Impact of permanent inundation on methane emissions from a Spartina alterniflora coastal salt marsh

Abstract

To understand the effect of water level on CH 4 emissions from an invasive Spartina alterniflora coastal brackish marsh, we measured CH 4 emissions from intermittently and permanently (5 cm water depth) inundated mesocosms with or without N fertilizer added at a rate of 2.7 g N m −2. Dissolved CH 4 concentrations in porewater and vertically-profiled sediment redox potential were measured, as were aboveground biomass and stem density of S. alterniflora. Mean CH 4 fluxes during the growing season in permanently inundated mesocosms without and with N fertilizer were 1.03 and 1.73 mg CH 4 m −2 h −1, respectively, which were significantly higher than in the intermittently inundated mesocosms. This response indicates that prolonged submergence of sediment, up to a water depth of 5 cm, stimulated CH 4 release. Inundation did not greatly affect aboveground biomass and stem density, but did significantly reduce redox potential in sediment, which in turn stimulated CH 4 production and increased the CH 4 concentration of porewater, resulting in higher CH 4 emission in the mesocosm. Our data showed that the stimulatory effect of shallow, permanent inundation on CH 4 emission in S. alterniflora marsh sediment was due primarily to an improved methanogenic environment rather than an increase in plant-derived substrates and/or the number of gas emission pathways through the plant’s aerenchymal system.