Warmed Plots Research Articles

Experimental warming alters the community composition, diversity, and N2 fixation activity of peat moss (Sphagnum fallax) microbiomes.

Sphagnum‐dominated peatlands comprise a globally important pool of soil carbon (C) and are vulnerable to climate change. While peat mosses of the genus Sphagnum are known to harbor diverse microbial communities that mediate C and nitrogen (N) cycling in peatlands, the effects of climate change on Sphagnum microbiome composition and functioning are largely unknown. We investigated the impacts of experimental whole‐ecosystem warming on the Sphagnum moss microbiome, focusing on N2 fixing microorganisms (diazotrophs). To characterize the microbiome response to warming, we performed next‐generation sequencing of small subunit (SSU) rRNA and nitrogenase (nifH) gene amplicons and quantified rates of N2 fixation activity in Sphagnum fallax individuals sampled from experimental enclosures over 2 years in a northern Minnesota, USA bog. The taxonomic diversity of overall microbial communities and diazotroph communities, as well as N2 fixation rates, decreased with warming (p < 0.05). Following warming, diazotrophs shifted from a mixed community of Nostocales (Cyanobacteria) and Rhizobiales (Alphaproteobacteria) to predominance of Nostocales. Microbiome community composition differed between years, with some diazotroph populations persisting while others declined in relative abundance in warmed plots in the second year. Our results demonstrate that warming substantially alters the community composition, diversity, and N2 fixation activity of peat moss microbiomes, which may ultimately impact host fitness, ecosystem productivity, and C storage potential in peatlands.

Responses of wetland soil carbon and nutrient pools and microbial activities after 7 years of experimental warming in the Yangtze Estuary

Wetlands play an important role in global carbon (C) cycling. However, little is known about the effects of climate warming on wetland C cycling, which may bias our prediction of wetland functioning under global climate change. This study was undertaken to examine the potential changes in soil C, nitrogen (N) and phosphorus (P) pools and the related microbial activities after 7 years of in-situ experimental warming by open-top chambers (OTCs) in a Phragmites wetland in the Yangtze Estuary. The mean soil temperature, moisture, and salinity in both the 0–10 cm and 10–40 cm soil layers were significantly increased by the warming treatment. In the upper 10 cm soil layer, significant increases in total organic C (TOC) and total N (TN) were observed under the warming treatment, while the phenomenon were further clarified by comparing the year-round data in the warming plots between 2008 and 2015; However, the molar C/molar N ratio (C/N) and total phosphorus (TP) were not affected. In both layers, microbial biomass C (MBC) and the molar MBC/molar MBN ratio (MBC/MBN) showed no significant responses, but microbial biomass N (MBN) and microbial biomass P (MBP) were increased by warming. Warming did not affect MBC/TOC, but it significantly increased both MBN/TN and MBP/TP throughout the 0–40 cm soil profile. Invertase activity was not affected in the 0–40 cm soil profile. The acid phosphatase, alkaline phosphatase, and urease activities in the upper 10 cm soil layer were increased by warming. Only alkaline phosphatase activity was increased by warming in the 10–40 cm soil layer. These field-experimental warming results suggested that in temperate P. australis wetlands without tidal impacts, climate warming might increase the soil organic C pools while exerting no effect on microbial C decomposition, whereas the N and P cycling that is mediated by soil microbes might be enhanced by moderate warming.

Warming increases microbial residue contribution to soil organic carbon in an alpine meadow

The contribution of microbial residues to soil organic carbon (SOC) is a process highly influenced by soil properties. We evaluated the presence of microbial amino sugar residues in soil (0–50 cm) of control and warmed plots in an alpine meadow on the Qinghai-Tibet Plateau. Alpine grasslands in the Qinghai-Tibet Plateau store large amounts of soil C and are highly vulnerable to climate change. Results showed that warming significantly increased total microbial residues across the 0–50 cm soil depth. The proportion of microbial-derived C to SOC significantly increased in warmed plots (52% on average) by soil depth compared to the control (38%). Higher microbial turnover and selective preservation into organo-mineral complexes likely explains the observed result. Given insignificant change in total SOC, our results infer an alteration of the SOC source configuration (microbial-derived vs. plant-derived). The observed greater magnitude of warming effects on fungal residues compared to bacterial illustrate a distinct community response to warming. We conclude that warming has the potential to influence soil C sequestration through increased microbial residue inputs, consequently altering its composition and source configuration. Our work provides valuable insights at the molecular level to identify mechanisms of microbial-mediated C processes that are influenced by climate change in high elevation ecosystems.

