Changes in soil mineralogy and exchangeable cation pools in stands of Norway spruce planted on former pasture land

Chemical and mineralogical properties of the soils in 35- and 70-year-old s tands of Norway spruce (Picea abies (L.) Karst), planted on former pasture and were studied at Asa Experimental Forest, southern Sweden. Remnant decid uous forests bordering the spruce stands were used as controls to assess po ssible tree-species-related effects on soil development. All soils are acid with little difference in soil pH between the spruce and deciduous stands. However, the saturation of the exchange complex with Mg is lower beneath s pruce and the total exchangeable Mg pool in the upper meter of these soils is one third of the Mg store beneath the deciduous stands. Amphibole, biotite and chlorite are the major sources of Mg in the parent s oil. The clay fraction of the topsoil beneath spruce has been depleted of a ll these easily weatherable ferromagnesian minerals. Apart from weathering- resistant primary silicates, the clay fraction consists almost exclusively of expandable, smectitic mixed-layer minerals, which are believed to be the products of advanced stages of biotite weathering. In contrast, vermiculit e is the dominant secondary mineral in the A-horizon in the deciduous stand s, and some chlorite has survived. Moreover, a greater depth of in situ wea thering is indicated for the soil of the old spruce stands where biotite/ve rmiculite mixed-layers have formed in the C-horizon as products of early st ages of biotite weathering. Thus, differences between the paired sites in s oil solution chemistry are supported by the qualitative differences in soil mineralogy, and are believed to reflect divergent biotic and/or abiotic pr ocesses in the different stand types.