Site-dependent N uptake from N-form mixtures by arctic plants,soil microbes and ectomycorrhizal fungi

Soil microbes constitute an important control on nitrogen (N) turnover and retention in arctic ecosystems where N availability is the main constraint on primary production. Ectomycorrhizal (ECM) symbioses may facilitate plant competition for the specific N pools available in various arctic ecosystems. We report here our study on the N uptake patterns of coexisting plants and microbes at two tundra sites with contrasting dominance of the circumpolar ECM shrub Betula nana. We added equimolar mixtures of glycine-N, NH₄⁺–N and NO₃⁻–N, with one N form labelled with ¹⁵N at a time, and in the case of glycine, also labelled with ¹³C, either directly to the soil or to ECM fungal ingrowth bags. After 2 days, the vegetation contained 5.6, 7.7 and 9.1% (heath tundra) and 7.1, 14.3 and 12.5% (shrub tundra) of the glycine-, NH₄⁺- and NO₃⁻–¹⁵N, respectively, recovered in the plant–soil system, and the major part of ¹⁵N in the soil was immobilized by microbes (chloroform fumigation-extraction). In the subsequent 24 days, microbial N turnover transferred about half of the immobilized ¹⁵N to the non-extractable soil organic N pool, demonstrating that soil microbes played a major role in N turnover and retention in both tundra types. The ECM mycelial communities at the two tundras differed in N-form preferences, with a higher contribution of glycine to total N uptake at the heath tundra; however, the ECM mycelial communities at both sites strongly discriminated against NO₃⁻. Betula nana did not directly reflect ECM mycelial N uptake, and we conclude that N uptake by ECM plants is modulated by the N uptake patterns of both fungal and plant components of the symbiosis and by competitive interactions in the soil. Our field study furthermore showed that intact free amino acids are potentially important N sources for arctic ECM fungi and plants as well as for soil microorganisms. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.