Abnormal Shoot Responses of Pinus densiflora Seedlings to Open-field Experimental Warming and Precipitation Manipulation

This study was conducted to investigate abnormal shoot responses in Pinus densiflora seedlings to open-field warming and precipitation manipulation. In May 2017, 18 plots (2 temperature levels × 3 precipitation levels × 3 replicates) were constructed and 90 seedlings of 1-year-old P. densiflora were planted in each plot. The temperature in the temperature warming (TW) plots was set to 3℃ higher than that in the temperature control (TC) plots, and precipitation was set to decrease (PD) or increase (PI) by 40% compared to the precipitation control (PC) plots. The manipulation experiment started in June, 2017. The abnormal shoot occurrence rates in September and November, 2017; seedling number in each needle length growth stage in September, 2017; and length of abnormal shoots in November, 2017 were measured. Warming significantly increased the occurrence rate and developed the needle length growth stage of abnormal shoots in P. densiflora seedlings, whereas precipitation and interaction effects between the two treatments were not significant. The mean occurrence rates in September and November were 86.17 ± 11.80% and 90.78 ± 7.92% in the TW plots and 58.59 ± 16.13% and 64.80 ± 14.71% in the TC plots, respectively. Thus, the abnormal conditions derived from increased temperature stimulated the occurrence of abnormal shoots. Warming also elongated the needle length of the abnormal shoots, which might affect future photosynthesis. However, there was no significant difference in abnormal shoot length under warming and precipitation manipulation. Thus, warming stimulated the occurrence and needle development of abnormal shoots in P. densiflora seedlings, which could lead to the inhibition of subsequent growth by the use of reserved nutrients and degradation of seedling quality.

Climate warming and plant biomechanical defences: Silicon addition contributes to herbivore suppression in a pasture grass

Abstract Plants, notably the Poaceae, often accumulate large amounts of silicon (Si) from the soil. Si has multiple functional roles, particularly for alleviating abiotic and biotic stresses (e.g., defence against herbivores). Recent evidence suggests that environmental change, including temperature changes, can diminish Si accumulation which could affect functions such as herbivore defence. Using a field warming experiment, we grew a pasture grass (Phalaris aquatica) that was either supplemented or untreated with Si (+Si and −Si, respectively) under ambient and elevated (+2.8°C above ambient) air temperatures. We quantified soil water, plant growth rates, Si accumulation, leaf biomechanical properties and in situ relative growth rates of a herbivorous global insect pest (Helicoverpa armigera). Si supplementation promoted shoot and root biomass by c. 48% and 61%, respectively under ambient temperatures, but these gains were not apparent under warmed conditions. Warmer temperatures reduced Si uptake by −Si plants by c. 17%, potentially due to the lower levels of soil water content in warmed plots. Si supplementation, however, increased Si accumulation in leaves by c. 24% in warmed plots restoring Si levels to those seen under ambient temperatures. Si supplementation enhanced biomechanical properties in the leaves, but this was only statistically significant under ambient temperatures; leaves of +Si plants required 42% more force to fracture and were 30% tougher at the midrib than leaves of −Si plants. The relative growth rates of H. armigera declined by 56% when feeding on +Si plants under ambient temperatures, and while Si supplementation caused a trend towards declining herbivore growth rates under warmer conditions, this was not statistically significant. We conclude that climate warming may mitigate the beneficial effects of Si on Phalaris aquatica in the short term, potentially by reducing Si uptake. While Si uptake can be restored with Si supplementation, Si‐enhanced biomechanical defences against a global pest may not be fully restored under warmer temperatures. A plain language summary is available for this article.

