Evidence and causes of recent decreases in nitrogen deposition in temperate forests in Northeast China

High reactive nitrogen (N) emissions due to anthropogenic activities in China have led to an increase in N deposition and ecosystem degradation. The Chinese government has strictly regulated reactive N emissions since 2010, however, determining whether N deposition has reduced requires long-term monitoring. Here, we report the patterns of N deposition at a rural forest site (Qingyuan) in northeastern China over the last decade. We collected 456 daily precipitation samples from 2014 to 2022 and analysed the temporal dynamics of N deposition. NH4+-N, NO3--N, and total inorganic N (TIN) deposition ranged from 10.5 ± 3.5 (mean ± SD), 6.1 ± 1.6, and 16.6 ± 4.7 kg N ha-1 year-1, respectively. Over the measurement period, TIN deposition at Qingyuan decreased by 55 %, whereas that in comparable sites in East Asia declined by 14-34 %. We used a random forest model to determine factors influencing the deposition of NH4+-N, NO3--N, and TIN during the study period. NH4+-N deposition decreased by 60 % because of decreased agricultural NH3 emissions. Furthermore, NO3--N deposition decreased by 42 %, due to reduced NOx emissions from agricultural soil and fossil fuel combustion. The steep decline in N deposition in northeastern China was attributed to reduced coal consumption, improved emission controls on automobiles, and shifts in agricultural practices. Long-term monitoring is needed to assess regional air quality and the impact of N emission control regulations.

Herbarium specimens reveal that mycorrhizal type does not mediate declining temperate tree nitrogen status over a century of environmental change

Rising atmospheric carbon dioxide concentrations (CO2) and atmospheric nitrogen (N) deposition have contrasting effects on ectomycorrhizal (EM) and arbuscular mycorrhizal (AM) symbioses, potentially mediating forest responses to environmental change. In this study, we evaluated the cumulative effects of historical environmental change on N concentrations and δ15N values in AM plants, EM plants, EM fungi, and saprotrophic fungi using herbarium specimens collected in Minnesota, USA from 1871 to 2016. To better understand mycorrhizal mediation of foliar δ15N, we also analyzed a subset of previously published foliar δ15N values from across the United States to parse the effects of N deposition and CO2 rise. Over the last century in Minnesota, N concentrations declined among all groups except saprotrophic fungi. δ15N also declined among all groups of plants and fungi; however, foliar δ15N declined less in EM plants than in AM plants. In the analysis of previously published foliar δ15N values, this slope difference between EM and AM plants was better explained by nitrogen deposition than by CO2 rise. Mycorrhizal type did not explain trajectories of plant N concentrations. Instead, plants and EM fungi exhibited similar declines in N concentrations, consistent with declining forest N status despite moderate levels of N deposition.

Mycena species can be opportunist-generalist plant root invaders

Traditional strict separation of fungi into ecological niches as mutualist, parasite or saprotroph is increasingly called into question. Sequences of assumed saprotrophs have been amplified from plant root interiors, and several saprotrophic genera can invade and interact with host plants in laboratory growth experiments. However, it is uncertain if root invasion by saprotrophic fungi is a widespread phenomenon and if laboratory interactions mirror field conditions. Here, we focused on the widespread and speciose saprotrophic genus Mycena and performed (1) a systematic survey of their occurrences (in ITS1/ITS2 datasets) in mycorrhizal roots of 10 plant species, and (2) an analysis of natural abundances of 13 C/15 N stable isotope signatures of Mycena basidiocarps from five field locations to examine their trophic status. We found that Mycena was the only saprotrophic genus consistently found in 9 out of 10 plant host roots, with no indication that the host roots were senescent or otherwise vulnerable. Furthermore, Mycena basidiocarps displayed isotopic signatures consistent with published 13 C/15 N profiles of both saprotrophic and mutualistic lifestyles, supporting earlier laboratory-based studies. We argue that Mycena are widespread latent invaders of healthy plant roots and that Mycena species may form a spectrum of interactions besides saprotrophy also in the field.

Can conservation agriculture mitigate climate change and reduce environmental impacts for intensive cropping systems in North China Plain?

Determining appropriate farming management practices to adapt to climate change with lower environmental costs is important for sustainable agricultural production. In this study, a long-term experiment (1985-2019) was conducted under different management practices combining fertilization rate (no, low and high N fertilizer, N₀, N₁ and N₂), straw additions (no, low and high addition, S₀, S₁ and S₂) with conservation tillage (no-tillage, NT) in the North China Plain (NCP). The Denitrification-Decomposition (DNDC) model was firstly evaluated using the experimental data, and then applied to simulate the changes of crop yields, soil organic carbon (SOC), and N₂O emissions under different management practices combined with climate change scenarios, under low and high emission scenarios of societal development pathways (SSP245 and SSP585, respectively) with climate projections from 2031 to 2100. Under the low emission scenario (SSP245), wheat yields were the highest with the NT-N₁-S₂ treatment (a 23% increase relative to the baseline (1981-2010)). For maize yields, the NT-N₁-S₁ treatment increased 46% relative to baseline under the SSP585, whereas, the yields increased less in all treatments under SSP245-2040s. The SOC was predicted to increase by 6-60% by 2100 under SSP245. Straw addition and tillage were the main factors influencing SOC. N fertilizer was the most important driver for wheat and maize yields, however, N₂O emissions from soil increased with increased application of N fertilizer. Therefore, the no-tillage method under low N fertilizer and high straw addition (NT-N₁-S₂) is recommended to promote crop yields and substantially increase SOC under SSP245 and SSP585. Conservation agriculture practices can potentially offset crop yield reductions, increase soil quality, and reduce greenhouse gas emissions in the NCP, and ensure crop production to meet the growing demand for food under future climate change.

