Plant and Soil 219: 243–250, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 243 Effects of inorganic nitrogen and phosphorus supply on nitrogen aquisition from alanine by birch seedlings in symbiosis with Paxillus involutus Minna-Maarit Kytöviita 1,2,3 and Kristina Arnebrant 1 1 Department of Microbial Ecology, Ecology Building, Lund University, S-223 62 Lund, Sweden. 2 Current address: Department of Biology, Oulu University, PB 3000, FIN 90401 Oulu, Finland. 3 Corresponding author ∗ Received 9 February 1999. Accepted in revised form 3 December 1999 Key words: ectomycorrhiza, nutrient interactions, organic N Abstract Northern forests are exposed to relatively high ammonia inputs due to high atmospheric deposition and the com- mon practise of forest fertilization. It is not known how increased soil NH 4 + concentrations affect acquisition of symbiosis-mediated N from organic sources. We examined the effect of inorganic N and P availability on N acquisition from alanine by 43 weeks old birch (Betula pendula) seedlings in symbiosis with the ectomycorrhizal fungus Paxillus involutus. The seedlings were exposed for 9 weeks to nutrient additions equivalent to 43 kg N and 6.4 kg P ha −1 (low N and P availability), 250 kg N and 38 kg P ha −1 (high N and P availability) or to 250 kg N and 6.4 kg P ha −1 (high N and low P availability). Carbon and nitrogen allocation between the symbionts was assessed by exposing the foliage to 14 CO 2 and the mycelium to 15 N-alanine or 15 NH 4 + simultaneously and measuring the distribution of the isotopic tracers after a three-day chase period. High inorganic N combined with low P availability did not have marked effect on symbiosis-mediated N uptake from alanine, whilst high N and P availability reduced alanine-derived 15 N translocation by the fungus to the plant. Shoot 15 N concentration and concentration of 14 C in the extramatrical mycelium correlated significantly across treatments pointing to controlled reciprocity of transactions between the partners. Introduction Ectomycorrhizal symbioses between coniferous or de- ciduous trees and soil-dwelling fungi are common in boreal forests. The ectomycorrhizal (EM) mycelium is thought to improve plant nutrient acquisition by providing a large surface area for nutrient absorption and by increasing the variety of forms of nitrogen readily taken up by the host tree (Marschner and Dell, 1994). EM mycelia and root tips are found mainly in the organic layers of soil in which most of the soil ni- trogen (N), phosphorus (P) and potassium are located. Most of the soil N is in organic forms: (e.g. Schul- ten and Schnitzer, 1998) and soluble organic N can be several-fold the amount of inorganic N in soil (Huang and Schoenau, 1998; Kielland, 1995; Northup et al., 1995). Approximately 40% of total soil N is in form of proteins (Schulten and Schnitzer, 1998), most of which is associated with humic substances (Qualls et al., 1991). Degradation of this proteinaceous matter is a gradual process involving several enzymes and in- termediate products such as polypeptides and amino acids. Free amino acid concentrations in soil can be as high as 8 μM for individual amino acids (Chapin et al., 1993), whilst total amino acid concentration may range between 15 and 200 μM (Kielland, 1994, 1995). Ammonium (NH 4 + ) is the dominant form of inorganic N in the relatively acidic boreal forest soils, its con- centrations ranging approximately from 2 to 200 μM (Smolander et al., 1995). Nitrate (NO 3 − ) may fluctu- ate within the same concentration range in fertilized forest soil, but is generally present at very low con- centrations under unfertilized conditions (Smolander et al., 1995).