The Chemical Weathering And Carbon Cycles Of Different Small Watersheds In Southwest China

The link between chemical weathering and the global carbon cycle and climate change is complicated by the combined influence of various factors such as geology,climate,and human activities.Therefore,the estimation of chemical weathering rates and the study of their controlling factors have attracted widespread attention from scholars worldwide.Sources of various forms of carbon(dissolved inorganic carbon(DIC),dissolved organic carbon(DOC),and particulate organic carbon(POC))in the basin and their migration and transformation are important parts of the carbon cycle,and their dynamics are influenced by many factors,such as watershed environment and climate.At present,domestic and foreign scholars have done a lot of research on large rivers,and the migration and transformation of carbon in large rivers are controlled by multiple factors,and their biogeochemical processes cannot be accurately distinguished.Therefore,it is urgent to explore these coupling mechanisms based on the research of small watersheds.In order to explore the chemical weathering rate and their influencing factors,sources of various carbons and their migration and transformation,and controlling factors in different small basins,river water samples were collected in the three small watersheds where the area of carbonate rock and silicate rock is quite different of the Yinjiang River basin,Shiqian River basin,and Yuqing River basin in the southwestern China from September 2018 to August 2019.Temporal and spatial sequences of river water samples were analyzed for hydrochemical characteristics,various forms of carbon(DIC,DOC,POC)concentrations and their stable isotopes(¹³C)and radioisotopes(¹⁴C).The results show that the average total dissolved solids(TDS)of river water is more than twice that of the world river average value(100 mg/L).The most abundant anions and cations in the basins are HCO₃^-and Ca²⁺,respectively,indicating that carbonate weathering in these basins plays a leading role in river water chemistry.The forward model was used to estimate the contribution of different endmembers(atmosphere,anthropogenic activities,silicate rock and carbonate rock)to the total dissolved cations in the river.The contributions of carbonate rocks in the tributaries show significant spatial variations(55.0%-93.9%),which is the result of the distribution of carbonate rocks.There are negative related power-law function relationships between most solutes and discharges for the time series samples in the Yinjiang River and Shiqian River,and the different power-law function relationships of different solutes reflect the different biogeochemical behaviours of each solute.The weathering rate of carbonate rocks in Yinjiang River,Shiqian River and Yuqing River is significantly higher than that of silicate.The results of the study show that the spatial differences in the weathering of the watershed indicated the important control effect of lithology on chemical weathering rate.Solutes fluxes simulated by LOADEST showed that DIC were the the highest solute flux of main solutes fluxes export from Yinjiang River and Shiqian River,followed by Ca²⁺solute flux.The solute flux is directly related to the solute concentration and discharge,indicating that the lithology and hydrological conditions of the basin itself play an important role in controlling the solutes fluxes of the river.The spatial variation of DIC content was affected by the lithology,human activities,temperature,and slope of the river basin,among which the basin lithologic influence is the dominant factor.While?¹³C_DIC is not significantly affected by lithology,and the outgassing effect of CO₂ and photosynthesis can significantly affect the?¹³C_DIC values of river water.?¹³C and?¹⁴C dual isotopes analysis suggested that phytoplankton and soil organic matter derived from C₃ plants were the main sources of DOC.The content of riverine DOC was affected by human activities,temperature,slope,and photosynthesis of algae in the watershed.Old DOC of some samples was affected by old POC decomposition and transformation in organic-rich lithological strata of rivers.C/N ratios and?¹³C_POC values indicated that photosynthesis of algae in rivers was an important source of POC.The increases of water retention time in river caused by damming and urbanization affect the trophic level of rivers,which may be one of the reasons for the strong photosynthesis in rivers.Both POC and TSM contents were affected by the slope,indicating the effect of soil erosion controlled by topographical factors on riverine material and carbon transport.Physical erosion was the most important controlling factor for POC transport in rivers.When the erosion was enhanced,the POC input in rivers mainly came from plant litter or organic-rich topsoil and soil-derived sediments.The results of the study show that the controlled factors of the riverine DIC,DOC and POC contents are not consistent with the controlled factors of their isotopes.In addition to the influence of terrestrial sources,isotopes are also obviously affected by multiple biogeochemical processes in the river.The exports and behaviors of DIC,DOC,and POC in rivers were closely related to climatic conditions.The DIC content in the Yinjiang and Shiqian rivers showed dilution effects that decreased with increasing discharge,but the DOC concentration may be affected by soil DOC caused by land scouring as a result of rainfall,without showing dilution effects.Rainfall can increase the physical erosion rate of basins,which caused the POC content of the Yinjiang and Shiqian rivers to increase with increasing discharge.DIC dominated the carbon output from rivers,followed by DOC,with the lowest POC.The DIC fluxes of the Yinjiang River and the Shiqian River were 14.92and 11.93 t C/km²/yr,which are much higher than the global average,highlighting the important controlling effect of lithology on DIC fluxes.The DOC fluxes of the Yinjiang and Shiqian rivers were 0.85 and 0.64 t C/km²/yr,respectively,and the POC fluxes were0.83 and 0.62 t C/km²/yr.The close relationships between carbon fluxes and discharges indicate the importance of high temporal resolution sampling,especially during the seasons when rainfall is concentrated.Overall,the research indicates that the geological and topographic background,climate and hydrological conditions in the study area are important factors influencing the chemical weathering and carbon dynamics in the basins.At the same time,the internal processes of the rivers have also greatly changed the sources and transport of carbons.Therefore,we should pay attention to the influence of these conditions when discussing regional/global chemical weathering and carbon cycle in the basins.