A farm-scale sustainability assessment of the anaerobic digestate application methods

Digestate is the anaerobic digestion by-product that can be used as an organic fertilizer, but some agronomic and environmental concerns still hinder its application on land. This study aims to evaluate the agronomic performances (i.e., silage maize dry yield, protein content, and N uptake) and environmental sustainability of solid and liquid digestate fraction application in a field experiment involving two farms with different textures covering about 47 ha in North Italy. The best available distribution methods included mineral fertilizer (MF), mineral fertilizer in variable rate application (VRA) (VRA-MF), liquid digestate with a nitrification inhibitor (LD+), liquid digestate in VRA (VRA-LD), liquid digestate with a nitrification inhibitor in VRA (VRA-LD+), and solid digestate (SD) and were applied to silage maize (Zea mays L.) in 2019 and 2020 cropping seasons. Results showed that both digestate fractions gave satisfying agronomic performances (i.e., dry biomass > 13 t ha-1 and protein content > 6.8%), comparable to those of mineral fertilizers, irrespective of soil type and application techniques. On the contrary, system sustainability investigated with a spatial evaluation of nitrogen use efficiency (NUE) revealed a strong interaction between NUE and soil texture. Indeed, in fine-textured soil only the adoption of both VRA and the nitrification inhibitor allowed the liquid digestate to reach a NUE between 50% and 90% while SD exhibited poor NUE (e.g., < 50%). In conclusion, liquid digestate fraction might be an effective substitute for mineral fertilizers in silty soils meeting also environmental criteria when VRA or nitrification inhibitors are applied. Contrarily, longer-term experiments are requested to evaluate SD fraction sustainability.