Nitrogen isotopes link mycorrhizal fungi and plants to nitrogen dynamics

Foliar uptake of atmospheric nitrate by two dominant subalpine plants: insights from in situ triple-isotope analysis

The significance of foliar uptake of nitrogen (N) compounds in natural conditions is not well understood, despite growing evidence of its importance to plant nutrition. In subalpine meadows, N-limitation fosters the dominance of specific subalpine plant species, which in turn ensures the provision of essential ecosystems services. Understanding how these plants absorb N and from which sources is important in predicting ecological consequences of increasing N deposition. Here, we investigate the sources of N to plants from subalpine meadows with distinct land-use history in the French Alps, using the triple isotopes (Δ¹⁷ O, δ¹⁸ O, and δ¹⁵ N) of plant tissue nitrate (NO₃^- ). We use this approach to evaluate the significance of foliar uptake of atmospheric NO₃^- (NO₃^-_atm ). The foliar uptake of NO₃^-_atm accounted for 4-16% of the leaf NO₃^- content, and contributed more to the leaf NO₃^- pool after peak biomass. Additionally, the gradual ¹⁵ N enrichment of NO₃^- from the soil to the leaves reflected the contribution of NO₃^-_atm assimilation to plants' metabolism. The present study confirms that foliar uptake is a potentially important pathway for NO₃^-_atm into subalpine plants. This is of major significance as N emissions (and deposition) are predicted to increase globally in the future.

Mycorrhizal types differ in ecophysiology and alter plant nutrition and soil processes

Mycorrhizal fungi benefit plants by improved mineral nutrition and protection against stress, yet information about fundamental differences among mycorrhizal types in fungi and trees and their relative importance in biogeochemical processes is only beginning to accumulate. We critically review and synthesize the ecophysiological differences in ectomycorrhizal, ericoid mycorrhizal and arbuscular mycorrhizal symbioses and the effect of these mycorrhizal types on soil processes from local to global scales. We demonstrate that guilds of mycorrhizal fungi display substantial differences in genome-encoded capacity for mineral nutrition, particularly acquisition of nitrogen and phosphorus from organic material. Mycorrhizal associations alter the trade-off between allocation to roots or mycelium, ecophysiological traits such as root exudation, weathering, enzyme production, plant protection, and community assembly as well as response to climate change. Mycorrhizal types exhibit differential effects on ecosystem carbon and nutrient cycling that affect global elemental fluxes and may mediate biome shifts in response to global change. We also note that most studies performed to date have not been properly replicated and collectively suffer from strong geographical sampling bias towards temperate biomes. We advocate that combining carefully replicated field experiments and controlled laboratory experiments with isotope labelling and -omics techniques offers great promise towards understanding differences in ecophysiology and ecosystem services among mycorrhizal types.

Leaf and Soil δ15N Patterns Along Elevational Gradients at Both Treelines and Shrublines in Three Different Climate Zones

The natural abundance of stable nitrogen (N) isotope (δ15N) in plants and soils can reflect N cycling processes in ecosystems. However, we still do not fully understand patterns of plant and soil δ15N at alpine treelines and shrublines in different climate zones. We measured δ15N and N concentration in leaves of trees and shrubs and also in soils along elevational gradients from lower altitudes to the upper limits of treelines and shrublines in subtropical, dry- and wet-temperate regions in China. The patterns of leaf δ15N in trees and shrubs in response to altitude changes were consistent, with lower values occurring at higher altitude in all three climate zones, but such patterns did not exist for leaf Δδ15N and soil δ15N. Average δ15N values of leaves (−1.2‰) and soils (5.6‰) in the subtropical region were significantly higher than those in the two temperate regions (−3.4‰ and 3.2‰, respectively). Significant higher δ15N values in subtro4pical forest compared with temperate forests prove that N cycles are more open in warm regions. The different responses of leaf and soil δ15N to altitude indicate complex mechanisms of soil biogeochemical process and N sources uptake with environmental variations.

High nitrogen contribution by Gunnera magellanica and nitrogen transfer by mycorrhizas drive an extraordinarily fast primary succession in sub-Antarctic Chile

Chronosequences at the forefront of retreating glaciers provide information about colonization rates of bare surfaces. In the northern hemisphere, forest development can take centuries, with rates often limited by low nutrient availability. By contrast, in front of the retreating Pia Glacier (Tierra del Fuego, Chile), a Nothofagus forest is in place after only 34 yr of development, while total soil nitrogen (N) increased from near zero to 1.5%, suggesting a strong input of this nutrient. We measured N-fixation rates, carbon fluxes, leaf N and phosphorus contents and leaf δ¹⁵ N in the dominant plants, including the herb Gunnera magellanica, which is endosymbiotically associated with a cyanobacterium, in order to investigate the role of N-fixing and mycorrhizal symbionts in N-budgets during successional transition. G. magellanica presented some of the highest nitrogenase activities yet reported (potential maximal contribution of 300 kg N ha^-1  yr^-1 ). Foliar δ¹⁵ N results support the framework of a highly efficient N-uptake and transfer system based on mycorrhizas, with c. 80% of N taken up by the mycorrhizas potentially transferred to the host plant. Our results suggest the symbiosis of G. magellanica with cyanobacteria, and trees and shrubs with mycorrhizas, to be the key processes driving this rapid succession.

