Fungal root colonization responses in natural grasslands after long-term exposure to elevated atmospheric CO2

Arbuscular mycorrhizae, ubiquitous mutualistic symbioses between plant roots and fungi in the order Glomales, are believed to be important controllers of plant responses to global change, in particular to elevated atmospheric CO₂. In order to test if any effects on the symbiosis can persist after long-term treatment, we examined root colonization by arbuscular mycorrhizal (AM) and other fungi of several plant species from two grassland communities after continuous exposure to elevated atmospheric CO₂ for six growing seasons in the field. For plant species from both a sandstone and a serpentine annual grassland there was evidence for changes in fungal root colonization, with changes occurring as a function of plant host species. We documented decreases in percentage nonmycorrhizal fungal root colonization in elevated CO₂ for several plant species. Total AM root colonization (%) only increased significantly for one out of the five plant species in each grassland. However, when dividing AM fungal hyphae into two groups of hyphae (fine endophyte and coarse endophyte), we could document significant responses of AM fungi that were hidden when only total percentage colonization was measured. We also documented changes in elevated CO₂ in the percentage of root colonized by both AM hyphal types simultaneously. Our results demonstrate that changes in fungal root colonization can occur after long-term CO₂ enrichment, and that the level of resolution of the study of AM fungal responses may have to be increased to uncover significant changes to the CO₂ treatment. This study is also one of the first to document compositional changes in the AM fungi colonizing roots of plants grown in elevated CO₂. Although it is difficult to relate the structural data directly to functional changes, possible implications of the observed changes for plant communities are discussed.     

Soil microbiota in two annual grasslands: responses to elevated atmospheric CO2

We measured soil bacteria, fungi, protozoa, nematodes, and biological activity in serpentine and sandstone annual grasslands after 4 years of exposure to elevated atmospheric CO₂. Measurements were made during the early part of the season, when plants were in vegetative growth, and later in the season, when plants were approaching their maximum biomass. In general, under ambient CO₂, bacterial biomass, total protozoan numbers, and numbers of bactivorous nematodes were similar in the two grasslands. Active and total fungal biomasses were higher on the more productive sandstone grassland compared to the serpentine. However, serpentine soils contained nearly twice the number of fungivorous nematodes compared to the sandstone, perhaps explaining the lower standing crop of fungal biomass in the serpentine and suggesting higher rates of energy flow through the fungal-based soil food web. Furthermore, root biomass in the surface soils of these grasslands is comparable, but the serpentine contains 6 times more phytophagous nematodes compared to the sandstone, indicating greater below-ground grazing pressure on plants in stressful serpentine soils. Elevated CO₂ increased the biomass of active fungi and the numbers of flagellates in both grasslands during the early part of the season and increased the number of phytophagous nematodes in the serpentine. Elevated CO₂ had no effect on the total numbers of bactivorous or fungivorous nematodes, but decreased the diversity of the nematode assemblage in the serpentine at both sampling dates. Excepting this reduction in nematode diversity, the effects of elevated CO₂ disappeared later in the season as plants approached their maximum biomass. Elevated CO₂ had no effect on total and active bacterial biomass, total fungal biomass, or the total numbers of amoebae and ciliates in either grassland during either sampling period. However, soil metabolic activity was higher in the sandstone grassland in the early season under elevated CO₂, and elevated CO₂ altered the patterns of use of individual carbon substrates in both grasslands at this time. Rates of substrate use were also significantly higher in the sandstone, indicating increased bacterial metabolic activity. These changes in soil microbiota are likely due to an increase in the flux of carbon from roots to soil in elevated CO₂, as has been previously reported for these grasslands. Results presented here suggest that some of the carbon distributed below ground in response to elevated CO₂ affects the soil microbial food web, but that these effects may be more pronounced during the early part of the growing season.     