Species specific physiological responses of Pinus densiflora and Larix kaempferi seedlings to open-field experimental warming and precipitation manipulation

Physiological responses of 1-year-old Pinus densiflora and Larix kaempferi seedlings were measured under open-field warming and precipitation manipulation. Air temperature of warming plots was 3 °C higher, while precipitation manipulation plots received ±40% of the precipitation than control plots. Seedlings were planted in May, and temperature and precipitation were manipulated from June 2017. Total chlorophyll content (Chlt), net photosynthetic rate (Pn), transpiration rate (E), and stomatal conductance (gs) were measured between July and September 2017. For P. densiflora and L. kaempferi, Chlt increased by 11.75% and 11.64%, and Pn decreased by 9.14% and 2.17% under warming, respectively. E and gs were lower under warming in P. densiflora, but higher in L. kaempferi. The lower Pn in P. densiflora resulted from stomatal closure, while that of L. kaempferi resulted from reduced vitality. Lower precipitation increased Chlt and Pn by 11.64% and 2.66% for P. densiflora, and by 6.40% and 4.32% for L. kaempferi, respectively. Conversely, higher precipitation decreased Pn of P. densiflora by 5.72%, and decreased Chlt and Pn of L. kaempferi by 8.24% and 4.55%, respectively. These results can be attributed to concentrated precipitation. In this study, two species responded differently even when they were exposed to the same environmental conditions, and this was due to the species-specific mechanisms to water stress derived from the high temperature.

Warming accelerates mangrove expansion and surface elevation gain in a subtropical wetland

Abstract Climatic warming can change how coastal wetland plants grow, thus altering their capacity to build land and keep pace with rising seas. As freeze events decline with climate change, mangroves expand their range to higher latitudes and displace salt marsh vegetation. Warmer air temperatures will likely alter above‐ and below‐ground plant dynamics as this dramatic coastal wetland biome shift proceeds, which in turn may result in changes in ecosystem function such as sediment building. We used a large scale in situ warming experiment in a subtropical wetland to increase both marsh and mangrove ecosystem air temperatures. We assessed how 2 years of continuous warming influenced above‐ and below‐ground plant growth and surface elevation relative to sea level. We found that chronic warming doubled plant height and accelerated the expansion of mangrove into salt marsh vegetation, as indicated by a sixfold greater increase in mangrove cover in warmed plots compared to ambient temperature plots and a corresponding loss in salt marsh cover. Surface elevation gain, a measure of soil‐building capacity, increased due to warming over a 2‐year period and these changes in surface elevation were driven by increased mangrove root production in warmed plots. Synthesis. Our findings suggest that, in some coastal wetlands, warming can facilitate plant community changes from marsh to mangrove, with corresponding increases in growth that help coastal wetlands to keep pace with sea‐level rise.

Effects of Seasonal Asymmetric Warming on Soil CO2 Release in Karst Region

Seasonal asymmetric warming is one of the distinguishing features of global warming. However, if this feature is not considered in studying the effects of global changes on terrestrial ecosystems, it might probably cause misunderstanding of these studies. The releasing features of soil CO2 in Karst regions under various warming scenarios were simulated following a four-year continuous warming period using infrared radiators. A total of six treatments was arranged:no warming (ambient temp, CK); symmetric warming (ambient plus 2.0℃ full year, SW); and, lowly, moderately, highly, and extremely asymmetric warming (ambient plus 2.5℃/1.5℃, 3.0℃/1.0℃, 3.5℃/0.5℃, and 4.0℃/0℃ in the winter-spring/summer-autumn seasons, respectively, LAW, MAW, HAW, and EAW). The results showed that compared to CK, soil CO2 efflux in all the warming plots significantly increased by 0.26 μmol·(m2·s)-1, or 17.41%. In the winter-spring seasons, soil CO2 efflux in the warming treatments increased by 0.23 μmol·(m2·s)-1. The Q10 values ranged from 1.53 to 3.24 with an average of 2.23 under the scenario of warming up by 2.0℃. The warming-induced contribution of CO2 efflux in the summer-autumn seasons (80%) was obviously higher than that in the winter-spring seasons (20%) in the SW treatment, whereas the mean contribution in the summer-autumn seasons (46%) was closer to that in the winter-spring seasons (54%) in the asymmetric warming treatments. Both soil CO2 efflux and Q10 showed a tendency towards decrease with the increase in the asymmetry of warming under the five warming scenarios. The soil CO2 efflux in the SW treatment was significantly (P<0.05) higher than those in the MAW, HAW, and EAW treatments. The Q10 values in the summer-autumn seasons was larger than those in the winter-spring seasons under each warming treatment or across all warming treatments, which was probably related to soil water content, soil microbe, dissolved inorganic carbon, and vegetation growth. The results revealed that it may potentially overestimate the effects of global warming on soil CO2 releasing subject to symmetric warming.