Atmospheric nitrogen enrichment changes nutrient stoichiometry and reduces fungal N supply to peatland ericoid mycorrhizal shrubs

Peatlands store one third of global soil carbon (C) and up to 15% of global soil nitrogen (N) but often have low plant nutrient availability owing to slow organic matter decomposition under acidic and waterlogged conditions. In rainwater-fed ombrotrophic peatlands, elevated atmospheric N deposition has increased N availability with potential consequences to ecosystem nutrient cycling. Here, we studied how 14 years of continuous N addition with either nitrate or ammonium had affected ericoid mycorrhizal (ERM) shrubs at Whim Bog, Scotland. We examined whether enrichment has influenced foliar nutrient stoichiometry and assessed using N stable isotopes whether potential changes in plant nutrient constraints are linked with plant N uptake through ERM fungi versus direct plant uptake. High doses of ammonium alleviated N deficiency in Calluna vulgaris and Erica tetralix, whereas low doses of ammonium and nitrate improved plant phosphorus (P) nutrition, indicated by the lowered foliar N:P ratios. Root acid phosphatase activities correlated positively with foliar N:P ratios, suggesting enhanced P uptake as a result of improved N nutrition. Elevated foliar δ¹⁵N of fertilized shrubs suggested that ERM fungi were less important for N supply with N fertilization. Increases in N availability in peat porewater and in direct nonmycorrhizal N uptake likely have reduced plant nitrogen uptake via mycorrhizal pathways. As the mycorrhizal N uptake correlates with the reciprocal C supply from host plants to the soil, such reduction in ERM activity may affect peat microbial communities and even accelerate C loss via decreased ERM activity and enhanced saprotrophic activity. Our results thus introduce a previously unrecognized mechanism for how anthropogenic N pollution may affect nutrient and carbon cycling within peatland ecosystems.

Quantifying nitrogen uptake and translocation for mature trees: an in situ whole-tree paired 15N labeling method

Nitrogen (N) is one of the major nutrients limiting plant growth in terrestrial ecosystems. To avoid plant-microbe competition, previous studies on plant N uptake preference often used hydroponic experiments on fine roots of seedlings and demonstrated ammonium preference for conifer species; however, we lack information about N uptake and translocation in the field. In this paper, we described a method of in situ paired 15N labeling and reported the rates and time course of N uptake and translocation by mature trees in situ. We added 15N-enriched ammonium or nitrate, together with the nitrification inhibitor dicyandiamide, to paired Larix kaempferi (Lamb.) Carr (larch) trees from 30-, 40- and 50-year-old plantations. Fine roots, coarse roots, leaves and small branches were collected 2, 4, 7, 14 and 30 days after labeling. Nitrate uptake and translocation averaged 1.59 ± 0.16 μg 15N g-1 day-1, which is slightly higher than ammonium (1.08 ± 0.10 μg 15N g-1 day-1), in all tree organs. Nitrate contributed 50-78% to N uptake and translocation, indicating efficient nitrate use by larch in situ. We observed no age effect. We suggest that sampling leaves after 4 days of 15N labeling is sufficient to detect mature tree N uptake preference in situ. Whole-tree 15N-ammonium recovery equaled that of 15N-nitrate 30 days after 15N addition, implying the importance of both ammonium and nitrate to mature larch N use in the long run. We conclude that our method is promising for studying mature tree N uptake preference in situ and can be applied to other conifer and broadleaf species. We suggest using highly enriched 15N tracer to overcome soil dilution and a nitrification inhibitor to minimize ammonium transformation to nitrate. Our study revealed mature tree N preference in situ and demonstrated the strong contribution of nitrate toward mature larch growth on soils rich in nitrate.

Dynamics and multi-annual fate of atmospherically deposited nitrogen in montane tropical forests

The effects of nitrogen (N) deposition on forests largely depend on its fate after entering the ecosystem. While several studies have addressed the forest fate of N deposition using ¹⁵ N tracers, the long-term fate and redistribution of deposited N in tropical forests remains unknown. Here, we applied ¹⁵ N tracers to examine the fates of deposited ammonium ( NH 4 + ) and nitrate ( NO 3 - ) separately over 3 years in a primary and a secondary tropical montane forest in southern China. Three months after ¹⁵ N tracer addition, over 60% of ¹⁵ N was retained in the forests studied. Total ecosystem retention did not change over the study period, but between 3 months and 3 years following deposition ¹⁵ N recovery in plants increased from 10% to 19% and 13% to 22% in the primary and secondary forests, respectively, while ¹⁵ N recovery in the organic soil declined from 16% to 2% and 9% to 2%. Mineral soil retained 50% and 35% of ¹⁵ N in the primary and secondary forests, with retention being stable over time. The total ecosystem retention of the two N forms did not differ significantly, but plants retained more 15 NO 3 - than 15 NH 4 + and the organic soil more 15 NH 4 + than NO 3 - . Mineral soil did not differ in 15 NH 4 + and 15 NO 3 - retention. Compared to temperate forests, proportionally more ¹⁵ N was distributed to mineral soil and plants in these tropical forests. Overall, our results suggest that atmospherically deposited NH 4 + and NO 3 - is rapidly lost in the short term (months) but thereafter securely retained within the ecosystem, with retained N becoming redistributed to plants and mineral soil from the organic soil. This long-term N retention may benefit tropical montane forest growth and enhance ecosystem carbon sequestration.

Mature conifers assimilate nitrate as efficiently as ammonium from soils in four forest plantations

Conifers are considered to prefer to take up ammonium (NH₄⁺ ) over nitrate (NO₃^- ). However, this conclusion is mainly based on hydroponic experiments that separate roots from soils. It remains unclear to what extent mature conifers can use nitrate compared to ammonium under field conditions where both roots and soil microbes compete for nitrogen (N). We conducted an in situ whole mature tree nitrogen-15 (¹⁵ N) labeling experiment (¹⁵ NH₄⁺ vs ¹⁵ NO₃^- ) over 15 d to quantify ammonium and nitrate uptake and assimilation rates in four 40-yr-old monoculture coniferous plantations (Pinus koraiensis, Pinus sylvestris, Picea koraiensis and Larix olgensis, respectively). For the whole tree, ¹⁵ NO₃^- contributed 39% to 90% to total ¹⁵ N tracer uptake among four plantations during the study period. At day 3, the ¹⁵ NO₃^- accounted for 77%, 64%, 62% and 59% by Larix olgensis, Pinus koraiensis, Pinus sylvestris and Picea koraiensis, respectively. Our study indicates that mature coniferous trees assimilated nitrate as efficiently as ammonium from soils even at low soil nitrate concentration, in contrast to the results from hydroponic experiments showing that ammonium uptake dominated over nitrate. This implies that mature conifers can adapt to increasing availability of nitrate in soil, for example, under the context of globalization of N deposition and global warming.

