石灰土对硫酸型酸雨缓冲过程模拟及碳汇效应研究

Simulation of buffering process and carbon sink effect of lime soil on sulfuric acid rain

硫酸型酸雨沉降至地表经石灰土缓冲后,参与碳酸盐岩溶蚀及对岩溶碳汇的影响尚不明确,严重制约了我国岩溶碳汇效应的准确评估。本研究通过设置不同土层厚度条件下pH4.5的硫酸型酸雨淋滤实验,以明确石灰土对硫酸型酸雨的缓冲过程及关键控制因素。结果表明:石灰土对酸雨的缓冲作用主要发生在表层(10 cm),淋出液中Ca^(2+)、Mg^(2+)、HCO_(3)^(-)含量在淋溶初期均表现快速降低,当淋溶量(土壤水达饱和后)为1020 mL时Ca^(2+)、Mg^(2+)、HCOj淋失量趋于稳定,稳定淋失量分别为20 mg·L^(-1)、6mgL、40mg·L^(-1)。淋出液中被酸雨H交换出的Ca^(2+)、Mg^(2+)仅占很小一部分,土壤水溶性Ca^(2+)、Mg^(2+)是淋出液中Ca^(2+)、Mg^(2+)的主要部分,开放系统中,大气和土壤CO_(2)溶于降雨形成H_(2)CO,不仅增加碳汇,且H,CO,解离产生的H'交换土壤中交换态Ca^(2+)、Mg^(2+),造成Ca^(2+)、Mg^(2+)的淋失量不容忽视。不同厚度.石灰土中交换性钙镁可缓冲酸雨容量均大于土壤碳酸钙矿物可缓冲容量,前者是后者的1.17-1.59倍。相同酸度、同一降雨量(土壤水达饱和后)下土壤盐基离子参与酸雨缓冲产生的碳汇量约为碳酸盐矿物风化缓冲产生碳汇量的2.1倍,不同厚度(≥10 cm)石灰土产生的碳汇量大致相等。根据本次实验及桂林市降雨数据计算,桂林市质纯石灰岩风化残积土壤区(土层厚度≥10cm),土壤盐基离子参与酸雨缓冲每年可产生0.59~0.93 mol-m^(-2)的碳汇通量。

After sulfuric acid rain settles to the surface and is buffered by lime soil,its participation in carbonate rock erosion and its impact on karst carbon sink are still unclear,which seriously restricts the accurate assessment of kars carbon sink effect in China.In order to clarify the buffering process and key control factors of lime soil to sulfuric acid rain,we conducted leaching experiments of sulfuric acid rain with pH=4.5 under different soil thicknesses.Results show that contents of Ca^(2+),Mg^(2+)and HCO_(3)^(-) in leaching solution decrease rapidly at the initial stage of leaching.When the leaching amount(after soil water reaches saturation)reaches 1,020 mL,the leaching loss of Ca^(2+),Mg^(2+)and HCO_(3)^(-) tends to be stable,and stable leaching amounts are 20 mg·L^(-1),6 mg·L^(-1) and 40 mg·L^(-1) respectively.The same ion in leaching solution of different thicknesses of soil columns shows the same trend,which indicates that the lime soil buffering of acid rain may mainly occur in the 10 cm(surface)soil layer.Ca^(2+)and Mg^(2+)exchanged by acid rain H+in leaching solution only accounts for a small part,and the soil water-soluble Ca^(2+)and Mg^(2+)is the main part of Ca^(2+)and Mg^(2+)in leaching solution.In an open system,atmospheric and soil CO_(2) dissolves in rainfall to form H_(2)CO_(3),which not only increases carbon sink,but also exchanges Ca^(2+)and Mg^(2+)in soil with H+generated by the dissociation of H_(2)CO_(3).The leaching amount of Ca^(2+)and Mg^(2+)as a result cannot be ignored.The special physical structure of topsoil may produce preferential flow,which will significantly reduce the exchangeable point of H+in leachate,causing dissolution of carbonate minerals to assist the buffering of H^(+) in leachate.The karst area is the main place for carbon loss in terrestrial ecosystems,and the most intense carbon leaching occurs in surface soil.Exchangeable calcium,exchangeable magnesium and carbonate minerals in lime soil are principal reactants for buffering acid rain.The buffering capacity of exchangeable calcium and magnesium in lime soil with different thicknesses is greater than that of soil calcium carbonate minerals,and the former is 1.17-1.59 times as large as that of the latter.Under the same acidity and the same rainfall(after the soil water reaches saturation),the carbon sink generated by soil base ions participating in acid rain buffering is about 2.1 times as large as that generated by weathering buffering of carbonate minerals.Under the same rainfall conditions,the existence of lime soil can significantly increase carbon sink,and the carbon sink generated by lime soil with different thicknesses (≥10 cm)is roughly equal.According to this experiment and the rainfall data of Guilin,in the weathered residual soil area of pure limestone in Guilin(soil thickness greater than 10 cm),the participation of soil base ions in acid rain buffering can produce 0.59-0.93 mol-m^(-2) carbon sink flux per year.