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Mycorrhizal Type Influences C Allocation and N Acquisition in Moist Tundra in Response to N Fertilization, P Fertilization, and WarmingErik Hobbie1, Laura Gough2, Sarah Hobbie3, Gaius Shaver4

1University of New Hampshire, 2University of Texas at Arlington, 3University of Minnesota, 4Ecosystems Center, Marine Biological Laboratory

Introduction

∙ Responses to climate change in the Arctic may be mediated through shifts in nutrient dynamics of plants and the associated mycorrhizal fungi that supply plants with N and P.

∙ Mycorrhizal fungi vary in enzymatic abilities and extent of spatial exploration; accordingly, the nutrient resources available for uptake vary among nonmycorrhizal (NON), arbuscular mycorrhizal (AM), ectomycorrhizal (ECM), and ericoid mycorrhizal fungi (ERM).

∙ Fertilization changes C allocation patterns in plants and often N and P supply can control allocation to mycorrhizal fungi.

∙ N isotope patterns (expressed as δ15N) are influenced by plant N sources and the linked partitioning of C and N in mycorrhizal symbioses.

∙ Soil δ15N at Toolik increases with depth (unpublished data, Michelle Mack). ∙ Based on responses to N fertilization in temperate and boreal regions, we predicted:

1) δ15N differences between NON and AM plants versus ECM and ERM plants should diminish with fertilization. This reflects decreased C allocation to ECM/ERM fungi, decreased importance of recalcitrant organic matter as a nutrient source, and diminished transfer of 15N-depleted N by ECM and ERM fungi.

2) Warming should increase SOM turnover but nutrient availability may not increase.3) δ15N in nonmycorrhizal plants should reflect bioavailable N and the depth of N acquisition.

∙ A warming (greenhouse, GH) and fertilization (N & P) experiment (Figure 2) in moist non-acidic tussock (MNT; Figure 3) and moist acidic tussock (MAT; Figure 4) tundra began in 1997; N and P were added at 10 g N/m2/yr (δ15N = 2.2‰) or 5 g P/m2/yr.

∙ A biomass harvest (pluck) in 2000 (Figure 5) was used to study plant responses to fertilization. ∙ Here, we report δ15N patterns across the treatments and species for insights into how warming

and fertilization altered C and N dynamics.

Methods ∙ Treatments: control, N, P, NP, GH, GHxNP in MNT plots (n=3); control and NP in MAT plots (n=4). ∙ Samples classified by tissue type, species, and probable functional/mycorrhizal type (Table 1). ∙ After measurement at KSU, δ15N patterns were analyzed using multiple regression with

functional type, species, tissue, and treatment (N, P, NP, or warming) as independent variables and interactive effects of functional type and treatment included.

Results ∙ Fertilization increased δ15N in ERM and ECM plants more than warming (Tables 2&3). In MNT

tundra, ECM plants were more sensitive than ERM plants to P and NP whereas ERM plants were more sensitive to warming than ECM plants (Table 3).

∙ In MAT tundra, δ15N of ECM and ERM plants increased similarly with NP fertilization (Figure 6). ∙ Nonmycorrhizal plants declined in δ15N if N was added but not in P only or warming only

treatments; nutrients, particularly N, decreased δ15N differences between ECM/ERM and AM/NON plants.

Discussion/Conclusions ∙ Increased δ15N with fertilization in ECM/ERM plants suggests that host plants reduce C flux to

their symbionts (as demonstrated in prior culture studies, Figure 7). These fungi transfer less 15N-depleted N to host plants, and retain less 15N-enriched N as their biomass declines.

∙ Results imply that source δ15N controls δ15N more in ERM than in ECM plants. Based on declining δ15N with P fertilization, ECM plants decrease C flux to ECM fungi in response to P fertilization but ERM plants do not.

∙ ERM plants may use N as the regulating nutrient whereas ECM plants use both. ∙ Graminoid δ15N declined with fertilization, indicating parallel declines in the δ15N of available N. ∙ Warming probably increased the uptake of deeper, 15N-enriched N by both ECM and ERM

fungi/plants but did not increase its bioavailability to nonmycorrhizal plants.

Table 1. Plants classified by probable functional/mycorrhizal type.(A), in MAT tundra; (N), in MNT tundra; (B), in both tundra types.