Does trait variation within broadly distributed species mirror patterns across species? A case study in Puerto Rico

Although populations are phenotypically diverse, the majority of trait-based studies have focused on examining differences among species. The justification for this broadly applied approach is based on the assumption that differences among species are always greater than within species. This is likely true for local communities, but species are often broadly distributed across a wide range of environments and patterns of intraspecific variation might surpass differences among species. Therefore, an appropriate interpretation of the functional diversity requires an assessment of patterns of trait variation across different ecological scales. In this study, we examine and characterize patterns of leaf trait variation for species that are broadly distributed along an elevational gradient. We focus on seven leaf traits that represent a main axis of functional differentiation in plants reflecting the balance between photosynthetic efficiency, display, and stomatal conductance. We evaluated patterns of trait variance across ecological scales (elevation, species, populations, and individuals) and examined trait covariance at both within species and across species levels, along the elevation gradient. Our results show three key patterns: (1) intraspecific leaf trait variation for broadly distributed species is comparable to the interspecific trait variation, (2) the trait variance structure is highly variable across species, and (3) trait coordination between pairs of leaf traits is evident across species along the gradient, but not always within species. Combined, our results show that trait coordination and covariance are highly idiosyncratic across broadly distributed and co-occurring species, indicating that species may achieve similar functional roles even when exhibiting different phenotypes. This result challenges the traditional paradigm of functional ecology that assumes single trait values as optimal solutions for environments. In conclusion, patterns of trait variation both across and within species should be considered in future studies that assess trade-offs among traits over environmental gradients.

Roots Mediate the Effects of Snowpack Decline on Soil Bacteria, Fungi, and Nitrogen Cycling in a Northern Hardwood Forest

Rising winter air temperature will reduce snow depth and duration over the next century in northern hardwood forests. Reductions in snow depth may affect soil bacteria and fungi directly, but also affect soil microbes indirectly through effects of snowpack loss on plant roots. We incubated root exclusion and root ingrowth cores across a winter climate-elevation gradient in a northern hardwood forest for 29 months to identify direct (i.e., winter snow-mediated) and indirect (i.e., root-mediated) effects of winter snowpack decline on soil bacterial and fungal communities, as well as on potential nitrification and net N mineralization rates. Both winter snowpack decline and root exclusion increased bacterial richness and phylogenetic diversity. Variation in bacterial community composition was best explained by differences in winter snow depth or soil frost across elevation. Root ingrowth had a positive effect on the relative abundance of several bacterial taxonomic orders (e.g., Acidobacterales and Actinomycetales). Nominally saprotrophic (e.g., Saccharomycetales and Mucorales) or mycorrhizal (e.g., Helotiales, Russalales, Thelephorales) fungal taxonomic orders were also affected by both root ingrowth and snow depth variation. However, when grouped together, the relative abundance of saprotrophic fungi, arbuscular mycorrhizal fungi, and ectomycorrhizal fungi were not affected by root ingrowth or snow depth, suggesting that traits in addition to trophic mode will mediate fungal community responses to snowpack decline in northern hardwood forests. Potential soil nitrification rates were positively related to ammonia-oxidizing bacteria and archaea abundance (e.g., Nitrospirales, Nitrosomondales, Nitrosphaerales). Rates of N mineralization were positively and negatively correlated with ectomycorrhizal and saprotrophic fungi, respectively, and these relationships were mediated by root exclusion. The results from this study suggest that a declining winter snowpack and its effect on plant roots each have direct effects on the diversity and abundance of soil bacteria and fungal communities that interact to determine rates of soil N cycling in northern hardwood forests.

Uptake of Soil-Derived Carbon into Plants: Implications for Disposal of Nuclear Waste

Radiocarbon (¹⁴C) is potentially significant in terms of release from deep geological disposal of radioactive waste and incorporation into the biosphere. In this study we investigated the transfer of soil-derived C into two plant species by using a novel approach, where the uptake of soil-derived C into newly cultivated plants was studied on 8000-year leftover peat in order to distinguish between soil-derived and atmospheric C. Two-pool isotope mixing model was used to reveal the fraction of soil C in plants. Our results indicated that although the majority of plant C was obtained from atmosphere by photosynthesis, a significant portion (up to 3-5%) of C in plant roots was derived from old soil. We found that uptake of soil C into roots was more pronounced in ectomycorrhizal Scots pine than in endomycorrhizal reed canary grass, but nonetheless, both species showed soil-derived C uptake in their roots. Although plenty of soil-derived C was available in canopy air for reassimilation by photosynthesis, no trace of soil-derived C was detected in aboveground parts, possibly due to the open canopy. The results suggest that the potential for contamination with ¹⁴C is higher for roots than for leaves.

N Isotope Fractionation in Tree Tissues During N Reabsorption and Remobilization in Fagus crenata Blume

Background and Motivation: Nitrogen content in tissues of Fagus crenata Blume is key for flowering and seed production. However, there is a lack of information on seasonal intra-plant nitrogen partitioning in this representative tree species typical of heavy snowfall regions in Japan. Therefore, the objective of this study was to elucidate Fagus crenata intra-plant nitrogen movement by means of nitrogen content, nitrogen isotope analysis, and amino acids temporal variability. Materials and Methods: Nitrogen content, isotope ratio, and free amino acids content were measured in coarse roots, sapwood, leaves, and litter in four phenological stages in nine adult Fagus crenata trees and upscaled to the whole-tree level.

Nutrient and Isotopic Dynamics of Litter Decomposition from Different Land Uses in Naturally Restoring Taihang Mountain, North China

Litter decomposition is a prominent pathway for nutrient availability and management in terrestrial ecosystems. An in-situ litter decomposition experiment was carried out for different land use types along an elevation gradient in the Taihang Mountain area restored after heavy forest degradation in the past. Four land use types, i.e., cropland, shrubland, grassland, and forest, selected randomly from a 300–700 m elevation were investigated for the experiment using the litter bag technique. Litter mass loss ranged from 26.9% (forest) to 44.3% (cropland) varying significantly among land use types. The initial litter quality, mainly N and C/N, had a significant effect on the litter loss rate. The interaction of elevation × land use types × time was significant (p < 0.001). Litter nutrient mobility (K > P ≈ N > C) of the decomposing litter was sporadic with substantial stoichiometric effects of C/N, N/P, and C/P. The residual litters were enriched in δ15N and depleted in 13C as compared to the initial litter. Increment of N, P, and 15N values in residual litter indicates that, even in the highly weathered substrate, plant litter plays a crucial role in conserving nutrients. This study is a strong baseline for monitoring the functioning of the Taihang Mountain ecosystem restored after the complete destruction in the early 1990s.

