserving fixed carbon. More advanced scientific studies conducted in both soil and marine ecosystems (Kögel-Knabner et al. 1992; Hedges and Keil 1995; Hedges et al. 2001) indicate that it is the preservation and modification of biopolymer that provides the humification pathway. Therefore it could be accepted the idea that plants, throughout CO2 fixation and biosynthesis of polymer represent the first step of humification process. The recent findings of humic-like material in plant (Adani and Ricca, 2004; Adani et al. 2006) seems to confirm this hypothesis. In this way, the plant assumes a main role in sequestration of atmospheric C in soil. A simple C-sequestration mechanism in soil can be proposed as in the following:

Atmosphere (CO2)→ Plant (plant biopolymer-HA)→ Soil(C-sequestered-HA)

In the last three years my research was focused to better understand the origin of HA-like material in plant (maize plant) and its role in the soil-C cycle. HA-likes were isolated directly form both plant and pre-isolated cell-wall, at different maturity stage and investigated by 13C-CPMAS NMR, surface area, porosity, wet analysis and complete mass balance. Moreover, maize plants were incubated in soil for long time (8 months) and HA-like material recovered and investigated. Results presented at this Conference suggest that the alkali-soluble part of the cell wall results in small domains formed by cross-linked polymers, mainly represented by lignin and cutin, and partially conserving the original cell wall-like structure, represented the HA-like fraction of plant. This HA is conserved in soil for long time representing a part of CO2 sequestered in soil. Plant maturation, i.e. lignification and cutinization processes, plays a main role in the definition of the structure of HA-like material, because these two processes well correlated with HA characteristics. Therefore, biochemical pathways forming cutin and above all lignin are implicated in the plant HA-like formation and its recalcitrant properties. To better investigate this aspect, mutant and wild type maize plants (W23: wild type and Brown MIBRID – Stock Center Resources of Maize GDB- USA) for lignin, are, now, under investigation in order to better understand the contribution of lignin to HA-like formation and C-sequestration in soil. Soil incubation of maize plant, shows low HA degradability that it seems caused by both chemical and physical properties of HA. High lignin content gives refractory properties to HA (Scobbie et al. 1993). On the other hand the low surface area (0.58 m2 g-1) that characterizes HA limits enzyme activity. Moreover, not all HA-surface area is available to enzymes because pores of diameters under 5 nm, representing 19 % of the total surface area of HA, are not accessible to enzymes (Chesson 2002). These results suggest the biochemical pathways to form HA play an important role in the C-sequestration and that more in deep studies need to better investigated i) the effect of atmospheric CO2 increasing on plant-HA balance; ii) to develop new strategies to increase HA-plant contents, and iii) to investigate the impact of the use of genetic modified plant on soil-C cycle.

Literature available by request to fabrizio.adani@unimi.it