Higher soil acidification risk in southeastern Tibetan Plateau

Stable soil pH is a key property in maintaining an ecosystem's structure, function, and sustainability. Increasing atmospheric deposition and grassland use on the Tibetan Plateau (TP) may increase the soil acidification risk, but we lack such information to date. Here, we evaluated the soil acidification risk in the TP, by comparing it with that in the Mongolia Plateau (MP) and applying the acid-base balance principles on atmospheric inputs, soils, and plants from 1980 to 2019. Cumulative acid input was lower in the TP than in the MP. Sulfur contributed more to acidity than nitrogen and atmospheric deposition contributed more to acidity than grassland use. Acid input was mainly influenced by local industry, animal husbandry and transportation in the MP, while in the TP it was also affected by the long-distance transportation of pollutants from South Asia and southern China. Overall, the TP was less acid-sensitive than the MP because of higher inorganic carbon content. However, soils in the southeastern TP, covering 21% of the total area, were acid-sensitive due to low levels of soil exchangeable base cation (EBCs) and lack of calcium carbonate. Coincidentally, the southeastern region has the highest concentration of acid input in the TP due to more rapid development and stronger influence of adjacent high acid deposition regions than others. Therefore, the acidification risk to the southeastern region is much higher than to other regions of the TP and the MP; in this region, the EBCs are likely to be depleted approximately 95 years earlier than in the MP. The findings of this study provide insights into the response of the TP to global change. For the ecosystem sustainability of southeastern TP, control of atmospheric acid deposition, especially sulfur deposition, in both local and adjacent regions and nations is required.

Peatland warming strongly increases fine-root growth

Belowground climate change responses remain a key unknown in the Earth system. Plant fine-root response is especially important to understand because fine roots respond quickly to environmental change, are responsible for nutrient and water uptake, and influence carbon cycling. However, fine-root responses to climate change are poorly constrained, especially in northern peatlands, which contain up to two-thirds of the world's soil carbon. We present fine-root responses to warming between +2 °C and 9 °C above ambient conditions in a whole-ecosystem peatland experiment. Warming strongly increased fine-root growth by over an order of magnitude in the warmest treatment, with stronger responses in shrubs than in trees or graminoids. In the first year of treatment, the control (+0 °C) shrub fine-root growth of 0.9 km m-2 y-1 increased linearly by 1.2 km m-2 y-1 (130%) for every degree increase in soil temperature. An extended belowground growing season accounted for 20% of this dramatic increase. In the second growing season of treatment, the shrub warming response rate increased to 2.54 km m-2 °C-1 Soil moisture was negatively correlated with fine-root growth, highlighting that drying of these typically water-saturated ecosystems can fuel a surprising burst in shrub belowground productivity, one possible mechanism explaining the "shrubification" of northern peatlands in response to global change. This previously unrecognized mechanism sheds light on how peatland fine-root response to warming and drying could be strong and rapid, with consequences for the belowground growing season duration, microtopography, vegetation composition, and ultimately, carbon function of these globally relevant carbon sinks.

Multiyear Measurements on Δ17O of Stream Nitrate Indicate High Nitrate Production in a Temperate Forest

Nitrification is a crucial step in ecosystem nitrogen (N) cycling, but scaling up from plot-based measurements of gross nitrification to catchments is difficult. Here, we employed a newly developed method in which the oxygen isotope anomaly (Δ¹⁷O) of nitrate (NO₃^-) is used as a natural tracer to quantify in situ catchment-scale gross nitrification rate (GNR) for a temperate forest from 2014 to 2017 in northeastern China. The annual GNR ranged from 71 to 120 kg N ha^-1 yr^-1 (average 94 ± 10 kg N ha^-1 yr^-1) over the 4 years in this forest. This result and high stream NO₃^- loss (4.2-8.9 kg N ha^-1 yr^-1) suggest that the forested catchment may have been N-saturated. At the catchment scale, the total N output of 10.7 kg N ha^-1 yr^-1, via leaching and gaseous losses, accounts for 56% of the N input from bulk precipitation (19.2 kg N ha^-1 yr^-1). This result indicates that the forested catchment is still retaining a large fraction of N from atmospheric deposition. Our study suggests that estimating in situ catchment-scale GNR over several years when combined with other conventional flux estimates can facilitate the understanding of N biogeochemical cycling and changes in the ecosystem N status.

Signaling from below: rodents select for deeper fruiting truffles with stronger volatile emissions

Many plant and fungal species use volatile organic compounds (VOCs) as chemical signals to convey information about the location or quality of their fruits or fruiting bodies to animal dispersers. Identifying the environmental factors and biotic interactions that shape fruit selection by animals is key to understanding the evolutionary processes that underpin chemical signaling. Using four Elaphomyces truffle species, we explored the role of fruiting depth, VOC emissions, and protein content in selection by five rodent species. We used stable isotope analysis of nitrogen (δ¹⁵ N) in truffles to estimate fruiting depth, proton-transfer-reaction mass spectrometry to determine volatile emission composition, and nitrogen concentrations to calculate digestible protein of truffles. We coupled field surveys of truffle availability with truffle spore loads in rodent scat to determine selection by rodents. Despite presumably easier access to the shallow fruiting species, E. americanus (0.5-cm depth) and E. verruculosus (2.5-cm depth), most rodents selected for truffles fruiting deeper in the soil, E. macrosporus (4.1-cm depth) and E. bartlettii (5.0-cm depth). The deeper fruiting species had distinct VOC profiles and produced significantly higher quantities of odiferous compounds. Myodes gapperi (southern red-backed vole), a fungal specialist, also selected for truffles with high levels of digestible protein, E. verruculosus and E. macrosporus. Our results highlight the importance of chemical signals in truffle selection by rodents and suggest that VOCs are under strong selective pressures relative to protein rewards. Strong chemical signals likely allow detection of truffles deep within the soil and reduce foraging effort by rodents. For rodents that depend on fungi as a major food source, protein content may also be important in selecting truffles.