Feast not famine: Nitrogen pools recover rapidly in 25-yr-old postfire lodgepole pine

The extent of young postfire conifer forests is growing throughout western North America as the frequency and size of high-severity fires increase, making it important to understand ecosystem structure and function in early seral forests. Understanding nitrogen (N) dynamics during postfire stand development is especially important because northern conifers are often N limited. We resampled lodgepole pine (Pinus contorta var. latifolia) stands that regenerated naturally after the 1988 fires in Yellowstone National Park (Wyoming, USA) to ask (1) How have N pools and fluxes changed over a decade (15 to 25 yr postfire) of very rapid forest growth? (2) At postfire year 25, how do N pools and fluxes vary with lodgepole pine density and productivity? Lodgepole pine foliage, litter (annual litterfall, forest-floor litter), and mineral soils were sampled in 14 plots (0.25 ha) that varied in postfire lodgepole pine density (1,500 to 344,000 stems/ha) and aboveground net primary production (ANPP; 1.4 to 16.1 Mg·ha^-1 ·yr^-1 ). Counter to expectation, foliar N concentrations in lodgepole pine current-year and composite needles (1.33 and 1.11% N, respectively) had not changed over time. Further, all measured ecosystem N pools increased substantially: foliar N increased to 89 kg N/ha (+93%), O-horizon N increased to 39 kg N/ha (+38%), and mineral soil percent total N (0-15 cm) increased to 0.08% (+33%). Inorganic N availability also increased to 0.69 μg N·[g resin]^-1 ·d^-1 (+165%). Thus, soil N did not decline as live biomass N pools increased. Among stands, biomass N pools at postfire year 25 remained strongly influenced by early postfire tree density: foliar and litterfall N concentrations declined with lodgepole pine density and ANPP, but the foliar N pool increased. Lodgepole pine ANPP correlated negatively with annual resin-sorbed N, and we found no indication of widespread N limitation. The large increases in N pools cannot be explained by atmospheric N deposition or presence of known N fixers. These results suggest an unmeasured N source and are consistent with recent reports of N fixation in young lodgepole pine.

Tricholoma matsutake may take more nitrogen in the organic form than other ectomycorrhizal fungi for its sporocarp development: the isotopic evidence

Tricholoma matsutake is an ectomycorrhizal (ECM) fungus capable of in vitro saprotrophic growth, but the sources of C and N used to generate sporocarps in vivo are not well understood. We examined natural abundance isotope data to investigate this phenomenon. For this purpose, C, N and their stable isotopes (13C, 15N) content of fungal sporocarps and their potential nutrient sources (i.e., foliage, litter, fine roots, wood, and soil) were investigated from two well-studied sites in Finland and Japan. Our results show that δ13C values of T. matsutake and other fungal groups are consistent with those of most studies, but a very high δ15N value (16.8‰ ± 2.3) is observed in T. matsutake. Such isotopic pattern of fungal δ15N suggests that matsutake has a greater proteolytic potential to digest chemically complex 15N-enriched organic matter and hydrophobic hyphae. This assumption is further supported by a significant and positive correlation between δ13Ccap-stipe and δ15Ncap-stipe exclusively in T. matsutake, which suggests common C and N sources (protein) possible for isotopically enriched cap. The 13C increase of caps relative to stipe presumably reflects greater contents of 13C-enriched protein than 13C-depleted chitin. We conclude that T. matsutake is a typical ECM fungus which obtains for its sporocarp development for both C and N from a common protein source (vs. photosynthetic carbon) present in soil organic matter.

Nitrogen isotope pattern in Mongolian larch stands at the southern Eurasian boreal forest boundary

In the last decades a drastic increase in air temperature but a stable precipitation regime in Mongolia has led to gradual drying conditions. Thus, we evaluated the effect of spatial and climatic characteristics on the soil-plant nitrogen dynamics in three representative larch stands (Larix sibirica) with different geographical and climatic conditions using stable nitrogen isotopes. The results showed significant differences in the soil inorganic N content among sites and consequently a different isotopic composition in the plant-soil system. Litter, bark and wood had the lowest δ¹⁵N values for all sites, slightly higher δ¹⁵N values for needles, while the highest δ¹⁵N values were observed for roots and soil. These differences could be the result of the larch stands age themselves, but were in agreement with the spatial and climatic characteristics of the sites. Based on the δ¹⁵N value a higher reliance on ectomycorrhizal fungi (ECMF) was observed in the warmest and driest site, while lower dependency was shown in the cooler northern site with higher soil inorganic N content. In both sites, the rate of air temperature increase has been similar in the last decades; however, their soil-plant N dynamics showed different characteristics.

Stable isotope analyses reveal previously unknown trophic mode diversity in the Hymenochaetales

PREMISE OF THE STUDY

The Hymenochaetales are dominated by lignicolous saprotrophic fungi involved in wood decay. However, the group also includes bryophilous and terricolous taxa, but their modes of nutrition are not clear. Here, we investigate patterns of carbon and nitrogen utilization in numerous non-lignicolous Hymenochaetales and provide a phylogenetic context in which these non-canonical ecological guilds arose.

METHODS

We combined stable isotope analyses of δ¹³ C and δ¹⁵ N and phylogenetic analyses to explore assignment and evolution of nutritional modes. Clustering procedures and statistical tests were performed to assign trophic modes to Hymenochaetales and test for differences between varying ecologies. Genomes of Hymenochaetales were mined for presence of enzymes involved in plant cell wall and lignin degradation and sucrolytic activity.

KEY RESULTS

Three different trophic clusters were detected - biotrophic, saprotrophic, and a second biotrophic cluster including many bryophilous Hymenochaetales and mosses. Non-lignicolous Hymenochaetales are generally biotrophic. All lignicolous Hymenochaetales clustered as saprotrophic and most terricolous Hymenochaetales clustered as ectomycorrhizal. Overall, at least 15 species of Hymenochaetales are inferred as biotrophic. Bryophilous species of Rickenella can degrade plant cell walls and lignin, and cleave sucrose to glucose consistent with a parasitic or endophytic life style.

