Field-derived nitrous oxide emission factors for fertilised smallholder oil palm systems on mineral soils in South-East Asia

Nitrous oxide (N2O) emissions from fertilised oil palm (OP) systems remain poorly quantified despite their importance for regional greenhouse gas inventories and life-cycle assessments. This study measured N2O emissions following 18 fertiliser application events across 12 smallholder OP farms in Indonesia and Malaysia between 2022 and 2025. Cumulative fertiliser-induced emissions were estimated over a 21-day measurement period and used to derive N2O emission factors (EFs) for individual fertilisation events. Individual EFs ranged from 0.01% to 1.43% of applied nitrogen fertiliser (mineral and organic). To place these measurements in a regional context, results were combined with published EFs from OP systems on mineral soils in Southeast Asia (SEA). Using log-normal statistical treatment to account for the highly skewed distribution of EFs, a regional mean EF of 0.82% of applied nitrogen was derived (95% confidence interval: 0.44–1.78%). Sensitivity analyses indicated that regional EF estimates varied according to study inclusion and analytical approach, with mean values ranging from 0.51% to 1.12% of applied nitrogen. While uncertainties remain and the observational study design does not allow the independent effects of site, year, fertiliser type and management to be disentangled, the derived EF is broadly consistent with the IPCC Tier 1 default value of 1% and represents a regional estimate based on the currently available field evidence. These results contribute to a limited empirical dataset for OP systems in SEA and provide a transparent evidence base for greenhouse gas inventories, life-cycle assessments, and future evaluations of mitigation opportunities within the sector.