Ecosystem service trade-off of salt marshes as coastal nutrient filters versus greenhouse gas sinks

• Tidal flooding was the main transport pathway for nitrate entering marsh porewater • Thirty-six percent of the nitrate transported into the marsh was attenuated by vegetation and microbial denitrification. • Nitrous oxide and methane fluxes were low, despite elevated nitrate. • Observed N 2 O and CH 4 fluxes offset the net CO 2 sequestration capacity of the two marshes by 8.3 and 7.2% Two important ecosystem services provided by salt marshes include attenuation of coastal nutrient pollution and carbon sequestration. However, an ecosystem service trade-off may be created where nitrogen loading to salt marshes can enhance marsh nitrous oxide emissions, limiting carbon sequestration. We characterize this potential ecosystem service trade-off by combing greenhouse gas flux measurements and hydrogeological modelling at a salt marsh on Canada’s east coast. Our objectives were to quantify the hydrologic transport and uptake of nitrate through the Basin Head salt marsh in Prince Edward Island and to quantify N 2 O and CH 4 fluxes from the Basin Head salt marsh and a secondary field site at Rushton’s Beach salt marsh in Nova Scotia. Our results indicate that nitrate enters the Basin Head salt marsh from the lagoon via tidal flooding and lateral porewater exchange. The average nitrate-N input is 0.06mmol N m −2 d −1 , and 36% of inputs are taken up by the marsh via assimilation into plant biomass, conversion to gaseous products, or conversion to ammonium, with the rest being re-exchanged with the lagoon. Average low and high marsh N 2 O fluxes (± standard error) were −0.44 ± 1.25 and 2.12 ± 1.53 µg m −2 h −1 , respectively, at Basin Head, and 0.94 ± 0.50 and 3.76 ± 0.25 µg m −2 h −1 , respectively, at Rushton’s Beach. These fluxes are on the low end of previous studies of nitrogen impacted salt marshes. Marsh N 2 O and CH 4 fluxes result in an 8.3% and 7.3% offset in the net climatic effect of marsh CO 2 sequestration at Basin Head and Rushton’s Beach, respectively. N 2 O fluxes were significantly (p < 0.05) positively correlated with porewater nitrate concentration at Basin Head only. We conclude that the Basin Head marsh functions as an effective nutrient filter with only a minor reduction in carbon sink capacity. While our findings have limited spatial scope, they represent a first combination of both gas flux and hydrogeological modelling methods to improve our understanding of the trade-offs between salt marsh ecosystem services under nutrient pollution.