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Title: DROUGHT-INDUCED N2O FLUX DYNAMICS IN AN ENCLOSED TROPICAL FOREST 1661

Author
item Van Haren, Joost
item HANDLEY, L.
item BIL, K.
item KUDEYAROV, V.
item McLain, Jean
item Martens, Dean
item COLODNER, D.

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/25/2005
Publication Date: 7/5/2005
Citation: Van Haren, J.L., Handley, L., Bil, K., Kudeyarov, V.N., Mclain, J.E., Martens, D.A., Colodner, D.C. 2005. Drought-induced N2o flux dynamics in an enclosed tropical forest. Global Change Biology. 11:1-11.

Interpretive Summary: Nitrous oxide (N2O) emissions from soils contribute more than 70% of the total global N2O emissions to the atmosphere. Understanding N2O emission pathways from soil is very important because N2O is a strong greenhouse gas with a warming potential of 310 times an equivalent concentration of carbon dioxide. In the enclosed climate of the Biosphere 2, we investigated the impacts of wetting and drying cycles on the emissions of N2O from soil and determined that the most discussed pathway of N20 production via anaerobic respiratory denitrification was not important as suggested in previous limited studies. The study also found that the aerobic surface of the soil was the most important source of N2O in the five wetting and drying cycles and not the deeper less aerobic soils as had been suggested by the previous studies. The whole system approach employed in this study from soil probes, surface fluxes and whole atmosphere provides scientists with the first complete soil to atmosphere investigation of this greenhouse gas pathway. The results suggest that the vast majority of N2O emissions are not controlled solely by environmental conditions such as anaerobic soils, but are due to predictable biological mechanisms.

Technical Abstract: El Niño-La Niña cycles strongly influence N2O emissions from tropical rainforests because they affect dry and wet seasons in the tropics. We measured whole system N2O fluxes during five month-long droughts in the Biosphere 2 tropical forest to determine how rainfall changes N2O production. A consistent pattern of N2O flux changes during drought and subsequent wetting emerged from our experiments. Nitrate change rate, nutrient, and soil air [N2O] and flux analyses suggest that denitrifiers dominated N2O production before drought. Drought caused an exponential decrease in N2O flux at a rate of 4.0±0.1% day-1. Throughout drought, N2O production occurred at a substantial rate because soil air N2O could sustain soil-to-atmosphere fluxes for up to 48 hours. Rains ending drought triggered a N2O pulse within hours, which amounted to 25% of drought-associated reduction in N2O flux and 1.3±0.4% of annual N2O emissions. Total amount of rainfall and soil [N2O] were used to rule out physical expulsion as source for the observed N2O pulse. During the pulse in soil [N2O] did not increase, so we presume N2O was produced at the litter-soil interface. Microbial biomass must have been the N-source for pulse N2O because soil [NH4-], [NO2-], [NO3-], or amino acid concentrations did not change significantly during drought. After the pulse, N2O fluxes were invariably 10% reduced from pre-drought values, which were reached after three weeks of continued wet conditions. Our drought experiments demonstrate that large-scale integration methods, such as flux towers, are paramount to improve ecosystem N2O flux estimates.