The invasive plant species Centaurea maculosa alters arbuscular mycorrhizal fungal communities in the field

While several recent studies have described changes in microbial communities associated with exotic plant invasion, how arbuscular mycorrhizal fungi (AMF) communities respond to exotic plant invasion is not well known, despite the salient role of this group in plant interactions. Here, we use molecular methods (terminal restriction fragment length polymorphism analyses based on the large subunit of the rRNA gene) to examine AMF community structure in sites dominated by the invasive mycorrhizal forb, Centaurea maculosa Lam. (spotted knapweed), and in adjacent native grassland sites. Our results indicate that significant AMF community alteration occurs following C. maculosa invasion. Moreover, a significant reduction in the number of restriction fragment sizes was found for samples collected in C. maculosa-dominated areas, suggesting reduced AMF diversity. Extraradical hyphal lengths exhibited a significant, on average 24%, reduction in C. maculosa-versus native grass-dominated sites. As both AMF community composition and abundance were altered by C.maculosa invasion, these data are strongly suggestive of potential impacts on AMF-mediated ecosystem processes. Given that the composition of AMF communities has the potential to differentially influence different plant species, our results may have important implications for site restoration after weed invasion.     

Spatial soil heterogeneity has a greater effect on symbiotic arbuscular mycorrhizal fungal communities and plant growth than genetic modification with Bacillus thuringiensis toxin genes

Maize, genetically modified with the insect toxin genes of Bacillus thuringiensis (Bt), is widely cultivated, yet its impacts on soil organisms are poorly understood. Arbuscular mycorrhizal fungi (AMF) form symbiotic associations with plant roots and may be uniquely sensitive to genetic changes within a plant host. In this field study, the effects of nine different lines of Bt maize and their corresponding non‐Bt parental isolines were evaluated on AMF colonization and community diversity in plant roots. Plants were harvested 60 days after sowing, and data were collected on plant growth and per cent AMF colonization of roots. AMF community composition in roots was assessed using 454 pyrosequencing of the 28S rRNA genes, and spatial variation in mycorrhizal communities within replicated experimental field plots was examined. Growth responses, per cent AMF colonization of roots and AMF community diversity in roots did not differ between Bt and non‐Bt maize, but root and shoot biomass and per cent colonization by arbuscules varied by maize cultivar. Plot identity had the most significant effect on plant growth, AMF colonization and AMF community composition in roots, indicating spatial heterogeneity in the field. Mycorrhizal fungal communities in maize roots were autocorrelated within approximately 1 m, but at greater distances, AMF community composition of roots differed between plants. Our findings indicate that spatial variation and heterogeneity in the field has a greater effect on the structure of AMF communities than host plant cultivar or modification by Bt toxin genes.     

Host plant differences in arbuscular mycorrhizae: Extra radical hyphae differences between an invasive forb and a native bunchgrass

Arbuscular mycorrhizae affect grassland plant community composition and host plant nutrient uptake, and can mediate shifts in competitive outcome between plant species. Centaurea maculosa, an invasive forb from Eurasia, dominates more than 4 million hectares in the Rocky Mountain region of North America. We examined the role of AM for phosphorus (P) acquisition from a distant source for C. maculosa and Festuca idahoensis, a native bunchgrass. Plants were grown individually in pots divided by a barrier that either excluded plant roots and AM hyphae, or only plant roots. In the half of the pot without a plant, 1 of 3 P treatments was applied: no P, phosphate rock (PR) or triple superphosphate (TSP), applied at a rate of 144 mg P kg^–1 soil. After 14 weeks of growth, C. maculosa was twice as large as F. idahoensis, and neither species biomass was affected by barrier type. Phosphorus fertilizer, and especially PR, moved across the barrier to the plant side of the pot. Tissue P concentration for C. maculosa was highest with the PR treatment, and was not affected by the barrier type. In contrast, F. idahoensis tissue P concentration did not vary with barrier or P treatments. There was more AM extra radical hyphae (ERH) associated with C. maculosa than F. idahoensis, suggesting that C. maculosa provides more carbon for the AM fungi, resulting in greater ERH production, ERH soil exploration and potential for soil nutrient pool exploitation. Although not tested in this study, differences between host plants may be the result of different physiological characteristics of the host plant or differences in AM fungal species that colonize the invader, with different fungal species accessing P from different distances.     

