Changes in carbon storage and macrobenthic communities in a mangrove-seagrass ecosystem after the invasion of smooth cordgrass in southern China.

The exotic smooth cordgrass (Spartina alterniflora) has invaded intertidal wetlands near the Ronggenshan tidal flats of Beihai, Guangxi, China, where historically seagrasses and mangroves coexisted. We investigated sediment organic carbon (SOC) storage and macrobenthic community structure in the existing mangroves (MG), S. alterniflora (SA), seagrass bed (SG), and unvegetated flat (UnV) habitats following the S. alterniflora invasion. SOC storage increased after S. alternifolia invasion in the SG and UnV habitats. Spartina alterniflora invasion changed the dominant species of the macrobenthos in the original habitats and reduced the diversity of macrobenthos in SG and UnV habitats. Clearly, S. alternifolia invasion can change the ecological functioning of south China's coastal ecosystems by altering carbon sequestration and affecting biodiversity.

Su Z ，Qiu G ，Fan H ，Li M ，Fang C ... -  《-》

Spartina alterniflora invasion controls organic carbon stocks in coastal marsh and mangrove soils across tropics and subtropics.

Coastal wetlands are among the most productive ecosystems and store large amounts of organic carbon (C)-the so termed "blue carbon." However, wetlands in the tropics and subtropics have been invaded by smooth cordgrass (Spartina alterniflora) affecting storage of blue C. To understand how S. alterniflora affects soil organic carbon (SOC) stocks, sources, stability, and their spatial distribution, we sampled soils along a 2500 km coastal transect encompassing tropical to subtropical climate zones. This included 216 samplings within three coastal wetland types: a marsh (Phragmites australis) and two mangroves (Kandelia candel and Avicennia marina). Using δ13 C, C:nitrogen (N) ratios, and lignin biomarker composition, we traced changes in the sources, stability, and storage of SOC in response to S. alterniflora invasion. The contribution of S. alterniflora-derived C up to 40 cm accounts for 5.6%, 23%, and 12% in the P. australis, K. candel, and A. marina communities, respectively, with a corresponding change in SOC storage of +3.5, -14, and -3.9 t C ha-1 . SOC storage did not follow the trend in aboveground biomass from the native to invasive species, or with vegetation types and invasion duration (7-15 years). SOC storage decreased with increasing mean annual precipitation (1000-1900 mm) and temperature (15.3-23.4℃). Edaphic variables in P. australis marshes remained stable after S. alterniflora invasion, and hence, their effects on SOC content were absent. In mangrove wetlands, however, electrical conductivity, total N and phosphorus, pH, and active silicon were the main factors controlling SOC stocks. Mangrove wetlands were most strongly impacted by S. alterniflora invasion and efforts are needed to focus on restoring native vegetation. By understanding the mechanisms and consequences of invasion by S. alterniflora, changes in blue C sequestration can be predicted to optimize storage can be developed.

Xia S ，Wang W ，Song Z ，Kuzyakov Y ，Guo L ，Van Zwieten L ，Li Q ，Hartley IP ，Yang Y ，Wang Y ，Andrew Quine T ，Liu C ，Wang H ... -  《-》

Seagrass beds store less carbon but support more macrobenthos than mangrove forests.

Seagrass beds (SG) have powerful ecosystem functions, but some seagrasses, e.g., Halophila beccarii, have been neglected because of their tiny plant structure and narrow and restricted distribution in the intertidal zone. In this study, we compared ecosystem structure and function between H. beccarii SG and adjacent coastal habitats through: (i) vegetation composition, (ii) carbon storage, and (iii) macrobenthic community. The vegetation carbon storage was only 0.1% of that in mangrove forests (MG). The sediment carbon storage (0-60 cm) of SG (67.4 ± 20.2 MgCorg·ha-1) was 74.31% of MG (90.7 ± 12.9 MgCorg·ha-1) and 13.51% higher than unvegetated flats (UnV) (55.1 ± 17.7 MgCorg·ha-1). The macrobenthic abundance, biomass and secondary productivity in SG were significantly higher than those in MG, indicating that SG support more macrobenthos than MG. However, SG experienced more disturbances than MG and UnV. Therefore, it is necessary to strengthen the protection of H. beccarii SG to preserve their essential functions and services.

Su Z ，Qiu G ，Fan H ，Fang C ... -  《-》

Effects of short-term invasion of Spartina alterniflora and the subsequent restoration of native mangroves on the soil organic carbon, nitrogen and phosphorus stock.

The exotic cordgrass Spartina alterniflora has severely invaded the mangrove wetlands in southern China and ecological restoration using native mangroves was conducted in an attempt to control this invasive species. In this study, the contents and pools of soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP) were quantified to investigate the invasive effects of S. alterniflora and then to evaluate whether the ecological restoration of native mangrove could reverse those effects. S. alterniflora only showed significantly higher organic carbon content in the surface 0-10 cm of soil than in the uninvaded mudflat. The high δ13C values in the surface soil of the invaded habitat demonstrated that S. alterniflora contributed 42.6-62.2% of the organic carbon. The SOC for invasive S. alterniflora and newly restored mangroves (4 years and 14 years) was not enhanced in comparison to the unvegetated mudflat. S. alterniflora significantly increased the surface soil TN content, but decreased the available phosphorus content and TP density. The TN densities increased gradually with the mangrove restoration, while the TP densities were only slightly influenced. The results suggested that short-term invasion of S. alterniflora and subsequent mangrove restoration did not alter SOC or TN pool sizes, but S. alterniflora was shown to affect the potential carbon storage capacity produced by the mangroves in the Zhangjiang Estuary.

Feng J ，Zhou J ，Wang L ，Cui X ，Ning C ，Wu H ，Zhu X ，Lin G ... -  《-》

Carbon sequestration potential of transplanted mangroves and exotic saltmarsh plants in the sediments of subtropical wetlands.

Coastal blue carbon ecosystems offer promising benefits for both climate change mitigation and adaptation. While there have been widespread efforts to transplant mangroves from the tropics to the subtropics and to introduce exotic saltmarsh plants like Spartina alterniflora in China, few studies have thoroughly quantified the chronological records of carbon sequestration with different organic carbon (OC) sources. To understand how variations in OC sources can affect the carbon sequestration potential of coastal wetland environment over time, we conducted a study on typical islands with two scenarios: S. alterniflora invasion and mangrove transplantation. Our study determined chronological records of carbon sequestration and storage from five sediment profiles and traced changes in the OC sources using carbon stable isotope (δ13C) and C:N ratios in response to these scenarios. The S. alterniflora invasion resulted in an 84 ± 19 % increase in the OC burial rate compared to unvegetated mudflats, while mangrove transplantation resulted in a 167 ± 74 % increase in the OC burial rate compared to unvegetated mudflats. S. alterniflora and mangroves showed greater carbon sequestration potential in areas with high supplies of suspended particulate matter, while mangroves needed to grow to a certain scale to display obvious carbon sequestration benefits. In the mangrove saltmarsh ecotone, mature mangrove habitats exhibited resistance to the S. alterniflora invasion, while mangrove transplantation in the environment invaded by S. alterniflora had a significant effect on OC contribution. Besides, plant-derived OC can be exported to the surrounding environment due to the rapid turnover of sediments. The blue carbon chronosequence-based estimation of OC sources and burial rates provides a useful reference for establishing carbon accounting policies.

Huang R ，He J ，Wang N ，Christakos G ，Gu J ，Song L ，Luo J ，Agusti S ，Duarte CM ，Wu J ... -  《-》