Chemical characteristics and chemical weathering of surface runoff in typical granitic watersheds of subtropical China

In order to study the effect of biogeochemical cycling processes on surface runoff and the consumption of CO2 by chemical weathering of granite at small-watershed scale,the surface runoffs in two small watersheds(F-forest,FA-forest/farmland) with different land uses,which located in the typical granitic region of subtropical China,were monitored continuously for three years,and the major ions and dissolved Si were analyzed.The results showed that the dissolved Si,Na+ and HCO-3 were the predominant ions in the chemical composition of surface runoffs,and the total dissolved salts of FA watershed were higher than those of F watershed,showing the contribution of agricultural activities in FA watershed to the chemical runoffs.The ratios of Sidiss/Na+ and NO-3/SO2-4 in typical granitic watersheds(F,FA) of south Anhui Province were much higher than those in the Taihu Basin,which was rich in carbonate rocks and located in the same bioclimatic zone,indicating that the lithology and the chemical composition of acid deposition caused by human activities were the primary factors controlling the chemical runoffs.The contribution of precipitation to the chemical composition of surface runoffs in the granitic watersheds(F,FA) of south Anhui Province was 43% and 38% respectively,and the contribution of rock weathering was 57% and 50% respectively,while the contribution of the agricultural activities in the FA watershed was 12%.The consumption of CO2 by chemical weathering of granite in the small watersheds(F,FA) of south Anhui Province was(0.67—0.96)×105 mol · km-2 · a-1 and(0.64—1.05)×105 mol · km-2 · a-1 respectively,which were much lower than those of watersheds with carbonate rocks.