桂林岩溶区石灰土壤对酸雨缓冲作用的观测及其对岩溶碳汇的指示意义

Buffering Effect of Lime Soil on Acid Rain and Its Influence on the Evaluation of the Karst Carbon Sink Effect

近期越来越多的研究显示硫酸、硝酸等外源酸会参与碳酸盐岩溶蚀,但酸雨在碳酸盐岩溶蚀中的作用与碳循环的关系并不明确,严重制约了我国岩溶碳汇效应的准确评估。本研究选择在酸雨强度大、频率高、岩溶发育典型的桂林丫吉岩溶泉野外科研实验场开展研究,对不同地貌部位土壤和丰水期(4月)、平水期(9月)降雨后土壤水进行分层取样测试。结果显示:土壤的pH范围为5.55~7.81,平均值为6.76±0.61,变异系数为9.11%,土壤水的pH范围为6.69~7.89,平均值为7.21±0.31,变异系数为4.43%,两者均接近中性和中性以上,并呈现出随深度增加而不断增大的趋势性变化规律,土壤pH主要受土壤有机质分解产酸的控制,土壤水pH主要受石灰土富钙偏碱的地球化学背景影响,酸雨对两者的影响较小。土壤阳离子交换量范围为302~423 mmol(+)/kg,位于酸沉降不敏感区的土壤范围内,土壤对酸雨来源的H+具有较好的缓冲能力。土壤水中碱性离子(Ca^(2+)+Mg^(2+))所占比例范围为23.66%~25.0%,平均值为24.25%±0.33%,变异系数为1.36%,含量稳定;酸性离子(SO_(4)^(2–)+NO_(3)^(-))所占比例范围为2.47%~18.94%,平均值为7.69%±5.09%,变异系数为66.28%,随着深度增加,土壤水中酸性离子浓度逐渐降低;土壤pH值呈碱性,可以认为,酸雨成分并没有穿透土壤层进入岩溶含水层,没有产生减碳汇效应。前期的研究由于没有考虑石灰土壤层对酸雨的缓冲作用,高估了酸雨的减汇效应。

Recent study results have shown that exogenous acids(e.g.,sulfuric and nitric acid)are increasingly involved in carbonate dissolution.However,the relationship between the role of acid rain in carbonate dissolution and carbon cycle is not clear,which seriously restricts the accurate evaluation of karst carbon sink effect in China.In this study,the soil water of different geomorphic parts and that after rainfall in wet season(April)and normal season(September)were sampled and tested in the Yaji Karst Spring field research and experimental site with high acid rain intensity,high frequency,and typical karst development.The results showed that the pH range of the soil was 5.55–7.81,with an average of 6.76±61.The coefficient of variation was 9.11%and the pH range of the soil water was 6.69–7.89.The average value was 7.21±31,and the coefficient of variation was 4.43%,with both of them close to neutral and above neutral;this showed a trend of increase with an increase in depth.Soil pH is mainly controlled by the decomposition of soil organic matter to produce acid.Soil water pH is mainly affected by the geochemical background of calcareous soil rich in calcium and alkaline;therefore,acid rain has little effect on both.The range of soil cation exchange capacity was 302-423 mmol(+)/kg,which was in the range for soil insensitive to acid deposition,indicating that the soil had a good buffer capacity for H+from acid rain.The proportion of alkaline ions(Ca^(2+)+Mg^(2+))in soil water ranged from 23.66%to 25.0%,with an average of 24.25%±0.33%,and the coefficient of variation was 1.36%,which indicated that the content was stable.The proportion of acid ions(SO_(4)^(2–)+NO_(3)^(-))ranged from 2.47%to 18.94%,with an average of 7.69%±5.09%,and the coefficient of variation was 66.28%,which indicated that the content changed greatly.The proportion of acid ions decreased with an increase in soil water circulation depth and path,indicating that acid ions are buffered in the soil layer and indicates that H+from acid rain will not have a sink reducing effect on carbonate rock dissolution in the lower karst aquifer after being buffered by the lime soil layer.In a previous study,because the buffering effect of lime soil layer on acid rain was not considered,Ca^(2+)and Mg^(2+)in groundwater in equilibrium with SO_(4)^(2–) and NO_(3)^(-) were attributed to carbonate rock dissolved by sulfuric acid and nitric acid in acid rain;therefore,the calculation results obviously overestimated the sink reduction effect of acid rain.