A05 - Nutrient use and retention efficiency, free-living N₂ fixation and soil and tree-stem N₂O and CH₄ fluxes in rainforest transformation systems

Results from the first phase of our project A05 have shown that of the four land-use systems (forest, jungle rubber, rubber and oil palm) for which we measured soil-N cycling rates, soil greenhouse gas (GHG) fluxes and nutrient leaching losses, oil palm showed intermediary soil N availability but reduced CH₄ uptake, increased N₂O emissions, and increased leaching losses of mineral N, P, Al, Na, Ca and Mg. These enhanced GHG fluxes and leaching losses were due to fertilization, liming and ashes from biomass burning in oil palm plantations. In the second phase, we propose firstly to investigate how management practices in oil palm affect the efficiencies with which soil nutrients are used and retained in this agro-ecosystem. This study will be conducted in the management experiment (project Z01), which will have fertilization rate and weed control as treatments. We will assess changes in nutrient response efficiency = yield / soil or plant-available nutrients, nutrient retention efficiency = leaching losses / nutrient availability in the soil, and free-living N₂ fixation which may compensate for harvest export in moderate fertilization rate. Secondly, in an oil palm plantation with an on-going eddy covariance flux tower measurements, we will contribute to an ecosystem-level GHG budget by providing the soil GHG flux budget. We will measure soil GHG fluxes at monthly interval for two years using chamber methods along the footprint of the tower. Thirdly, we will carry out another year of soil GHG flux measurements at the core plots of the first phase in order to quantify the inter-annual variability and contribute to focus 2 as well as use these core sites as reference for the landscape-scale variability assessment that will include riparian areas of the landscape. We will also assess the contributions of denitrification and nitrification to N₂O emissions, using ¹⁵N tracing technique. Fourthly, we will conduct soil GHG measurements at the 12 new core plots on riparian areas of the landscape to assess their contributions to landscapescale GHG fluxes. For this last objective, we will additionally evaluate the importance of tree-stem emissions on the soil GHG budget. Tree stems can function as an effective conduit for transport of N₂O and CH₄ from the soil (where they are produced) to the atmosphere. At present, it is unknown how important this tree-stem GHG transport is in the full soil GHG budget.