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Background and Aims: To sustainably manage N in oil palm systems quantities of N fixed by cover legumes need to be understood. Current values are scarce, based on shoot N measures and do not include litter which releases nitrate as it decomposes. We aimed to quantify N2 fixed by legumes under oil palm systems in PNG and to determine if soil nitrate influenced dependence on N2 fixation (Ndfa). Methods: The ureide technique for estimating tropical legume Ndfa was calibrated for Calapogonium mucunoides and Pueraria phaseoloides using 15N isotope dilution, and then used to assess Ndfa for legume cover under oil palms (2 to 25 years old) in Papua New Guinea. Amounts of fixed N in above-ground legume biomass (shoot plus litter) were calculated incorporating % groundcover. Soil nitrate under the legume litter was also measured. Results: Legume Ndfa was highly negatively correlated with soil nitrate concentration but independent of palm age. Legume groundcover, shoot and litter dry matter, and quantity of fixed N were greater under oil palms less than 5 years old, decreasing under older plantations where solely C. caeruleum was present. DM and N content of litter were similar to shoots for legumes in plantations less than 6 years old. Conclusion: The calibrated ureide technique can be used, together with estimates of annual legume N accumulation, to quantify N input from legume groundcover during the life cycle of oil palm plantations and other tropical ecosystems, in order to support more sustainable management of N. [ABSTRACT FROM AUTHOR]