CONCLUSIONS

Most non-lignicolous Hymenochaetales are biotrophic. Stable isotope values of many bryophilous Hymenochaetales cluster as ectomycorrhizal or in a biotrophic cluster indicative of parasitism or an endophytic life style. Overall, trophic mode diversity in the Hymenochaetales is greater than anticipated, and non-lignicolous ecological traits and biotrophic modes of nutrition are evolutionarily derived features.

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.

Groundwater drawdown drives ecophysiological adjustments of woody vegetation in a semi-arid coastal ecosystem

Predicted droughts and anthropogenic water use will increase groundwater lowering rates and intensify groundwater limitation, particularly for Mediterranean semi-arid ecosystems. These hydrological changes may be expected to elicit differential functional responses of vegetation either belowground or aboveground. Yet, our ability to predict the impacts of groundwater changes on these ecosystems is still poor. Thus, we sought to better understand the impact of falling water table on the physiology of woody vegetation. We specifically ask (a) how is woody vegetation ecophysiological performance affected by water table depth during the dry season? and (b) does the vegetation response to increasing depth to groundwater differ among water-use functional types? We examined a suite of physiological parameters and water-uptake depths of the dominant, functionally distinct woody vegetation along a water-table depth gradient in a Mediterranean semi-arid coastal ecosystem that is currently experiencing anthropogenic groundwater extraction pressure. We found that groundwater drawdown did negatively affect the ecophysiological performance of the woody vegetation. Across all studied environmental factors, depth to groundwater was the most important driver of ecophysiological adjustments. Plant functional types, independent of groundwater dependence, showed consistent declines in water content and generally reduced C and N acquisition with increasing depths to groundwater. Functional types showed distinct operating physiological ranges, but common physiological sensitivity to greater water table depth. Thus, although differences in water-source use exist, a physiological convergence appeared to happen among different functional types. These results strongly suggest that hydrological drought has an important impact on fundamental physiological processes, constraining the performance of woody vegetation under semi-arid conditions. By disentangling the functional responses and vulnerability of woody vegetation to groundwater limitation, our study establishes the basis for predicting the physiological responses of woody vegetation in semi-arid coastal ecosystems to groundwater drawdown.

Disentangling the effects of acidic air pollution, atmospheric CO2 , and climate change on recent growth of red spruce trees in the Central Appalachian Mountains

In the 45 years after legislation of the Clean Air Act, there has been tremendous progress in reducing acidic air pollutants in the eastern United States, yet limited evidence exists that cleaner air has improved forest health. Here, we investigate the influence of recent environmental changes on the growth and physiology of red spruce (Picea rubens Sarg.) trees, a key indicator species of forest health, spanning three locations along a 100 km transect in the Central Appalachian Mountains. We incorporated a multiproxy approach using 75-year tree ring chronologies of basal tree growth, carbon isotope discrimination (∆¹³ C, a proxy for leaf gas exchange), and δ¹⁵ N (a proxy for ecosystem N status) to examine tree and ecosystem level responses to environmental change. Results reveal the two most important factors driving increased tree growth since ca. 1989 are reductions in acidic sulfur pollution and increases in atmospheric CO₂ , while reductions in pollutant emissions of NO_x and warmer springs played smaller, but significant roles. Tree ring ∆¹³ C signatures increased significantly since 1989, concurrently with significant declines in tree ring δ¹⁵ N signatures. These isotope chronologies provide strong evidence that simultaneous changes in C and N cycling, including greater photosynthesis and stomatal conductance of trees and increases in ecosystem N retention, were related to recent increases in red spruce tree growth and are consequential to ecosystem recovery from acidic pollution. Intrinsic water use efficiency (iWUE) of the red spruce trees increased by ~51% across the 75-year chronology, and was driven by changes in atmospheric CO₂ and acid pollution, but iWUE was not linked to recent increases in tree growth. This study documents the complex environmental interactions that have contributed to the recovery of red spruce forest ecosystems from pervasive acidic air pollution beginning in 1989, about 15 years after acidic pollutants started to decline in the United States.

Effects of environmental variation on stable isotope abundances during typical seasonal floodplain dry season litter decomposition

Unique hydrological characteristics and complex topography can create wide-ranging dry season environmental heterogeneity in response to groundwater level across China's Jiangxi Province Poyang Lake wetland. Soil traits are one of several fluctuating environmental variables. To determine the effects of soil variables on stable isotope (δ¹³C and δ¹⁵N) abundances during decomposition, we performed a field experiment using Carex cinerascens along a groundwater level gradient (GT-L: -25 to -50cm, GT-LM: -15 to -25cm, GT-MH: -5 to -15cm, GT-H: 5 to -5cm) in a shallow lake. Twelve soil properties-including total organic carbon (TOC), nitrogen (N), pH, moisture, bulk density, clay, silt, sand, peroxidase, cellulase, microbial biomass carbon (MBC), and microbial biomass nitrogen-were measured in surface soil samples to assess soil environmental conditions. Analyses were performed to determine the effects of soil traits and lignin degradation on changes in stable isotope abundances. This study revealed that stable isotope abundances were significantly lower at high groundwater levels than at low groundwater levels. Lignin degradation was associated with a decrease in both δ¹³C and δ¹⁵N abundances. These two stable isotopes were positively related with soil N and bulk density, but negatively with pH and microbial quotient (MBC/TOC). Variation partitioning analysis (VPA) showed that soil variables and lignin decay rates explained 80.1% of the δ¹³C variation and 42.8% of the δ¹⁵N variation. Soil chemical and biological variables exhibited significant interactions with lignin decay rates, indicating they may affect stable isotope abundances via complex mechanisms. Our results indicate that the change in stable isotope abundances during decomposition may be affected directly by soil variables or indirectly through lignin degradation. Our results provide useful insight for understanding the roles of litter decomposition and soil traits in changing environmental conditions of seasonal floodplain wetlands.