Table 3. δ15N shifts relative to control treatments in MNT tundra of NxP treatments, NPxGH (greenhouse) treatments, and in MAT tundra of NP fertilization. Calculations from multiple regression analyses. Units in ‰.

Figure 6. δ15N values for plant foliage from MAT tundra. Mycorrhizal type: blue, ERM; pink, ECM; green, AM; red, nonmycorrhizal.

AcknowledgementsThis study was supported by NSF OPP-1108074, OPP-0909441, OPP-0909507, OPP-0312186, DRL-0832173, and the NSF LTER

network. We thank all the volunteer teachers and technicians who participated in the plant harvest.

Figure 2. Study site, Toolik LTER.

Ericoid mycorrhizal: Andromeda polifolia (A), Cassiope tetragona (B), Ledum palustre (A), Vaccinium uligonosum (B), V. vitis-idaea (A)Ectomycorrhizal: Betula nana (A), Salix pulchra (N), Dryas integrifolia (N)Arbuscular mycorrhizal: Polygonum bistorta (B), Rubus chamaemorus (A) Nonmycorrhizal: Carex bigelowii (B), Eriophorum angustifolium (B), Eriophorum angustifolium (A), Eriophorum vaginatum (B) (graminoids)Moss: Tomenthypnum (N), Sphagnum (A), “other moss” (B)Lichens: Mixed (B)

Treatment AM ECM ERM Lichen Moss NonN -2.3 1.5 2.1 0.3 0.5 -2.2P -0.5 1.0 -0.1 0.3 -0.4 -0.3GH -3.8 0.8 1.6 0.2 1.2 0.0GHxNP -1.6 2.3 1.8 -1.1 1.5 -3.0NP -0.8 3.2 1.5 -3.3 0.3 -0.9NP (MAT) -1.2 1.1 1.0 0.0 1.0 -1.6

Table 2. Mean δ15N values for foliage of plants from MNT tundra after four years of treatment. ulig. = uligonosum, ang. = angustifolium, and vag. = vaginatum. Treatments: GH, greenhouse; N, N fertilization; P, P fertilization; NP, fertilization with both. Values are ± SE (‰), n is usually 3. 1appeared nonmycorrhizal in Denali (Treu et al. 1996).

Figure 4. MAT tundra, NP plot 1.

Figure 3. MNT tundra, NP plot 1.

Figure 5. Happy days during the harvest of 2012.

Figure 7. δ15N values for plant foliage correlate with allocation to ECM fungi in culture (Hobbie EA, Colpaert JV. 2003. New Phytologist 157: 115-126).

Species Type Control GH GH-NP N P NP Dryas integrifolia ecm -6.5±0.2 -2.9±3.6 0.6±0.7 0.5±0.6 -4.1±1.6 0.0±0.6 Salix reticulata ecm -6.4±1.3 -7.4±0.6 -1.1±0.5 -5.0±0.8 -3.4±2.0 -1.4±0.9 Cassiope tetragona erm -6.2±0.2 -5.7±0.4 -0.1±0.3 0.4±0.5 -- -1.1±0.7 Vaccinium uligonosum erm -5.8±1.4 -4.7 -1.6 -2.4±0.1 -4.5 -3.3±0.5 Polygonum bistorta herb1 -0.4±0.5 -4.9 -0.2 -1.0±1.1 -0.1±0.8 0.2±0.4 Mixed lichen lichen -2.3±0.2 -3.2±0.7 -1.6±2.3 -0.9±0.3 -1.6±0.9 -4.7±0.4 Other mosses moss -2.3±0.3 -2.3±0.1 0.9±1.7 -1.2±0.2 -2.1±0.5 -0.9±0.6 Tomenthypnum moss -2.2±0.2 -2.1±0.2 1.4±0.3 -0.1±0.9 -2.3±0.1 -1.3±0.1 Carex bigelowii non 1.9±0.4 0.1±1.5 -0.6±1.0 0.1±0.3 2.0±0.2 1.3±0.5 Equisetum spp. non1 1.0±0.2 -0.4±0.5 1.1±0.3 -0.3±1.1 1.7±1.0 2.8±0.5 Eriophorum angustifolium non 2.1±0.3 2.0±0.3 1.1±0.1 0.7±1.3 2.3±0.4 2.5±0.8 Eriophorum vaginatum non 2.3±0.2 1.1±0.9 0.8±0.5 1.7±0.1 2.5±0.1 2.0±0.7