典型喀斯特河流二氧化碳分压及交换通量季节变化

河流水-气界面二氧化碳(CO2)交换是全球及区域碳收支平衡的重要组成部分。然而, 当前对喀斯特水体CO2吸收及排放特征的研究仍然有限。为探究喀斯特河流CO2分压(pCO2)及水-气界面CO2交换特征, 本研究以我国典型喀斯特河流綦江为对象, 调查了气象水文条件、碳酸盐平衡参数(pH、T、Alk、DIC、HCO3-、CO32-、CO2)、水-气界面CO2交换通量(F)和环境变量(EC、DOC、TDN和TDP)的季节变化特征, 分析了pCO2的调控因素及其与环境变量的耦合关系, 比较了不同河流的水-气CO2交换现状。结果显示, 綦江原位水体pCO2在1.3~7205.2 μatm范围内变化, 其中有47%样品高于大气CO2平衡浓度值。受雨季降水稀释作用和旱季呼吸矿化作用及人为输入复合影响, 旱季水体pCO2(1549.5±1786.8 μatm)显著高于雨季初期(448.9±184.0 μatm)和雨季后期(83.8±166.0 μatm)(P < 0.001)。河流F值在-213.6~5526.6 mmol/(m2· d)范围内, 其平均值为202.4±907.8 mmol/(m2·d)。其中, 旱季碳源(F>0)样点比例较多为85%, 次之为雨季初期(48%), 雨季后期样点碳源比例较少(6%)。水体pCO2与环境变量EC和TDN具有显著相关关系(P < 0.05)。喀斯特河流水-气界面可同时作为大气CO2的源和汇, 掌握其变化规律对我国生态系统"碳达峰、碳中和"目标的实现具有重要理论和现实意义。

Partial pressure of carbon dioxide and its water-air exchange in a typical karst rive

Riverine water-air carbon dioxide(CO2) emission is regarded as an important component in global and regional carbon budget. However, there is a knowledge gap in how aqueous CO2 uptake and emission in karst waters. In order to explore seasonal patterns and drivers of CO2 partial pressure(pCO2) and the corresponding water-air interface CO2 exchange in karst rivers, our investigations were performed in a typical karst river system(28°11'~29°18'N, 106°19'~107°15'E). The catchment area is 7046 km2 with a river length of 222.3 km. Annual mean temperature is 18.7 with the highest instantaneous temperature of 42.2 ℃. About 70% ~80% of annual precipitation(1000~1300 mm) is mainly concentrated in the rainy season(April~August). Five land-use types extracted from Landsat TM and Digital Elevation Model(DEM) data were distinguished: forest(68.20%), farmland(23.40%), grass(3.80%), urban(1.92%) and water(1.15%). The catchment geology is characterized by dolomite, calcite and limestone, and thus carbonate dissolution, chemical weathering and precipitation greatly affect the surface water. The watershed was revisited three times in 2016 based on the hydrological rhythm(April: initial-rainy season; August: post-rainy season; December: dry season). A total of 89 samples were collected from surface waters at a depth of 10~20cm. To incorporate full spectrum of stream order(the main stem and major tributaries), 31 sampling sites were selected in the Qijiang River. Water samples were filtered through glass microfiber filters(GF/F 47 mm, 0.7-μm, Whatman) and preserved in 50-ml and 100-ml high-density polyethylene(HDPE) plastic containers. Water temperature (T), pH and electrical conductivity(EC) were in-situ measured using a portable CyberScan PCD 650multi-parameters system. Current velocity(w) was determined using float method. Wind speed(UZ) and air temperature(Tair) were measured with a Testo 410-1 anemometer. Total alkalinity(Alk) was titrated with 0.02mol/L HCl(1~2 drops of 0.1% helianthine B as the acid-base indicator). Dissolved organic carbon(DOC) was detected by an automated organic carbon analyzer. Total dissolved phosphorus(TDP) and total dissolved nitrogen(TND) were determined using ultraviolet-visible spectrophotometric method. The CO2 SYS software and theoretical model(thin boundary layer) were employed to estimate dissolved inorganic carbon(DIC) species(HCO3-, CO32- and CO2) and CO2 exchange, respectively. In this study, we explored temporal patterns of hydrometeorological parameters, carbonate equilibria and water-air CO2 flux(F), as well as further linked pCO2 to environmental variables i.e., EC, DOC, TDP and TDN. The results indicated that riverine DIC concentrations ranged between 328.4~4621.4μmol/L and were largely composed of HCO3- (86.5% contribution). Surface water pCO2 varied between 1.3~7205.2 μatm with 47% samples being in CO2 oversaturation. Coupled carbonate dissolution with photosynthesis consumed CO2 in the rainy season, whereas in-situ respiration and human inputs contributed to aquatic CO2 in the dry season, resulting in notable seasonal variability of riverine pCO2 in the Qijiang River: dry season(1549.5±1786.8 μatm) >initial-rainy season(448.9±184.0 μatm) and post-rainy season(83.8±166.0 μatm). Riverine F varied from -213.6 mmol/(m2·d) to 5526.6 mmol/(m2·d) with an average of 202.4±907.8 mmol/(m2·d). Approximately 85%, 48% and 6% samples were supposed to be carbon sources to the atmosphere in the dry, initial-rainy and post-rainy seasons, respectively. Environmental variables EC and TDN concentrations ranged respectively between 170.2~1251.0 μS/cm and 0.013~2.64mg/L, which tightly liked to aquatic pCO2 in the karst river(P < 0.05). These suggested that carbonate dissolution/precipitation and biological processes greatly regulated the magnitude of aquatic CO2 exchange. Karst rivers can serve as both CO2 source and sink with respect to the atmosphere, highlighting their implications for China's carbon neutrality and emission peak.