Invasion chronosequence of Spartina alterniflora on methane emission and organic carbon sequestration in a coastal salt marsh

Spartina alterniflora was intentionally introduced to China in 1979 for the purpose of sediment stabilization and dike protection, and has continuously replaced native plants or invaded bare mudflats in the coastal marsh. To evaluate the spatial variation of CH4 emission and soil organic carbon (SOC) storage along the invasion chronosequence, we selected four sites including bare mudflat (Control, as first year invasion), and marshes invaded by S. alterniflora in 2002 (SA-1), 1999 (SA-2) and 1995 (SA-3), respectively, in Sheyang county, Jiangsu, China and set up the marsh mesocosm system for flux measurement. The mean accumulation rate of SOC in the 0-30 cm layer exponentially increased with the invasion time, ranging from 1.08 (over the first 9 years) to 2.35 Mg C ha−1 yr−1 (over the period of 12-16 years). The cumulative CH4 emission during the growth season was 20.5, 75.4, 81.0 and 92.2 kg CH4 ha−1 in Control, SA-1, SA-2 and SA-3, respectively, and there was a binomial relationship between CH4 emission and invasion time. Cumulative CH4 emission was logarithmically increased with SOC concentration; however the ratio of CH4 emission to SOC concentration was inversely correlated with the invasion time in the S. alterniflora marsh, suggesting that the less increased SOC in the S. alterniflora marsh was converted into CH4. The net global warming potential (GWP) was estimated at 733 kg CO2-eq ha−1 yr−1 in the tidal mudflat and reduced to −1273 (SA-1) to −2233 kg CO2-eq ha−1 yr−1 (SA-3) in the S. alterniflora marsh. Our results indicated that S. alterniflora invasion effectively sequestrated atmospheric CO2 and mitigated GWP in the coastal marsh of China.