Arbuscular mycorrhizal fungal community differs between a coexisting native shrub and introduced annual grass

Arbuscular mycorrhizal fungi (AMF) have been implicated in non-native plant invasion success and persistence. However, few studies have identified the AMF species associating directly with plant invaders, or how these associations differ from those of native plant species. Identifying changes to the AMF community due to plant invasion could yield key plant–AMF interactions necessary for the restoration of native plant communities. This research compared AMF associating with coexisting Bromus tectorum, an invasive annual grass, and Artemisia tridentata, the dominant native shrub in western North America. At three sites, soil and root samples from Bromus and Artemisia were collected. Sporulation was induced using trap cultures, and spores were identified using morphological characteristics. DNA was extracted from root and soil subsamples and amplified. Sequences obtained were aligned and analyzed to compare diversity, composition, and phylogenetic distance between hosts and sites. Richness of AMF species associated with Artemisia in cultures was higher than AMF species associated with Bromus. Gamma diversity was similar and beta diversity was higher in AMF associated with Bromus compared to Artemisia. AMF community composition differed between hosts in both cultures and roots. Two AMF species (Archaeospora trappei and Viscospora viscosum) associated more frequently with Artemisia than Bromus across multiple sites. AMF communities in Bromus roots were more phylogenetically dispersed than in Artemisia roots, indicating a greater competition for resources within the invasive grass. Bromus associated with an AMF community that differed from Artemisia in a number of ways, and these changes could restrict native plant establishment.     

Root responses along a subambient to elevated CO2 gradient in a C3–C4 grassland

Atmospheric CO₂ (C_a) concentration has increased significantly during the last 20 000 years, and is projected to double this century. Despite the importance of belowground processes in the global carbon cycle, community‐level and single species root responses to rising C_a are not well understood. We measured net community root biomass over 3 years using ingrowth cores in a natural C₃–C₄ grassland exposed to a gradient of C_a from preglacial to future levels (230–550 μmol mol^?1). Root windows and minirhizotron tubes were installed below naturally occurring stands of the C₄ perennial grass Bothriochloa ischaemum and its roots were measured for respiration, carbohydrate concentration, specific root length (SRL), production, and lifespan over 2 years. Community root biomass increased significantly (P<0.05) with C_a over initial conditions, with linear or curvilinear responses depending on sample date. In contrast, B. ischaemum produced significantly more roots at subambient than elevated C_a in minirhizotrons. The lifespan of roots with five or more neighboring roots in minirhizotron windows decreased significantly at high C_a, suggesting that after dense root growth depletes soil resource patches, plants with carbon surpluses readily shed these roots. Root respiration in B. ischaemum showed a curvilinear response to C_a under moist conditions in June 2000, with the lowest rates at C_a<300 μmol mol^?1 and peak activity at 450 μmol mol^?1 in a quadratic model. B. ischaemum roots at subambient C_a had higher SRLs and slightly higher carbohydrate concentrations than those at higher C_a, which may be related to drier soils at low C_a. Our data emphasize that belowground responses of plant communities to C_a can be quite different from those of the individual species, and suggest that complex interactions between and among roots and their immediate soil environment influence the responses of root physiology and lifespan to changing C_a.     

Structural analysis of the phytophagous insect guilds associated with the roots of Centaurea maculosa Lam. C. diffusa Lam., and C. vallesiaca Jordan in Europe:

Summary During extensive field surveys in central and eastern Europe, 21 herbivorous root insect species were found on Centaurea maculosa ssp. rhenana Boreau, 12 species on C. diffusa Lam. and 11 species on C. vallesiaca Jordan, representing 12 families in 4 orders. The large geographic distribution (species-area function), the high number of Centaurea spp. present (host speciation rate), and the high apparency of the rosettes and the rich food resources offered by the roots during winter, together with their poor accessibility, correlate with the high number of specialist feeders associated with the roots of C. maculosa and C. diffusa. The members of the taxonomically diverse root entomofauna exploit specific structures of the tap root (food niches). Interspecific competition among members of food niches, as well as species-specific responses to different phenological stages (for oviposition) and tissues (for larval development) are thought to be responsible for the high predictability in guild structure. The relatively low levels of host plant attack (two thirds of the roots were unattacked) and the fact that food niches remained unoccupied in most of the regions suggest, however, that the majority of the studied guilds do not represent equilibrium assemblages. Ecological (different habitats), climatic (transitional zone) and historical (ancient pre-Pleistocene communities) factors could account for the highest values of species diversity, infestation levels, species packing and food niche utilization, which are found on C. maculosa in E. Austria/NW. Hungary, compared to other regions. A positive correlation between species packing (number of root-feeding species per population) and infestation rates (percent of roots attacked) was only found for the more stable, semi-natural habitats. A comparative analysis of the regional root insect guilds of C. maculosa with corresponding data for the phytophagous insects associated with the flower heads revealed distinct taxonomical differences, but a high degree of numerical and structural similarity. The different geographical regions are similarly ranked for host plant attack, herbivore pressure, average species packing and level of food niche utilization.     

