Temporal–spatial variation and source analysis of carbon and nitrogen in a tidal wetland of Luoyuan Bay

Abstract

Carbon and nitrogen are important elements in biogeochemical studies of tidal wetlands. Three wetland zones in Luoyuan Bay in the Fujian province were chosen for this study; the Spartina alterniflora flat zone with Spartina alterniflora growing, the silt zone with no Spartina alterniflora growing and the Spartina alterniflora-silt flat zone – a transition zone between the two. The spatial and seasonal variations of total organic carbon (TOC), total nitrogen (TN), stable isotopes of organic material (δ13C, δ15N), C/N ratio, average particle size and sediment composition in surface and vertical sediments of different ecological zones were analyzed. Carbon and nitrogen accumulation and particle size effects in the different ecological zones were discussed and the indicators of δ13C and C/N ratios were also compared. TOC, TN, δ13C contents, C/N ratios, and average particle size varied within the ranges of 0.611–1.133%, 0.053–0.090%, −22.60 to −18.92‰, 12.3–15.7, and 6.4–8.7μm, respectively. Sediments were mainly silt-sized. Besides δ15N values, the other parameters, such as TOC, TN, δ13C contents, C/N ratios, and average particle size showed an obvious zonal distribution in surface sediments. The distribution of TOC and TN contents reflected the distribution of Spartina alterniflora within the bay. The profile and seasonal variations of these parameters in different ecological zones indicated that variations in the Spartina alterniflora flat and transition zones were complex because of the effect of Spartina alterniflora. Vertical and seasonal variations were sampled in the silt flat area. The profile and seasonal variations of TOC, TN and δ13C were similar in the transition zone and the Spartina alterniflora flat zone. Seasonal concentrations of TOC, TN and δ13C decreased from autumn>spring>winter>summer. The seasonal variation of carbon and nitrogen in the sediments may be influenced by temperature, particle size, plankton and benthos. The particle size effect was significant in the surface sediments and profile sediments of the transition zone. However, other factors had a greater effect on the distributions of TOC and TN in the Spartina alterniflora flat and silt flat zones. C/N ratios in sediments of the Spartina alterniflora flat, transition zone and silt flat were close to or>12, indicating that the organic material source was dominated by terrestrial inputs. However, δ13C values decreased from the Spartina alterniflora flat zone>transition zone>silt flat zone indicating that the organic material source was predominantly from marine inputs. Thus the indications from C/N ratios and δ13C were different. There was no clear relationship between C/N ratios and δ13C values and a better relationship between δ13C values and TOC concentrations suggested that δ13C values provided a better indication of the organic source. Limited amounts of organic material came from Spartina alterniflora. This study has provided basic data for researching biogeochemical processes of biogenic elements in tidal wetlands and vegetation restoration, and has also provided a reference for assessing and protecting the environment and ecological systems in wetlands.