Nitrous oxide emissions could reduce the blue carbon value of marshes on eutrophic estuaries

Abstract

The supply of nitrogen to ecosystems has surpassed the Earth's Planetary Boundary and its input to the marine environment has caused estuarine waters to become eutrophic. Excessive supply of nitrogen to salt marshes has been associated with shifts in species' distribution and production, as well as marsh degradation and loss. Our study of salt marshes in agriculturally intensive watersheds shows that coastal eutrophication can have an additional impact. We measured gas fluxes from marsh soils and verified emissions of nitrous oxide (N 2 O) in nitrogen-loaded marshes while the reference marsh was a sink for this gas. Salt marsh soils are extremely efficient carbon sinks, but emissions of N 2 O, a greenhouse gas 298 times more potent than CO 2 , reduces the value of the carbon sink, and in some marshes, may counterbalance any value of stored carbon towards mitigation of climate change. Although more research is merited on the nitrogen transformations and carbon storage in eutrophic marshes, the possibility of significant N 2 O emissions should be considered when evaluating the market value of carbon in salt marshes subject to high levels of nitrogen loading.

Greenhouse gas flux with reflooding of a drained salt marsh soil

“…Static chamber measurements are thus often taken during low tide when nutrient concentrations and N 2 O fluxes are likely to be lower ( Adams, Andrews & Jickells, 2012 ). Roughan et al (2018) found N 2 O flux varied with the tidal cycle and observed the highest N 2 O fluxes immediately after the high tide receded. Simply using an average of high tide and low tide fluxes extrapolated over 24 hours may thus overestimate or underestimate daily N 2 O fluxes ( Roughan et al, 2018 ).…”

“… Roughan et al (2018) found N 2 O flux varied with the tidal cycle and observed the highest N 2 O fluxes immediately after the high tide receded. Simply using an average of high tide and low tide fluxes extrapolated over 24 hours may thus overestimate or underestimate daily N 2 O fluxes ( Roughan et al, 2018 ). To accurately model the soil’s response to flooding, it is important to verify the effect of flooding on N 2 O emissions at high tide, low tide, and mid tide (halfway between high tide and low tide).…”

“…Although restoring marsh habitat can help reduce atmospheric concentrations of CO 2 , under some conditions salt marshes can be sources of N 2 O and CH 4 . In marshes with elevated nutrient inputs, for example, N 2 O emission can be substantial and could reduce the value of soil C storage from the perspective of climate change mitigation ( Moseman-Valtierra et al, 2011 ; Chmura et al, 2016 ; Roughan et al, 2018 ). Roughan et al (2018) observed significant N 2 O emissions from Prince Edward Island marshes in watersheds subject to N runoff from intensively farmed watersheds.…”

“…In marshes with elevated nutrient inputs, for example, N 2 O emission can be substantial and could reduce the value of soil C storage from the perspective of climate change mitigation ( Moseman-Valtierra et al, 2011 ; Chmura et al, 2016 ; Roughan et al, 2018 ). Roughan et al (2018) observed significant N 2 O emissions from Prince Edward Island marshes in watersheds subject to N runoff from intensively farmed watersheds. N 2 O flux is of particular concern as it has a global warming potential (GWP) 265 times that of CO 2 over a 100-year time horizon ( Myhre et al, 2013 ).…”

A Guide to International Climate Mitigation Policy and Finance Frameworks Relevant to the Protection and Restoration of Blue Carbon Ecosystems

“…Methane and nitrous oxide are released by microbial decomposition of organic matter, and are influenced by conditions such as temperature, salinity, and the amount of organic matter. Methane emissions tend to be more pronounced where salinity is low, but can sometimes be high in mangrove and tidal marsh ecosystems (Rosentreter et al, 2021), while nitrous oxide emissions can be high in places where nitrogen inputs have been substantial (Roughan et al, 2018). Methane emissions in seagrasses tend to be low, and although nitrous oxide emissions can sometimes be high, one study showed that net abatement of seagrass restoration was still substantial (Oreska et al, 2020).…”