Geochemistry Character Of Carbon (Nitrogen, Phosphorus, Silicon) In Lakes In West Inner Mongolia Plateau

Carbon (C) shares with nitrogen (N) and phosphorus (P) the characteristic of being the major chemical elements in the global biosphere, and the photosynthetic fixation of C by phytoplankton is accompanied by the uptake of major chemical elements such as N and P for diatoms. So the chemical, geological, biological, and physical processes of carbon are important to many fundamental geochemical processes. Interest in the biogeochemical cycles of important elements like C, N, P and trace elements has been heightened by issues associated with global, regional and local environmental problems caused by increased fluxes of C, N and P compounds arising from anthropogenic activities. Due to concern associated with the increase of atmospheric CO₂ concentrations and a future enhanced greenhouse effect, numerous studies have focused on the global carbon cycling. This increase has led researchers worldwide to examine the sources and sinks of carbon and to calculate carbon balances. Apart from the binding of carbon in peat lands, mineral soils and the tree layer vegetation, it is also present in large quantities in lake sediments, which constitute one of the few permanent sinks in the global carbon cycle.Therefore, this study was carried out on the Wuliangsuhai Lake (WLSH) and the Daihai Lake (DH) based on their representativeness on the mechanism and characteristic of eutrophication, and on the different mechanism of carbon form distribution in the water/sediment system. Based on the theories and methods of environmental geochemistry, biogeochemistry, pollution ecology and limnology, the form distribution, the geochemistry characters and the contribution to carbon cycling of carbon in the Wuliangsuhai Lake and the Daihai Lake are researched in this paper.To say concretely, the patter of horizontal distribution of dissolved inorganic carbon (DIC), total inorganic carbon (TIC), dissolved organic carbon (DOC) and total organic carbon (TOC) in overlying water and the form distribution of inorganic carbon (IC) in sediment of the two lakes are studied, which includes the impacts of environmental factors on the form distribution of inorganic carbon, the pulling function to the carbon cycling of the lake ecosystem by the eutrophication status and types. Meanwhile, the form transformation of IC and their contribution to carbon cycling in sediment, and the DIC fluxes across water-sediment interface in the two lakes are also discussed. In addition, based on accurate interpretation of the information of IC form distribution, allochthonous organic carbon (OC) and BSi archived in sediment cores, the palaeoenvironment and palaeoclimate of the two lakes and their catchments basins were well reconstructed, which may provide valuable database for the studies of environmental geochemical cycle and function transformation of source/sink of carbon in lakes sediment. The results are described as follows:1. Based on the calculation of fluxes of DIC across the water-sediment interface, there about 2.64×10⁴t DIC is released from sediment to overlying water in WLSH in summer (90 days); there about 1.70×10⁴t DIC is released from sediment to overlying water in DH's shoal water zone, while there about 2.00×10⁴t DIC transfers into sediment from overlying water across the water-sediment interface in DH's deepwater area, thus there about 0.30×10⁴t DIC transfers into sediment in DH.2. For the first time, based on the accurate interpretation of the information of allochthonous OC and NH₂OH-HC1 form IC archived in sediment cores, the palaeoenvironment and palaeoclimate of the Daihai Lake and its catchment basin in recent 250 and 550 years are well reconstructed respectively. So the research does some significative works in rinding new effective indexes of palaeoenvironment and palaeoclimate studies.3. Because of the impacts of lake types especially eutrophication level and character, the key factors affecting the form distribution of IC are TOC, TN, Org-N, NH₄⁺-N and NO₃^--N in WLSH sediment, while TOC, TN, Org-N, water ratio, BSi, TP and inorganic phosphorus (IP) in DH sediment.The content of IC from biogenic origin and organic matter decomposition controlled by TOC, TN and Org-N, the pulling function to carbon cycling by coupling effects of C-N-P-Si and the exchange function of pore water are the key mechanisms affecting the forms distribution in sediments and the fluxes and processes of IC across the water-sediment interface.The nitrification and denitrification in sediment of shallow lake, the eutrophication character, the disturbance intension of water-sediment interface, the primary conductivity and the mass action law also play important roles in affecting the forms distribution in sediments and the fluxes and processes of IC across the water-sediment interface.4. The contributions to carbon cycling of IC forms rank as: NaCl form > NaOH form and NH₃-H₂O form > NH₂OH-HC1 form > HC1 form.Further, at a certain extent, the HCl form of IC also contributes to carbon cycling of IC, which is related to the rhizosphere environment of submerged macrophyte and emergent plant in WLSH; while the HCl form of IC does not contribute to carbon cycling of IC because of DH's sediment lacking of impacts by aquatic organisms.5. The dominant forms of IC in sediment from WLSH and DH are NH₂OHHCl form and HCl form indicating that calcite and aragonite, which are the main source of IC, are the dominant carbonate minerals in the two lake sediment.The differences of NaCl form, NH₃·H₂O form and NaOH form IC between WLSH and DH sediment reveal that form distribution of the three IC forms are the key controlled factors of the fluxes and processes of carbon across the water-sediment interface, which further affect the function transformation of sediment as carbon source/sink. Meanwhile, the difference of vertical distribution of the three IC forms of DH indicate that the sediment in different lake areas play different role in functioning as carbon source/sink. 6. The DIC and TIC distributions in overlying water show that the contents of DIC and TIC in DH are higher markedly than those in WLSH, which mainly results from the photosynthetic fixation of DIC and TIC by submerged macrophyte of WLSH, stronger chemical weathering, more IC input by surface runoff and the enclosed character of DH and its catchment basin.The correlation between NH₄⁺-N, NO₃^--N, DTP, DRP, SiO₃^-2 and TIC, DIC, TOC, DOC indicate that the photosynthesis and respiration of phytoplankton in DH overlying water, which is related to the bio-geochemical cycling of carbon, is controlled by NH₄⁺-N, DTP, DRP and SiO₃^2- revealing the pulling function to carbon cycling by eutrophication level.7. The concentration levels of SiO₃^2- and BSi in the overlying water and surface sediment reflect the difference of eutrophication character between the two lakes. Further, based on the vertical distribution of BSi, TN and TOC in the sediment cores, the past eutrophication trends of WLSH and DH is well reconstructed.