Pulsed resource availability changes dietary niche breadth and partitioning between generalist rodent consumers

Identifying the mechanisms that structure niche breadth and overlap between species is important for determining how species interact and assessing their functional role in an ecosystem. Without manipulative experiments, assessing the role of foraging ecology and interspecific competition in structuring diet is challenging. Systems with regular pulses of resources act as a natural experiment to investigate the factors that influence the dietary niches of consumers. We used natural pulses of mast-fruiting of American beech (Fagus grandifolia) to test whether optimal foraging or competition structure the dietary niche breadth and overlap between two congener rodent species (Peromyscus leucopus and P. maniculatus), both of which are generalist consumers. We reconstructed diets seasonally over a 2-year period using stable isotope analysis (δ13C, δ15N) of hair and of potential dietary items and measured niche dynamics using standard ellipse area calculated within a Bayesian framework. Changes in niche breadth were generally consistent with predictions of optimal foraging theory, with both species consuming more beechnuts (a high-quality food resource) and having a narrower niche breadth during masting seasons compared to nonmasting seasons when dietary niches expanded and more fungi (a low-quality food source) were consumed. In contrast, changes in dietary niche overlap were consistent with competition theory, with higher diet overlap during masting seasons than during nonmasting seasons. Overall, dietary niche dynamics were closely tied to beech masting, underscoring that food availability influences competition. Diet plasticity and niche partitioning between the two Peromyscus species may reflect differences in foraging strategies, thereby reducing competition when food availability is low. Such dietary shifts may have important implications for changes in ecosystem function, including the dispersal of fungal spores.

Uptake Patterns of Glycine, Ammonium, and Nitrate Differ Among Four Common Tree Species of Northeast China

Fundamental questions of how plant species within secondary forests and plantations in northeast China partition limited nitrogen (N) resource remain unclear. Here we conducted a 15N tracer greenhouse study to determine glycine, ammonium, and nitrate uptake by the seedlings of two coniferous species, Pinus koraiensis (Pinus) and Larix keampferi (Larix), and two broadleaf species, Quercus mongolica (Quercus) and Juglans mandshurica (Juglans), that are common in natural secondary forests in northeast China. Glycine contributed 43% to total N uptake of Pinus, but only 20, 11, and 21% to N uptake by Larix, Quercus, and Juglans, respectively (whole plant), whereas nitrate uptake was 24, 74, 88, and 68% of total uptake for these four species, respectively. Retention of glycine carbon versus nitrogen in Pinus roots indicated that 36% of glycine uptake was retained intact. Nitrate was preferentially used by Larix, Quercus, and Juglans, with nitrate uptake constituting 68∼88% of total N use by these three species. These results demonstrated that these dominant tree species in secondary forests in northeast China partitioned limited N resource by varying uptake of glycine, ammonium and nitrate, with all species, except Pinus, using nitrate that are most abundant within these soils. Such N use pattern may also provide potential underlying mechanisms for the higher retention of deposited nitrate than ammonium into aboveground biomass in these secondary forests.

Atmospheric nitrogen deposition impacts on the structure and function of forest mycorrhizal communities: A review

Humans have dramatically increased atmospheric nitrogen (N) deposition globally. At the coarsest resolution, N deposition is correlated with shifts from ectomycorrhizal (EcM) to arbuscular mycorrhizal (AM) tree dominance. At finer resolution, ectomycorrhizal fungal (EcMF) and arbuscular mycorrhizal fungal (AMF) communities respond strongly to long-term N deposition with the disappearance of key taxa. Conifer-associated EcMF are more sensitive than other EcMF, with current estimates of critical loads at 5-6 kg ha^-1 yr^-1 for the former and 10-20 kg ha^-1 yr^-1 for the latter. Where loads are exceeded, strong plant-soil and microbe-soil feedbacks may slow recovery rates after abatement of N deposition. Critical loads for AMF and tropical EcMF require additional study. In general, the responses of EcMF to N deposition are better understood than those of AMF because of methodological tractability. Functional consequences of EcMF community change are linked to decreases by fungi with medium-distance exploration strategies, hydrophobic walls, proteolytic capacity, and perhaps peroxidases for acquiring N from soil organic matter. These functional losses may contribute to declines in forest floor decomposition under N deposition. For AMF, limited capacity to directly access complexed organic N may reduce functional consequences, but research is needed to test this hypothesis. Mycorrhizal biomass often declines with N deposition, but the relative contributions of alternate mechanisms for this decline (lower C supply, higher C cost, physiological stress by N) have not been quantified. Furthermore, fungal biomass and functional responses to N inputs probably depend on ecosystem P status, yet how N deposition-induced P limitation interacts with belowground C flux and mycorrhizal community structure and function is still unclear. Current 'omic analyses indicate potential functional differences among fungal lineages and should be integrated with studies of physiology, host nutrition, growth and health, fungal and plant community structure, and ecosystem processes.

Isotopic evidence of biotrophy and unusual nitrogen nutrition in soil-dwelling Hygrophoraceae

Several lines of evidence suggest that the agaricoid, non-ectomycorrhizal members of the family Hygrophoraceae (waxcaps) are biotrophic with unusual nitrogen nutrition. However, methods for the axenic culture and lab-based study of these organisms remain to be developed, so our current knowledge is limited to field-based investigations. Addition of nitrogen, lime or organophosphate pesticide at an experimental field site (Sourhope) suppressed fruiting of waxcap basidiocarps. Furthermore, stable isotope natural abundance in basidiocarps were unusually high in 15 N and low in 13 C, the latter consistent with mycorrhizal nutritional status. Similar patterns were found in waxcap basidiocarps from diverse habitats across four continents. Additional data from 14 C analysis of basidiocarps and 13 C pulse label experiments suggest that these fungi are not saprotrophs but rather biotrophic endophytes and possibly mycorrhizal. The consistently high but variable δ15 N values (10-20‰) of basidiocarps further indicate that N acquisition or processing differ from other fungi; we suggest that N may be derived from acquisition of N via soil fauna high in the food chain.

Isotopic Analysis of Sporocarp Protein and Structural Material Improves Resolution of Fungal Carbon Sources

Fungal acquisition of resources is difficult to assess in the field. To determine whether fungi received carbon from recent plant photosynthate, litter or soil-derived organic (C:N bonded) nitrogen, we examined differences in δ13C among bulk tissue, structural carbon, and protein extracts of sporocarps of three fungal types: saprotrophic fungi, fungi with hydrophobic ectomycorrhizae, or fungi with hydrophilic ectomycorrhizae. Sporocarps were collected from experimental plots of the Duke Free-air CO2 enrichment experiment during and after CO2 enrichment. The differential 13C labeling of ecosystem pools in CO2 enrichment experiments was tracked into fungi and provided novel insights into organic nitrogen use. Specifically, sporocarp δ13C as well as δ15N of protein and structural material indicated that fungi with hydrophobic ectomycorrhizae used soil-derived organic nitrogen sources for protein carbon, fungi with hydrophilic ectomycorrhizae used recent plant photosynthates for protein carbon and both fungal groups used photosynthates for structural carbon. Saprotrophic fungi depended on litter produced during fumigation for both protein and structural material.

