Swedish University of Agricultural Sciences, SLU, Department of Aquatic Sciences and Assessment, P.O. Box 7050, SE-75007 Uppsala, Sweden.
Sci Total Environ. 2011 Apr 15;409(10):1916-26. doi: 10.1016/j.scitotenv.2011.02.017. Epub 2011 Mar 5.
During the last two decades, there is a common trend of increasing concentrations of dissolved organic carbon (DOC) in streams and lakes in Europe, Canada and the US. Different processes have been proposed to explain this trend and recently a unifying hypothesis was presented, concluding that declining sulphur deposition and recovery from acidification, is the single most important factor for the long-term DOC concentration trends in surface waters. If this recovery hypothesis is correct, the soil water DOC concentrations should increase as well. However, long-term soil water data from Sweden and Norway indicate that there are either decreasing or indifferent DOC concentrations, while positive DOC trends have been found in the Czech Republic. Based on the soil water data from two Swedish integrated monitoring sites and geochemical modelling, it has been shown that depending on changes in pH, ionic strength and soil Al pools, the DOC solubility might be positive, negative or indifferent. In this study, we test the acidification recovery hypothesis on long-term soil water data (25 and 50cm soil depth) from 68 forest covered sites in southern Sweden, showing clear signs of recovery from acidification. The main aim was to identify potential drivers for the DOC solubility in soil solution by comparing trends in DOC concentrations with observed changes in pH, ionic strength and concentrations of Al(n+). As in earlier Swedish and Norwegian studies, the DOC concentrations in soil water decreased or showed no trend. The generally small increases in pH (median <0.3 pH units) during the investigation period seem to be counterbalanced by the reduced ionic strength and diminished Al concentrations, increasing the organic matter coagulation. Hence, opposite to the conclusion for surface waters, the solubility of organic matter seems to decrease in uphill soils, as a result of the acidification recovery.
在过去的二十年中，欧洲、加拿大和美国的溪流和湖泊中溶解有机碳 (DOC) 的浓度呈上升趋势。人们提出了不同的过程来解释这一趋势，最近提出了一个统一的假设，即硫沉降减少和酸化恢复是地表水 DOC 浓度长期趋势的最重要因素。如果这个恢复假设是正确的，那么土壤水中的 DOC 浓度也应该增加。然而，来自瑞典和挪威的长期土壤水数据表明，DOC 浓度要么在下降，要么没有变化，而在捷克共和国则发现了正的 DOC 趋势。根据瑞典两个综合监测点的土壤水数据和地球化学模型，结果表明，取决于 pH 值、离子强度和土壤 Al 库的变化，DOC 的溶解度可能为正、负或无变化。在这项研究中，我们利用来自瑞典南部 68 个森林覆盖站点的长期土壤水数据（25 和 50cm 土壤深度）来检验酸化恢复假设，这些站点显示出明显的酸化恢复迹象。主要目的是通过比较 DOC 浓度的趋势与观察到的 pH 值、离子强度和 Al(n+)浓度的变化，确定土壤溶液中 DOC 溶解度的潜在驱动因素。与早期瑞典和挪威的研究一样，土壤水中的 DOC 浓度下降或没有趋势。在研究期间，pH 值普遍仅略有增加（中位数 <0.3 pH 单位），似乎被离子强度降低和 Al 浓度减少所抵消，这增加了有机物的凝聚。因此，与地表水的结论相反，由于酸化恢复，上坡土壤中有机物的溶解度似乎下降。