Effects of the mycorrhizal fungus Glomus intraradices on uranium uptake and accumulation by Medicago truncatula L. from uranium-contaminated soil

Phytostabilization strategies may be suitable to reduce the dispersion of uranium (U) and the overall environmental risks of U-contaminated soils. The role of Glomus intraradices, an arbuscular mycorrhizal (AM) fungus, in such phytostabilization of U was investigated with a compartmented plant cultivation system facilitating the specific measurement of U uptake by roots, AM roots and extraradical hyphae of AM fungi and the measurement of U partitioning between root and shoot. A soil-filled plastic pot constituted the main root compartment (C_A) which contained a plastic vial filled with U-contaminated soil amended with 0, 50 or 200 mg KH₂PO₄−P kg^–1soil (C_B). The vial was sealed by coarse or fine nylon mesh, permitting the penetration of both roots and hyphae or of just hyphae. Medicago truncatula plants grown in C_A were inoculated with G. intraradices or remained uninoculated. Dry weight of shoots and roots in C_A was significantly increased by G. intraradices, but was unaffected by mesh size or by P application in C_B. The P amendments decreased root colonization in C_B, and increased P content and dry weight of those roots. Glomus intraradices increased root U concentration and content in C_A, but decreased shoot U concentrations. Root U concentrations and contents were significantly higher when only hyphae could access U inside C_B than when roots could also directly access this U pool. The proportion of plant U content partitioned to shoots was decreased by root exclusion from C_B and by mycorrhizas (M) in the order: no M, roots in C_B > no M, no roots in C_B > M, roots in C_B > M, no roots in C_B. Such mycorrhiza-induced retention of U in plant roots may contribute to the phytostabilization of U contaminated environments.     

Differences in arbuscular mycorrhizal fungal communities associated with sugar maple seedlings in and outside of invaded garlic mustard forest patches

Garlic mustard (Alliaria petiolata) is a Eurasian native that has become invasive in North America. The invasive success of A. petiolata has been partly attributed to its production of allelopathic compounds that can limit the growth of arbuscular mycorrhizal fungi (AMF). Although such effects are well known, specific effects on the richness and community composition of AMF associated with woody species have not been explored. We collected sugar maple (Acer saccharum) seedlings from eight natural forest sites in Ohio and Massachusetts, containing areas either invaded or uninvaded by A. petiolata. We measured AMF root colonization of seedlings, isolated DNA from the roots and performed PCR-TRFLP analysis to assess the richness and community composition of AMF. As expected, we found reduced AMF colonization in A. petiolata invaded patches. A. petiolata did not alter the detected TRF richness, but was associated with significant changes in the composition of AMF communities in half of the sites monitored in each region. Our results suggest that although AMF colonization was reduced in A. petiolata patches, many indigenous AMF communities include AMF that are tolerant to allelopathic effects of A. petiolata.     

The composition of arbuscular mycorrhizal fungal communities differs among the roots,spores and extraradical mycelia associated with five Mediterranean plant species

Arbuscular mycorrhizal fungi (AMF) are essential constituents of most terrestrial ecosystems. AMF species differ in terms of propagation strategies and the major propagules they form. This study compared the AMF community composition of different propagule fractions – colonized roots, spores and extraradical mycelium (ERM) – associated with five Mediterranean plant species in Sierra de Baza Natural Park (Granada, Spain). AMF were identified using 454 pyrosequencing of the SSU rRNA gene. A total of 96 AMF phylogroups [virtual taxa (VT)] were detected in the study site, including 31 novel VT. After per‐sample sequencing depth standardization, 71 VT were recorded from plant roots, and 47 from each of the spore and ERM fractions. AMF communities differed significantly among the propagule fractions, and the root‐colonizing fraction differed among host plant species. Indicator VT were detected for the root (13 Glomus VT), spore (Paraglomus VT281, VT336, Pacispora VT284) and ERM (Diversispora VT62) fractions. This study provides detailed evidence from a natural system that AMF taxa are differentially allocated among soil mycelium, soil spores and colonized root propagules. This has important implications for interpreting AMF diversity surveys and designing applications of AMF in vegetation restoration.     

