Ecology, 95(7), 2014, pp. 1960–1971 Ó 2014 by the Ecological Society of America Tidal pulsing alters nitrous oxide fluxes in a temperate intertidal mudflat A. M. VIEILLARD 1,3 AND R. W. FULWEILER 1,2 1 Boston University Department of Earth and Environment, 685 Commonwealth Avenue, Boston, Massachusetts 02215 USA 2 Boston University Department of Biology, 5 Cummington Mall, Boston, Massachusetts 02215 USA Abstract. Environmental pulses, or sudden, marked changes to the conditions within an ecosystem, can be important drivers of resource availability in many systems. In this study, we investigated the effect of tidal pulsing on the fluxes of nitrous oxide (N 2 O), a powerful greenhouse gas, from a marine intertidal mudflat on the north shore of Massachusetts, USA. We found these tidal flat sediments to be a sink of N 2 O at low tide with an average uptake rate of 6.7 6 2 lmolm 2 h 1 . Further, this N 2 O sink increased the longer sediments were tidally exposed. These field measurements, in conjunction with laboratory nutrient additions, revealed that this flux appears to be driven primarily by sediment denitrification. Additionally, N 2 O uptake was most responsive to dissolved inorganic nitrogen with phosphorus (DIN þ DIP) addition, suggesting that the N 2 O consumption process may be P limited. Furthermore, nutrient addition experiments suggest that dissimilatory nitrate reduction to ammonium (DNRA) releases N 2 O at the highest levels of nitrate fertilization. Our findings indicate that tidal flats are important sinks of N 2 O, potentially capable of offsetting the release of this potent greenhouse gas by other, nearby ecosystems. Key words: biogeochemistry; denitrification; greenhouse gas; intertidal; nitrogen; nitrous oxide; tidal flat. INTRODUCTION Pulses in ecosystems can be both internal (e.g., predator–prey or herbivore cycles) and external (e.g., tides, hurricanes, floods; Odum et al. 1995). At a fundamental level, these pulses determine resource availability (e.g., water, nutrients, carbon, oxygen), and thus, ecosystem structure and function (Junk et al. 1989, Odum et al. 1995, Kessavalou et al. 1998, and many others). In both terrestrial and aquatic systems, climate and/or environmental conditions are commonly the most important drivers of resource pulses (Nowlin et al. 2008). In turn, these pulsed events help to explain numerous ecosystem-level dynamics from internal pro- ductivity and accretion dynamics in wetlands (Odum et al. 1995, Hensel et al. 1999), to sediment transport and nutrient cycling in estuarine and deep-sea ecosystems (Karl 2002, Davis et al. 2004). In many cases, pulsed events have had long-lasting consequences for the ecosystems in which they were studied. For instance, in desert grasses, Huxman et al. (2004) measured increased carbon accumulation for up to 15 days after a pulsed rain event (Huxman et al. 2004). Additionally, Davis et al. (2004) found that two storm events delivered 60–65% of annual river N and P export to northeast Florida Bay (Davis et al. 2004). Tidal flat and other intertidal sediments experience daily, pulsed events with the tidal cycle. This external pulsing, the ebbing and flooding of the tide, is a major driver regulating the availability of resources within these sediments. Pulsed tidal dynamics in intertidal ecosystems can therefore lead to rapid changes in sediment nutrient and gas availability. For instance, the incoming tide may replenish porewater nutrients, while the ebbing tide leaves sediments exposed to the atmosphere, and may increase sediment oxygen pene- tration. These kinds of rapid changes in resource availability likely play a significant role in the way in which tidal flat ecosystems process nutrients, and exchange atmospheric greenhouse gases. Many studies have focused on the impact of pulsed events on greenhouse gas emissions from terrestrial soils (e.g., Scholes et al. 1997, Harper et al. 2005, Kim et al. 2012). Both laboratory and field studies have shown that pulsed wetting events, such as rapid rainfall following drought, in particular, lead to a significant increase in the production of various greenhouse gases including nitrous oxide (N 2 O; e.g., Scholes et al. 1997, Martin et al. 2003, Butterbach-Bahl et al. 2004, Kim et al. 2010). N 2 O has shown especially remarkable responses to rewetting, exhibiting from 450% to over 9700% increases in N 2 O flux rates in field studies, and as much as 80 000% increases in laboratory manipulations (Kim et al. 2012). Additionally, Nobre et al. (2001) found that one moderate rain event could account for 15–90% of the total weekly production of N 2 O in tropical soils Manuscript received 10 July 2013; revised 15 November; accepted 10 January 2014. Corresponding Editor: B. Z. Houlton. 3 E-mail: amariev@bu.edu 1960