Studies On Distribution Patterns Of And Controlling Factors For Soil Carbon Pools Of Selected Mangrove Wetlands In China

Mangrove forest is an important type of coastal wetland.Due to their high productivity and sedimentation rate,mangrove wetlands have a very high marine carbon sink capacity,and are key ecosystems for blue carbon sink.Generally,mangrove wetlands have vegetation and soil?sediment?carbon stocks.Besides the mangrove source,algae and Spartina alterniflora could be other potential sources for mangrove soil carbon.Although many previous studies have assessed the stocks of soil organic carbon?SOC?of individual mangrove forests in China,no study has comprehensively investigated mangrove SOC and quantitatively estimated the carbon sources.Moreover,there is still an issue around the controlling mechanisms for the effects of algal bloom and S.alterniflora invasions on mangrove soil carbon stocks.In this study,the mangrove wetlands in the southeastern coasts of China?Yunxiao in Fujian Province,Gaoqiao and Futian in Guangdong Province,Dongzhaigang and Wenchang in Hainan Province?were selected for the investigation.Comparisons were made on soil physicochemical properties,soil organic carbon densities,stable carbon isotopic values(?¹³C),relative contributions of possible carbon sources to mangrove soil carbon,and diversity of algae communities,in order to explore the distribution patterns of SOC pools as influenced by latitude,mangrove communities,and soil depth.The results from this study may provide new insights into effective mechanisms against S.alterniflora invasions and algal blooms on mangrove soil carbon stocks.The main results and conclusions are listed below:?1?Mangrove SOC contents varied with sampling sites,mangrove communities,and depth of sediments.The SOC contents in the four sampling sites are?1.19±0.06?%,?2.52±0.11?%,?0.81±0.10?%and?4.78±0.35?%for Yunxiao,Gaoqiao,Dongzhaigang and Wenchang,respectively.Apart from Dongzhaigang,the values in the other three sites were generally decreasing with latitude.The SOC contents in the Bruguiera sexangula and Heritiera littoralis communities were relatively higher than in other communities.The SOC contents of the top soil fraction in the four sampling sites showed higher values than those of the deeper fractions.?2?The soil carbon stocks of the four mangrove sites varied with location,and the variation trends were similar to those of the SOC contents.The total carbon stock density in Wenchang??421.8±66.0?Mg C/ha?was higher than in the other three mangrove sites??260.3±28.0?Mg C/ha in Gaoqiao,?95.9±11.1?Mg C/ha in Yunxiao,and?84.8±2.6?Mg C/ha in Dongzhaigang?.The regression analysis results revealed that soil p H,salinity,bulk density,and moisture were significantly correlated with the SOC contents in all four sites.?3?The?¹³C values of soil profiles at different sites showed distinct increases with soil depth,and a decreased tendency from north to south with average values of?–25.37±0.52?‰in Yunxiao,?-26.68±0.38?‰in Gaoqiao,?-24.87±1.76?‰in Dongzhaigang,and?-26.29±1.02?‰in Wenchang.The analysis of soil carbon sources indicated that the major sources of SOC were all from mangrove plants?61.56-79.92%?.In Yunxiao and Dongzhaigang,the contribution values from mangroves were lower?61.56%and 65.88%,respectively?,while the algae contribution values were higher?24.86%and 24.38%,respectively?,as compared to the other two sites,which could be attributed to the biological invasion of S.alterniflora and algal blooms caused by water eutrophication,due to the heavy aquaculture in these two mangrove wetlands.?4?Diatoms are dominant both in the planktonic and benthic algae communities in mangrove areas.It was found that the high species diversity of algae communities might have an effect on the quantity and source of carbon stocks in mangrove nature reserves in Futian,Guangdong.The value of benthic microalgae biomass,carbon content and carbon contribution to SOC in Yuanxiao was higher than that in Gaoqiao,which was coupled with the carbon contribution patterns of algae to soil organic carbon resulting from carbon sources analysis.Our results suggested that changes in the composition of microalgal species and biomass could influence the carbon content in the soil organic carbon in mangrove wetlands,especially at the surface level.The above results from this study provide new insights into the distribution patterns,formation process and controlling mechanisms of mangrove carbon stocks,which have significant implications for mangrove wetland conservation and blue carbon sink management in China.