Changes in soil internal N cycle and trace gas fluxes under elevated nutrient input

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Although projections are that N deposition will increase strongly in the tropics, its effects on tropical forests are still largely descriptive. In the proposed project we plan to continue on-going measurements of soil-atmosphere fluxes of N2O, NO, CH4 and CO2 (NUMEX experiment). Our early results suggest that after the first year of flux measurements there are already substantial changes in the microbial processes nitrification and denitrification. Furthermore there are indications that N cycling rates have increased in plots that receive N addition at moderate rates. We plan to conduct measurements using 15N pool dilution and a 15N pulse chase experiment to assess quantitatively how microbial N cycling has changed in forest soils and to translate the short-term N fluxes to long-term fates of NH4 + and NO3 - in forest soils exposed to chronic N addition. Another focus will be potential feedbacks of N deposition on the soil carbon cycling by measuring heterotrophic CO2 respiration. During the first phase, our group discovered in cooperation with A5 that tank bromeliads are a substantial source of Methane. We plan a series of measurement to determine the size of these CH4 emissions from different elevations.