Article pubs.acs.org/est
Controls on Nitrous Oxide Emissions from the Hyporheic Zones of Streams Annika M. Quick,*,†,∥ W. Jeffery Reeder,‡ Tiffany B. Farrell,†,§ Daniele Tonina,‡ Kevin P. Feris,§ and Shawn G. Benner*,† †
Department of Geosciences, Boise State University, Boise, Idaho 83725, United States Center for Ecohydrdaulics Research, University of Idaho, Boise, Idaho 83702, United States § Department of Biological Sciences, Boise State University, Boise, Idaho 83725, United States ‡
S Supporting Information *
ABSTRACT: The magnitude and mechanisms of nitrous oxide (N2O) release from rivers and streams are actively debated. The complex interactions of hydrodynamic and biogeochemical controls on emissions of this important greenhouse gas preclude prediction of when and where N2O emissions will be significant. We present observations from column and large-scale flume experiments supporting an integrative model of N2O emissions from stream sediments. Our results show a distinct, replicable, pattern of nitrous oxide generation and consumption dictated by subsurface (hyporheic) residence times and biological nitrogen reduction rates. Within this model, N2O emission from stream sediments requires subsurface residence times (and microbially mediated reduction rates) be sufficiently long (and fast reacting) to produce N2O by nitrate reduction but also sufficiently short (or slow reacting) to limit N2O conversion to dinitrogen gas. Most subsurface exchange will not result in N2O emissions; only specific, intermediate, residence times (reaction rates) will both produce and release N2O to the stream. We also confirm previous observations that elevated nitrate and declining organic carbon reactivity increase N2O production, highlighting the importance of associated reaction rates in controlling N2O accumulation. Combined, these observations help constrain when N2O release will occur, providing a predictive link between stream geomorphology, hydrodynamics, and N2O emissions.
1. INTRODUCTION Streams are an important, but poorly constrained, source of the greenhouse gas nitrous oxide (N2O). N2O has approximately 300 times the warming potential of CO21 and is the dominant ozone-depleting anthropogenic substance.2 Most anthropogenic emissions are related, directly or indirectly, to agricultural practices,3 with nitrogen fertilizer stimulating N2O production in soils and in downstream systems subjected to fertilizer runoff.3−5 The United States Environmental Protection Agency (EPA)6 estimates that natural nitrous oxide emissions from rivers are 0.1 Tg N−N2O yr−1, while a recent large scale tracer study suggests that rivers account for at least 0.68 Tg N−N2O yr−1, representing up to 10% of global anthropogenic N2O emissions.7 Despite the potential importance of streams to the global N2O budget, there is considerable uncertainty regarding the mechanisms and controls on its release. While nitrogen transformations can occur in the water column of a stream, many biogeochemical reactions, including denitrification, take place in the saturated sediments beneath and immediately adjacent to streams. This area of active exchange and transformation of surface and groundwater, the hyporheic zone,8 operates at a range of spatiotemporal scales.9,10 It is useful to conceptualize hyporheic flow in © XXXX American Chemical Society
terms of hyporheic residence time (τHZ), the time a packet of water spends in the subsurface before returning to the stream. The hyporheic residence time is a function of flow path length and flow velocity, which are dictated by stream geomorphology and hydraulics.11 Stream bedform morphology influences hyporheic residence time by modulating pressure differentials that drive water into and out of the hyporheic zone;12 the greater the produced hydraulic gradients, the higher the flow velocities. Dissolved gases, including N2O, produced along these hyporheic flow paths can be released to the atmosphere after returning to the stream, a process that is potentially an important global source of N2O.7,13,14 Although a variety of processes can lead to N2O generation under varying conditions,7,15,16 it is generally believed that most N2O in saturated sediments is the product of denitrification17−19 because of the potential for favorable anaerobic conditions. Denitrification is the sequential reduction of NO3− Received: May 28, 2016 Revised: September 30, 2016 Accepted: October 3, 2016
A
DOI: 10.1021/acs.est.6b02680 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology Table 1. Experimental Setup
to NO2−, NO, N2O, and finally to N2.20 This multistep process can be simplified to two reactions: 2NO3− + 2CH 2O + 2H+ → N2O + 2CO2 + 3H 2O
(1)
2N2O + CH 2O → 2N2 + CO2 + H 2O
(2)
in which we monitored nitrogen transformations as water followed hyporheic flow paths. We documented the conditions under which N2O was generated and the specific hydrologic and biogeochemical conditions that led to its release or consumption. 2.1. Column Experiments. Column experiments evaluated nitrous oxide generation and consumption in a quasi-onedimensional system mimicking a single hyporheic flowline. The primary variable was the initial particulate organic carbon content (Table 1). PVC pipes (10 cm diameter) were fitted with water-tight caps on both ends and filled with sediment. Each column was filled with a mixture of 90% quarry sand (sieved to