Arbuscular mycorrhizae respond to plants exposed to elevated atmospheric CO2 as a function of soil depth

The importance of arbuscular mycorrhizae (AM) in plant and ecosystem responses to global changes, e.g. elevated atmospheric CO₂, is widely acknowledged. Frequently, increases in AM root colonization occur in response to increased CO₂, but also the lack of significant changes has been reported. The goal of this study was to test whether arbuscular mycorrhizae (root colonization and composition of root colonization) respond to plants grown in elevated CO₂ as a function of soil depth. We grew Bromus hordeaceus L. and Lotus wrangelianus Fischer & C. Meyer monocultures in large pots with a synthetic serpentine soil profile for 4 yr in an experiment, in which CO₂ concentration was crossed factorially with NPK fertilization. When analyzing root infection separately for topsoil (0–15 cm) and subsoil (15–45 cm), we found large (e.g., about 5-fold) increases of AM fungal root colonization in the subsoil in response to CO₂, but no significant changes in the corresponding topsoil of Bromus. Only the coarse endophyte AM fungi, not the fine endophyte AM fungi, were responsible for the observed increase in the bottom soil layer, indicating a depth-dependent shift in the AM community colonizing the roots, even at this coarse morphological level. Other response variables also had significant soil layer ^* CO₂ interaction terms. The subsoil response would have been hidden in an unstratified assessment of the total root system, since most of the root length was concentrated in the top soil layer. The increased presence of mycorrhizae in roots deeper in the soil should be considered in sampling protocols, as it may be indicative of changed patterns of nutrient acquisition and carbon sequestration.     

Respiratory responses of arbuscular mycorrhizal roots to short-term alleviation of P deficiency

Arbuscular mycorrhizal (AM) root respiration can impose a respiratory sink on host reserves under low P conditions, but it is not known how AM roots respond to short-term supply of sufficient P. Therefore, the effect of P stress alleviation on the respiration of AM roots was investigated in 5-week-old tomato plants. Plants were inoculated with Glomus mosseae in sand culture and grown hydroponically in a low P (2 μM) nutrient medium for 3 weeks. P stress was alleviated by the supply of 2 mM P for 72 h. With P stress alleviation, the improved root P status coincided with a decline in AM fungal activity and a reduction in root CO₂ and O₂ fluxes of the AM plants. During P stress alleviation, the AM roots had lower concentrations of organic acids, derived from root-zone CO₂ assimilation, in their root exudates. These results show that short-term alleviation of low P conditions in AM roots rapidly affects AM fungal symbiont activity, AM root respiration, and root-zone CO₂-derived organic acid metabolism.     

Ecosystem water fluxes for two grasslands in elevated CO2: a modeling analysis

The need to combine data from CO₂ field experiments with climate data remains urgent, particularly because each CO₂ experiment cannot run for decades to centuries. Furthermore, predictions for a given biome need to take into account differences in productivity and leaf area index (LAI) independent of CO₂-derived changes. In this study, we use long-term weather records and field data from the Jasper Ridge CO₂ experiment in Palo Alto, California, to model the effects of CO₂ and climate variability on ecosystem water fluxes. The sandstone and serpentine grasslands at Jasper Ridge provide a range of primary productivity and LAI, with the sandstone as the more productive system. Modeled soil water availability agreed well with published observations of time-domain reflectometry in the CO₂ experiment. Simulated water fluxes based on 10-year weather data (January 1985–December 1994) showed that the sandstone grassland had a much greater proportion of water movement through plants than did the serpentine; transpiration accounted for approximately 30% of annual fluxes in the sandstone and only 10% in the serpentine. Although simulated physiological and biomass changes were similar in both grasslands, the consequences of elevated CO₂ were greater for the sandstone water budget. Elevated CO₂ increased soil drainage by 20% in the sandstone, despite an approximately one-fifth increase in plant biomass; in the serpentine, drainage increased by <10% and soil evaporation was unchanged for the same simulated biomass change. Phenological changes, simulated by a 15-day lengthening of the growing season, had minimal impacts on the water budget. Annual variation in the timing and amount of rainfall was important for water fluxes in both grasslands. Elevated CO₂ increased sandstone drainage >50 mm in seven of ten years, but the relative increase in drainage varied from 10% to 300% depending on the year. Early-season transpiration in the sandstone decreased between 26% and 41%, with elevated CO₂ resulting in a simulated water savings of 54–76 mm. Even in years when precipitation was similar (e.g., 505 and 479 mm in years 3 and 4), the effect of CO₂ varied dramatically. The response of grassland water budgets to CO₂ depends on the productivity and structure of the grassland, the amount and timing of rainfall, and CO₂-induced changes in physiology. In systems with low LAI, large physiological changes may not necessarily alter total ecosystem water budgets dramatically. Received: 11 March 1997 / Accepted: 23 September 1997     

