| Abstract: | Data on accumulated exchangeable H, Al, Fe and Mn (Ma) cations in rock fractions in German soil profiles are scarce. The objective of this study was to describe the sum of accumulated Ma cations of fine earth and rock fragments in 11 deep soil profiles of varying genesis. Soil profiles were laid out at the sites Solling, Eifel, Harz mountains and the Erzgebirge and the parent materials included sandstones, siltstones, quartzite, slate, greywacke, diabase, gneiss and quartz porphyry. Exchangeable cations in the fine earth and rock fragments were measured in depths down to 6 m. Additionally, effective porosity and specific surface of rock fragments were determined. The effective porosity of the different rock fragments ranged from 4 to 28% (v/v), indicating that the rocks were accessible to solutions. For most samples, the cation exchange capacities (CEC) of the fine earth fractions were larger than those of the rock fragments, and the CEC (fine earth)/CEC (rock) ratios decreased with depth. All 11 profiles had small (<40%) amounts of exchangeable Na, K, Mg and Ca (Mb) cations in the fine earth fraction. Exchangeable Ma and Mb cations in the rock fragments changed similarly with depth as in the fine earth fractions for all profiles. Cumulative (rock + fine earth) Ma cations from 0—200 cm ranged from 474 to 1592 kmolc ha−1. The contribution of the rock fraction to the cumulative exchangeable Ma cations accounted for 13 to 85% of the total. The sum of exchangeable Ma cations was much higher than the cumulative acid deposition in western Germany since the beginning of industrialization, suggesting that carbonic acid and organic acids contributed largely to soil acidification. The rocks contribute significantly to buffering the acidity of the seepage water by silicate weathering and cation exchange. Therefore, acidification models which consider the fine earth fraction only, may lead to an overestimation of the rate of soil and groundwater acidification. |
