PhD defence by Pietro Sica

Optimizing use and application method of waste and biobased fertilizer: Plant and rhizosphere response to fertilizer placement

The European Union is highly dependent on the import of mineral fertilizers and, at the same time, generates huge volumes of urban and animal-derived wastes with high phosphorus contents. However, the phosphorus contained in these wastes is poorly soluble and available to plants.

Based on this background, this thesis aimed to formulate an efficient P-rich biobased fertilizer with enhanced phosphorus availability. To achieve this, two approaches were proposed: fertilizer placement and pre-treatments to increase their phosphorus solubility. We found that the placement of biowastes creates a nutrient-rich zone in the soil with intensified physical, chemical, and biological processes. Pre-treatments can significantly affect these processes, for example, by increasing the phosphorus diffusion rates, changing the soil pH, and inhibiting microbial activity. In certain cases, such as meat and bone meal and acidified sewage sludge, we found that placement can have negative effects on plants, causing ammonium and aluminum toxicity, respectively.

Among different biowastes and pre-treatments, the placement of acidified digestate solid fraction and meat and bone meal showed promising results in terms of increasing phosphorus uptake and plant growth, compared to the placement of their respective untreated biomaterial. In the last study of this thesis, we formulated a biobased fertilizer with acidified digestate solid fraction as a pellet that yielded similar results to a commercial fertilizer reference (triple superphosphate), indicating that it could be an equal replacement for mineral fertilizers.

Therefore, application methods and pre-treatments are strategies that can increase the phosphorus fertilizer value of biowastes and reduce the European Union's dependency on the imports of non-renewable fertilizers.

Assessment Committee

Professor Torsten Müller, University of Hohenheim, Germany
Professor Peter Sørensen, Aarhus University, Denmark
Professor Lars Stoumann Jensen (Chair), PLEN, University of Copenhagen, Denmark

Supervisors

Associate Professor Jakob Magid, PLEN, University of Copenhagen, Denmark
Associate Professor Dorette Sophie Müller-Stöver, PLEN, University of Copenhagen, DK

The defence is followed by a reception in meeting room R322/23 on 3rd floor