Hot moments of N2O transformation and emission in tropical soils from the Pantanal and the Amazon (Brazil)

Abstract

Tropical wetland soils emit large amounts of nitrous oxide (N2O), 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 N2O 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 O2 and N2O, ii) monitor the accumulation of N2O in the anoxic soil volume, and iii) quantify the N2O efflux out of the soil. Flux chamber measurements of N2O emission confirmed the effluxes calculated from microsensor measurements.The N2O concentration dynamics in the soil cores were characterized by three distinct phases: 1) an initial slow N2O production, 2) a higher N2O production ending abruptly when the supply of NO3− and NO2− (NOx−) was exhausted, and 3) a final phase where the accumulated N2O was reduced to N2. This evolution of the N2O pool in an intact soil core could be accurately simulated by a simple diffusion-reaction model with the presence of O2 and NOx− as determining factors.Approximately one third of the initial NO3− present in the soil was lost as N2O or N2. As the soil was depleted for NO3− by the end of the experiment we suggest that dissimilatory nitrate reduction to ammonia (DNRA) was responsible for reducing the remaining NO3−.