We present the carbon isotopic composition of the total organic carbon(TOC) and fine roots in the sedimentary profile from the underground ancient forest in Sihui to study the climatic and environmental changes from 4.5 ka BP to 0.6 ka BP.Results show that C3 plant was the main vegetation from 4.5 ka BP to 0.6 ka BP in this region.The ancient forest began to develop in the wetland at around 4 ka BP and disappeared together with the wetland at about 3.0 ka BP,implying that the climate had changed greatly at around 3.0 ka BP.As indicated by the simulation results,the content of atmospheric CO2 increased slightly during 3.5 ka BP and 3.0 ka BP,implying climate warming during that period.The interval of radiocarbon age between 3.0 ka BP and 1.2 ka BP was possibly caused by the strong erosion when the block was lifted in the neotectonic movement.From 1.2 ka BP to 0.6 ka BP,the region remained in terrestrial sedimentary environment,and the surface plant biomass declined gradually.Drought caused by the climate change was the likely cause for the disappearance of the ancient forest.South transition of Intertropical Convergence Zone(ITCZ) was probably the main mechanism for the climate change.
Ping Ding ChengDe Shen Ning Wang WeiXi Yi KeXin Liu XingFang DingDongPo Fu
Carbon isotopic compositions of soil CO2 in rainy season (July) from two natural soil profiles (DHLS & DHS) in the monsoon evergreen broadleaf forest in the Dinghushan Biosphere Reservoir (DBR),South China,are presented.Turnover and origins of soil CO2 are preliminarily discussed in this paper.Results show that the content of soil CO2 varies between 6120 and 18718 ppmv,and increases with increasing depth until 75 cm,and then it declines.In DHLS,soil CO2 δ 13C ranges from -24.71‰ to -24.03‰,showing a significant inverse correlation (R2=0.91) with the soil CO2 content in the same layer.According to a model related to soil CO2 δ 13C,the soil CO2 is mainly derived from the root respiration (>80%) in DHLS.While in DHS,where soil CO2 δ 13C ranges from -25.19‰ to -22.82‰,soil CO2 is primarily originated from the decomposition of organic matter (51%–94%),excluding the surface layer (20 cm,90%).Radiocarbon data suggest that the carbon in soil CO2 is modern carbon in both DHLS and DHS.Differences in 14C ages between the "oldest" and "youngest" soil CO2 in DHLS and DHS are 8 months and 14 months,respectively,indicating that soil CO2 in DHLS has a faster turnover rate than that in DHS.The Δ14C values of soil CO2,which range between 100.0‰ and 107.2‰ and between 102.5‰ and 112.1‰ in DHLS and DHS,respectively,are obviously higher than those of current atmospheric CO2 and SOC in the same layer,suggesting that soil CO2 is likely an important reservoir for Bomb-14C in the atmosphere.
High-resolution sampling,measurements of organic carbon contents and 14C signatures of selected four soil profiles in the Haibei Station situated on the northeast Tibetan Plateau,and application of 14C tracing technology were conducted in an attempt to investigate the turnover times of soil organic car-bon and the soil-CO2 flux in the alpine meadow ecosystem. The results show that the organic carbon stored in the soils varies from 22.12×104 kg C hm-2 to 30.75×104 kg C hm-2 in the alpine meadow eco-systems,with an average of 26.86×104 kg C hm-2. Turnover times of organic carbon pools increase with depth from 45 a to 73 a in the surface soil horizon to hundreds of years or millennia or even longer at the deep soil horizons in the alpine meadow ecosystems. The soil-CO2 flux ranges from 103.24 g C m-2 a-1 to 254.93 gC m-2 a-1,with an average of 191.23 g C m-2 a-1. The CO2 efflux produced from microbial decomposition of organic matter varies from 73.3 g C m-2 a-1 to 181 g C m-2 a-1. More than 30% of total soil organic carbon resides in the active carbon pool and 72.8%―81.23% of total CO2 emitted from or-ganic matter decomposition results from the topsoil horizon (from 0 cm to 10 cm) for the Kobresia meadow. Responding to global warming,the storage,volume of flow and fate of the soil organic carbon in the alpine meadow ecosystem of the Tibetan Plateau will be changed,which needs further research.
TAO Zhen1,2,SHEN ChengDe2,GAO QuanZhou1,SUN YanMin2,YI WeiXi2 & LI YingNian3 1 School of Geography and Planning,Sun Yat-sen University,Guangzhou 510275,China
The history of natural fire and its rela- tionship with climate and vegetation are revealed from the content of elemental carbon and associated pollen data and paleoclimatic substitutive indicators for the loess of Lingtai Section in the last 370 ka BP. The study indicates that intense episodes of vegeta- tion fires occurred during the interim especially when the climate was changing from wet to drought. The average content of elemental carbon in the intergla- ciers is higher than that in the glaciers, which coin- cides with the biomass change locally (or globally). The content of elemental carbon increases in the stage around 130 ka BP, indicating that the vegeta- tion and climate pattern have changed, which may contribute to the variation of CO2. As a whole, the content of elemental carbon increasing with the time reflects the increasing aridity trend to some degree. In addition, the occurrence of the maximum peak and the highest average content of elemental carbon in the Holocene reflects the occurrence of a rapid cli- mate event in 5900 a BP and more frequent fires caused by anthropic activities.
