Digestate from farm biogas plants as a possible fertiliser for major crops in northern Italy

Abstract

Aim of study: Efficient recycling of digestates in livestock systems is a way for maintaining satisfactory crop yields and reducing environmental pollution. Substitution of mineral N fertilisers with digestates was demonstrated under field conditions for silage maize (Zea mays L.), while studies evaluating digestate’s fertiliser efficacy for legume crops relevant for the livestock sector, namely alfalfa (Medicago sativa L.) and soybean (Glycine max (L.) Merr.), remain limited. This study evaluated digestate’s efficacy in enhancing crop yield of silage maize, alfalfa forage and soybean grain under irrigated field conditions in northern Italy. Area of study: Mediterranean North climate (Lombardy Region, northern Italy). Material and methods: During the cropping seasons 2020 to 2022, field experiments were conducted involving the application of digestates, mineral fertilisers, and digestates enriched with microbial-based biostimulants. The effect of each treatment was evaluated considering two varieties of each crop, measuring the yield of maize silage, alfalfa forage and soybean grain. Main results: Digestates providing 250 kg N ha–1 to maize, and 50 kg N ha–1 to soybean resulted in equal yield compared to mineral fertiliser. The response of the three crops to fertilisation treatments was consistent between years and varieties. The lack of response of alfalfa to digestate did not support the application of digestates to this crop. Finally, the addition of biostimulant increased maize yield in only one location, requiring further evaluation under field conditions. Conclusions: Liquid digestate can effectively substitute urea in maize fertilization, is beneficial for soybean at low application rates, but is not recommended for alfalfa under high-fertility conditions.