Solid and Liquid Fraction of Digestate as an Alternative Mineral Nitrogen Source: Two-Year Field Research in Croatia

This research aimed to assess the effect of applying digestate fractions and conventional mineral nitrogen (N) fertilizers on plant and soil properties, as well as potential nitrate leaching, in acidic soil over two consecutive years (2018 and 2019). The digestate fractions were obtained after the anaerobic co-digestion of maize silage and liquid cattle manure (LCM). Seven different treatments were applied to the maize crop in four replicates: unfertilized control (C), mineral fertilizer (MF), LCM, solid fraction of digestate (SFD), liquid fraction of digestate (LFD) and a mixture of SFD and LFD with MF (50% of total N from digestate fractions + 50% of total N from MF). The highest maize yields were achieved on average in 2018. Statistically, the highest dry grain yield was observed in the MF treatment (12.1 t ha−1) and in the mixtures of MF+SFD (11.0 t ha−1) and MF+LFD (11.8 t ha−1), while the lowest yield was achieved in both years in the C treatment (7.9 t ha−1). The N fertilizer replacement value was statistically highest on average in the MF treatment (100%) and in the mixtures of MF+LFD (80.5%) for both years. The experiment results could inform legal guidelines and standardize digestate application on agricultural land in Croatia and the European Union (EU).