Mycorrhizal dynamics under elevated CO2 and nitrogen fertilization in a warm temperate forest

We examined the response of mycorrhizal fungi to free-air CO₂ enrichment (FACE) and nitrogen (N) fertilization in a warm temperate forest to better understand potential influences over plant nutrient uptake and soil carbon (C) storage. In particular, we hypothesized that mycorrhizal fungi and glomalin would become more prevalent under elevated CO₂ but decrease under N fertilization. In addition, we predicted that N fertilization would mitigate any positive effects of elevated CO₂ on mycorrhizal abundance. Overall, we observed a 14% increase in ectomycorrhizal (ECM) root colonization under CO₂ enrichment, which implies that elevated CO₂ results in greater C investments in these fungi. Arbuscular mycorrhizal (AM) hyphal length and glomalin stocks did not respond substantially to CO₂ enrichment, and effects of CO₂ on AM root colonization varied by date. Nitrogen effects on AM fungi were not consistent with our hypothesis, as we found an increase in AM colonization under N fertilization. Lastly, neither glomalin concentrations nor ECM colonization responded significantly to N fertilization or to an N-by-CO₂ interaction. A longer duration of N fertilization may be required to detect effects on these parameters.     

Comparisons of AM fungal spore communities with the same hosts but different soil chemistries over local and geographic scales

Arbuscular mycorrhizal (AM) fungi are ubiquitous and ecologically important microbes in grasslands. Both the host plant species and soil properties have been suggested as potentially important factors structuring AM fungal communities based on studies within local field sites. However, characterizations of the communities in relation to both host plant identity and soil properties in natural plant communities across both local and broader geographic scales are rare. We examined the AM fungal spore communities associated with the same C₄ grasses in two Eastern serpentine grasslands, where soils have elevated heavy metals, and two Iowa tallgrass prairie sites. We compared AM fungal spore communities among host plants within each site, looked for correlations between fungal communities and local soil properties, and then compared communities among sites. Spore communities did not vary with host plant species or correlate with local soil chemical properties at any site. They did not differ between the two serpentine sites or between the two prairie sites, despite geographic separation, but they did differ between serpentine and prairie. Soil characteristics are suggested as a driving force because spore communities were strongly correlated with soil properties when data from all four sites are considered, but climatic differences might also play a role.     

Growth,photosynthesis and storage of carbohydrates and nitrogen in Phaseolus lunatus in relation to resource availability

External hyphae of vesicular-arbuscular mycorrhizal (VAM) fungi were quantified over a growing season in a reconstructed tallgrass prairie and an ungrazed cool-season pasture. In both sites, hyphal lengths increased throughout the growing season. Peak external hyphal lengths were 111 m cm^–3 of soil in the prairie and 81 m cm^–3 of soil in the pasture. These hyphal lengths calculate to external hyphal dry weights of 457 g cm^–3 and 339 g cm^–3 of soil for prairie and pasture communities, respectively. The relationships among external hyphal length, root characteristics, soil P and soil moisture were also determined. Measures of gross root morphology [e.g., specific root length (SRL) and root mass] have a strong association with external hyphal length. Over the course of the study, both grassland communities experienced a major drought event in late spring. During this period a reduction in SRL occurred in both the pasture and prairie without a measured reduction in external hyphal length. Recovery for both the pasture and prairie occurred not by increasing SRL, but rather by increasing external hyphal length. This study suggests that growth is coordinated between VAM hyphae and root morphology, which in turn, are constrained by plant community composition and soil nutrient and moisture conditions.     

