Salt marshes can attenuate nutrient pollution and store large amounts of ‘blue carbon’ in their soils, however, the value of sequestered carbon may be partially offset by nitrous oxide (N₂O) emissions. Global climate and land use changes result in higher temperatures and inputs of reactive nitrogen (Nr) into coastal zones. Here, we investigated the combined effects of elevated temperature (ambient + 5℃) and Nr (double ambient concentrations) on nitrogen processing in marsh soils from two climatic regions (Quebec, Canada and Louisiana, U.S.) with two vegetation types, Sporobolus alterniflorus (= Spartina alterniflora) and Sporobolus pumilus (= Spartina patens), using 24-h laboratory incubation experiments. Potential N₂O fluxes increased from minor sinks to major sources following elevated treatments across all four marsh sites. One day of potential N₂O emissions under elevated treatments (representing either long-term sea surface warming or short-term ocean heatwaves effects on coastal marsh soil temperatures alongside pulses of N loading) offset 15–60% of the potential annual ambient N₂O sink, depending on marsh site and vegetation type. Rates of potential denitrification were generally higher in high latitude than in low latitude marsh soils under ambient treatments, with low ratios of N₂O:N₂ indicating complete denitrification in high latitude marsh soils. Under elevated temperature and Nr treatments, potential denitrification was lower in high latitude soil but higher in low latitude soil as compared to ambient conditions, with incomplete denitrification observed except in Louisiana S. pumilus. Overall, our findings suggest that a combined increase in temperature and Nr has the potential to reduce salt marsh greenhouse gas (GHG) sinks under future global change scenarios.

盐沼可以减轻养分污染并在土壤中储存大量“蓝碳”，但是，固碳的价值可能会被一氧化二氮（N ₂ O）排放部分抵消。全球气候和土地利用变化导致气温升高，并向沿海地区输入活性氮 (Nr)。在这里，我们研究了升高温度（环境 + 5℃）和 Nr（双倍环境浓度）对来自两个气候地区（加拿大魁北克省和美国路易斯安那州）的沼泽土壤中氮处理的综合影响，该沼泽土壤有两种植被类型：互花孢子菌（ = Spartina alterniflora) 和 Sporobolus pumilus (= Spartina patens)，采用 24 小时实验室培养实验。在所有四个沼泽地进行处理后，潜在的 N ₂ O 通量从较小的汇增加到主要源。在升高处理下一天潜在的 N ₂ O 排放（代表长期海面变暖或短期海洋热浪对沿海沼泽土壤温度的影响以及氮负荷脉冲）抵消了 15-60%潜在的年度环境 N ₂ O 汇，取决于沼泽地点和植被类型。常温处理下高纬度沼泽土壤的潜在反硝化率普遍高于低纬度沼泽土壤，其中N ₂ O:N ₂ 比例较低，表明高纬度沼泽土壤已完全反硝化。在升高温度和Nr处理下，与环境条件相比，高纬度土壤的潜在反硝化作用较低，而低纬度土壤的反硝化作用较高，除路易斯安那州短小短须草外，观察到反硝化不完全。 总体而言，我们的研究结果表明，在未来全球变化情景下，温度和 Nr 的联合增加有可能减少盐沼温室气体 (GHG) 汇。