Fenton reaction facilitates organic nitrogen acquisition by an ectomycorrhizal fungus

Boreal trees rely on their ectomycorrhizal fungal symbionts to acquire growth-limiting nutrients, such as nitrogen (N), which mainly occurs as proteins complexed in soil organic matter (SOM). The mechanisms for liberating this N are unclear as ectomycorrhizal fungi have lost many genes encoding lignocellulose-degrading enzymes present in their saprotrophic ancestors. We hypothesized that hydroxyl radicals (^˙ OH), produced by the ectomycorrhizal fungus Paxillus involutus during growth on SOM, are involved in liberating organic N. Paxillus involutus was grown for 7 d on N-containing or N-free substrates that represent major organic compounds of SOM. ^˙ OH production, ammonium assimilation, and proteolytic activity were measured daily. ^˙ OH production was strongly induced when P. involutus switched from ammonium to protein as the main N source. Extracellular proteolytic activity was initiated shortly after the oxidation. Oxidized protein substrates induced higher proteolytic activity than unmodified proteins. Dynamic modeling predicted that ^˙ OH production occurs in a burst, regulated mainly by ammonium and ferric iron concentrations. We propose that the production of ^˙ OH and extracellular proteolytic enzymes are regulated by similar nutritional signals. Oxidation works in concert with proteolysis, improving N liberation from proteins in SOM. Organic N mining by ectomycorrhizal fungi has, until now, only been attributed to proteolysis.

Accounting for the effect of temperature in clarifying the response of foliar nitrogen isotope ratios to atmospheric nitrogen deposition

Atmospheric nitrogen deposition affects nitrogen isotope composition (δ¹⁵N) in plants. However, both negative effect and positive effect have been reported. The effects of climate on plant δ¹⁵N have not been corrected for in previous studies, this has impeded discovery of a true effect of atmospheric N deposition on plant δ¹⁵N. To obtain a more reliable result, it is necessary to correct for the effects of climatic factors. Here, we measured δ¹⁵N and N contents of plants and soils in Baiwangshan and Mount Dongling, north China. Atmospheric N deposition in Baiwangshan was much higher than Mount Dongling. Generally, however, foliar N contents showed no difference between the two regions and foliar δ¹⁵N was significantly lower in Baiwangshan than Mount Dongling. The corrected foliar δ¹⁵N after accounting for a predicted value assumed to vary with temperature was obviously more negative in Baiwangshan than Mount Dongling. Thus, this suggested the necessity of temperature correction in revealing the effect of N deposition on foliar δ¹⁵N. Temperature, soil N sources and mycorrhizal fungi could not explain the difference in foliar δ¹⁵N between the two regions, this indicated that atmospheric N deposition had a negative effect on plant δ¹⁵N. Additionally, this study also showed that the corrected foliar δ¹⁵N of bulk data set increased with altitude above 1300m in Mount Dongling, this provided an another evidence for the conclusion that atmospheric N deposition could cause ¹⁵N-depletion in plants.

Symbiosis constraints: Strong mycobiont control limits nutrient response in lichens

Symbioses such as lichens are potentially threatened by drastic environmental changes. We used the lichen Peltigera aphthosa-a symbiosis between a fungus (mycobiont), a green alga (Coccomyxa sp.), and N2-fixing cyanobacteria (Nostoc sp.)-as a model organism to assess the effects of environmental perturbations in nitrogen (N) or phosphorus (P). Growth, carbon (C) and N stable isotopes, CNP concentrations, and specific markers were analyzed in whole thalli and the partners after 4 months of daily nutrient additions in the field. Thallus N was 40% higher in N-fertilized thalli, amino acid concentrations were twice as high, while fungal chitin but not ergosterol was lower. Nitrogen also resulted in a thicker algal layer and density, and a higher δ13C abundance in all three partners. Photosynthesis was not affected by either N or P. Thallus growth increased with light dose independent of fertilization regime. We conclude that faster algal growth compared to fungal lead to increased competition for light and CO 2 among the Coccomyxa cells, and for C between alga and fungus, resulting in neither photosynthesis nor thallus growth responded to N fertilization. This suggests that the symbiotic lifestyle of lichens may prevent them from utilizing nutrient abundance to increase C assimilation and growth.

Stable isotopes of carbon and nitrogen help to predict the belowground communities at a regional scale

At the regional scale, although environmental factors are known to shape the distributions of belowground communities in terrestrial ecosystems, these environmental factors account for relatively low percentages of the variation in belowground communities. More of this variation might be explained by considering ecosystem stable isotopic values, which can provide insight into environmental conditions. Here, we investigated ecosystem (plant and soil) δ¹³C and δ¹⁵N values and belowground communities (microbes and nematodes) as well as environmental factors (climates, soils, and plants) across the Mongolian Plateau. The regression analyses showed that plant isotopic values were more closely associated with belowground communities than soil isotopic values, while ecosystem δ¹³C values were more closely associated with the belowground communities than ecosystem δ¹⁵N values. We also found isotopic values were more closely associated with nematode communities than microbial communities. Variation partioning analyses indicated that environmental variables together explained 16–45% of total variation in belowground communities. After isotopic variables were added as predictors to the variation partition analyses, the explanation of the variance was improved by14–24% for microbial communities and was improved by 23–44% for nematode communities. These findings indicate that isotopic values could be used to predict the properties of belowground communities at a regional scale.

