Biological nitrogen fixation by legume cover plants in oil palm plantations: calibration of the ureide technique and effects of plantation age and soil nitrate.

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]