This study investigates the potential of Mg isotopes as tracers of biogeochemical processes in a small-forested catchment located on sandstones extremely poor in Mg-bearing minerals. The average delta Mg-26 is -0.63 +/- 0.12 parts per thousand and 0 +/- 0.14 parts per thousand for local rainwater and bedrock, respectively. From the C horizon to the upper eluvial (E) horizon, soil delta Mg-26 (from 0.0 +/- 0.14 parts per thousand to 0.25 +/- 0.14 parts per thousand) is close to the bedrock value, while more than 70% of Mg is lost, suggesting a small isotopic shift during illite dissolution. The surface soil horizon (A(h)) delta Mg-26 is close to plant delta Mg-26, and especially to the grass delta Mg-26 value (-0.49 parts per thousand). The bulk delta Mg-26 of trees and grass (-0.32 parts per thousand and -0.41 parts per thousand, respectively) are higher than the average delta Mg-26 values of the soil exchangeable fraction (-0.92 parts per thousand to -0.42 parts per thousand), and of rainwater (-0.65 parts per thousand). Within plants, roots are enriched in heavy isotopes, whereas light isotopes are preferentially translocated and stored in the above ground parts. In Norway spruce, the older needles, forming the annual litterfall, are isotopically lighter and strongly depleted in Mg compared to more recent needles. Soil solution delta Mg-26 shifts seasonally, from low values, lower than rainwater and close to litterfall during a high rainfall period in spring, to higher values, close to soil delta Mg-26 in dryer periods of winter or summer. At the watershed scale, streamwater delta Mg-26 varies between -0.85 perpendicular to 0.14 parts per thousand and -0.08 perpendicular to 0.14 parts per thousand and delta Mg-26 values decrease linearly with discharge. The high streamwater delta Mg-26 at low flow, close to bedrock delta Mg-26, most likely reflects dissolution processes in the deep saprolite in relation to the very long water residence time. Conversely, we suggest that low stream level delta Mg-26 values are at least partly related to the contribution of surface flows from wet areas. Using a simple mass and isotopic balance approach, we compute that mineral dissolution rates in the soil (0.35 kg Mg ha(-1) year(-1)) presently compensate for Mg losses from the soil. (C) 2012 Elsevier Ltd. All rights reserved.