The ectomycorrhizal status of a tropical black bolete, Phlebopus portentosus, assessed using mycorrhizal synthesis and isotopic analysis

Phlebopus portentosus is one of the most popular wild edible mushrooms in Thailand and can produce sporocarps in the culture without a host plant. However, it is still unclear whether Phlebopus portentosus is a saprotrophic, parasitic, or ectomycorrhizal (ECM) fungus. In this study, Phlebopus portentosus sporocarps were collected from northern Thailand and identified based on morphological and molecular characteristics. We combined mycorrhizal synthesis and stable isotopic analysis to investigate the trophic status of this fungus. In a greenhouse experiment, ECM-like structures were observed in Pinus kesiya at 1 year after inoculation with fungal mycelium, and the association of Phlebopus portentosus and other plant species showed superficial growth over the root surface. Fungus-colonized root tips were described morphologically and colonization confirmed by molecular methods. In stable isotope measurements, the δ(13)C and δ(15)N of natural samples of Phlebopus portentosus differed from saprotrophic fungi. Based on the isotopic patterns of Phlebopus portentosus and its ability to form ECM-like structures in greenhouse experiments, we conclude that Phlebopus portentosus could be an ECM fungus.

Redefining fine roots improves understanding of below-ground contributions to terrestrial biosphere processes

Fine roots acquire essential soil resources and mediate biogeochemical cycling in terrestrial ecosystems. Estimates of carbon and nutrient allocation to build and maintain these structures remain uncertain because of the challenges of consistently measuring and interpreting fine-root systems. Traditionally, fine roots have been defined as all roots ≤ 2 mm in diameter, yet it is now recognized that this approach fails to capture the diversity of form and function observed among fine-root orders. Here, we demonstrate how order-based and functional classification frameworks improve our understanding of dynamic root processes in ecosystems dominated by perennial plants. In these frameworks, fine roots are either separated into individual root orders or functionally defined into a shorter-lived absorptive pool and a longer-lived transport fine-root pool. Using these frameworks, we estimate that fine-root production and turnover represent 22% of terrestrial net primary production globally - a c. 30% reduction from previous estimates assuming a single fine-root pool. Future work developing tools to rapidly differentiate functional fine-root classes, explicit incorporation of mycorrhizal fungi into fine-root studies, and wider adoption of a two-pool approach to model fine roots provide opportunities to better understand below-ground processes in the terrestrial biosphere.

Fungal functioning in a pine forest: evidence from a ¹⁵N-labeled global change experiment

• We used natural and tracer nitrogen (N) isotopes in a Pinus taeda free air CO₂ enrichment (FACE) experiment to investigate functioning of ectomycorrhizal and saprotrophic fungi in N cycling. • Fungal sporocarps were sampled in 2004 (natural abundance and (15) N tracer) and 2010 (tracer) and δ(15)N patterns were compared against litter and soil pools. • Ectomycorrhizal fungi with hydrophobic ectomycorrhizas (e.g. Cortinarius and Tricholoma) acquired N from the Oea horizon or deeper. Taxa with hydrophilic ectomycorrhizas acquired N from the Oi horizon (Russula and Lactarius) or deeper (Laccaria, Inocybe, and Amanita). (15)N enrichment patterns for Cortinarius and Amanita in 2010 did not correspond to any measured bulk pool, suggesting that a persistent pool of active organic N supplied these two taxa. Saprotrophic fungi could be separated into those colonizing pine cones (Baeospora), wood, litter (Oi), and soil (Ramariopsis), with δ(15)N of taxa reflecting substrate differences. (15)N enrichment between sources and sporocarps varied across taxa and contributed to δ(15)N patterns. • Natural abundance and (15)N tracers proved useful for tracking N from different depths into fungal taxa, generally corresponded to literature estimates of fungal activity within soil profiles, and provided new insights into interpreting natural abundance δ(15)N patterns.

Microbes in nature are limited by carbon and energy: the starving-survival lifestyle in soil and consequences for estimating microbial rates

Understanding microbial transformations in soils is important for predicting future carbon sequestration and nutrient cycling. This review questions some methods of assessing one key microbial process, the uptake of labile organic compounds. First, soil microbes have a starving-survival life style of dormancy, arrested activity, and low activity. Yet they are very abundant and remain poised to completely take up all substrates that become available. As a result, dilution assays with the addition of labeled substrates cannot be used. When labeled substrates are transformed into (14)CO2, the first part of the biphasic release follows metabolic rules and is not affected by the environment. As a consequence, when identical amounts of isotopically substrates are added to soils from different climate zones, the same percentage of the substrate is respired and the same half-life of the respired (14)CO2 from the labeled substrate is estimated. Second, when soils are sampled by a variety of methods from taking 10 cm diameter cores to millimeter-scale dialysis chambers, amino acids (and other organic compounds) appear to be released by the severing of fine roots and mycorrhizal networks as well as from pressing or centrifuging treatments. As a result of disturbance as well as of natural root release, concentrations of individual amino acids of ~10 μM are measured. This contrasts with concentrations of a few nanomolar found in aquatic systems and raises questions about possible differences in the bacterial strategy between aquatic and soil ecosystems. The small size of the hyphae (2-10 μm diameter) and of the fine roots (0.2-2 mm diameter), make it very difficult to sample any volume of soil without introducing artifacts. Third, when micromolar amounts of labeled amino acids are added to soil, some of the isotope enters plant roots. This may be an artifact of the high micromolar concentrations applied.

Carbon sources for the Palaeozoic giant fungus Prototaxites inferred from modern analogues

A wide range of carbon isotope values in the Devonian fossil Prototaxites has been interpreted to support heterotrophy and the classification of Prototaxites as a giant fungus. This inference remains controversial because of the huge size of Prototaxites relative to co-occurring terrestrial vegetation and the lack of existing fungal analogues that display equally broad isotopic ranges. Here, we show wide isotopic variability in the modern saprotrophic fungus Arrhenia obscurata collected adjacent to shallow meltwater pools of a sparsely vegetated glacial succession in the Washington Cascades, USA. Soils collected specifically around the edges of these pools were up to 5 per thousand higher in delta(13)C than adjacent soils consistent with C(3) origin. Microbial sources of primary production appear to cause these high delta(13)C values, and the environment may be analogous to that of the Early Devonian landscapes, where Prototaxites individuals with extreme isotopic variance were found. Carbon isotopes are also compared in Prototaxites, Devonian terrestrial vascular plants, and Devonian algal-derived lake sediments. Prototaxites isotopic values show little correspondence with those of contemporaneous tracheophytes, providing further evidence that non-vascular land plants or aquatic microbes were important contributors to its carbon sources. Thus, a saprotrophic fungal identity is supported for Prototaxites, which may have relied on deposits of algal-derived organic matter in floodplain environments that were less dominated by vascular plants than a straight reading of the macrofossil record might suggest.