Species-specific responses of plant communities to altered carbon and nutrient availability

In a field microcosm experiment, species‐specific responses of aboveground biomass of two California annual grassland communities to elevated CO₂ and nutrient availability were investigated. One community grows on shallow, nutrient‐poor serpentine‐derived soil whereas the other occurs on deeper, modestly fertile sandstone/greenstone‐derived substrate. In most species, CO₂ effects did not appear until late in the growing season, probably because the elevated CO₂ increased water‐use‐efficiency easing, the onset of the summer drought. Responses of aboveground biomass to elevated CO₂ differed depending on nutrient availability. Similarly, biomass responses to nutrient treatments differed depending on the CO₂ status. For the majority of the species, production increased most under elevated CO₂ with added nutrients (N,P,K, and micro nutrients). Some species were losers under conditions that increased overall community production, including Bromus hordeaceus in the serpentine community (negative biomass response under elevated CO₂) and Lotus wrangelianus in both communities (negative biomass response with added nitrogen). Treatment and competitive effects on species‐specific biomass varied in both magnitude and direction, especially in the serpentine community, significantly affecting community structure. Individual resource environments are likely to be affected by neighbouring plants, and these competitive interactions complicate predictions of species' responses to elevated CO₂.     

Enkianthus campanulatus (Ericaceae) is commonly associated with arbuscular mycorrhizal fungi

Enkianthus is the most basal extant genus in the phylogeny of ericaceous plants. Its members harbor arbuscular mycorrhiza (AM)-like hyphal structures in their roots but, as yet, no study has surveyed the AM fungal species component. Roots from six species of Enkianthus were collected from five distantly located sites in Japan. Intracellular hyphal coils were observed in the root cortical cells of all species. Fungal DNA sequences of the small subunit ribosomal RNA gene were obtained from 73 of 75 segments of Enkianthus campanulatus roots by PCR using either AML2 or NS31/AM1primer pairs. Results indicated that all E. campanulatus trees were extensively associated with Glomus spp. A phylogenetic analysis showed that 71 root segments harbored fungi belonging to Glomus group A. Among eight delineated clades, seven did not nest with any known AM fungal species. One clade was detected in all roots at all sites at relatively high frequencies, but the rest were detected sporadically at each site. The placement of sequences from distantly located sites into a single clade without known AM fungal species suggests the common association of E. campanulatus with particular AM fungal taxa.     

Relationships between soil heavy metal concentration and mycorrhizal colonisation in<Emphasis Type="Italic"> Thymus polytrichus</Emphasis> in northern England

A study was conducted to establish whether the wild thyme [Thymus polytrichus A. Kerner ex Borbás ssp. britannicus (Ronn.) Kerguelen (Lamiaceae)] growing in the metal-contaminated soils along the River South Tyne, United Kingdom, is colonised by arbuscular mycorrhizal (AM) fungi, and whether the degree of colonisation increases (perhaps suggesting increasing mycorrhizal dependence) or decreases (indicating possible inhibition of AM growth) with increasing degree of soil contamination. Seasonal changes in AM colonisation were also assessed. The AM fungal communities colonising T. polytrichus were also investigated, using the polymerase chain reaction with restriction fragment length polymorphism and sequencing of fungal DNA to establish whether AM species richness varied between sites, and whether fungal ecotypes specific to sites with different amounts of metal contamination could be identified. All plants examined were heavily colonised by AM fungi, and mean percentage root length colonised did not increase significantly with increasing soil metal contamination. However, AM vesicle abundance (percentage of mycorrhizal root length containing vesicles) at the most contaminated site was significantly greater than at the other sites. No significant seasonal variation in degree of colonisation or vesicle abundance was found. Glomus was the predominant AM genus detected at all sites. The number of AM genotypes colonising T. polytrichus roots was similar at all sites but, although some were common to all sites, certain strains appeared to be specific to either the most- or the least-contaminated site. This variation in species may account for the difference in vesicle abundance between sites. The consistently heavy AM colonisation of T. polytrichus found suggests that these fungi are not inhibited by soil heavy metals at these sites, and that the host derives some benefit from its AM symbiont.     

Root dynamics and demography in shortgrass steppe under elevated CO2, and comments on minirhizotron methodology

