| Since the industrial revolution, the rising of atmospheric greenhouse gases (GHG) has led to climate change and resulted in significant impacts on the global environment. Coastal wetlands are located in the transition zone between land and ocean, the frequent material exchange among these adjacent ecosystems determined that the coastal wetlands are the important regulator in carbon and nitrogen cycling, they are efficient in absorbing atmosphere GHG (primarily in CO2) through the photosynthesis of plants and can sequestered it in biomass and sediment over long time scales. The characteristics of high productivity, low decomposition and sediment accretion of the coastal wetlands enabled themselves as the efficient sinks for carbon and nitrogen, being supposed to the effective way in mitigating the rising atmospheric GHG concentration. Recently, some researchers propose the 'blue carbon'strategy in the case of climate change, which utilizes the high capacity of carbon sequestration and explores the potentiality of coastal wetlands to mitigate the climate change influences. Therefore, the carbon and nitrogen cycle of coastal wetlands attract more and more attention.Taking the Chongming Dongtan wetlands in the Yangtze estuary, China as a case study. Based on the interpretation of remote sensing data, the spatio-temporal distribution (during the last 20 years) of the main vegetation types that influenced by human activities and invasion of exotic species were analyzed. Then, three typical transects and sampling sites were set up on the basis of vegetation distribution status which obtained from integrated data of remote sensing data interpretation and field investigation. By the periodic measurement of sediment accretion/erosion and the collection and analysis of exogenous sediment samples, the monthly dynamics of sedimentation and TOC (Total organic carbon, TOC) and TN (Total nitrogen, TN) stocks in the exogenous sediment were determined at Dongtan wetlands during 2013. Through the seasonal sampling and analysis of biomass and sediment (0-50 cm) samples, the dynamics of biomass, TOC and TN stocks in the biomass, SOC (Sediment TOC, SOC) and TN stocks in the sediment (0-50 cm) were determined in the Dongtan wetlands during 2013. Based on these data, the interactive effect between vegetation and sedimentation on SOC and TN stocks in the sediment (0-50 cm) were analyzed. The main results of the present study are summerised as follows:1. The spatio-temporal dynamic of the main saltmarsh vegetationThe main saltmarsh vegetation types are P. australis, S. alterniflora and S. mariqueter in Chongming Dongtan wetlands, and presented regularly distribution along the elevation gradient. During the last 20 years, the total vegetation area was firstly decreased and then increased under the combined effects of sedimentation, introduction of exotic species and human activities (reclamation). The species of S. alterniflora was introduced in the late 1990s, and it has rapidly expanded due to the broader niche and stronger competitive capacity, resulting in profound impacts on the plant community types and structures. To 2011, the total saltmarh vegetation area of Dongtan wetlands was 5079.2 hm2, including 1684.2 hm2 of P. australis community, 1487.3 hm2 of S. alterniflora community and 1907.7 hm2 of S. mariqueter community.2. The dynamic of biomass and TOC and TN stocks in biomass in the main saltmarsh vegetationIn the P. australis, S. alterniflora and S. mariqueter communities, the biomass dynamic showed a similar trend among each other. The aboveground biomass was the lowest in spring and rapidly increased in summer and autumn, and generally reached the maximum in autumn while decreased in winter. Whereas, the belowground biomass constantly increased through 2013 and reached the maximum in winter. The per unit area biomass (aboveground and belowground) were in the order of S. alterniflora community (55.1±5.7 t-hm-2)>P. australis community (49.3±3.8 t·hm-2) >S. mariqueter community (11.7±0.8 t·hm-2). In 0-50 cm sediment profiles, the belowground biomass distribution pattern of three main vegetation types all showed a trend that firstly increased and then