Fungal root symbionts and their relationship with fine root proportion in native plants from the Bolivian Andean highlands above 3,700?m elevation

Here, we examined the colonization by fungal root symbionts in the cultivated Andean grain Chenopodium quinoa and in 12 species that dominate plant communities in the Bolivian Altiplano above 3,700 m elevation and explore for the possible relationships between fungal colonization and fine root proportion. The 12 most abundant species in the study area were consistently colonized by AMF and DSE. In contrast, the annual Andean grain C. quinoa showed negligible or absence of mycorrhizal fungi colonizing roots. On the other hand, C. quinoa, Junelia seriphioides and Chersodoma jodopappa were infected to a varying degree by the root pathogen Olpidium sp. We observed no relationship between AMF and DSE colonization and proportion of fine roots in the root system, but instead, the ratio between DSE and AMF colonization (ratio DSE/AMF) negatively related with proportion of fine roots. Our findings support the hypothesis regarding the importance of DSE at high altitudes and suggest a functional relationship between the rate of DSE/AMF and proportion of fine roots. The colonization by the root pathogen Olpidium sp. in C. quinoa deserves further study since this Andean grain is increasingly important for the local economy in these marginal areas.     

Interactive effects of mycorrhizae and a root hemiparasite on plant community productivity and diversity

Plant communities can be affected both by arbuscular mycorrhizal fungi (AMF) and hemiparasitic plants. However, little is known about the interactive effects of these two biotic factors on the productivity and diversity of plant communities. To address this question, we set up a greenhouse study in which different AMF inocula and a hemiparasitic plant (Rhinanthus minor) were added to experimental grassland communities in a fully factorial design. In addition, single plants of each species in the grassland community were grown with the same treatments to distinguish direct AMF effects from indirect effects via plant competition. We found that AMF changed plant community structure by influencing the plant species differently. At the community level, AMF decreased the productivity by 15–24%, depending on the particular AMF treatment, mainly because two dominant species, Holcus lanatus and Plantago lanceolata, showed a negative mycorrhizal dependency. Concomitantly, plant diversity increased due to AMF inoculation and was highest in the treatment with a combination of two commercial AM strains. AMF had a positive effect on growth of the hemiparasite, and thereby induced a negative impact of the hemiparasite on host plant biomass which was not found in non-inoculated communities. However, the hemiparasite did not increase plant diversity. Our results highlight the importance of interactions with soil microbes for plant community structure and that these indirect effects can vary among AMF treatments. We conclude that mutualistic interactions with AMF, but not antagonistic interactions with a root hemiparasite, promote plant diversity in this grassland community. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Mycorrhizae transfer carbon from a native grass to an invasive weed: evidence from stable isotopes and physiology

Invasive exotic weeds pose one of the earth's most pressing environmental problems. Although many invaders completely eliminate native plant species from some communities, ecologists know little about the mechanisms by which these exotics competitively exclude other species. Mycorrhizal fungi radically alter competitive interactions between plants within natural communities, and a recent study has shown that arbuscular mycorrhizal (AM) fungi provide a substantial competitive advantage to spotted knapweed, Centaurea maculosa, a noxious perennial plant that has spread throughout much of the native prairie in the northwestern U.S. Here we present evidence that this advantage is potentially due to mycorrhizally mediated transfer of carbon from a native bunchgrass, Festuca idahoensis, to Centaurea. Centaurea maculosa, Festuca idahoensis (Idaho fescue, C₃), and Bouteloua gracilis (blue gramma, C₄) were grown in the greenhouse either alone or with Centaurea in an incomplete factorial design with and without AM fungi. Centaurea biomass was 87–168% greater in all treatments when mycorrhizae were present in the soil (P < 0.0001). However, Centaurea biomass was significantly higher in the treatment with both mycorrhizae and Festuca present together than in any other treatment combination (P < 0.0001). This high biomass was attained even though Centaurea photosynthetic rates were 14% lower when grown with Festuca and mycorrhizae together than when grown with Festuca without mycorrhizae. Neither biomass nor photosynthetic rates of Centaurea were affected by competition with the C₄ grass Bouteloua either with or without mycorrhizae. The stable isotope signature of Centaurea leaves grown with Festuca and mycorrhizae was more similar to that of Festuca, than when Centaurea was grown alone with mycorrhizae (P = 0.06), or with Festuca but without mycorrhizae (P = 0.09). This suggests that carbon was transferred from Festuca to the invasive weed. We estimated that carbon transferred from Festuca by mycorrhizae contributed up to 15% of the aboveground carbon in Centaurea plants. Our results indicate that carbon parasitism via AM soil fungi may be an important mechanism by which invasive plants out compete their neighbors, but that this interaction is highly species-specific.     

