Estimating the Impacts of Organic Manures on Grass Yield, Soil Carbon Storage and N2O Emissions Using the DNDC Model and the Impact of Future Climate Change Scenarios

Livestock manure management plays a critical role in sustaining agricultural productivity, in the absence of no or a limited supply of inorganic fertilizers, while also enhancing soil organic carbon (SOC) storage. Yet, its effects on greenhouse gas (GHG) emissions remain uncertain. Using the Denitrification-Decomposition (DNDC) model, this study assessed the long-term impacts of pig and cattle manure applications on grass silage yield, SOC, and nitrous oxide (N₂O) emissions at a grassland site in Northern Ireland. The model was calibrated using SOC observations from 2012 to 2013 and grass yield observations from 2010 to 2013, and validated against 2016 measurements; N₂O performance was evaluated using annual cumulative emissions measured in 2016. It was then applied under future climate scenarios (SSP1–2.6, SSP3–7.0, SSP5–8.5) projected by the IPSL-CM6A-LR model. The DNDC model reproduced observed SOC, grass silage yield and annual N₂O emissions with acceptable performance, although N₂O emissions from the high cattle slurry treatment were underestimated. Across future SSP scenarios, high manure inputs increased silage yield and SOC storage, but also strongly enhanced N₂O-derived GWP estimates. The N₂O-derived GWP exceeded the SOC sequestration benefit by 2.0–5.7 times in several manure treatments. High pig manure produced the greatest yield and GHGI, whereas high cattle manure resulted in the largest SOC accumulation and N₂O emissions. Increasing manure loading enhanced SOC storage but also disproportionately increased N₂O-derived GWP and GHGI, particularly in the high-input treatments. These results indicate that manure-derived climate benefits are conditional rather than universal and depend strongly on manure type, application rate and future climate forcing.