North Central Forest Experiment Station, USDA Forest Service, 1831 Highway 169 East, 55744, Grand Rapids, Minnesota, USA.
Environ Monit Assess. 1989 Apr;12(1):61. doi: 10.1007/BF00396731.
Lakes and streams are acidified by direct precipitation and water channeled through nearby soils, but water in low base-saturation soils can produce highly acidic percolate after prolonged contact and subsequent degassing in surface waters. Theories advanced by Reuss (1983), Reuss and Johnson (1985), and Seip and Rustad (1984) suggest that soils with less than 15% base saturation are susceptible to soil-water pH depression of up to 0.4 unit, which is sufficient to cause negative alkalinity in soil solutions. High concentrations of mobile anions (notably sulfate) are responsible for the negative alkalinity and these solutions on CO2 degassing in surface waters can retain acidities equivalent to a pH value of 5.0 or less. This mechanism purports to explain why some lakes acidify when they are surrounded by acid soils and cation leaching is not required.Ambient precipitation set to pH 5.4 and pH 4.2 was applied to columns of low base-saturated, sand, soils, starting in 1985. The columns (15 cm diameter and 150 cm long) were collected from soils with base saturations falling into one of three groups (0-10, 10-20, and 20-40%) from national forests in the Superior Uplands area (includes Boundary Waters Canoe Area, Rainbow Lakes, Sylvania, Moquah Barrens, and other Wilderness and Natural areas). The soils were Haplorthods and Udipsamments mainly from outwash plains.The soil columns were instrumented and reburied around a subterranean structure used to collect leachate water and to maintain natural temperature, air, and light conditions. Three humus treatments were applied to soil column (none, northern hardwood, and jack pine) to measure the effect of natural acidification compared to acidification by acid precipitation. The cores were treated with precipitation buffered to pH 5.4 to simulate natural rain and pH 4.2 to simulate acid rain.Columns were treated in 1985 and 1986 with approximately 200 cm of buffered precipitation each year over the frost-free season. Data is now being analyzed for the 1986 treatment year. In leachate collected from the upper horizons of the soil colums, there was a significant difference in pH, alkalinity, nitrate, and sulfate concentrations between the pH 5.4 and pH 4.2 precipitation treatments. This difference, however, disappears at the bottom of the columns. This could be partly due to exchange reactions in the B horizon. The pH and alkalinities are higher in bottom leachate. Chloride and nitrate also increased significantly due mainly to concentrating effects. Even with a pickup of sulfate in the B horizon, sulfate adsorption decreased bottom leachate concentrations well below surface values.Alkalinity, pH, and sulfate concentration in the leachate decreased over the treatment season. Nitrate concentration increased by 4- to 5-fold over the season. Leachate from the bottom of the soil columns is becoming more acidic with time with negative alkalinities appearing more frequently in columns with soils of lower base saturation. There were some significant alkalinity differences due to humus treatments; however, these were not consistent between pH treatments, and need further study. This research will eventually answer whether soil processes can be important to the acidification of lakes in poor, sandy, outwash plains of the Superior Uplands, and whether a reduction in acid sulfate deposition will reverse the percolate alkalinity from negative to positive.
湖泊和溪流受到直接降水和流经附近土壤的水道酸化的影响，但在低基础饱和度的土壤中，水在长时间接触和随后在地表水中脱气后，会产生高度酸性的渗滤液。Reuss（1983）、Reuss 和 Johnson（1985）以及 Seip 和 Rustad（1984）提出的理论表明，基础饱和度低于 15%的土壤容易受到土壤-水 pH 值降低 0.4 个单位的影响，这足以导致土壤溶液中的负碱度。高浓度的可移动阴离子（特别是硫酸盐）是负碱度的原因，这些溶液在地表水中脱气 CO2 时可以保留相当于 pH 值为 5.0 或更低的酸度。这一机制旨在解释为什么一些湖泊在周围是酸性土壤的情况下会酸化，而不需要阳离子淋失。
从 1985 年开始，将设定为 pH 5.4 和 pH 4.2 的环境降水应用于低基础饱和度的砂质土壤柱中。这些柱子（直径 15 厘米，长 150 厘米）是从苏必利尔高地地区（包括边界水域独木舟地区、彩虹湖、锡尼亚、莫夸巴贫瘠地区以及其他荒野和自然地区）国家森林中基饱和度分为三组（0-10%、10-20%和 20-40%）的土壤中采集的。土壤主要为冰碛平原上的 Haplorthods 和 Udipsamments。
土壤柱被仪器化并重新掩埋在一个用于收集渗滤液和维持自然温度、空气和光照条件的地下结构周围。对三个腐殖质处理（无、北方硬木和杰克松）应用于土壤柱，以测量与自然酸化相比，酸化对土壤的影响。将土壤柱用缓冲至 pH 5.4 的降水处理，以模拟自然降雨，用 pH 4.2 的降水处理，以模拟酸雨。
1985 年和 1986 年，在无霜期内，每年对土壤柱进行约 200 厘米的缓冲降水处理。现在正在分析 1986 年的处理年数据。在从土壤柱上层收集的渗滤液中，pH 值、碱度、硝酸盐和硫酸盐浓度在 pH 值为 5.4 和 pH 值为 4.2 的降水处理之间存在显著差异。然而，这种差异在柱底消失。这可能部分是由于 B 层的交换反应。底部渗滤液的 pH 值和碱度较高。氯和硝酸盐也显著增加，主要是由于浓缩效应。即使 B 层中硫酸盐的吸收增加，硫酸盐吸附也使底部渗滤液的浓度大大低于表面值。
渗滤液的碱度、pH 值和硫酸盐浓度在处理季节内下降。硝酸盐浓度在整个季节增加了 4-5 倍。随着时间的推移，土壤柱底部的渗滤液变得更加酸性，基础饱和度较低的土壤柱中出现的负碱度越来越频繁。由于腐殖质处理，存在一些显著的碱度差异；然而，这些差异在 pH 处理之间并不一致，需要进一步研究。这项研究最终将回答土壤过程是否对苏必利尔高地贫瘠、砂质、冰碛平原上的湖泊酸化很重要，以及减少酸性硫酸盐沉积是否会使渗滤液的碱度从负变为正。
