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

Abstract

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 (-) -(15)N, 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.

Depleted 15N in hydrolysable-N of arctic soils and its implication for mycorrhizal fungi–plant interaction

“…Taylor et al (1997), however, suggested that most of the N taken up by plants of northern Sweden boreal forests must have passed through fungi, judging from the fact that most fine root tips (98%) were mycorrhizal. In contrast, Clemmensen et al (2008) observed in their isotope tracer experiment in the field that 87-99% of added 15 N was immediately incorporated into microbial biomass and that half of the biomass turned over to form soil N during a 26-day period, whereas accumulation of 15 N in ectomycorrhizal plant was slow (5-14% of added 15 N) during the same period. Based on this and a poor relationship between 15 N uptake by mycelia and host plants (Betula nana), they concluded that N transfer between fungi and host plants would be determined by the strength of N sinks (plant vs. fungi).…”

Nitrogen competition between three dominant plant species and microbes in a temperate grassland

“…The N uptake patterns of plants varied with seasons and soil depths (McKane et al 2002;Xu et al 2011a). And which could be affected by plant species, habitat conditions, mycorrhizal status, different physi-chemical and biological characteristics of N (Kaye and Hart 1997;Sorensen et al 2008;Clemmensen et al 2008;Näsholm et al 2009;Wu et al 2013).…”

Two Decades of Experimental Manipulations of Heaths and Forest Understory in the Subarctic

“…The availability of N in different forms (NO 3 -, NH 4 ? , amino acids), and the cycling between plants, microbes, fungal mycelium and soil has been intensively investigated at Abisko, both in experiments designed to address fundamental questions on plant N acquisition and mycorrhizal function (Andresen et al 2008;Clemmensen et al 2008;Krab et al 2008), and in climate change related experiments (Sorensen et al 2008a, b;Olsrud and Michelsen 2009).…”

“…Hence, as multiple reasons may explain 15 N natural abundance patterns, studies with 15 N enrichment may often provide clearer information on N transfers within ecosystem compartments and exchange with the environment. For instance, by injection of 15 N-enriched compounds separately in soil and in root-free soil compartments with mycorrhizal fungal access solely, it has been shown that ectomycorrhizal dwarf birch (Betula nana) roots showed highest preference for ammonium while the fungal component showed low nitrate uptake (Clemmensen et al 2008). Furthermore, ecto-and ericoid mycorrhizal plants show higher N uptake than non-mycorrhizal plants in longterm studies, with uptake of N in both inorganic and organic form, as amino acids (Andresen et al 2008).…”