Increasing Microbial Stability of SOM by In Situ Catalyzed Polymerization of Humic Components

Understanding humus as a supramolecular association of small molecules means overcoming the limitations imposed by the paradigmatic polymeric model. In fact, another important implication of the supramolecular view of SOM resides in the fact that some self-assembled small soil humic molecules can then be coupled together into larger molecular weight materials, thereby increasing the amount of energy-rich intermolecular interactions (covalent bonds) in the superstructures. This would signify an enhancement of the energetic level that soil microbes must overcome to make use of the chemical energy needed for their own metabolism, thereby inhibiting their OM degradation capacity and consequent reduction of GHG emission.

The oligo-/poly-merization of humic heterogeneous molecules can be achieved by applying current chemical technologies based on oxidative catalysis promoted by specific enzymes (Piccolo et al. 2000; Peralta-Zamora et al. 2003). Piccolo (2002) showed that by subjecting some synthetic clay-humic complexes to an oxidative reaction with a peroxidase enzyme as a catalyst and H2O as oxidant, the extraction of humic matter from the treated material was lower than from control. The yield of extraction from humic-clay complexes decreased by more than 40% after the oxidative coupling reaction. These results indicated that polymerization of humic molecules occurred also at the interphase of the solid clay-humic complexes, and the increase in molecular size of the humic materials was the most probable cause for the reduction in extraction yields. It seemed then possible to induce the polymerization of HS in natural soil samples in order to control or change the properties of native SOM.

However, the use of enzymes to promote oxidative coupling among humic molecules cannot be recommended as a field practice due to adsorption of enzymes on soil particles, and consequent denaturation and loss of catalytic activity.

Soil Oxidative Enzyme Image

Fig. 1.5 Mean Weight Diameter in water (MWDw) for the three soils, Porrara, Colombaia, Itri, before and after the photo-polymerization treatment for 5 days incubation, and 15 and 30 wetting and drying cycles (w/d). Error bars indicate standard error (n = 3). The asterisks denote significant differences between control and treatment at the level of P < 0.05

Fig. 1.5 Mean Weight Diameter in water (MWDw) for the three soils, Porrara, Colombaia, Itri, before and after the photo-polymerization treatment for 5 days incubation, and 15 and 30 wetting and drying cycles (w/d). Error bars indicate standard error (n = 3). The asterisks denote significant differences between control and treatment at the level of P < 0.05

Conversely, a biomimetic catalyst, such as biocompatible metal-porphyrins that mimic the activity of the heme prosthetic group of oxidative enzymes (e.g., laccase, peroxidase) without its cumbersome protein envelope, can be made adequately water soluble to maintain its catalytic activity in the soil environment (Sheldon 1994).

While soil treatment with mature compost is increasingly recognized as a valuable practice for SOM stabilization, the use of biomimetic catalyst represents an absolutely innovative technology to increase the SOC sequestration process. Humic molecules in solution were found to oxidatively polymerize and provide more rigid products under the catalysis of a water-soluble iron-porphyrin (Piccolo et al. 2005b). This polymerization reaction was shown to also occur under photo-oxidation without the need of a chemical oxidant (Smejkalova and Piccolo 2005). The oligomers formed during the biomimetic catalyzed reaction were isolated and characterized by Smejkalova et al. (2006, 2007). It was found that up to pentamers were formed during the oligomerization of phenolic precursors.

The results obtained on the polymerization of HS by both enzymatic and biomimetic catalysis encouraged to step further in the research. A laboratory incubation on three Mediterranean soils was conducted to verify the impact of an

I I Control after 5 d ^^ Control after 15 w/d ^^ Control after 30 w/d

I I Control after 5 d ^^ Control after 15 w/d ^^ Control after 30 w/d

Fig. 1.6 Soil respiration (mg CO2 g_1 of soil) from the three soils, Porrara, Colombaia, Itri, before and after photo-polymerization treatment for 5 days incubation, and 15 and 30 wetting and drying cycles (w/d). Error bars indicate standard error (n = 3). The asterisks denote significant differences between control and treatment at the level of P < 0.05

PORRARA

COLOMBAIA

ITRI

Fig. 1.6 Soil respiration (mg CO2 g_1 of soil) from the three soils, Porrara, Colombaia, Itri, before and after photo-polymerization treatment for 5 days incubation, and 15 and 30 wetting and drying cycles (w/d). Error bars indicate standard error (n = 3). The asterisks denote significant differences between control and treatment at the level of P < 0.05

in situ photo-polymerization of soil OM catalyzed by the biomimetic iron-porphyrin under solar irradiation, on the physical status of the soils, their organic carbon distribution in soil particle sizes, and the reduction of microbial respiration (Piccolo et al. 2011). These soils were also subjected to long period of wetting and drying (w/d) to assess the capacity of the photo-polymerized SOM to sustain soil processes which affect both soil physical and biological quality. The in situ photo-polymerization reaction increased water stability of soil aggregates both after 5 days incubation and 15 w/d cycles, but not after 30 w/d cycles (Fig. 1.5).

The gain in soil physical quality was reflected by the shift of organic carbon content from small to large soil aggregates, thereby suggesting that the photo-polymerization stabilized OC by both chemical and physical processes. Finally, a further evidence that the photo-catalytic treatment enables an effective sequestration of carbon in soil was provided by the significant reduction of CO2 respired by all soils after both 5 days incubation and w/d cycles (Fig. 1.6).

Both mechanisms of hydrophobic protection by humified matter such as compost and in situ catalyzed photo-polymerization of SOM received a solid scientific basis that pointed out how they were both effective in favoring carbon sequestration in soil. Both mechanisms were chemically and physical-chemically based and were far different than any other method so far attempted to increase the role of OC sink of soils.

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