Greenhouse gas fluxes and their determining factors in salt marsh wetlands along the south shore of Hangzhou Bay, East China Sea.

This study employed in-situ online monitoring to assess the impact of Spartina alterniflora harvesting on greenhouse gas emissions. Their fluxes and δ13C values were measured in unvegetated tidal flat, low and medium vegetation coverage areas of the salt marsh wetlands along the south shore of Hangzhou Bay about a month after harvest. The objective was to clarify fluxes changes and interactions with environmental factors. The results indicated that methane (CH4) emissions were highest in the unvegetated tidal flat area, reaching 230.27 nmol m-2·s-1, whereas carbon dioxide (CO2) emissions peaked at 2.16 μmol m-2·s-1 in the low vegetation coverage area. Increased vegetation coverage was associated with decreased CH4 emissions, however, CO2 emissions showed inconsistent trends across different levels of vegetation. During the daytime, average CH4 emissions from the unvegetated tidal flat, low and medium vegetation coverage areas were 214.00, 100.26, and 77.54 nmol m-2·s-1, respectively. In contrast, nighttime emissions from these areas were 207.18, 81.00, and 69.90 nmol m-2·s-1, respectively. The average CO2 emissions during the daytime were 2.18, 1.56, and 0.23 μmol m-2·s-1 for the same three areas, while nighttime emissions were 1.94, 2.52, and 2.88 μmol m-2·s-1, respectively. CH4 emissions were elevated during the daytime under vegetative cover, whereas CO2 emissions peaked at night. Tidal influences led to increased CH4 fluxes before and after high tide, while CO2 emissions reached their nadir during high tide. Soil exposure coupled with increased organic matter from harvested Spartina alterniflora significantly elevated CH4 emissions. However, CO2 emissions did not exhibit a significant increase compared to other regions.