Tracing metabolic pathways of lipid biosynthesis in ectomycorrhizal fungi from position-specific 13C-labelling in glucose

Six position-specific (13)C-labelled isotopomers of glucose were supplied to the ectomycorrhizal fungi Suillus pungens and Tricholoma flavovirens. From the resulting distribution of (13)C among fungal PLFAs, the overall order and contribution of each glucose atom to fatty acid (13)C enrichment was: C6 (approximately 31%) > C5 (approximately 25%) > C1 (approximately 18%) > C2 (approximately 18%) > C3 (approximately 8%) > C4 (approximately 1%). These data were used to parameterize a metabolic model of the relative fluxes from glucose degradation to lipid synthesis. Our data revealed that a higher amount of carbon is directed to glycolysis than to the oxidative pentose phosphate pathway (60% and 40% respectively) and that a significant part flows through these pathways more than once (73%) due to the reversibility of some glycolysis reactions. Surprisingly, 95% of carbon cycled through glyoxylate prior to incorporation into lipids, possibly to consume the excess of acetyl-CoA produced during fatty acid turnover. Our approach provides a rigorous framework for analysing lipid biosynthesis in fungi. In addition, this approach could ultimately improve the interpretation of isotopic patterns at natural abundance in field studies.

Global patterns of foliar nitrogen isotopes and their relationships with climate, mycorrhizal fungi, foliar nutrient concentrations, and nitrogen availability

Ratios of nitrogen (N) isotopes in leaves could elucidate underlying patterns of N cycling across ecological gradients. To better understand global-scale patterns of N cycling, we compiled data on foliar N isotope ratios (delta(15)N), foliar N concentrations, mycorrhizal type and climate for over 11,000 plants worldwide. Arbuscular mycorrhizal, ectomycorrhizal, and ericoid mycorrhizal plants were depleted in foliar delta(15)N by 2 per thousand, 3.2 per thousand, 5.9 per thousand, respectively, relative to nonmycorrhizal plants. Foliar delta(15)N increased with decreasing mean annual precipitation and with increasing mean annual temperature (MAT) across sites with MAT >or= -0.5 degrees C, but was invariant with MAT across sites with MAT < -0.5 degrees C. In independent landscape-level to regional-level studies, foliar delta(15)N increased with increasing N availability; at the global scale, foliar delta(15)N increased with increasing foliar N concentrations and decreasing foliar phosphorus (P) concentrations. Together, these results suggest that warm, dry ecosystems have the highest N availability, while plants with high N concentrations, on average, occupy sites with higher N availability than plants with low N concentrations. Global-scale comparisons of other components of the N cycle are still required for better mechanistic understanding of the determinants of variation in foliar delta(15)N and ultimately global patterns in N cycling.

Mycorrhizal fungi supply nitrogen to host plants in Arctic tundra and boreal forests: 15N is the key signalThis article is one of a selection of papers in the Special Issue on Polar and Arctic Microbiology

Symbiotic fungi’s role in providing nitrogen to host plants is well-studied in tundra at Toolik Lake, Alaska, but little-studied in the adjoining boreal forest ecosystem. Along a 570 km north–south transect from the Yukon River to the North Slope of Alaska, the 15N content was strongly reduced in ectomycorrhizal and ericoid mycorrhizal plants including Betula , Salix , Picea mariana (P. Mill.) B.S.P., Picea glauca Moench (Voss), and ericaceous plants. Compared with the 15N content of soil, the foliage of nonmycorrhizal plants ( Carex and Eriophorum ) was unchanged, whereas content of the ectomycorrhizal fungi was very much higher (e.g., Boletaceae, Leccinum and Cortinarius ). It is hypothesized that similar processes operate in tundra and boreal forest, both nitrogen-limited ecosystems: (i) mycorrhizal fungi break down soil polymers and take up amino acids or other nitrogen compounds; (ii) mycorrhizal fungi fractionate against 15N during production of transfer compounds; (iii) host plants are accordingly depleted in 15N; and (iv) mycorrhizal fungi are enriched in 15N. Increased N availability for plant roots or decreased light availability to understory plants may have decreased N allocation to mycorrhizal partners and increased δ15N by 3‰–4‰ for southern populations of Vaccinium vitis-idaea L. and Salix. Fungal biomass, measured as ergosterol, correlated strongly with soil organic matter and attained amounts similar to those in temperate forest soils.

The ectomycorrhizal status of Calostoma cinnabarinum determined using isotopic, molecular, and morphological methods

Calostoma cinnabarinum Corda belongs to the suborder Sclerodermatineae (Boletales), which includes many well-known ectomycorrhizal basidiomycetes, but the genus Calostoma has been described as saprotrophic. This study combines isotopic, molecular, and morphological techniques to determine the mode of nutrition of C. cinnabarinum. δ13C and δ15N measurements were compared among co-occurring C. cinnabarinum, ectomycorrhizal fungi, saprotrophic fungi, and ectomycorrhizal plants. Isotopic profiles of C. cinnabarinum resembled those of ectomycorrhizal fungi but not those of saprotrophic fungi. For molecular analyses, ectomycorrhizal root tips were extracted from soil cores collected beneath C. cinnabarinum fruit bodies. Nuclear ribosomal internal transcribed spacer (nrITS) sequences were obtained from ectomycorrhizal root tips using fungal-specific primers and screened against C. cinnabarinum nrITS sequences. Ectomycorrhizal root tips had nrITS sequences that matched C. cinnabarinum fruiting bodies. Root tips colonized by C. cinnabarinum were also described morphologically. Several morphological characters were shared between fruiting bodies and ectomycorrhizal root tips of C. cinnabarinum. Results of isotopic, molecular, and morphological analyses indicate that C. cinnabarinum is ectomycorrhizal. Molecular analysis and observations of plant associations suggest that C. cinnabarinum forms ectomycorrhizae with Quercus .

