MINERAL NITROGEN AND BIOCHAR: ISOTHERM ADSORPTION AND SUBSTRATE-PLANT INTERACTIONS STUDIES

The general purpose of this thesis was to focus on biochar and N chemical interactions, particularly referring to ammonium-nitrogen (NH4+-N) adsorption onto biochar, in order to clarify whether this mechanism could be responsible for the biochar effects on the N cycle in the soil eventually altering the N use efficiency of plants. The first specific objective was to elucidate the mechanisms that control the adsorption of NH4+-N onto biochar. For this purpose the effect of biochar ashes on NH4+-N adsorption was tested under controlled pH conditions and at different temperatures. Ammonium was selected instead of nitrate because: i) it is the main form of N adsorbed by biochar, since biochar is mainly characterized by the presence of negative charges; ii) it is the first mineral N form produced from soil organic matter mineralization; iii) it is the first form of N derived from urea degradation and urea is the N fertilizer most used worldwide; iv) it is the substrate used by nitrifying bacteria for the production of nitrate. This research aims to understand if the cation exchange capacity of biochar or its surface acidic groups are useful indicator to predict the biochar NH4+-N adsorption potential. A second scope of this research was to test if the N use efficiency of plant is affected by the enrichment of biochar with N fertilizer. The adsorption of NH4+-N fertilizer in a suspension with BC was compared with the conventional practice of applying N fertiliser directly on substrate. The underlying hypothesis was that the adsorption of NH4+ ions could negatively affect the N availability for plants, by subtracting mineral N to the cultivation system. Moreover biochar was tested as substitute of the most commonly used natural, but not eco-friendly, pot growing substrates: peat, perlite and zeolite. If biochar demonstrated to be suitable as amendment for soilless substrate it might contribute to the conservation of non-renewable peatland ecosystems, which are largely recognized for their role in biodiversity and C conservation.