Heterogeneous environments shape invader impacts: integrating environmental, structural and functional effects by isoscapes and remote sensing

Spatial heterogeneity of ecosystems crucially influences plant performance, while in return plant feedbacks on their environment may increase heterogeneous patterns. This is of particular relevance for exotic plant invaders that transform native ecosystems, yet, approaches integrating geospatial information of environmental heterogeneity and plant-plant interaction are lacking. Here, we combined remotely sensed information of site topography and vegetation cover with a functional tracer of the N cycle, δ15N. Based on the case study of the invasion of an N2-fixing acacia in a nutrient-poor dune ecosystem, we present the first model that can successfully predict (R 2 = 0.6) small-scale spatial variation of foliar δ15N in a non-fixing native species from observed geospatial data. Thereby, the generalized additive mixed model revealed modulating effects of heterogeneous environments on invader impacts. Hence, linking remote sensing techniques with tracers of biological processes will advance our understanding of the dynamics and functioning of spatially structured heterogeneous systems from small to large spatial scales.

Below-ground frontiers in trait-based plant ecology

Contents 1597 I. 1597 II. 1597 III. 1598 IV. 1598 V. 1600 VI. 1601 VII. 1601 VIII. 1601 1602 References 1602 SUMMARY: Trait-based approaches have led to significant advances in plant ecology, but are currently biased toward above-ground traits. It is becoming clear that a stronger emphasis on below-ground traits is needed to better predict future changes in plant biodiversity and their consequences for ecosystem functioning. Here I propose six 'below-ground frontiers' in trait-based plant ecology, with an emphasis on traits governing soil nutrient acquisition: redefining fine roots; quantifying root trait dimensionality; integrating mycorrhizas; broadening the suite of root traits; determining linkages between root traits and abiotic and biotic factors; and understanding ecosystem-level consequences of root traits. Focusing research efforts along these frontiers should help to fulfil the promise of trait-based ecology: enhanced predictive capacity across ecological scales.

Spatial variations in larch needle and soil δ15N at a forest-grassland boundary in northern Mongolia

The spatial patterns of plant and soil δ¹⁵N and associated processes in the N cycle were investigated at a forest-grassland boundary in northern Mongolia. Needles of Larix sibirica Ledeb. and soils collected from two study areas were analysed to calculate the differences in δ¹⁵N between needle and soil (Δδ¹⁵N). Δδ¹⁵N showed a clear variation, ranging from -8 ‰ in the forest to -2 ‰ in the grassland boundary, and corresponded to the accumulation of organic layer. In the forest, the separation of available N produced in the soil with ¹⁵N-depleted N uptake by larch and ¹⁵N-enriched N immobilization by microorganisms was proposed to cause large Δδ¹⁵N, whereas in the grassland boundary, small Δδ¹⁵N was explained by the transport of the most available N into larch. The divergence of available N between larch and microorganisms in the soil, and the accumulation of diverged N in the organic layer control the variation in Δδ¹⁵N.

Soil warming opens the nitrogen cycle at the alpine treeline

Climate warming may alter ecosystem nitrogen (N) cycling by accelerating N transformations in the soil, and changes may be especially pronounced in cold regions characterized by N-poor ecosystems. We investigated N dynamics across the plant-soil continuum during 6 years of experimental soil warming (2007-2012; +4 °C) at a Swiss high-elevation treeline site (Stillberg, Davos; 2180 m a.s.l.) featuring Larix decidua and Pinus uncinata. In the soil, we observed considerable increases in the NH4+ pool size in the first years of warming (by >50%), but this effect declined over time. In contrast, dissolved organic nitrogen (DON) concentrations in soil solutions from the organic layer increased under warming, especially in later years (maximum of +45% in 2012), suggesting enhanced DON leaching from the main rooting zone. Throughout the experimental period, foliar N concentrations showed species-specific but small warming effects, whereas δ¹⁵ N values showed a sustained increase in warmed plots that was consistent for all species analysed. The estimated total plant N pool size at the end of the study was greater (+17%) in warmed plots with Pinus but not in those containing Larix, with responses driven by trees. Irrespective of plot tree species identity, warming led to an enhanced N pool size of Vaccinium dwarf shrubs, no change in that of Empetrum hermaphroditum (dwarf shrub) and forbs, and a reduction in that of grasses, nonvascular plants, and fine roots. In combination, higher foliar δ¹⁵ N values and the transient response in soil inorganic N indicate a persistent increase in plant-available N and greater cumulative plant N uptake in warmer soils. Overall, greater N availability and increased DON concentrations suggest an opening of the N cycle with global warming, which might contribute to growth stimulation of some plant species while simultaneously leading to greater N losses from treeline ecosystems and possibly other cold biomes.

Functional community structure of African monodominant Gilbertiodendron dewevrei forest influenced by local environmental filtering

Monodominant patches of forest dominated by Gilbertiodendron dewevrei are commonly found in central African tropical forests, alongside forests with high species diversity. Although these forests are generally found sparsely distributed along rivers, their occurrence is not thought to be (clearly) driven by edaphic conditions but rather by trait combinations of G. dewevrei that aid in achieving monodominance. Functional community structure between these monodominant and mixed forests has, however, not yet been compared. Additionally, little is known about nondominant species in the monodominant forest community. These two topics are addressed in this study. We investigate the functional community structure of 10 one-hectare plots of monodominant and mixed forests in a central region of the Congo basin, in DR Congo. Thirteen leaf and wood traits are measured, covering 95% (basal area weighted) of all species present in the plots, including leaf nutrient contents, leaf isotopic compositions, specific leaf area, wood density, and vessel anatomy. The trait-based assessment of G. dewevrei shows an ensemble of traits related to water use and transport that could be favorable for its location near forest rivers. Moreover, indications have been found for N and P limitations in the monodominant forest, possibly related to ectomycorrhizal associations formed with G. dewevrei. Reduced leaf N and P contents are found at the community level for the monodominant forest and for different nondominant groups, as compared to those in the mixed forest. In summary, this work shows that environmental filtering does prevail in the monodominant G. dewevrei forest, leading to lower functional diversity in this forest type, with the dominant species showing beneficial traits related to its common riverine locations and with reduced soil N and P availability found in this environment, both coregulating the tree community assembly.

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.