15N IN SYMBIOTIC FUNGI AND PLANTS ESTIMATES NITROGEN AND CARBON FLUX RATES IN ARCTIC TUNDRA

When soil nitrogen is in short supply, most terrestrial plants form symbioses with fungi (mycorrhizae): hyphae take up soil nitrogen, transport it into plant roots, and receive plant sugars in return. In ecosystems, the transfers within the pathway fractionate nitrogen isotopes so that the natural abundance of 15N in fungi differs from that in their host plants by as much as 12% per hundred. Here we present a new method to quantify carbon and nitrogen fluxes in the symbiosis based on the fractionation against 15N during transfer of nitrogen from fungi to plant roots. We tested this method, which is based on the mass balance of 15N, with data from arctic Alaska where the nitrogen cycle is well studied. Mycorrhizal fungi provided 61-86% of the nitrogen in plants; plants provided 8-17% of their photosynthetic carbon to the fungi for growth and respiration. This method of analysis avoids the disturbance of the soil-microbe-root relationship caused by collecting samples, mixing the soil, or changing substrate concentrations. This analytical technique also can be applied to other nitrogen-limited ecosystems, such as many temperate and boreal forests, to quantify the importance for terrestrial carbon and nitrogen cycling of nutrient transfers mediated by mycorrhizae at the plant-soil interface.

CARBON ALLOCATION TO ECTOMYCORRHIZAL FUNGI CORRELATES WITH BELOWGROUND ALLOCATION IN CULTURE STUDIES

Ectomycorrhizal fungi form symbioses with most temperate and boreal tree species, but difficulties in measuring carbon allocation to these symbionts have prevented the assessment of their importance in forest ecosystems. Here, I surveyed allocation patterns in 14 culture studies and five field studies of ectomycorrhizal plants. In culture studies, allocation to ectomycorrhizal fungi (NPPf) was linearly related to total belowground net primary production (NPPb) by the equation NPPf = 41.5% x NPPb - 11.3% (r2 = 0.55, P < 0.001) and ranged from 1% to 21% of total net primary production. As a percentage of NPP, allocation to ectomycorrhizal fungi was highest at lowest plant growth rates and lowest nutrient availabilities. Because total belowground allocation can be estimated using carbon balance techniques, these relationships should allow ecologists to incorporate mycorrhizal fungi into existing ecosystem models. In field studies, allocation to ectomycorrhizal fungi ranged from 0% to 22% of total allocation, but wide differences in measurement techniques made intercomparisons difficult. Techniques such as fungal in-growth cores, root branching-order studies, and isotopic analyses could refine our estimates of turnover rates of fine roots, mycorrhizae, and extraradical hyphae. Together with ecosystem modeling, such techniques could soon provide good estimates of the relative importance of root vs. fungal allocation in belowground carbon budgets.

Foliar and fungal 15 N:14 N ratios reflect development of mycorrhizae and nitrogen supply during primary succession: testing analytical models

Nitrogen isotopes (15N/14N ratios, expressed as delta15N values) are useful markers of the mycorrhizal role in plant nitrogen supply because discrimination against 15N during creation of transfer compounds within mycorrhizal fungi decreases the 15N/14N in plants (low delta15N) and increases the 15N/14N of the fungi (high delta15N). Analytical models of 15N distribution would be helpful in interpreting delta15N patterns in fungi and plants. To compare different analytical models, we measured nitrogen isotope patterns in soils, saprotrophic fungi, ectomycorrhizal fungi, and plants with different mycorrhizal habits on a glacier foreland exposed during the last 100 years of glacial retreat and on adjacent non-glaciated terrain. Since plants during early primary succession may have only limited access to propagules of mycorrhizal fungi, we hypothesized that mycorrhizal plants would initially be similar to nonmycorrhizal plants in delta15N and then decrease, if mycorrhizal colonization were an important factor influencing plant delta15N. As hypothesized, plants with different mycorrhizal habits initially showed similar delta15N values (-4 to -6 per thousand relative to the standard of atmospheric N2 at 0 per thousand), corresponding to low mycorrhizal colonization in all plant species and an absence of ectomycorrhizal sporocarps. In later successional stages where ectomycorrhizal sporocarps were present, most ectomycorrhizal and ericoid mycorrhizal plants declined by 5-6 per thousand in delta15N, suggesting transfer of 15N-depleted N from fungi to plants. The values recorded (-8 to -11 per thousand) are among the lowest yet observed in vascular plants. In contrast, the delta15N of nonmycorrhizal plants and arbuscular mycorrhizal plants declined only slightly or not at all. On the forefront, most ectomycorrhizal and saprotrophic fungi were similar in delta15N (-1 to -3 per thousand), but the host-specific ectomycorrhizal fungus Cortinarius tenebricus had values of up to 7 per thousand. Plants, fungi and soil were at least 4 per thousand higher in delta15N from the mature site than in recently exposed sites. On both the forefront and the mature site, host-specific ectomycorrhizal fungi had higher delta15N values than ectomycorrhizal fungi with a broad host range. From these isotopic patterns, we conclude: (1) large enrichments in 15N of many ectomycorrhizal fungi relative to co-occurring ectomycorrhizal plants are best explained by treating the plant-fungal-soil system as a closed system with a discrimination against 15N of 8-10 per thousand during transfer from fungi to plants, (2) based on models of 15N mass balance, ericoid and ectomycorrhizal fungi retain up to two-thirds of the N in the plant-mycorrhizal system under the N-limited conditions at forefront sites, (3) sporocarps are probably enriched in 15N by an additional 3 per thousand relative to available nitrogen, and (4) host-specific ectomycorrhizal fungi may transfer more N to plant hosts than non-host-specific ectomycorrhizal fungi. Our study confirms that nitrogen isotopes are a powerful tool for probing nitrogen dynamics between mycorrhizal fungi and associated plants.

