ENVIRONMENTAL IMPACTS IN THE USE OF DIGESTATE AS SUBSTITUTE OF CHEMICAL FERTILIZERS

Digestate has potentially the characteristics of a low environmental impact fertilizer, which allows the recovery of nutrients in a circular economy perspective. However, the effects of an agronomic use of the digestate depend on its characteristics, which on turn depend on the composition of the starting substrates and on the process conditions, as well as on the methods of application. The aim of this work was to evaluate the impacts resulting from the use of a highly stabilized digestate and a digestate-derived fertilizer (ammonium sulphate) obtained from anaerobic digestion of sewage sludge, as a crop fertilizer, in various environmental compartments, namely soil, water and atmosphere, and on the cycle of nutrients, in particular on the nitrogen cycle. This study has been carried on for three consecutive crop seasons, on an open field maize crop. The agronomic and environmental performances of the digestate were evaluated with respect to those of a synthetic fertilizer, through open field measurements and monitoring, chemical-physical analysis of the soil and digestate characteristics, and molecular biology studies, in order to obtain a complete and exhaustive view. Application of digestate as crop fertilizer showed no negative impacts on soil quality, nor on the accumulation of organic and inorganic pollutants. Instead, content of organic carbon in soil seems to increase after repeated use of digestate, confirming its characteristics of soil amendment. Nitrogen applied via organic fertilization with digestate is efficiently used by crops, as demonstrated by the yields of maize grain, which were equivalent to those obtained using a similar dose of urea N from a synthetic fertilizer, with very similar fertilizer use efficiencies (FUE). Digestate fertilization seems to have not a higher environmental impact in terms of GHG and ammonia emissions, compared to conventional chemical fertilization. This can be mainly imputed to the use of soil injection application technique, which reduces ammonia emission, down to levels comparable to those typical of surface fertilization with urea. In addition, for what concerns the emission of odour, it has been observed that with digestate injection, emission was reduced to a level that was no longer distinguishable from that of non-fertilized soil. Concentrations of nitrate in deep soil layers seems to be equal between digestate fertilization and chemical fertilization, indicating that there are no differences for what concerns losses of nitrogen by leaching. The addition of digestate seems to have no influence on the balance of soil microbial communities, at least in terms of metabolism. The value of digestate as a substitute for synthetic fertilizers is also confirmed by the LCA analysis, which indicates that producing and using renewable fertilizers led to a strong environmental impact reduction, compared to producing and using synthetic fertilizers. LCA approach also confirmed the importance of assuring high biological stability of digestate, and using it at the right time and according to crop requirements, as well as the distribution by injection, in order to avoid losing all the advantages because of impacts derived from their incorrect use. This thesis is part of EU H2020 Nutri2Cycle project (Transition towards a more carbon and nutrient efficient agriculture in Europe, grant agreement No 773682), which aims to propose various solutions to reduce the use of non-renewable resources and close the nutrient cycle (N, P and C) of various European agricultural systems.