decreased, and the biomass increment in P. australis and S. alterniflora communities were higher than that in S. mariqueter community. In 2013, the total biomass of P. australis, S. alterniflora and S. mariqueter communities in the study area of Chongming Dongtan were estimated to (83.0±6.4) 103t, (81.9±8.5) 103t and (22.3±1.5) 103t, respectively.The TOC and TN stocks in biomass, annual TOC and TN increment and distribution pattern for P. australis, S. alterniflora and S. mariqueter communities were similar to the biomass dynamics. The per unit area of biomass TOC and TN stocks in P. australis, S. alterniflora and S. mariqueter communities were 17.3±3.9t·hm2 and 0.4±0.04t-hm-2,26.1±1.8t-hm-2 and 0.6±0.04t·hm-2,4.6±1.5t·hm-2 and 0.1±0.01t·hm-2, respectively. In 2013, the TOC and TN stocks in the biomass of P. australis, S. alterniflora and S. mariqueter community in the study area of Chongming Dongtan were estimated to (29.1±6.6) 103t and (6.7±0.7) 102t, (38.8±2.7) 103t and (8.9±0.6) 102t, (8.8±0.3) 103t and (1.9±0.2) 102t, respectively.3. The dynamic of sedimentation and TOC and TN stocks in surface exogenous sedimentDuring 2013, the one-year measurement revealed an accretion situation occurred in the northern and middle transects and high marsh (P. australis community) in the southern transect, while erosion happened in the low-lying marsh (S. mariqueter community) and bare mudflat in the southern transect. The high marsh dominated by P. australis and S. alterniflora communities had relatively stable sedimentary rate in comparison with S. mariqueter marsh and bare mudflat. The per unit area surface exogenous sediment TOC and TN stocks in P. australis, S. alterniflora, S. mariqueter community and bare mudflat region were (2.4±1.9) 10-2 t·hm-2 and (0.4±0.4) 10-2 t·hm-2, (6.4±3.2) 10-2 t·hm-2 and (0.8±0.4) 10-2 t·hm-2, (2.5±0.8) 10-2 t·hm-2 and (0.4±0.1) 10-2 t·hm-2, (2.0±1.5) 10-2 t·hm-2 and (0.3±0.1) 10-2 t±hm-2, respectively.4. The dynamic of SOC and TN stocks in sediment (0-50 cm)In the P. australis, S. alterniflora and S. mariqueter marsh and bare mudflat, the SOC stock all presented similar seasonal dynamics in 2013, showing an increasing trend from spring to winter. While the sediment TN stock was seasonally fluctuated, decreasing from spring to summer and increasing in autumn and winter. The per unit area SOC and TN stocks both were in the order of P. australis marsh (SOC:30.3±3.4 t·hm-2; TN:3.1±0.3 t-hm-2)>S. alterniflora marsh (SOC:22.6±1.9 t·hm-2; TN:3±0.2 t·hm-2)>S. mariqueter marsh (SOC:9.3±0.7 t·hm-2; TN:2.1±0.1 t·hm-2)>bare mudflat (SOC:2.6±0.1 t·hm-2; TN:0.9±0.1 t·hm-2).In 2013, the SOC and soil TN stocks at the P. australis, S. alterniflora and mariqueter marshes in the study area of Chongming Dongtan were estimated to (51.0±5.2) 103 t and (5.2±0.5) 103 t, (33.6±2.8) 103 t and (4.8±0.3) 103 t, and (17.7±1.3) 103t and (4.0±0.2) 103t, respectively.5. Interactive effect between vegetation and sedimentation on SOC and TN stocks (0-50 cm) in sedimentAnalysis based on the above results indicated that the SOC and TN stocks in sediment (0-50 cm) were influenced by the interactive effect between saltmarsh vegetation and sedimentation at Chongming Dongtan wetland, especially in 0-10 cm sediment layer (p<0.05). Vegetation was the dominant regulator in 0-50 cm SOC dynamic, reaching significant level in each sediment layer (p<0.01). But, this significant level did not appeared in sediment TN dynamic. The TOC and TN stocks was very low in surface exogenous sediment, and the significant influence of surface exogenous sediment on SOC and TN dynamic only appeared in 0-5 cm sediment layer.Based on the analysis of tidal flat development and vegetation succession, the tidal flat of Chongming Dongtan wetlands was expanded and elevated by the deposition of large quantity of sediments which were loaded by Yangtze River, and this process provided suitable habitat for vegetation succession. During the expansion of saltmarsh vegetation, the biomass was continuously buried by sediment in the form of litter and exudates, which promoted the accumulation of sediment organic carbon and total nitrogen. |
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