The dynamics and demography of roots were followed for 5 years that spanned wet and drought periods in native, semiarid shortgrass steppe grassland exposed to ambient and elevated atmospheric CO₂ treatments. Elevated compared with ambient CO₂ concentrations resulted in greater root‐length growth (+52%), root‐length losses (+37%), and total pool sizes (+41%). The greater standing pool of roots under elevated compared with ambient CO₂ was because of the greater number of roots (+35%), not because individuals were longer. Loss rates increased relatively less than growth rates because life spans were longer (+41%). The diameter of roots was larger under elevated compared with ambient CO₂ only in the upper soil profile. Elevated CO₂ affected root architecture through increased branching. Growth‐to‐loss ratio regressions to time of equilibrium indicate very long turnover times of 5.8, 7.0, and 5.3 years for control, ambient, and elevated CO₂, respectively. Production was greater under elevated compared with ambient CO₂ both below‐ and aboveground, and the above‐ to belowground ratios did not differ between treatments. However, estimates of belowground production differed among methods of calculation using minirhizotron data, as well as between minirhizotron and root‐ingrowth methods. Users of minirhizotrons may need to consider equilibration in terms of both new growth and disappearance, rather than just growth. Large temporal pulses of root initiation and termination rates of entire individuals were observed (analogous to birth–death rates), and precipitation explained more of the variance in root initiation than termination. There was a dampening of the pulsing in root initiation and termination under elevated CO₂ during both wet and dry periods, which may be because of conservation of soil water reducing the suddenness of wet pulses and duration and severity of dry pulses. However, a very low degree of synchrony was observed between growth and disappearance (production and decomposition).     

Soil disturbance alters plant community composition and decreases mycorrhizal carbon allocation in a sandy grassland

We have studied how disturbance by ploughing and rotavation affects the carbon (C) flow to arbuscular mycorrhizal (AM) fungi in a dry, semi-natural grassland. AM fungal biomass was estimated using the indicator neutral lipid fatty acid (NLFA) 16:1ω5, and saprotrophic fungal biomass using NLFA 18:2ω6,9. We labeled vegetation plots with ¹³CO₂ and studied the C flow to the signature fatty acids as well as uptake and allocation in plants. We found that AM fungal biomass in roots and soil decreased with disturbance, while saprotrophic fungal biomass in soil was not influenced by disturbance. Rotavation decreased the ¹³C enrichment in NLFA 16:1ω5 in soil, but ¹³C enrichment in the AM fungal indicator NLFA 16:1ω5 in roots or soil was not influenced by any other disturbance. In roots, ¹³C enrichment was consistently higher in NLFA 16:1ω5 than in crude root material. Grasses (mainly Festuca brevipila) decreased as a result of disturbance, while non-mycorrhizal annual forbs increased. This decreases the potential for mycorrhizal C sequestration and may have been the main reason for the reduced mycorrhizal C allocation found in disturbed plots. Disturbance decreased the soil ammonium content but did not change the pH, nitrate or phosphate availability. The overall effect of disturbance on C allocation was that more of the C in AM fungal mycelium was directed to the external phase. Furthermore, the functional identity of the plants seemed to play a minor role in the C cycle as no differences were seen between different groups, although annuals contained less AM fungi than the other groups.     

Effect of forest fire on number, viability and post-fire re-establishment of arbuscular mycorrhizae

 Forest fire can affect arbuscular mycorrhizal (AM) fungi by changing the soil conditions and by directly altering AM proliferation. We studied the effects of a severe forest fire at Margalla Hills near Islamabad on the number and viability of AM fungal propagules in the burnt soil and their role in the re-establishment of post-fire infection in colonized plants. Compared with a nearby control area, the burnt site had a similar number of total spores but a lower number of viable AM fungal propagules. The roots of the two most frequent species at the burnt site, Dodonaea viscosa and Aristida adscensionis, showed a gradual increase in percentage root length colonized by AM fungi in general and hyphal infection in particular. Our results indicate resumption of mycorrhizal activity following the fire, probably from AM hyphae in the roots of these dominant shrubs. Accepted: 18 July 1997     

Species of plants and associated arbuscular mycorrhizal fungi mediate mycorrhizal responses to CO2 enrichment

