Carrino, Linda (2023) Phytomanagement of contaminated and marginal lands with Ricinus communis L. [Tesi di dottorato]

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Abstract

Soils are a non-renewable resource increasingly endangered by human activities at an unsustainable rate. Suffice it to say that the extent of all forms of soil degradation is estimated at 10 million hectares per year. Biophysical degradation of soils caused by land misuse and/or overuse and its consequential economic effects lead to the generation of the so-called marginal lands. According to global demographic estimates, there will be a minimum of nine billion people worldwide by the year 2050, leading to an increase in the demand for food and energy production. As follows, this entails that marginal soils must be recovered, reclaimed, and utilized in the best possible way while restoring ecosystem services. However, marginal areas' biophysical limitations might reduce crop yield, being frequently tackled by low fertility often coupled with a shortage of freshwater supplies, and contaminations with potentially toxic elements (PTEs) that can limit crop growth. With this in mind, the present work looks at the phytomanagement of marginal areas using the multipurpose crop Ricinus communis L. and analyzes different techniques that can be employed to increase its production and remediation efficacy. In order to accomplish these claims, the trials here reported were carried out testing a variety of organic amendments (waste by-products, i.e., compost obtained from the solid fraction of urban waste, anaerobically digested municipal sewage sludge, biochar produced from lignocellulosic biomasses employed for phytoremediation purposes) and remediation methods at different scales, such as field, greenhouse, growth chamber, and laboratory. Ricinus communis L. was tested for its tolerance to extremely low fertility and saline conditions, as well as soil contamination (specifically soil contaminated with Pb, As, and Sb due to shooting activities), and qualified for its suitability for phytomanagement of marginal lands, through the measurement of several physiological and biochemical indices. The results obtained emphasize the viability of using Ricinus communis L. for the phytomanagement of marginal lands. When grown on extremely poor substrates, some cultivars of castor bean were able to germinate, grow and produce oil seeds and residual biomass, even under saline irrigation. As an example, the germination of one of the tested cultivars was enhanced by saline irrigation at 4 dS/m (+ 3.3% more than the control at 0 dS/m). Nevertheless, castor bean production was increased when supported by using a combination of compost and biochar as organic amendments (up to 5.4-fold, compared to the unamended control). This demonstrates how by-products of organic waste can be used to restore marginal land while improving the growth of bioenergy crops, enhancing carbon sequestration, and disposing of organic waste in an environmentally responsible manner. Moreover, the use of organic amendments proved the immobilization of some PTEs and the mobilization of others. As an example, the reduction of the exchangeable soil fraction of Sb was achieved with the application of digestate produced from the anaerobic digestion of municipal sewage sludge, while biochar application enhanced its bioavailability. The application of arbuscular mycorrhiza fungi limited the accumulation of Pb, As, and Sb in Ricinus plants (on average -14% in leaves, stem and roots for all the PTEs). The findings of this work contribute to the possibilities of the eco-restoration of degraded, contaminated and marginal lands, through the adaptative capacities of Ricinus communis L.

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