The fertilising potential of manure-based biogas fermentation residues: pelleted vs. liquid digestate

Spreading of manure on agricultural soils is a main source of ammonia emissions and/or nitrate leaching. It has been addressed by the European Union with the Directives 2001/81/EC and 91/676/EEC to protect the environment and the human health. The disposal of manure has therefore become an economic and environmental challenge for farmers. Thus, the conversion of manure via anaerobic digestion in a biogas plant could be a sustainable solution, having the byproducts (solid and liquid digestates) the potential to be used as fertilizers for crops. This work aimed at characterizing and assessing the effect of digestates obtained from a local biogas plant (Biogas Wipptal, Gmbh), either in the form of liquid fraction or as a solid pellet on: (i) the fertility of the soils during an incubation experiment; (ii) the plant growth and nutritional status of different species (maize and cucumber). Moreover, an extensive characterization of the pellet was performed via X-ray microanalytical techniques. The data obtained showed that both digestates exhibit a fertilizing potential for crops, depending on the plant species and the fertilizer dose: the liquid fraction increases the shoot fresh weight at low dose in cucumber, conversely, the solid pellet increases the shoot fresh weight at high dose in maize. The liquid digestate may have the advantage to release nutrients (i.e. nitrogen) more rapidly to plants, but its storage represents the main constraint (i.e. ammonia volatilization). Indeed, pelleting the digestates could improve the storability of the fertilizer besides enhancing plant nutrient availability (i.e. phosphate and potassium), plant biomass and soil biochemical quality (i.e. microbial biomass and activity). The physical structure and chemical composition of pellet digestates allow nutrients to be easily mobilized over time, representing a possible source of mineral nutrients also in long-term applications.