Carbon and Nitrogen Mineralisation in Soils and Nutrient Efficiency of Digestates from Fruit and Vegetable Wastes

A significant amount of fruit and vegetable wastes is generated annually in markets, supermarkets, restaurants and households. Anaerobic digestion allows their appropriate management and helps to complete the cycle of the circular economy as it converts wastes into resources: biogas, a renewable source of energy, and digestate, rich in nutrients of interest for agriculture. The aim of this study was to define the agronomic value of digestate from fruit and vegetable wastes. Two digestates from fruit and vegetable residues were used in incubation experiments for C and N mineralisation in the soil, and in a pot experiment with crops (cardoon and maize), to calculate their fertiliser potential in comparison with a mineral fertiliser. The digestate quickly mineralised in the soil and nitrification processes led to fast formation of NO 3 − -N. However, increasing the digestate application rate enhanced N-immobilisation and reduced N-mineralisation in the soil. The addition of digestates to the soil resulted in adequate plant growth and nutrient composition, without any negative effects on the plants or soil. However, special attention should be paid to the salt accumulation in the soil for long-term digestate application. The nutrient recovery efficiency indicated that digestate could replace mineral fertiliser completely in cardoon crops and partially (44.5–82.6%) for maize, with an associated economic benefit. The salinity of the digestates limits their quality and their agricultural use to salt-sensitive crops should be limited.