Greenhouse gas fluxes from salt marshes exposed to chronic nutrient enrichment
We assessed the impact of nutrient additions on greenhouse gas fluxes using dark static chambers in a microtidal and a macrotidal marsh along the coast of New Brunswick, Canada approximately monthly over a year. Both were experimentally fertilized for six years with varying levels of N and P. For unfertilized, N and NPK treatments, average yearly CO 2 emissions (which represent only respiration) at the microtidal marsh (13, 19, and 28 mmoles CO 2 m -2 hr -1 , respectively) were higher than at the macrotidal marsh (12, 15, and 19 mmoles m -2 hr -1 , respectively, with a flux under the additional high N/low P treatment of 21 mmoles m -2 hr -1 ). Response of CH 4 to fertilization was more variable. At the macrotidal marsh average yearly fluxes were 1.29, 1.26, and 0.77 μmol CH 4 m -2 hr -1 with control, N, and NPK treatments, respectively and 1.21 μmol m -2 hr -1 under high N/low P treatment. At the microtidal marsh CH 4 fluxes were 0.23, 0.16, and -0.24 μmol CH 4 m -2 hr -1 in control, N, and NPK and treatments, respectively. Fertilization changed soils from sinks to sources of N 2 O. Average yearly N 2 O fluxes at the macrotidal marsh were -0.07, 0.08, and 1.70, μmol N 2 O m -2 hr -1 in control, N, NPK and treatments, respectively and 0.35 μmol m -2 hr -1 under high N/low P treatment. For the control, N, and NPK treatments at the microtidal marsh N 2 O fluxes were -0.05, 0.30, and 0.52 μmol N 2 O m -2 hr -1 , respectively. Our results indicate that N 2 O fluxes are likely to vary with the source of pollutant nutrients but emissions will be lower if N is not accompanied by an adequate supply of P (e.g., atmospheric deposition vs sewage or agricultural runoff). With chronic fertilization the global warming potential of the increased N 2 O emissions may be enough to offset the global cooling potential of the C sequestered by salt marshes.