Effects of grassland community composition and digestate fertiliser on nitrous oxide emissions from soil

The agriculture sector is under considerable pressure to reduce greenhouse gas (GHG) emissions. The use of grass from farmland as a feedstock for anaerobic digestion (AD) can contribute to the development of biomethane as a renewable energy. Digestate, a by-product of AD, can be recycled as a source of N fertilizer on grassland, but little is known about its nitrous oxide (N₂O) emissions (a potent greenhouse gas) following land-spreading. In a two-year field experiment, we investigated the effects of two fertilisers (digestate and calcium ammonium nitrate), nitrogen (N) application rate and grassland species composition on N₂O emissions, emission intensity, and N₂O emission factors. Application of digestate resulted in lower N₂O emissions than the application of calcium ammonium nitrate (CAN). Increasing N application rates led to higher emissions for both fertilizers, with a more pronounced effect for CAN. Grassland composition affected N₂O emissions intensity where diverse swards had lower emissions intensity compared to perennial ryegrass monoculture. The N₂O emission factors (EFs) for digestate ranged from 0.28 to 1 %, which is lower than those of CAN and those of the IPCC default Tier 1 EF of 1 %. These results support the case for differentiating EFs by N source (e.g. digestate) in N₂O emissions inventories. Our results suggest that digestate can lower N₂O emissions while supporting the circular economy objectives of nutrient recycling. Further long-term studies should refine emissions estimates, optimise legume inclusion, and assess the agronomic and biomethane potential of the forage from grassland systems under varying N inputs.