Author's personal copy Using 15 N tracers to estimate N 2 O and N 2 emissions from nitrification and denitrification in coastal plain wetlands under contrasting land-uses Jennifer L. Morse * , Emily S. Bernhardt Department of Biology, Box 90338, Duke University, Durham, NC 27708, USA article info Article history: Received 4 May 2012 Received in revised form 19 July 2012 Accepted 31 July 2012 Available online 25 August 2012 Keywords: Nitrification Denitrification Nitrous oxide Stable isotope tracer Restored wetland Nitrous oxide yield abstract Microbial nitrification and denitrification both can emit nitrous oxide (N 2 O), a major greenhouse gas, and the relative contribution of each pathway depends strongly on soil moisture conditions. We conducted a stable isotope tracer experiment to determine the contribution of nitrification and denitrification to N 2 O and dinitrogen (N 2 ) fluxes in coastal plain wetlands, and to determine the response of these processes to changing soil moisture. We added 15 N-labeled nitrate (NO  3 ) or ammonium (NH þ 4 ) to intact soil cores collected from an agricultural field, a restored wetland, and a preserved forested wetland, and subjected the cores to a drying or wetting hydrologic manipulation. Across all soils and treatments, the combined fluxes of N 2 O and N 2 ranged widely, between 0.23 and 2900 mgNm 2 h 1 , and N 2 O accounted for as little as 0% to as much as 43% of the total gaseous nitrogen (N) fluxes. Fluxes of both gases increased with increasing soil moisture in all soils and tracer treatments, but the relative enhancement of the two gases varied by soil type and N source. The N 2 O yields [N 2 O/(N 2 O þ N 2 )] derived from both nitrification and denitrification were low (1e3%) in five of eight soils in each tracer experiment. Surprisingly, nitrification-derived N 2 O yields were highest (13e31%) in soils with the highest organic matter and soil moisture (restored wetland under simulated rain and forested wetland under drained and simulated rain), while denitrification-derived N 2 O yields (12e36%) were highest under simulated rain in the two mineral soils (agricultural field and mineral soils of the restored wetland), and under drained conditions in the forested wetland. These results are consistent with field-measured N 2 O fluxes in our previous work in these sites. We suggest that nitrification plays an important and underappreciated role in contributing to N 2 O fluxes from freshwater wetlands with often-saturated, acid-organic soils, while incomplete denitrification is the likely source of N 2 O following rain events in agricultural soils in southeastern U.S. coastal plain wetlands. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Wetland restoration, especially in agricultural watersheds where nitrogen (N) availability is high, has the potential to remove significant quantities of N from surface waters and soil (Ardón et al., 2010; Zedler, 2003), with microbial denitrification (DNF) of nitrate (NO  3 ) to gaseous N being the major permanent N removal mechanism. Nitrification (NF), the production of NO  3 from ammonium (NH þ 4 ), is a necessary precursor to DNF in wetlands without external supplies of NO  3 . Both DNF and NF have global relevance, in part through their production of nitrous oxide (N 2 O), a trace gas which both contributes to greenhouse warming and stratospheric ozone destruction (Forster et al., 2007; Ravishankara et al., 2009). Emissions of N 2 O by microbial processes represent 70% of annual N 2 O fluxes to the atmosphere (Mosier, 1998), and are primarily generated in waterlogged or periodically saturated environments, as typically found in wetland and stream ecosys- tems, and through agricultural practices (Conrad, 1995). If NF and DNF yield a substantial fraction of their products as N 2 O rather than N 2 , this could offset the local or regional water quality benefits of DNF, by exacerbating global warming (Schlesinger et al., 2006; Verhoeven et al., 2006). Measuring DNF rates and products can be difficult: DNF is highly variable spatially and temporally, and fluxes of its major product, N 2 , are generally against the atmospheric background of 78% N 2 (Davidson and Seitzinger, 2006). Measuring N 2 O against trace levels in the atmosphere is easier, but determining the relative importance of NF and DNF as sources of that N 2 O is difficult since both processes co-occur under many environmental conditions (Stevens et al., 1997). * Corresponding author. Present address: Cary Institute of Ecosystem Studies, Box AB, 2801 Sharon Turnpike, Millbrook, NY 12545, USA. Tel.: þ1 845 677 7600234; fax: þ1 845 677 5976. E-mail addresses: morsej@caryinstitute.org, jlmorse@gmail.com (J.L. Morse). Contents lists available at SciVerse ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.soilbio.2012.07.025 Soil Biology & Biochemistry 57 (2013) 635e643