西南喀斯特流域土地利用对河流溶解无机碳及其同位素的影响

Effects of Land Use on Riverine Dissolved Inorganic Carbon(DIC)and δ^(13) C_(DIC) in a Karst River Basin,Southwestern China

河流溶解无机碳(DIC)作为流域碳循环的重要部分一直备受关注,其稳定同位素(δ^(13)C_(DIC))能够反映DIC来源和转化过程,而其受土地利用变化的影响的研究还较为缺乏.为了研究喀斯特河流水体DIC和δ^(13)C_(DIC)的影响因素,在典型喀斯特流域赤水河进行了河水样品采集与分析.赤水河流域上游主要分布碳酸盐岩而下游分布硅酸盐岩,且下游林地覆盖率较高,河水的元素组成揭示了流域上游河流水化学主要受碳酸盐岩风化作用控制,而下游部分支流主要受硅酸盐岩风化的影响.丰水期河流c(DIC)和δ^(13)C_(DIC)的平均值分别为(1940±493)μmol·L^(-1)和(-9±1)‰;枯水期c(DIC)偏高,δ^(13)C_(DIC)值更偏正,二者平均值分别为(2334±626)μmol·L^(-1)和(-7.3±1)‰.河流DIC主要受开放体系中碳酸盐岩风化作用控制,来源于碳酸盐矿物以及大气和土壤CO_(2).c(DIC)和δ^(13)C_(DIC)值的季节差异反映了气候、水文和生物作用对DIC的调控,夏季生物碳的汇入和有机质降解是δ^(13)C_(DIC)偏负的主要控制因素,而较高径流导致的稀释效应是c(DIC)偏低的主要原因.流域下游硅酸盐岩和林地面积占比均最高的3个支流流域的河水在丰水期具有最高的溶解有机碳浓度[c(DOC)]、最低的c(DIC)和最偏负的δ^(13)C_(DIC)值.碳酸盐岩面积占比和丰水期(R^(2)=0.52,P<0.01)以及枯水期(R^(2)=0.58,P<0.01)的c(DIC)均具有较好的正相关关系,表明岩性是控制河水c(DIC)的主要因素.流经碳酸盐岩地区的河段,丰水期具有比枯水期低的c(DIC)和偏负的δ^(13)C_(DIC)值.而对于流经林地面积占比较高、植被覆盖较好的非碳酸盐岩地区的河段,丰水期具有比枯水期高的c(DIC)和显著偏负的δ^(13)C_(DIC)值,表明DIC和δ^(13)C_(DIC)在受岩性影响较小的情况下,受土地利用影响较大.

As an important part of the riverine carbon cycle,dissolved inorganic carbon(DIC)has attracted continuous attention.The stable carbon isotope(δ^(13)C_(DIC))of DIC can reflect its sources and transformations.However,the effects of land use on DIC andδ^(13)C_(DIC) are rarely investigated.To study the influencing factors of DIC andδ^(13)C_(DIC),river water samples were collected and analyzed from the Chishui River basin,a typical karst river basin in southwestern China.The upper reaches of the Chishui River basin were predominantly underlain by carbonate sedimentary rocks,whereas the lower reaches contained mostly siliciclastic sedimentary rocks,and the forest coverage in the lower reaches was relatively high.The elemental compositions of the river water revealed that the hydrochemistry of rivers in the upper reaches of the basin was mainly controlled by carbonate weathering,whereas the hydrochemistry of some tributaries in the lower reaches was mainly affected by silicate weathering.During the wet season,the average values of c(DIC)and theδ^(13)C_(DIC) value were(1940±493)μmol·L^(-1) and(-9±1)‰,respectively,whereas the c(DIC)was relatively high,and theδ^(13)C_(DIC) value was more positive in the dry season,with the average values of(2334±626)μmol·L^(-1) and(-7.3±1)‰,respectively.The DIC of most samples was mainly controlled by carbonate weathering under an open system and was derived from carbonate minerals and atmospheric and soil CO_(2).The seasonal variations in the c(DIC)andδ^(13)C_(DIC) values suggested that c(DIC)was regulated by climate,hydrology,and biology.The increasing contribution of biological carbon to DIC in summer was the main cause of the more negativeδ^(13)C_(DIC) value in the wet season,whereas the dilution effect of higher discharge was the main cause of the low c(DIC)in the wet season.The samples from three tributaries in the lower reaches with the highest proportion of silicate and forest distribution had the highest dissolved organic carbon concentration[c(DOC)],the lowest c(DIC),and the most negativeδ^(13)C_(DIC) value in the wet season.The proportion of carbonate distribution had positive correlations with c(DIC)in the wet and dry seasons,indicating that lithology was the main controlling factor of c(DIC).The rivers draining the carbonate areas had a lower c(DIC)and a more negativeδ^(13)C_(DIC) value in the wet season than those in the dry season,whereas for the rivers draining non-carbonate areas with high forest coverage,the c(DIC)was higher and theδ^(13)C_(DIC) value was significantly more negative in the wet season than those in the dry season.This implies that c(DIC)andδ^(13)C_(DIC) are significantly affected by land use when they are less affected by lithology.