Distributions of the capacity to take up nutrients by Betula spp. and Picea abies in mixed stands

We examined the distribution of nutrient uptake (i.e. uptake of radioactive isotopes of nutrients) in the forest floor and lower part of the rooting zone (mineral soil) for birch (Betula pendula Roth and B. pubescens Ehr.) and Norway spruce (Picea abies (L.) Karst) in mixed stands on different soils by applying radioactive P-32, Rb-86 and Ca-45 around individual trees at two depths. One month after application the current year foliage of the tree around which the application had been done and of an adjacent tree was sampled and analyzed for radioactivity (Bq kg(-1)). Birch and spruce had a similar proportion of uptake of P-32 from the lower application depth. In two of the stands, application of Ca-45 and subsequent foliar analyses did not support the hypothesis that birch and spruce differ in their capacity to take up Ca from organic (forest floor) and upper mineral soil horizons. The analogue of potassium, Rb-86 was applied in the upper mineral soil on two different occasions (June, July/August) in one stand. The uptake of Rb-86 from the mineral soil by spruce did not differ when neighbouring trees consisted of birch and spruce (mixed neighborhood) or of spruce only (spruce neighborhood). The total uptake, from forest floor and mineral soil, of radioactive phosphorus (P-32/P-33) in current year foliage was in general higher for birch than for spruce indicating a higher demand for P by birch. The distribution of uptake of radiotracers did not match data on the distribution of fine roots at the investigated sites which indicated that spruce generally had a more shallow distribution of fine roots compared to birch. Our results do not support the hypothesis that broad-leaved tree species and conifers generally are complementary in nutrient uptake from the forest floor and mineral soil in mixed stands. Neither do the nutrient uptake from the mineral soil by spruce generally change with an admixture of a broad-leaved tree species. (C) 2004 Elsevier B.V. All rights reserved.