It has been suggested that enrichment of atmospheric CO₂ should alter mycorrhizal function by simultaneously increasing nutrient‐uptake benefits and decreasing net C costs for host plants. However, this hypothesis has not been sufficiently tested. We conducted three experiments to examine the impacts of CO₂ enrichment on the function of different combinations of plants and arbuscular mycorrhizal (AM) fungi grown under high and low soil nutrient availability. Across the three experiments, AM function was measured in 14 plant species, including forbs, C₃ and C₄ grasses, and plant species that are typically nonmycorrhizal. Five different AM fungal communities were used for inoculum, including mixtures of Glomus spp. and mixtures of Gigasporaceae (i.e. Gigaspora and Scutellospora spp.). Our results do not support the hypothesis that CO₂ enrichment should consistently increase plant growth benefits from AM fungi, but rather, we found CO₂ enrichment frequently reduced AM benefits. Furthermore, we did not find consistent evidence that enrichment of soil nutrients increases plant growth responses to CO₂ enrichment and decreases plant growth responses to AM fungi. Our results show that the strength of AM mutualisms vary significantly among fungal and plant taxa, and that CO₂ levels further mediate AM function. In general, when CO₂ enrichment interacted with AM fungal taxa to affect host plant dry weight, it increased the beneficial effects of Gigasporaceae and reduced the benefits of Glomus spp. Future studies are necessary to assess the importance of temperature, irradiance, and ambient soil fertility in this response. We conclude that the affects of CO₂ enrichment on AM function varies with plant and fungal taxa, and when making predictions about mycorrhizal function, it is unwise to generalize findings based on a narrow range of plant hosts, AM fungi, and environmental conditions.     

Effect of drought and season on arbuscular mycorrhizal fungi in a subtropical secondary forest

Arbuscular mycorrhizal (AM) fungi in both soil and roots were examined in May (summer) and December (winter) under a 4-y drought experiment in a Chinese subtropical secondary forest. Drought significantly decreased AM fungal extra-radical hyphal density, spore density, and root colonization rate in both seasons. These AM parameters were significantly higher in summer than in winter in the control treatment, but only AM fungal extra-radical hyphal density exhibited the same seasonal trend in the drought treatment. In total, 45 AM fungal operational taxonomic units (OTUs) were obtained at a 97% sequence similarity level using Illumina sequencing of 18S rDNA. Drought and season had no significant effects on AM fungal OTU richness in soil and roots. AM fungal community composition in soil and roots was significantly affected by season but not by drought. This finding enhances our understanding of the response of AM fungi to global climate change in subtropical forest ecosystems.     

Soil nutritional status, not inoculum identity, primarily determines the effect of arbuscular mycorrhizal fungi on the growth of Knautia arvensis plants

Arbuscular mycorrhizal (AM) symbiosis is among the factors contributing to plant survival in serpentine soils characterised by unfavourable physicochemical properties. However, AM fungi show a considerable functional diversity, which is further modified by host plant identity and edaphic conditions. To determine the variability among serpentine AM fungal isolates in their effects on plant growth and nutrition, a greenhouse experiment was conducted involving two serpentine and two non-serpentine populations of Knautia arvensis plants grown in their native substrates. The plants were inoculated with one of the four serpentine AM fungal isolates or with a complex AM fungal community native to the respective plant population. At harvest after 6-month cultivation, intraradical fungal development was assessed, AM fungal taxa established from native fungal communities were determined and plant growth and element uptake evaluated. AM symbiosis significantly improved the performance of all the K. arvensis populations. The extent of mycorrhizal growth promotion was mainly governed by nutritional status of the substrate, while the effect of AM fungal identity was negligible. Inoculation with the native AM fungal communities was not more efficient than inoculation with single AM fungal isolates in any plant population. Contrary to the growth effects, a certain variation among AM fungal isolates was revealed in terms of their effects on plant nutrient uptake, especially P, Mg and Ca, with none of the AM fungi being generally superior in this respect. Regardless of AM symbiosis, K. arvensis populations significantly differed in their relative nutrient accumulation ratios, clearly showing the plant’s ability to adapt to nutrient deficiency/excess.     

Seasonality of arbuscular mycorrhizal hyphae and glomalin in a western Montana grassland

