Land use drives nitrous oxide dynamics in estuaries on regional and global scales

Abstract

Urban and agricultural development of coastal catchments is known to increase dissolved nitrogen inputs into estuaries; however, much less is known about how land use influences the production of the powerful greenhouse gas nitrous oxide (N 2 O). Here, we assess dissolved N 2 O dynamics in four nearby estuaries across a regional land use gradient and summarize the literature to put the observations into global perspective. During summer dry conditions, N 2 O saturation ranged from 131.4% AE 45.0% in the most pristine system (28% modified) to 198.6% AE 52.3% within the most modified urban system (91% modified). The N 2 O saturation in the wetter winter campaign was higher and more variable than the summer dry campaign (range 84.7-677.7%) likely due to direct transport of N 2 O into the estuaries from catchment runoff and/or produced through denitrification fueled by high nitrate inputs. During both seasons, N 2 O was lowest in areas adjacent to fringing mangroves and highest in upstream fresh/saltwater mixing areas. Coupling our results with previously published N 2 O data from 50 estuarine systems worldwide revealed that estuarine N 2 O increases concomitantly with catchment modification, dissolved inorganic nitrogen availability, and decreasing oxygen concentrations. Based on these results, a 1% increase in anthropogenic modification to global catchments (i.e., agricultural development and/or urbanization) may increase estuarine N 2 O saturation by 2.6% AE 1.2%. These findings indicate that future estuarine N 2 O emissions are likely to increase as anthropogenic modification of coastal catchments intensifies.

Ideas and perspectives: A strategic assessment of methane and nitrous oxide measurements in the marine environment

“…A recent study demonstrated that CH 4 production by cyanobacteria is linked to general cell metabolism and does not rely on the presence of methylated precursor compounds (Bižić et al, 2020). Deep within the ocean's pelagic interior, CH 4 is weakly undersaturated, reflecting depletion via microbial oxidation (Reeburgh, 2007;Weber et al, 2019). Towards the coastline, CH 4 supersaturation increases by orders of magnitude ( Fig.…”

“…For the coastal and open ocean, the dominant driver of gas exchange is wind speed (e.g., Nightingale et al, 2000;Wanninkhof, 2014) whereas in nearshore, shallow water environments the interaction of water, depth, and tidal current speeds may be a major contributor to near-surface turbulence. Several k w parameterizations are now in use for coastal waters (e.g., Raymond and Cole, 2001;Kremer et al, 2003;Zappa et al, 2003;Borges and Abril, 2011;Ho et al 2011;Rosentreter et al, 2017;Jeffrey et al, 2018), which increases the uncertainties associated with CH 4 and N 2 O emissions. For example, a 5-fold variation in CH 4 emissions from a single system occurred when applying different parameterizations to the measured gradients in CH 4 (Ferrón et al, 2007).…”

“…Such data are important because for example, the magnitude of CH 4 and N 2 O fluxes varies over a diel period depending on the redox environment as a result of tidal effects and changes in inorganic N and O 2 availability (Seitzinger and Kroeze, 1998;Call et al, 2015;Vieillard and Fulweiler, 2014;Maher et al, 2015;Murray et al, 2015;Foster and Fulweiler, 2019). The magnitude of CH 4 and N 2 O fluxes also varies over longer temporal scales (seasonally to yearly) due to additional factors such as groundwater inputs, adjacent land use, dissolved O 2 , organic matter content and quality, and macrofaunal distributions (Barnes and Upstill-Goddard, 2011; Upstill-Goddard and Barnes, 2016;Gelesh et al, 2016;Bonaglia et al, 2017;Borges et al, 2018;Wells et al, 2018;Ray et al, 2019;Al-Haj and Fulweiler, 2020;Reading et al, 2020). To determine the contributing factors and resolve the spatial distributions, mobile sampling platforms such as small vessels (Müller et al, 2016;Brase et al, 2017;Tait et al, 2017) and autonomous vehicles (Manning et al, 2019) are essential.…”

Ideas and perspectives: A strategic assessment of methane and nitrous oxide measurements in the marine environment

“…Such data are important because for example, the magnitude of CH 4 and N O fluxes vary over a diel period depending on the redox environment as a result of tidal effects and changes in inorganic N and O 2 availability (Seitzinger and Kroeze, 1998;Call et al, 2015;Vieillard and Fulweiler, 2014;Maher et al, 2015;Murray et al, 2015;Foster and Fulweiler, 2019). The magnitude of CH 4 and N 2 O fluxes also varies over longer temporal scales (seasonally to yearly) due to additional factors such as groundwater inputs, adjacent land-use, dissolved O 2 , organic matter content and quality, and macrofaunal distributions (Barnes and Upstill-Goddard, 2011; Upstill-Goddard and Barnes, 2016;Gelesh et al, 2016;Bonaglia et al, 2017;Borges et al, 2018;Wells et al, 2018;Ray et al, 2019;Al-Haj and Fulweiler, 2020;Reading et al, 2020). To determine the contributing factors and resolve the spatial distributions, mobile sampling platforms such as small vessels (Müller et al, 2016;Brase et al, 2017;Tait et al, 2017), and autonomous vehicles (Manning et al, 2019) are essential.…”

Shifts in methanogenic archaea communities and methane dynamics along a subtropical estuarine land use gradient

“…Estuaries contribute between 1 and 7 Tg of CH 4 and 0.1 to 0.25 Gt of CO 2 to the atmosphere each year [9], with the global flux of CO 2 to the atmosphere from estuaries comparable to the uptake of the entire continental shelf, despite estuaries representing only 5% of the continental shelf equivalent surface area [10]. Increasing inputs of anthropogenic pollutants stemming from urban, industrial and agricultural runoff into adjacent estuarine ecosystems have been reported to elevate GHG fluxes [11][12][13]. CH 4 emissions originating from microbial sources have been suggested to contribute about 70% of all global methane emissions [14][15][16].…”