Soil respiration and extracellular enzyme production respond differently across seasons to elevated temperatures

The activity of extracellular enzymes is one control on soil organic matter decomposition and serves as a driver of heterotrophic soil respiration. To understand how temperature sensitive enzyme reactions will influence the processing of soil carbon substrates at elevated temperatures, we deployed a passive warming experiment in a deciduous forest with highly invaded understory plant communities and high soil nitrogen concentrations. We seasonally assessed six extracellular enzyme activities and measured soil respiration in the field and in laboratory incubations, to determine if soil carbon use and nutrient cycling responded to 3.5 years of warming. Field measurements indicate soil respiration was 24% lower and the production of the recalcitrant C-acquiring enzyme phenoloxidase was also lower in warmed plots during fall and winter. In the spring, phosphate (P) acquiring enzyme activity increased in response to warming. In lab incubations, soil respiration was not different between warmed and control soils. Despite minimal changes to C stores after 3.5 years of warming, recalcitrant C-use and nutrient processing enzymes responded differentially to higher temperatures in fall, winter, and spring. This suggests that C- and nutrient cycle responses to warming can change throughout the seasons, perhaps mediated by plant phenological changes.

Long-Term Warming Shifts the Composition of Bacterial Communities in the Phyllosphere of Galium album in a Permanent Grassland Field-Experiment.

Global warming is currently a much discussed topic with as yet largely unexplored consequences for agro-ecosystems. Little is known about the warming effect on the bacterial microbiota inhabiting the plant surface (phyllosphere), which can have a strong impact on plant growth and health, as well as on plant diseases and colonization by human pathogens. The aim of this study was to investigate the effect of moderate surface warming on the diversity and composition of the bacterial leaf microbiota of the herbaceous plant Galium album. Leaves were collected from four control and four surface warmed (+2°C) plots located at the field site of the Environmental Monitoring and Climate Impact Research Station Linden in Germany over a 6-year period. Warming had no effect on the concentration of total number of cells attached to the leaf surface as counted by Sybr Green I staining after detachment, but changes in the diversity and phylogenetic composition of the bacterial leaf microbiota analyzed by bacterial 16S rRNA gene Illumina amplicon sequencing were observed. The bacterial phyllosphere microbiota were dominated by Proteobacteria, Bacteroidetes, and Actinobacteria. Warming caused a significant higher relative abundance of members of the Gammaproteobacteria, Actinobacteria, and Firmicutes, and a lower relative abundance of members of the Alphaproteobacteria and Bacteroidetes. Plant beneficial bacteria like Sphingomonas spp. and Rhizobium spp. occurred in significantly lower relative abundance in leaf samples of warmed plots. In contrast, several members of the Enterobacteriaceae, especially Enterobacter and Erwinia, and other potential plant or human pathogenic genera such as Acinetobacter and insect-associated Buchnera and Wolbachia spp. occurred in higher relative abundances in the phyllosphere samples from warmed plots. This study showed for the first time the long-term impact of moderate (+2°C) surface warming on the phyllosphere microbiota on plants. A reduction of beneficial bacteria and an enhancement of potential pathogenic bacteria in the phyllosphere of plants may indicate that this aspect of the ecosystem which has been largely neglected up till now, can be a potential risk for pathogen transmission in agro-ecosystems in the near future.

