| The atmospheric CO2 concentration is a primary driver of global climate change.Many researchers have done lots of studies on the natural processes of carbon cycle,especially on the rock weathering.Since the industrial revolution,human activities have been dramatically affecting all aspects of the natural environment.The most direct impact on the atmospheric carbon pool is a rapid anthropogenic injection of CO2 in a short time.However,the importance and different processes of human activities to carbon cycle have not received much attention.In order to study the influence of human activities on the rock weathering in the river basin,one large river basin and two urban-karst(or urban-impervious-surfaces)small watersheds were investigated in a silimar way to those applied in the natural process research.For the large river basin,the hydrochemical composition of Liao River was measured.Using the forward model,it is estimated that the chemical composition of the whole basin was dominated by carbonate weathering(65.74%),though was strongly influenced by the human activities(10.90%)including industry,agriculture,stockbreeding,urban construction and so on.For the small urban watersheds,the Lichuan and Jinxi County,Fuzhou,Jiangxi Province,were selected as the studied sites.We collected water samples from the surface runoff formed on the urban cement pavement during the rainfall as the representative of the urban water sample,and compared with the natural water samples in the nearby granite bedrock watershed.The hydrochemical analysis and the strontium isotopic composition analysis were conducted for both urban water sample and natural water sample.Results showed that compared to natural water samples,the pH and TDS of urban water samples were higher,and the composition was manly Ca2+ and DIC,which were closer to those of the water samples characterized by carbonate weathering.It showed that the composition of the input of the river solute changes after the urban construction covers the original bedrock.Compared with the strontium isotopic compositions of the concrete samples,the soil samples and the laboratory water samples,the strontium isotopic composition of the urban water samples was lower than that of the two solid samples in the field,and it was consistent with the laboratory water samples produced from leaching the cement with weak acid.It is presumed that when the pavement concrete is in contact with the rainfall,the part with lower strontium isotopic composition is preferentially weathered.The DIC concentrations of urban water samples were similar to those of the basalt bedrock watersheds from previous literatures,which were much higher than those of natural water samples and granite bedrock watersheds.The carbon sequestration rate of the two urban watersheds estimated by DIC concentration of urban water samples was 0.20-2.35×106mol/km2a,with the average value of 0.997×106mol/km2a.Combined with the area of 1008 urban built-up regions with the population of>50 million from the statistics data in 2015,the estimated carbon sequestration flux of these areas was 20.425×1012g/a,which would account for 0.786%of the global carbon missing sink.These built-up regions are only a small part of urban construction,thus the potential of carbon sequestration from urban karst maybe enormous by the increasingly rapid expansion of human cities. |
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