Carbon use, nitrogen use, and isotopic fractionation of ectomycorrhizal and saprotrophic fungi in natural abundance and 13C-labelled cultures

Stable isotopes in fruit bodies from field studies have been used to infer ectomycorrhizal or saprotrophic status and to understand carbon and nitrogen use, but few controlled culture studies have correlated source and fungal isotopic patterns. Here, we measured natural abundances of 15N and 13C in ten strains of ectomycorrhizal fungi and seven strains of saprotrophic fungi grown on agar with three different primary carbon sources: glucose, glucose plus malt extract, and potato dextrose agar. Eight fungal strains were also grown using position-specific, 13C-labelled glucose (C-1 through C-6 labelled). Most fungi resembled nitrogen sources in delta 15N, suggesting that growth on agar media minimizes isotopic fractionation on uptake compared to growth on liquid media, and that in general saprotrophic and mycorrhizal fungi process nitrogen similarly. Saprotrophic fungi were more depleted in 13C than ectomycorrhizal fungi on all media, presumably because of assimilation of 13C-depleted, agar-derived carbon. Results on 13C-enriched glucose indicated that saprotrophic fungi obtained up to 45 % of their carbon from the agar substrate. Fungi generally incorporated the individual carbon atoms of glucose in the order, C-4 < C-1 < C-2, C-3, C-5 < C-6, ranging from a mean of 9 % for the C-4 atom to 21 % for the C-6 atom. Based on these incorporation patterns and intramolecular 13C patterns within glucose, differential incorporation of carbon atoms within glucose among fungal taxa contributed less than 1% to isotopic differences among taxa, whereas isotopic fractionation among taxa during metabolism varied up to 4%. Parallel studies of 13C-enriched and natural abundance substrates were crucial to interpreting our results.

Intramolecular, compound-specific, and bulk carbon isotope patterns in C3 and C4 plants: a review and synthesis

Studies using carbon isotope differences between C₃ and C₄ photosynthesis to calculate terrestrial productivity or soil carbon turnover assume that intramolecular isotopic patterns and isotopic shifts between specific plant components are similar in C₃ and C₄ plants. To test these assumptions, we calculated isotopic differences in studies measuring components from C₃ or C₄ photosynthesis. Relative to source sugars in fermentation, C₃ -derived ethanol had less ¹³ C and C₃ -derived CO₂ had more ¹³ C than C₄ -derived ethanol and CO₂ . Both results agreed with intramolecular isotopic signatures in C₃ and C₄ glucose. Isotopic shifts between plant compounds (e.g. lignin and cellulose) or tissues (e.g. leaves and roots) also differed in C₃ and C₄ plants. Woody C₃ plants allocated more carbon to ¹³ C-depleted compounds such as lignin or lipids than herbaceous C₃ or C₄ plants. This allocation influenced ¹³ C patterns among compounds and tissues. Photorespiration and isotopic fractionation at metabolic branch points, coupled to different allocation patterns during metabolism for C₃ vs C₄ plants, probably influence position-specific and compound-specific isotopic differences. Differing ¹³ C content of mobile and immobile compounds (e.g. sugars vs lignin) may then create isotopic differences among plant pools and along transport pathways. We conclude that a few basic mechanisms can explain intramolecular, compound-specific and bulk isotopic differences between C₃ and C₄ plants. Understanding these mechanisms will improve our ability to link bulk and compound-specific isotopic patterns to metabolic pathways in C₃ and C₄ plants. Contents Summary 371 I. Introduction 372 II. Methods and terminology 373 III. Results 373 IV. Discussion 376 V. Conclusions 382 Acknowledgements 382 References 382.

Nitrogen availability and colonization by mycorrhizal fungi correlate with nitrogen isotope patterns in plants

•  Nitrogen isotope (δ ¹⁵ N) patterns in plants may provide insight into plant N dynamics. Here, two analytical models of N-isotope cycling in plants and mycorrhizal fungi were tested, as dominant plants in many forest ecosystems obtain most of their N through intereactions with mycorrhizal fungi. •  Fungi were treated either as a single well-mixed N pool, or as two N pools (one available, plus one not available, for transfer to the host). Models were compared against complete biomass and ¹⁵ N budgets from culture studies of nonmycorrhizal and ectomycorrhizal Pinus sylvestris (colonized with Suillus luteus or Thelephora terrestris ) grown exponentially at low and high N supply. •  Fungal biomass and N increased at low N relative to high N supply, whereas needle δ ¹⁵ N decreased. Needle δ ¹⁵ N correlated strongly and negatively with biomass of extraradical hyphae. Our data and models suggest that low plant δ ¹⁵ N values in low productivity and N-limited environments result partly from high retention of ¹⁵ N-enriched N by mycorrhizal fungi; this retention was driven by increased C flux to fungi under N-limited conditions. The two-pool model of fungal N accounted for greater variability in plant δ ¹⁵ N than the one-pool model. •  Plant δ ¹⁵ N patterns may indicate relative allocation of fixed C from plants to mycorrhizal fungi under some conditions. Studies are needed on whether patterns observed in culture can be applied to interpret field measurements of δ ¹⁵ N.

Foliar nitrogen concentrations and natural abundance of (15)N suggest nitrogen allocation patterns of Douglas-fir and mycorrhizal fungi during development in elevated carbon dioxide concentration and temperature

Pseudotsuga menziesii (Mirb.) Franco (Douglas-fir) seedlings were grown in a 2 x 2 factorial design in enclosed mesocosms at ambient temperature or 3.5 degrees C above ambient, and at ambient CO2 concentration ([CO2]) or 179 ppm above ambient. Two additional mesocosms were maintained as open controls. We measured the extent of mycorrhizal infection, foliar nitrogen (N) concentrations on both a weight basis (%N) and area basis (Narea), and foliar delta15N signatures (15N/14N ratios) from summer 1993 through summer 1997. Mycorrhizal fungi had colonized nearly all root tips across all treatments by spring 1994. Elevated [CO2] lowered foliar %N but did not affect N(area), whereas elevated temperature increased both foliar %N and Narea. Foliar delta15N was initially -1 per thousand and dropped by the final harvest to between -4 and -5 per thousand in the enclosed mesocosms, probably because of transfer of isotopically depleted N from mycorrhizal fungi. Based on the similarity in foliar delta15N among treatments, we conclude that mycorrhizal fungi had similar N allocation patterns across CO2 and temperature treatments. We combined isotopic and Narea data for 1993-94 to calculate fluxes of N for second- and third-year needles. Yearly N influxes were higher in second-year needles than in third-year needles (about 160 and 50% of initial leaf N, respectively), indicating greater sink strength in the younger needles. Influxes of N in second-year needles increased in response to elevated temperature, suggesting increased N supply from soil relative to plant N demands. In the elevated temperature treatments, N effluxes from third-year needles were higher in seedlings in elevated [CO2] than in ambient [CO2], probably because of increased N allocation below ground. We conclude that N allocation patterns shifted in response to the elevated temperature and [CO2] treatments in the seedlings but not in their fungal symbionts.