The Vertical Distribution of Roots, Mycorrhizal Mycelia and Nutrient Acquisition in Mature Forest Trees

Sammanfattning: The vertical distribution of the nutrient uptake of Norway spruce (Picea abies (L.) Karst.), European beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.) has been investigated in southern Scandinavia. Two approaches were employed. The first involved estimation of the nutrient uptake capacity of the trees at different soil depths by determining the distributions of roots, external ectomycorrhizal mycelia (EEM) and the nutrient uptake capacity of the roots located at different soil depths. The fine root biomass and length (Ø<1 mm) were determined down to a soil depth of 55 cm. The amount of EEM was estimated by measurements of the PLFA 18.2w6,9 using a new incubation technique. It was thus possible to separate the ectomycorrhizal and saprophytic mycelia. The uptake capacity of fine roots was determined by root bioassays using labelled rubidium, 86Rb+ (an analogue to potassium (K)), and ammonium 15NH4+. In the second method, direct measurements of the relative uptake capacity of the trees from different soil depths were made by injection of 15NH4+, labelled phosphorus (H232PO4- and H233PO4-) and caesium (another analogue to K) into the soil, and after 21-339 days the tracers were recovered in the foliage. Generally, the amount of EEM seemed to follow the root distribution. The uptake capacity of 86Rb+ by fine roots decreased with soil depth for oak, but was similar in beech and Norway spruce irrespective of soil depth. The nutrient uptake capacity of the tree was estimated by multiplying the root weight by the uptake capacity of the roots at the different soil horizons, as EEM followed the root distribution. In oak, the estimated uptake of K from 50 cm soil depth relative to 5 cm was lower than in beech and Norway spruce due to the low uptake capacity of the oak's fine roots in deep soil layers. Direct measurements of the K uptake capacity of trees confirmed that the oaks had a lower uptake capacity at greater soil depth than beech and Norway spruce. The relative uptake of K and N from 50 cm soil depth was higher using direct measurements than estimates using the first method. This was probably due to extensive overlapping of the soil volumes around the roots and hyphae from which nutrients can diffuse in the top layer, which decreases the uptake per unit root length. The results show that the nutrient uptake dose not always follow the root and EEM distribution in forest soils. This may be due to an overcapacity of nutrient uptake for mobile ions in the top layer and/or differences in the nutrient uptake capacity of roots at different soil depths.