Soil and vegetation sampling
Samples representing the organic horizon (O) and the uppermost mineral soil (0 – 5 cm) were taken in late August and early September 2019 from sampling plots (10×10 m) covered by pure stands of black locust (Robinia pseudoacacia ), black alder (Alnus glutinosa ), Scots pine (Pinus sylvestris ) and silver birch (Betula pendula ). The age of forest stands was 18 – 20 years for those growing on Ashes, 30 – 35 years for the stands growing on Sands and 38 – 44 years for the stands growing on Clays. At each sampling site one composite sample was taken from the O horizon and from the mineral soil. Each composite sample consisted of five subsamples (area of each subsample = 1.0 m2 for the O horizon and 0.16 m2 for the mineral soil) located at the corners and in the middle of the sampling plot. The samples were sieved (10 mm mesh for the O horizon and 2 mm mesh for the mineral soil). The mineral soil samples were divided into two parts. One part was air-dried and used for physical, physicochemical, and chemical analyses, and the other one was stored field-moist at 4°C and used for organic P fractionation. The samples of O horizon were air-dried and used for chemical analyses.
Leaves and needles were taken from trees growing within the sampling sites in direct vicinity of the O horizon and mineral soil sampling plots following Rautio and Fürst (2013) methodology. Within each sampling site the foliage material was taken from five trees. Leaves and needles were collected from the upper third of the crown with SW exposition. In case of Scots pine current year needles were collected. The collected leaves and needles from each sampling site were combined to produce a composite sample representative for the site. The foliage samples were dried to constant weight at 40 °C, homogenized and analyzed for C and nutrient contents.