亚热带花岗岩地区土壤矿物风化过程中盐基离子的释放特征

矿物风化过程中盐基离子释放遵从一定的化学计量关系,这种化学计量关系一般只能通过模拟实验来获取。本研究采用pH 7.0的EDTA-乙酸铵溶液将土壤中的交换性盐基离子完全洗脱出来,然后用Batch方法模拟不同pH溶液淋溶洗脱盐基和未洗脱盐基土壤,旨在消除土壤中交换性盐基离子的影响后更为准确地判断土壤矿物风化的盐基离子释放特征。结果表明:未洗脱盐基土壤的淋出液pH由3.73±0.14逐渐上升到4.23±0.06,主要原因是淋溶液中有高浓度的NH_4~+;洗脱盐基土壤矿物风化后淋出液pH从7.39±0.02逐渐下降到5.39±0.17,主要是由于土壤中可风化矿物减少。土壤交换性盐基离子会改变盐基离子释放特征、释放总量:未洗脱盐基土壤经酸雨淋溶后,各盐基离子释放均呈现急速下降后逐渐平缓的趋势,洗脱盐基土壤矿物风化后,K~+及盐基离子释放总量呈波动上升趋势,且盐基离子释放总量比未洗脱盐基土壤低。土壤交换性盐基离子的存在还会改变淋出液中的盐基离子化学计量关系:未洗脱盐基土壤的K~+︰Ca~(2+)︰Mg~(2+)︰Na+化学计量关系为11︰13︰4︰1(当量比),而洗脱盐基土壤为7︰2︰2︰1。K~+是盐基离子中风化释放量最多的,大部分K~+来自于土壤中云母的风化。因此,只有利用洗脱盐基土壤的盐基离子释放量才能准确计算矿物风化速率并获得准确的化学计量关系。土壤矿物风化作用随着淋溶液酸度增大而增强,但模拟一年降雨量的情况下,p H 3.5、4.5和5.5三种不同p H溶液对矿物风化后盐基离子的释放在实验期间没有显著性影响,较长时间后的差异性有待观察。本研究表明,可以通过预洗脱盐基土壤然后模拟酸雨淋溶的方法,观察矿物风化特征,特别是盐基离子释放的化学计量特征。

Release Characteristics of Base Cations During Soil Mineral Weathering in Subtropical Granite Area

Release of base cations follow a stoichiometric relationship in the process of soil mineral weathering, different minerals have different ions release characteristics, and the stoichiometric relationship must be obtain by using simulated experiments. This study used EDTA-Ammonium acetate at pH 7.0 to remove soil base cations (which called treated soil), and then used the Batch method to simulate leaching at different pH values in treated and untreated soil, the release characteristics of base cations were then observed after soil mineral weathering. Results showed that leaching solution pH of the soil without first removing base cations raised from 3.73±0.14 to 4.23±0.06, attributed to high concentration NH4+ in the leaching solution. Decrease of the weathered minerals reduced the leaching solution pH from 7.39±0.02 to 5.39±0.17 within leaching time. Exchangeable base cations of soil without removing base cations could change the release characteristics and increase the release amount of base cations. Base cations in leaching solution declined firstly, and then remained steady. However, K+ and total base cations increased with times in the leaching solution for the soil removed base cations. The total released amount of base cations was lower for the soil removed base cations than without removing base cations. Soil exchangeable base cations could change the stoichiometric relationship of base cations in leaching solution. The ratio of K+:Ca2+:Mg2+:Na+ was 11:13:4:1 (equivalent charge)in the leaching solution for soil without removing base cations, while it was is 7:2:2:1 for soil removed base cations. K+ was the most release amount of base cations and most of K+ came from weathering of mica. Therefore only by using the release amount of base cations of soil which have removed base cations can we estimate mineral weathering rate and obtain the accurate base cations stoichiometric relationship. The more acid the leaching solution, the more the release of base cations, and the stronger the soil mineral weathering. At pH 3.5, 4.5 and 5.5, there was no significant effect on the release of base cations within the leaching amount of one year, and the differences remains to be seen over longer duration of leaching. This study showed that we can observe the release characteristics and stoichiometric characteristics of base cations by simulating acid rain at soil which has been removed base cations.