Fractionation and assimilation of Mg isotopes by fungi is species dependent

Symbiotic ectomycorrhizal fungi mobilize nutrients from both organic and inorganic substrates and supply them to their host plants. Their role in mobilizing base cations and phosphorus from mineral substrates through weathering has received increasing attention in recent years but the processes involved remain to be elucidated. We grew selected ectomycorrhizal and nonmycorrhizal fungi in axenic systems containing mineral and organic substrates and examined their capacity to fractionate and assimilate stable isotopes of magnesium. The mycorrhizal fungi were significantly depleted in heavy isotopes with the lowest Δ²⁶ Mg values (the difference between δ²⁶ Mg in fungal tissue and δ²⁶ Mg in the substrate) compared with nonmycorrhizal fungi, when grown on mineral substrates containing granite particles. The ectomycorrhizal fungi accumulated significantly higher concentrations of Mg, K and P than the nonmycorrhizal fungi. There was a highly significant statistical relationship between δ²⁶ Mg tissue signature and mycelial concentration of Mg, with a clear separation between most ectomycorrhizal fungi and the nonmycorrhizal fungi. These results are consistent with the idea that ectomycorrhizal fungi have evolved efficient mechanisms to mobilize, transport and store Mg within their mycelia.

Plant δ15 N reflects the high landscape-scale heterogeneity of soil fertility and vegetation productivity in a Mediterranean semiarid ecosystem

We investigated the magnitude and drivers of spatial variability in soil and plant δ¹⁵ N across the landscape in a topographically complex semiarid ecosystem. We hypothesized that large spatial heterogeneity in water availability, soil fertility and vegetation cover would be positively linked to high local-scale variability in δ¹⁵ N. We measured foliar δ¹⁵ N in three dominant plant species representing contrasting plant functional types (tree, shrub, grass) and mycorrhizal association types (ectomycorrhizal or arbuscular mycorrhizal). This allowed us to investigate whether δ¹⁵ N responds to landscape-scale environmental heterogeneity in a consistent way across species. Leaf δ¹⁵ N varied greatly within species across the landscape and was strongly spatially correlated among co-occurring individuals of the three species. Plant δ¹⁵ N correlated tightly with soil δ¹⁵ N and key measures of soil fertility, water availability and vegetation productivity, including soil nitrogen (N), organic carbon (C), plant-available phosphorus (P), water-holding capacity, topographic moisture indices and normalized difference vegetation index. Multiple regression models accounted for 62-83% of within-species variation in δ¹⁵ N across the landscape. The tight spatial coupling and interdependence of the water, N and C cycles in drylands may allow the use of leaf δ¹⁵ N as an integrative measure of variations in moisture availability, biogeochemical activity, soil fertility and vegetation productivity (or 'site quality') across the landscape.

Provenancing Archaeological Wool Textiles from Medieval Northern Europe by Light Stable Isotope Analysis (δ13C, δ15N, δ2H)

We investigate the origin of archaeological wool textiles preserved by anoxic waterlogging from seven medieval archaeological deposits in north-western Europe (c. 700–1600 AD), using geospatial patterning in carbon (δ¹³C), nitrogen (δ¹⁵N) and non-exchangeable hydrogen (δ²H) composition of modern and ancient sheep proteins. δ¹³C, δ¹⁵N and δ²H values from archaeological wool keratin (n = 83) and bone collagen (n = 59) from four sites were interpreted with reference to the composition of modern sheep wool from the same regions. The isotopic composition of wool and bone collagen samples clustered strongly by settlement; inter-regional relationships were largely parallel in modern and ancient samples, though landscape change was also significant. Degradation in archaeological wool samples, examined by elemental and amino acid composition, was greater in samples from Iceland (Reykholt) than in samples from north-east England (York, Newcastle) or northern Germany (Hessens). A nominal assignment approach was used to classify textiles into local/non-local at each site, based on maximal estimates of isotopic variability in modern sheep wool. Light element stable isotope analysis provided new insights into the origins of wool textiles, and demonstrates that isotopic provenancing of keratin preserved in anoxic waterlogged contexts is feasible. We also demonstrate the utility of δ²H analysis to understand the location of origin of archaeological protein samples.

Nitrogen isotope fractionation during N uptake via arbuscular mycorrhizal and ectomycorrhizal fungi into grey alder

Arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi affect plant nitrogen (N) dynamics. Plant N isotope patterns have been used to characterise the contribution of ECM fungi to plant N uptake. By quantifying and comparing the effects of an AM and an ECM fungus on growth, N uptake and isotopic composition of one host plant grown at different relative N supply levels, the aim of this study was to improve the mechanistic understanding of natural ¹⁵N abundance patterns in mycorrhizal plants and their underlying causes. Grey alders were inoculated with one ECM fungus or one AM fungus or left non-mycorrhizal. Plants were grown under semi-hydroponic conditions and were supplied with three rates of relative N supply ranging from deficient to luxurious. Neither mycorrhizal fungus increased plant growth or N uptake. AM root colonisation had no effect on whole plant δ¹⁵N and decreased foliar δ ¹⁵N only under N deficiency. The roots of these plants were ¹⁵N-enriched. ECM root colonisation consistently decreased foliar and whole plant δ¹⁵N. It is concluded, that both mycorrhizal fungi contributed to plant N uptake into the shoot. Nitrogen isotope fractionation during N assimilation and transformations in fungal mycelia is suggested to have resulted in plants receiving ¹⁵N-depleted N via the mycorrhizal uptake pathways. Negative mycorrhizal growth effects are explained by symbiotic resource trade on carbon and N and decreased direct plant N uptake.

Red deer bone and antler collagen are not isotopically equivalent in carbon and nitrogen

RATIONALE

Bone and antler collagen δ(13) C and δ(15) N values are often assumed to be equivalent when measured in palaeodietary, palaeoclimate and palaeocological studies. Although compositionally similar, bone grows slowly and is remodelled whereas antler growth is rapid and remodelling does not occur. These different patterns of growth could result in isotopic difference within antler and between the two tissue types. Here we test whether red deer (Cervus elaphus) bone and antler δ(13) C and δ(15) N values are equivalent, and whether intra-antler isotopic values are uniform.

