Hot moments of N 2 O transformation and emission in tropical soils from the Pantanal and the Amazon (Brazil) Lars Liengaard a, * , Viviane Figueiredo b , Rikke Markfoged c , Niels Peter Revsbech c , Lars Peter Nielsen c , Alex E. Prast b , Michael Kühl a, d a Department of Biology, University of Copenhagen, Helsingør, Denmark b Laboratory of Biogeochemistry, University Federal of Rio de Janeiro, Brazil c Department of Bioscience, Aarhus University, Denmark d Plant Functional Biology and Climate Change Cluster, University of Technology Sydney, Australia article info Article history: Received 22 November 2013 Received in revised form 28 February 2014 Accepted 9 March 2014 Available online 3 April 2014 Keywords: Denitrification Tropical wetland Nitrous oxide emission Microbial nitrogen cycling Oxiceanoxic transition DNRA abstract Tropical wetland soils emit large amounts of nitrous oxide (N 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 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 2 and N 2 O, ii) monitor the accumulation of N 2 O in the anoxic soil volume, and iii) quantify the N 2 O efflux out of the soil. Flux chamber measurements of N 2 O emission confirmed the effluxes calculated from microsensor measurements. The N 2 O concentration dynamics in the soil cores were characterized by three distinct phases: 1) an initial slow N 2 O production, 2) a higher N 2 O production ending abruptly when the supply of NO 3  and NO 2  (NO x  ) was exhausted, and 3) a final phase where the accumulated N 2 O was reduced to N 2 . This evolution of the N 2 O pool in an intact soil core could be accurately simulated by a simple diffusion- reaction model with the presence of O 2 and NO x  as determining factors. Approximately one third of the initial NO 3  present in the soil was lost as N 2 O or N 2 . As the soil was depleted for NO 3  by the end of the experiment we suggest that dissimilatory nitrate reduction to ammonia (DNRA) was responsible for reducing the remaining NO 3  . Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The increasing atmospheric concentration of nitrous oxide (N 2 O) is problematic as N 2 O is a powerful greenhouse gas and ozone depleter (IPCC, 2007; Ravishankara et al., 2009). Soil is known to be a major source of N 2 O contributing w50% of the global N 2 O emission (Seitzinger et al., 2000). The most important N 2 O producing processes in soil are microbial nitrification, nitrifier denitrification and denitrification, with denitrification considered the main process (Dobbie et al., 1999; Abbasi and Adams, 2000; Zhu, 2013). Uncultivated tropical soils are estimated to contribute one third of the total soil-related N 2 O emission, but the exact locations and controlling mechanisms of such N 2 O emission remain uncertain (Smith, 1997; D’Amelio et al., 2009). Tropical forest soils have been shown to emit significant amounts of N 2 O(Davidson et al., 2001; Garcia-Montiel et al., 2002; Houlton et al., 2006), especially when receiving elevated N inputs (Hall and Matson, 1999; Steudler et al., 2002; Koehler et al., 2009). Drained tropical soils in seasonally flooded areas in the Pantanal have recently been shown to emit very high amounts of N 2 O, with peak emissions being closely associated with precipitation events and temporary waterlogging of the soil (Liengaard et al., 2013). These findings indicate that tropical wetlands may be a globally significant but hitherto over- looked source of N 2 O. Soil studies of N-rich soils as different as highly productive, temperate agricultural soil (Markfoged et al., 2011) and natural tropical wetland soil (Liengaard et al. 2013), show N 2 O increasing upon sudden soil O 2 depletion. In both soil types a general pattern * Corresponding author. Tel.: þ45 3077 1718. E-mail address: lalj@novozymes.com (L. Liengaard). Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio http://dx.doi.org/10.1016/j.soilbio.2014.03.015 0038-0717/Ó 2014 Elsevier Ltd. All rights reserved. Soil Biology & Biochemistry 75 (2014) 26e36