Contrasting above- and belowground organic matter decomposition and carbon and nitrogen dynamics in response to warming in High Arctic tundra.

Tundra regions are projected to warm rapidly during the coming decades. The tundra biome holds the largest terrestrial carbon pool, largely contained in frozen permafrost soils. With warming, these permafrost soils may thaw and become available for microbial decomposition, potentially providing a positive feedback to global warming. Warming may directly stimulate microbial metabolism but may also indirectly stimulate organic matter turnover through increased plant productivity by soil priming from root exudates and accelerated litter turnover rates. Here, we assess the impacts of experimental warming on turnover rates of leaf litter, active layer soil and thawed permafrost sediment in two high-arctic tundra heath sites in NE-Greenland, either dominated by evergreen or deciduous shrubs. We incubated shrub leaf litter on the surface of control and warmed plots for 1 and 2years. Active layer soil was collected from the plots to assess the effects of 8years of field warming on soil carbon stocks. Finally, we incubated open cores filled with newly thawed permafrost soil for 2years in the active layer of the same plots. After field incubation, we measured basal respiration rates of recovered thawed permafrost cores in the lab. Warming significantly reduced litter mass loss by 26% after 1year incubation, but differences in litter mass loss among treatments disappeared after 2years incubation. Warming also reduced litter nitrogen mineralization and decreased the litter carbon to nitrogen ratio. Active layer soil carbon stocks were reduced 15% by warming, while soil dissolved nitrogen was reduced by half in warmed plots. Warming had a positive legacy effect on carbon turnover rates in thawed permafrost cores, with 10% higher respiration rates measured in cores from warmed plots. These results demonstrate that warming may have contrasting effects on above- and belowground tundra carbon turnover, possibly governed by microbial resource availability.

Warming, shading and a moth outbreak reduce tundra carbon sink strength dramatically by changing plant cover and soil microbial activity

Future increases in temperature and cloud cover will alter plant growth and decomposition of the large carbon pools stored in Arctic soils. A better understanding of interactions between above- and belowground processes and communities of plants and microorganisms is essential for predicting Arctic ecosystem responses to climate change. We measured ecosystem CO2 fluxes during the growing season for seven years in a dwarf-shrub tundra in West Greenland manipulated with warming and shading and experiencing a natural larvae outbreak. Vegetation composition, soil fungal community composition, microbial activity, and nutrient availability were analyzed after six years of treatment. Warming and shading altered the plant community, reduced plant CO2 uptake, and changed fungal community composition. Ecosystem carbon accumulation decreased during the growing season by 61% in shaded plots and 51% in warmed plots. Also, plant recovery was reduced in both manipulations following the larvae outbreak during the fifth treatment year. The reduced plant recovery in manipulated plots following the larvae outbreak suggests that climate change may increase tundra ecosystem sensitivity to disturbances. Also, plant community changes mediated via reduced light and reduced water availability due to increased temperature can strongly lower the carbon sink strength of tundra ecosystems.

Digging into the roots of belowground carbon cycling following seven years of Prairie Heating and CO2 Enrichment (PHACE), Wyoming USA

Grassland soils are significant carbon (C) sinks as more than half of grassland plant biomass is belowground and roots are the main source of soil C. It is uncertain if grassland soils will continue as C sinks in the future because climate change may affect the dynamic, belowground relationships among crown and root biomass, root chemistry and morphology, and root and soil decomposition, all of which influence C sequestration potential. To better understand future belowground C cycling in semiarid grasslands we analyzed three native species (Bouteloua gracilis, Carex eleocharis, and Pascopyrum smithii) and mixed-grass community crown and root biomass, root chemistry, morphology, and decomposability, and soil organic carbon (SOC) priming following seven years of simulated climate change at the Prairie Heating and CO2 Enrichment (PHACE) experiment in Wyoming, USA. We found that individual species and the community respond uniquely to the climate change field treatments, indicating that species composition is important when analyzing climate change effects on grassland C cycling. Root biomass in the C3 sedge, C. eleocharis, increased under elevated CO2, especially when combined with warming. Decomposition rates of roots from warming plots were higher than those from ambient plots for B. gracilis and P. smithii. Across species, root decomposition rates increased with C and N concentrations. Root morphology was altered as well: B. gracilis root diameter increased under warming, and P. smithii specific root length and surface area increased under elevated CO2. P. smthii roots induced short-term, negative SOC priming across all field treatments. Together, our results indicate that grass roots may play a critical role in maintaining soil C stocks in grasslands in the future.

