Conclusions

Porphyrins and metal-porphyrins represent a vast class of chemically reactive compounds that are being used in several biotechnological applications (Smith 1975; Lesage et al. 1993). In particular, a synthetic iron-porphyrin was recently used as a biomimetic catalyst to promote oxidative coupling reactions in soil, in order to increase both the chemical protection of organic C pool (Smejkalova et al. 2006; Fukushima et al. 2010; Piccolo et al., 2011) and the removal of toxicants from the soil environment (Hahn et al. 2007). Within this context, the functional transformation of soil humic matter is a novel and, apparently, effective strategy in the current strive to control organic matter in ecosystems.

The results from microcosm experiments presented here suggest that, when applied to bare soil, the synthetic iron-porphyrin is able to reduce CO2 emission from soil. On the contrary, compost amendments stimulated CO2 emission from soil. However, the combined addition of compost with the iron-porphyrin strongly depressed the compost-induced CO2 release over the entire experimental period.

In planted microcosms, the contribution of the rhizosphere-derived CO2 flux markedly increased the total soil respiration and the biomimetic catalyst addition further stimulated CO2 release from soil. This finding suggests that iron-porphyrin, growth of maize root, and CO2 release are functionally interconnected, though the mechanism of such interconnection in not clear yet (see Chaps. 4, 6, and 9). The increased total soil respiration observed in planted systems may be due to a larger contribution of rhizosphere-derived CO2 fluxes, as a consequence of a secondary action of iron-porphyrin on plant root systems. However, it is also possible that the occurred catalyst-assisted photo-polymerization of SOM molecules altered the humic conformation towards an increased bio-accessibility of previously protected humic molecules. A deeper knowledge of the complex relationship existing among biomimetic catalyst, maize root, and CO2 fluxing mechanisms may be better achieved by further testing soil-plant microcosms filled with contrasting soil types and SOM qualities.

Laboratory studies concerning direct effects on model plant species, by using CAT concentrations similar to those used for microcosm experiments, revealed a complex pattern of rate-dependent and, remarkably, species-specific responses, as observed in both root systems and aerial plant parts. This, together with the ostensibly promoting effects on the synthesis of photosynthetic pigments, encourages to imagine a potential for an in planta uptake and translocation of the iron-porphyrin in a perspective of widening its potential usefulness for a range of possible applications, even beyond soil-related biotechnologies.

Acknowledgments The MESCOSAGR Project contributes to the Strategic Programme "Sustainable Development and Climate Changes", funded through the Integrative Special Fund for Research by the Italian Ministry for Education, University and Research. The authors gratefully acknowledge the dedicated and competent scientific support from Demetrio Tortorella, Barbara Logoteta, Beatrix Petrovicova, and Giuseppe Princi. The technical assistance from Vincenzo Cianci was highly appreciated.

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