In order to more fully understand the basic biology of arbuscular mycorrhizal fungi (AMF), and their role in natural ecosystems, it is necessary to document seasonal changes of various aspects of the life history of these fungi. Due to their unique position at the root-soil interface, AMF have been described as `keystone mutualists' in ecosystems. Despite the importance of AMF in ecosystems, few studies exist that examine the seasonality of external hyphae and their exuded products (e.g. glomalin), the AMF variables directly related to ecosystem function through their contributions to soil aggregation. This study examined seasonal dynamics of several soil variables, with a specific interest in the seasonality of external hyphae and glomalin, a glycoprotein produced by AMF, which is correlated with soil aggregate stability. Here we measured glomalin concentrations and external AMF and non-AMF hyphal length, as well as soil moisture, percent fungal root colonization (AMF and non-AMF), and root length in soil in an intermountain grassland in western Montana over one growing season (13 time points). Of the glomalin pools and hyphal lengths measured, significant seasonal changes occurred for total glomalin (TG; 24.5% change), immunoreactive easily extractable glomalin (IREEG; 53.8% change), and AM hyphal length (107% change). Prior studies on glomalin in natural systems have not considered seasonal effects. The small seasonal change in glomalin pools lends further support to the hypothesis that glomalin is relatively stable in soils, and suggests that one-time sampling may be sufficient to satisfactorily capture this response variable. However, the generality of this observation has yet to be tested in a wider range of ecosystems.     

Impact of soil warming and shading on colonization and community structure of arbuscular mycorrhizal fungi in roots of a native grassland community

Arbuscular mycorrhizal (AM) fungi have a major influence on the structure, responses and below‐ground C allocation of plant communities. Our lack of understanding of the response of AM fungi to factors such as light and temperature is an obstacle to accurate prediction of the impact of global climate change on ecosystem functioning. In order to investigate this response, we divided a grassland site into 24 plots, each either unshaded or partly shaded with soil either unheated or heated by 3°C at 2 cm depth. In both short‐term studies in spring and autumn, and in a 1‐year‐long study, we measured root length colonization (L_RC) by AM and non‐AM fungi. For selected root samples, DNA sequences were amplified by PCR with fungal‐specific primers for part of the small sub‐unit (SSU) rRNA gene. In spring, the total L_RC increased over 6 weeks from 12% to 25%. Shading significantly reduced AM but increased non‐AM fungal colonization, while soil warming had no effect. In the year‐long study, colonization by AM fungi peaked in summer, whereas non‐AM colonization peaked in autumn, when there was an additive effect of shading and soil warming that reduced AM but increased non‐AM fungi. Stepwise regression revealed that light received within the 7 days prior to sampling was the most significant factor in determining AM L_RC and that mean temperature was the most important influence on non‐AM L_RC. Loglinear analysis confirmed that there were no seasonal or treatment effects on the host plant community. Ten AM fungal sequence types were identified that clustered into two families of the Glomales, Glomaceae and Gigasporaceae. Three other sequence types were of non‐AM fungi, all Ascomycotina. AM sequence types showed seasonal variation and shading impacts: loglinear regression analysis revealed changes in the AM fungal community with time, and a reduction of one Glomus sp. under shade, which corresponded to a decrease in the abundance of Trifolium repens. We suggest that further research investigating any impacts of climate change on ecosystem functioning must not only incorporate their natural AM fungal communities but should also focus on niche separation and community dynamics of AM fungi.     

Disentangling root responses to climate change in a semiarid grassland

Future ecosystem properties of grasslands will be driven largely by belowground biomass responses to climate change, which are challenging to understand due to experimental and technical constraints. We used a multi-faceted approach to explore single and combined impacts of elevated CO₂ and warming on root carbon (C) and nitrogen (N) dynamics in a temperate, semiarid, native grassland at the Prairie Heating and CO₂ Enrichment experiment. To investigate the indirect, moisture mediated effects of elevated CO₂, we included an irrigation treatment. We assessed root standing mass, morphology, residence time and seasonal appearance/disappearance of community-aggregated roots, as well as mass and N losses during decomposition of two dominant grass species (a C₃ and a C₄). In contrast to what is common in mesic grasslands, greater root standing mass under elevated CO₂ resulted from increased production, unmatched by disappearance. Elevated CO₂ plus warming produced roots that were longer, thinner and had greater surface area, which, together with greater standing biomass, could potentially alter root function and dynamics. Decomposition increased under environmental conditions generated by elevated CO₂, but not those generated by warming, likely due to soil desiccation with warming. Elevated CO₂, particularly under warming, slowed N release from C₄—but not C₃—roots, and consequently could indirectly affect N availability through treatment effects on species composition. Elevated CO₂ and warming effects on root morphology and decomposition could offset increased C inputs from greater root biomass, thereby limiting future net C accrual in this semiarid grassland.