Leaf 15N abundance of subarctic plants provides field evidence that ericoid, ectomycorrhizal and non-and arbuscular mycorrhizal species access different sources of soil nitrogen

Abstract

The natural abundance of the nitrogen isotope 15, δN, was analysed in leaves of 23 subarctic vascular plant species and two lichens from a tree-line heath at 450 m altitude and a fellfield at 1150 m altitude close to Abisko in N. Sweden, as well as in soil, rain and snow. The aim was to reveal if plant species with different types of mycorrhizal fungi also differ in their use of the various soil N sources. The dwarf shrubs and the shrubs, which in combination formed more than 65% of the total above-ground biomass at both sites, were colonized by ericoid or ectomycorrhizal fungi. Their leaf δN was between-8.8 and-5.5‰ at the heath and between-6.1 and -3.3‰ at the fellfield. The leaf δN of non- or arbuscular mycorrhizal species was markedly different, ranging from -4.1 to -0.4‰ at the heath, and from -3.4 to+2.2‰ at the fellfield. We conclude that ericoid and ectomycorrhizal dwarf shrubs and shrubs utilize a distinct N source, most likely a fraction of the organic N in fresh litter, and not complexed N in recalcitrant organic matter. The latter is the largest component of soil total N, which had a δN of -0.7‰ at the heath and +0.5‰ at the fellfield. Our field-based data thus support earlier controlled-environment studies and studies on the N uptake of excised roots, which have demonstrated protease activity and amino acid uptake by ericoid and ectomycorrhizal tundra species. The leaves of ectomycorrhizal plants had slightly higher δN (fellfield) and N concentration than leaves of the ericoids, and Betula nana, Dryas octopetala and Salix spp. also showed NO reductase activity. These species may depend more on soil inorganic N than the ericoids. The δN of non- or arbuscular mycorrhizal species indicates that the δN of inorganic N available to these plants was higher than that of average fresh litter, probably due to high microbial immobilization of inorganic N. The δN of NH -N was +12.3‰ in winter snow and +1.9‰ in summer rain. Precipitation N might be a major contributer in species with poorly developed root systems, e.g. Lycopodium selago. Our results show that coexisting plant species under severe nutrient limitation may tap several different N sources: NH , NO and organic N from the soil, atmospheric N, and N in precipitation. Ericoid and ectomycorrhizal fungi are of major importance for plant N uptake in tundra ecosystems, and mycorrhizal fungi probably exert a major control on plant δN in organic soils.

Ectomycorrhizal fungal taxa differing in response to nitrogen deposition also differ in pure culture organic nitrogen use and natural abundance of nitrogen isotopes

“…This suggests either N-source differences in isotopic signature, or differences in the plantfungal system with respect to physiological processes that alter the isotopic signature of N (Nadelhoffer et al, 1996). Michelson et al (1996Michelson et al ( , 1998 argue that this pattern is most likely a function of differences in pool use rather than fractionation processes. If it is a function of differences in pool use, then it runs contrary to the pattern of higher isotopic enrichment for protein-users in the present study, and counter to the expected pattern of isotopic signature of labile soil N pools under low N conditions, i.e.…”

Stable Nitrogen Isotopes in a Forested Watershed in Taiwan

“…In a few studies, natural abundance of 15 N of soil and foliage has been used to identify areas with higher rates of nitrification (Garten 1993;Garten and Van Miegroet 1994;Emmett et al 1998). Several studies have suggested that among non-mycorrhizal plants, VA (vesicular-arbuscular) plants are generally higher than in 15 N than ectomycorrhizal plants (Michelsen et al 1996;Schimdt and Stewart 1997). Hobbie et al (2000) reported that foliar N concentration and δ 15 N were correlated at nitrogen limited sites, and attributed this pattern to differences in plant and mycorrhizal interactions.…”

Differential responses of grass and a dwarf shrub to long‐term changes in soil microbial biomass C, N and P following factorial addition of NPK fertilizer, fungicide and labile carbon to a heath

“…Furthermore, a large proportion of total soil N and P is tied up in the microbial biomass (Cheng & Virginia, 1993 ;Jonasson et al, 1996a ;Michelsen et al, 1996b). In the light of the potential effects of CO # enhancement and warming on microbial biomass and the possible consequences for tundra ecosystem function (Nadelhoffer et al, 1992), it is urgent to integrate investigations of plant and microbial responses to environmental changes.…”