Warmer winters increase the rhizosphere carbon flow to mycorrhizal fungi more than to other microorganisms in a temperate grassland.

A decisive set of steps in the terrestrial carbon (C) cycle is the fixation of atmospheric C by plants and the subsequent C-transfer to rhizosphere microorganisms. With climate change winters are expected to become milder in temperate ecosystems. Although the rate and pathways of rhizosphere C input to soil could be impacted by milder winters, the responses remain unknown. To address this knowledge-gap, a winter-warming experiment was established in a seminatural temperate grassland to follow the C flow from atmosphere, via the plants, to different groups of soil microorganisms. In situ 13 CO2 pulse labelling was used to track C into signature fatty acids of microorganisms. The winter warming did not result in any changes in biomass of any of the groups of microorganisms. However, the C flow from plants to arbuscular mycorrhizal (AM) fungi, increased substantially by winter warming. Saprotrophic fungi also received large amounts of plant-derived C-indicating a higher importance for the turnover of rhizosphere C than biomass estimates would suggest-still, this C flow was unaffected by winter warming. AM fungi was the only microbial group positively affected by winter warming-the group with the closest connection to plants. Winter warming resulted in higher plant productivity earlier in the season, and this aboveground change likely induced plant nutrient limitation in warmed plots, thus stimulating the plant dependence on, and C allocation to, belowground nutrient acquisition. The preferential C allocation to AM fungi was at the expense of C flow to other microbial groups, which were unaffected by warming. Our findings imply that warmer winters may shift rhizosphere C-fluxes to become more AM fungal-dominated. Surprisingly, the stimulated rhizosphere C flow was matched by increased microbial turnover, leading to no accumulation of soil microbial biomass.

Enhanced decomposition of stable soil organic carbon and microbial catabolic potentials by long-term field warming.

Quantifying soil organic carbon (SOC) decomposition under warming is critical to predict carbon-climate feedbacks. According to the substrate regulating principle, SOC decomposition would decrease as labile SOC declines under field warming, but observations of SOC decomposition under warming do not always support this prediction. This discrepancy could result from varying changes in SOC components and soil microbial communities under warming. This study aimed to determine the decomposition of SOC components with different turnover times after subjected to long-term field warming and/or root exclusion to limit C input, and to test whether SOC decomposition is driven by substrate lability under warming. Taking advantage of a 12-year field warming experiment in a prairie, we assessed the decomposition of SOC components by incubating soils from control and warmed plots, with and without root exclusion for 3years. We assayed SOC decomposition from these incubations by combining inverse modeling and microbial functional genes during decomposition with a metagenomic technique (GeoChip). The decomposition of SOC components with turnover times of years and decades, which contributed to 95% of total cumulative CO2 respiration, was greater in soils from warmed plots. But the decomposition of labile SOC was similar in warmed plots compared to the control. The diversity of C-degradation microbial genes generally declined with time during the incubation in all treatments, suggesting shifts of microbial functional groups as substrate composition was changing. Compared to the control, soils from warmed plots showed significant increase in the signal intensities of microbial genes involved in degrading complex organic compounds, implying enhanced potential abilities of microbial catabolism. These are likely responsible for accelerated decomposition of SOC components with slow turnover rates. Overall, the shifted microbial community induced by long-term warming accelerates the decomposition of SOC components with slow turnover rates and thus amplify the positive feedback to climate change.

