Closing the Loop: Biogas Residues as a Sustainable Alternative to Chemical Nitrogen Fertilizers in European Grasslands

Nitrogen fertilization sustains food and feed production, but the reactive nitrogen that escapes from agricultural soils pollutes groundwater, acidifies soils and erodes biodiversity. Anaerobic digestion converts manure, crop residues and agro-food waste into biogas and into a nutrient rich co-product known as biogas residues, or digestate. Between 2013 and 2019, the Luxembourg Institute of Science and Technology (LIST) ran a coordinated field experiment on three grasslands of the Greater Region to showcase whether biogas residues can safely and effectively replace chemical fertilizers. Up to 17 fertilization treatments were compared at nitrogen rates of 230 and 350 kg N ha−1 yr−1, the higher rate deliberately testing the residues beyond the legal ceiling for organic nitrogen. Biogas residues matched chemical fertilizers in yield and, after 5 years, in nitrogen uptake, while soil residual nitrate matched the level of the unfertilized control even under over-application. They retarded organic matter decomposition and demonstrated a potential to promote carbon sequestration in grassland soils, as opposed to urea that accelerated decomposition. Finally, soil bacterial and fungal communities were not altered, and no pathogens were established under biogas residue application. Ammonium sulfate, in contrast, reduced bacterial diversity. This trial provides evidence that farmers and regulators can use.