METHODS

Bone and antler were isotopically analysed from six stags that lived in a temperate maritime climate on the Isle of Rum, Scotland. Multiple antlers from different years were sampled per individual, together with a single bone sample per individual. Up to 12 samples were taken along the length of each antler (total of 25 antlers, 259 samples) so that a chronological record of the isotopic composition during antler growth could be obtained. Collagen was extracted and its δ(13) C and δ(15) N values were measured by continuous-flow isotope ratio mass spectrometry.

RESULTS

Intra-antler collagen isotope signatures vary, and show that not all antlers from an individual or a growth year are equivalent in carbon and nitrogen isotopic ratios. δ(15) N values typically increase with distance along antler length, but no overall trend is observed in δ(13) C values. An isotopic offset is visible between bone and antler, with bone δ(13) C and δ(15) N values being higher in most cases.

CONCLUSIONS

Bone and antler collagen δ(13) C and δ(15) N values are not isotopically equivalent and are therefore not directly comparable in palaeodietary, palaeoclimate and palaeocological studies. Bone and antler collagen isotopic differences probably relate to differential metabolic processes during the formation of the two tissues. Intra- and inter-antler isotopic variations probably reflect the isotopic composition of an individual's diet rather than physiological parameters, and may have the potential to provide high-resolution individual-specific information in modern and ancient cervid populations. Copyright © 2016 John Wiley & Sons, Ltd.

Geographical variability in northern European sheep wool isotopic composition (δ(13) C, δ(15) N, δ(2) H values)

RATIONALE

Light stable isotopic analysis of herbivore proteinaceous tissues (hair, muscle, milk) is critical for authenticating the point of origin of finished agricultural or industrial products in both ancient and modern economies. This study examined the distribution of light stable isotopes in herbivores in northern Europe (Iceland to Finland), which is expected to depend on regional-level environmental inputs (precipitation, temperature) and local variables (vegetation type, fodder type, soil type).

METHODS

Sheep wool was obtained from animals managed using traditional methods and located across a gradient of northern European environments. Defatted whole-year samples were analysed by isotope ratio mass spectrometry (IRMS) for carbon (δ(13) C values), nitrogen (δ(15) N values) and un-exchangeable hydrogen (δ(2) H values) isotopic composition.

RESULTS

Wool δ(13) C, δ(15) N and δ(2) H values showed the same correlations to local mean annual precipitation and temperature as were expected for graze plants. Wool δ(2) H values were correlated with local modelled meteoric water δ(2) H values, mediated by plant solid tissue and leaf water fractionations. Cluster analysis distinguished wool from Sweden and the Baltic region from more western material. Local variation in vegetation or soil type did not disrupt dependence on climatic variables but did affect geospatial discrimination.

CONCLUSIONS

Wool isotopic composition in northern Europe is controlled by the effects of local precipitation and temperature on graze plant inputs, and is only weakly affected by pasture type. Copyright © 2016 John Wiley & Sons, Ltd.

Early-stage changes in natural (13)C and (15)N abundance and nutrient dynamics during different litter decomposition

Decomposition, nutrient, and isotopic (δ(13)C and δ(15)N) dynamics during 1 year were studied for leaf and twig litters of Pinus densiflora, Castanea crenata, Erigeron annuus, and Miscanthus sinensis growing on a highly weathered soil with constrained nutrient supply using litterbags in a cool temperate region of South Korea. Decay constant (k/year) ranged from 0.58 to 1.29/year, and mass loss ranged from 22.36 to 58.43 % among litter types. The results demonstrate that mass loss and nutrient dynamics of decomposing litter were influenced by the seasonality of mineralization and immobilization processes. In general, most nutrients exhibited alternate phases of rapid mineralization followed by gradual immobilization, except K, which was released throughout the field incubation. At the end of study, among all the nutrients only N and P showed net immobilization. Mobility of different nutrients from decomposing litter as the percentage of initial litter nutrient concentration was in the order of K > Mg > Ca > N ≈ P. The δ(13)C (0.32-6.70 ‰) and δ(15)N (0.74-3.90 ‰) values of residual litters showed nonlinear increase and decrease, respectively compared to initial isotopic values during decomposition. Litter of different functional types and chemical quality converged toward a conservative nutrient use strategy through mechanisms of slow decomposition and slow nutrient mobilization. Our results indicate that litter quality and season, are the most important regulators of litter decomposition in these forests. The results revealed significant relationships between litter decomposition rates and N, C:N ratio and P, and seasonality (temperature). These results and the convergence of different litters towards conservative nutrient use in these nutrient constrained ecosystems imply optimization of litter management because litter removal can have cascading effects on litter decomposition and nutrient availability in these systems.

Chemical and Isotopic Thresholds in Charring: Implications for the Interpretation of Charcoal Mass and Isotopic Data

Charcoal plays a significant role in the long-term carbon cycle, and its use as a soil amendment is promoted as a C sequestration strategy (biochar). One challenge in this research area is understanding the heterogeneity of charcoal properties. Although the maximum reaction temperature is often used as a gauge of pyrolysis conditions, pyrolysis duration also changes charcoal physicochemical qualities. Here, we introduce a formal definition of charring intensity (CI) to more accurately characterize pyrolysis, and we document variation in charcoal chemical properties with variation in CI. We find two types of responses to CI: either linear or threshold relationships. Mass yield decreases linearly with CI, while a threshold exists across which % C, % N, and δ(15)N exhibit large changes. This CI threshold co-occurs with an increase in charcoal aromaticity. C isotopes do not change from original biomass values, supporting the use of charcoal δ(13)C signatures to infer paleoecological conditions. Fractionation of N isotopes indicates that fire may be enriching soils in (15)N through pyrolytic N isotope fractionation. This influx of "black N" could have a significant impact on soil N isotopes, which we show theoretically using a simple mass-balance model.