Drought‐induced nitrous oxide flux dynamics in an enclosed tropical forest

Abstract El Niño–La Niña cycles strongly influence dry and wet seasons in the tropics and consequently nitrous oxide (N 2 O) emissions from tropical rainforest soils. We monitored whole‐system and soil chamber N 2 O fluxes during 5‐month‐long droughts in the Biosphere 2 tropical forest to determine how rainfall changes N 2 O production. A consistent pattern of N 2 O flux changes during drought and subsequent wetting emerged from our experiments. Soil surface drying during the first days of drought, presumably increased gas transport out of the soil, which increased N 2 O fluxes. Subsequent drying caused an exponential decrease in whole‐system (4.0±0.1% day −1 ) and soil chamber N 2 O flux (8.9±0.8% day −1 ; south chamber; and 13.7±1.1% day −1 ; north chamber), which was highly correlated with soil moisture content. Soil air N 2 O concentration ([N 2 O]) and flux measurements revealed that surface N 2 O production persisted during drought. The first rainfall after drought triggered a N 2 O pulse, which amounted to 25% of drought‐associated reduction in N 2 O flux and 1.3±0.4% of annual N 2 O emissions. Physical displacement of soil air by water and soil chemistry changes during drought could not account for the observed N 2 O pulse. We contend that osmotic stress on the microbial biomass must have supplied the N source for pulse N 2 O, which was produced at the litter–soil interface. After the pulse, N 2 O fluxes were consistently 90% of predrought values. Nitrate change rate, nutrient, [N 2 O], and flux analyses suggested that nitrifiers dominated N 2 O production during the pulse and denitrifiers during wet conditions. N 2 O flux measurements in Biosphere 2, especially during the N 2 O pulse, demonstrate that large‐scale integration methods, such as flux towers, are essential for improving ecosystem N 2 O flux estimates.