Community and species-specific responses of plant traits to 23 years of experimental warming across subarctic tundra plant communities

To improve understanding of how global warming may affect competitive interactions among plants, information on the responses of plant functional traits across species to long-term warming is needed. Here we report the effect of 23 years of experimental warming on plant traits across four different alpine subarctic plant communities: tussock tundra, Dryas heath, dry heath and wet meadow. Open-top chambers (OTCs) were used to passively warm the vegetation by 1.5–3 °C. Changes in leaf width, leaf length and plant height of 22 vascular plant species were measured. Long-term warming significantly affected all plant traits. Overall, plant species were taller, with longer and wider leaves, compared with control plots, indicating an increase in biomass in warmed plots, with 13 species having significant increases in at least one trait and only three species having negative responses. The response varied among species and plant community in which the species was sampled, indicating community-warming interactions. Thus, plant trait responses are both species- and community-specific. Importantly, we show that there is likely to be great variation between plant species in their ability to maintain positive growth responses over the longer term, which might cause shifts in their relative competitive ability.

The fate of nitrogen inputs in a warmer alpine treeline ecosystem: a 15N labelling study

Summary Global warming may accelerate nitrogen (N) transformations in the soil, with potentially large effects in N‐poor high‐elevation ecosystems. To gain insight into the partitioning of inorganic and organic N inputs within the plant–soil system and how warming influences these patterns, we applied a 15N label (15NH4Cl or 15N‐glycine) shortly after snowmelt during the sixth year of experimental soil warming (+4 °C) at treeline in the Swiss Alps. Seven weeks after labelling, approximately 60% of the applied label remained in the soil organic layer to 10 cm depth, whereas label recovery summed over all measured plant pools was &lt;10% of the added label. Soil warming led to a weaker Δ15N signal in plants and no change in the amount of added label recovered in plants. This 15N dilution resulted from a greater N pool size of some plant species in warmed plots as well as enhanced availability of (unlabelled) N under warming. Temporal dynamics of foliar Δ15N in dominant dwarf shrub species suggested that these plants primarily take up N early in the season. In a subset of plots labelled with 13C‐enriched glycine (U‐13C2‐15N‐glycine), the labelled glycine was mineralized rapidly, with approximately 50% of the applied 13C respired as CO2 during the first 99 h, suggesting that effects of warmer soils on N dynamics in this treeline system are only slightly modulated by the preferences of different plant species for inorganic and organic N forms. Synthesis. Plants growing in warmer soils acquired more unlabelled, soil‐derived N in the sixth year of treatment, implying a sustained increase in N mineralization and availability in alpine treeline ecosystems with higher soil temperatures predicted for the future. Wide variation in the ability of plant species and functional groups to compete for early‐summer N inputs means that there is a feedback between plant community shifts and N dynamics under environmental change at the treeline.

Experimental Air Warming of a Stylosanthes capitata, Vogel Dominated Tropical Pasture Affects Soil Respiration and Nitrogen Dynamics.

Warming due to global climate change is predicted to reach 2°C in tropical latitudes. There is an alarming paucity of information regarding the effects of air temperature on tropical agroecosystems, including foraging pastures. Here, we investigated the effects of a 2°C increase in air temperature over ambient for 30 days on an established tropical pasture (Ribeirão Preto, São Paulo, Brazil) dominated by the legume Stylosanthes capitata Vogel, using a T-FACE (temperature free-air controlled enhancement) system. We tested the effects of air warming on soil properties [carbon (C), nitrogen (N), and their stable isotopic levels (δ13C and δ15N), as well as soil respiration and soil enzymatic activity] and aboveground characteristics (foliar C, N, δ13C, δ15N, leaf area index, and aboveground biomass) under field conditions. Results show that experimental air warming moderately increased soil respiration rates compared to ambient temperature. Soil respiration was positively correlated with soil temperature and moisture during mid-day (when soil respiration was at its highest) but not at dusk. Foliar δ13C were not different between control and elevated temperature treatments, indicating that plants grown in warmed plots did not show the obvious signs of water stress often seen in warming experiments. The 15N isotopic composition of leaves from plants grown at elevated temperature was lower than in ambient plants, suggesting perhaps a higher proportion of N-fixation contributing to tissue N in warmed plants when compared to ambient ones. Soil microbial enzymatic activity decreased in response to the air warming treatment, suggesting a slower decomposition of organic matter under elevated air temperature conditions. Decreased soil enzyme capacity and increases in soil respiration and plant biomass in plots exposed to high temperature suggest that increased root activity may have caused the increase seen in soil respiration in this tropical pasture. This response along with rapid changes in soil and plant 15N may differ from what has been shown in temperate grasslands.

