Hot moments of N sub(2)O transformation and emission in tropical soils from the Pantanal and the Amazon (Brazil)

Tropical wetland soils emit large amounts of nitrous oxide (N sub(2)O), especially following wetting of drained soil. We investigated seasonally drained wetland soils from the Pantanal and the Amazon, both with a natural high nitrate content and low pH. Here we report the effect of wetting on the production, emission and consumption of N sub(2)O on these soils. Intact soil cores were wetted to simulate natural water logging events, and microsensor measurements were used to i) characterize the vertical microscale distribution of O sub(2) and N sub(2)O, ii) monitor the accumulation of N sub(2)O in the anoxic soil volume, and iii) quantify the N sub(2)O efflux out of the soil. Flux chamber measurements of N sub(2)O emission confirmed the effluxes calculated from microsensor measurements. The N sub(2)O concentration dynamics in the soil cores were characterized by three distinct phases: 1) an initial slow N sub(2)O production, 2) a higher N sub(2)O production ending abruptly when the supply of NO sub(3) super(-) and NO sub(2) super(-) (NO sub(x) super(-)) was exhausted, and 3) a final phase where the accumulated N sub(2)O was reduced to N sub(2). This evolution of the N sub(2)O pool in an intact soil core could be accurately simulated by a simple diffusion-reaction model with the presence of O sub(2) and NO sub(x) super(-) as determining factors. Approximately one third of the initial NO sub(3) super(-) present in the soil was lost as N sub(2)O or N sub(2). As the soil was depleted for NO sub(3) super(-) by the end of the experiment we suggest that dissimilatory nitrate reduction to ammonia (DNRA) was responsible for reducing the remaining NO sub(3) super(-).