Short-term herbivory has long-term consequences in warmed and ambient high Arctic tundra

Climate change is occurring across the world, with effects varying by ecosystem and region but already occurring quickly in high-latitude and high-altitude regions. Biotic interactions are important in determining ecosystem response to such changes, but few studies have been long-term in nature, especially in the High Arctic. Mesic tundra plots on Svalbard, Norway, were subjected to grazing at two different intensities by captive Barnacle geese from 2003–2005, in a factorial design with warming by Open Top Chambers. Warming manipulations were continued through 2014, when we measured vegetation structure and composition as well as growth and reproduction of three dominant species in the mesic meadow. Significantly more dead vascular plant material was found in warmed compared to ambient plots, regardless of grazing history, but in contrast to many short-term experiments no difference in the amount of living material was found. This has strong implications for nutrient and carbon cycling and could feed back into community productivity. Dominant species showed increased flowering in warmed plots, especially in those plots where grazing had been applied. However, this added sexual reproduction did not translate to substantial shifts in vegetative cover. Forbs and rushes increased slightly in warmed plots regardless of grazing, while the dominant shrub, Salix polaris, generally declined with effects dependent on grazing, and the evergreen shrub Dryas octopetala declined with previous intensive grazing. There were no treatment effects on community diversity or evenness. Thus despite no changes in total live abundance, a typical short-term response to environmental conditions, we found pronounced changes in dead biomass indicating that tundra ecosystem processes respond to medium- to long-term changes in conditions caused by 12 seasons of summer warming. We suggest that while high arctic tundra plant communities are fairly resistant to current levels of climate warming, underlying ecosystem processes are beginning to change. In addition, even short bouts of intense herbivory can have long-term consequences for some species in these communities.

Experimental climate warming alters the relationship between fungal root symbiosis and Sphagnum litter phenolics in two peatland microhabitats

Belowground interactions between plants and microorganisms are involved in numerous ecosystems processes such as carbon and nutrient cycling. Understanding their responses to on-going climate warming is thus of paramount importance to better predict future ecosystem functioning. We hypothesized that climate warming alters the interactions between Sphagnum litter phenolics and the fungal root symbiosis of the Ericale plant Andromeda polifolia in a Jura mountain peatland (France). We initiate a climate warming treatment (+1 °C) in April 2008 in two microhabitats (lawns and hummocks). We measured polyphenolic contents, mycorrhizal and dark septate endophyte (DSE) root colonization, phenoloxidase and peroxidase activities in the A. polifolia rhizosphere from 2010 to 2012. We found that four years of warming modulated rhizospheric parameters (fungal root symbiosis and enzyme activities) of A. polifolia, but the response differed between microhabitats. Enzyme activities and fungal root colonization displayed different responses to warming according to the microhabitats. DSE root colonization increased and peroxidase activities decreased in warmed Hummocks while they remained stable in warmed lawns. We found a significant positive correlation between DSE root colonization and litter phenolics recovered in both microhabitats but only under in ambient conditions. On the contrary, significant negative correlation was observed between mycorrhizae and litter phenolics recovered in warmed plots while no relationship was found in control plots. Our work thus provides evidences that interplay between phenolics from Sphagnum litter and fungal symbiosis of an Ericale species in peatlands was affected by climate warming. It highlights different plant biological responses to climate warming according to peatland microhabitats and confirm that belowground activities are crucial to understand the response of peatlands to climate change.