Differential host plant performance as a function of soil arbuscular mycorrhizal fungal communities: experimentally manipulating co-occurring Glomus species

In order to evaluate host plant performance relative to different soil arbuscular mycorrhizal fungal (AMF) communities, Andropogon gerardii seedlings were grown with nine different AMF communities. The communities consisted of 0, 10, or 20 spores of Glomus etunicatum and 0, 10, or 20 spores of Glomus intraradices in all possible combinations. Spores were produced by fungal cultures originating on A. gerardii in a serpentine plant community; seeds of A. gerardii were collected at the same site. The experiment was performed in the greenhouse using a mixture of sterilized serpentine soil and sand to which naturally occurring non-mycorrhizal microbes were added. There was no difference in root AMF colonization rates between single species communities of either G. etunicatum or G. intraradices, but G. intraradices enhanced plant growth and G. etunicatum did not. However, plants grew larger with some combinations of G.␣intraradices plus G. etunicatum than with the same quantity of G. intraradices alone. These results suggest the potential for niche complementarity in the mycorrhizal fungi. That G. etunicatum only increased plant growth in the presence of G. intraradices could be illustrative of why AMF that appear to be parasitic or benign when examined in isolation are maintained within multi-species mycorrhizal communities in nature.     

Grassland invaders and their mycorrhizal symbionts: a study across climate and invasion gradients

Controlled experiments show that arbuscular mycorrhizal fungi (AMF) can increase competitiveness of exotic plants, potentially increasing invasion success. We surveyed AMF abundance and community composition in Centaurea stoebe and Potentilla recta invasions in the western USA to assess whether patterns were consistent with mycorrhizal-mediated invasions. We asked whether (1) AMF abundance and community composition differ between native and exotic forbs, (2) associations between native plants and AMF shift with invading exotic plants, and (3) AMF abundance and/or community composition differ in areas where exotic plants are highly invasive and in areas where they are not. We collected soil and roots from invaded and native forb communities along invasion gradients and in regions with different invasion densities. We used AMF root colonization as a measure of AMF abundance and characterized AMF communities in roots using 454-sequencing of the LSU-rDNA region. All plants were highly colonized (>60%), but exotic forbs tended to be more colonized than natives (P < 0.001). We identified 30 AMF operational taxonomic units (OTUs) across sites, and community composition was best predicted by abiotic factors (soil texture, pH). Two OTUs in the genera Glomus and Rhizophagus dominated in most communities, and their dominance increased with invasion density (r = 0.57, P = 0.010), while overall OTU richness decreased with invasion density (r = −0.61, P = 0.006). Samples along P. recta invasion gradients revealed small and reciprocal shifts in AMF communities with >45% fungal OTUs shared between neighboring native and P. recta plants. Overall, we observed significant, but modest, differences in AMF colonization and communities between co-occurring exotic and native forbs and among exotic forbs across regions that differ in invasion pressure. While experimental manipulations are required to assess functional consequences, the observed patterns are not consistent with those expected from strong mycorrhizal-mediated invasions.     

Effects of Alliaria petiolata (garlic mustard; Brassicaceae) on mycorrhizal colonization and community structure in three herbaceous plants in a mixed deciduous forest

Herbaceous plant species are important components of forest ecosystems, and their persistence in forests may be affected by invasive plant species that reduce mycorrhizal colonization of plant roots. I examined the effect of the invasive plant Alliaria petiolata on arbuscular mycorrhizal fungi (AMF) colonizing the roots of three forest plant species. AMF root colonization and community structure was examined from plants that were growing either in the absence or presence of Alliaria under natural forest conditions. AMF root colonization varied among the plant species but was not significantly affected by Alliaria. With molecular methods, ～12 different taxa of AMF could be distinguished among the root samples, and these taxa belonged to the genera Acaulospora and Glomus, with Glomus dominating AMF communities. There were significant differences between the community of AMF colonizing roots of Maianthemum racemosum and Trillium grandiflorum, but only AMF communities of Maianthemum roots were significantly affected by Alliaria. Indicator species analysis found that an Acaulospora species type was a significant indicator of Maianthemum plants grown in the absence of Alliaria. These results suggest invasive plants like Alliaria may selectively suppress AMF fungi, and this suppression can affect AMF communities colonizing the roots of some native plant species.     

连作花魔芋软腐病株与健株根域丛枝菌根真菌群落多样性

【目的】解析不同连作年限花魔芋软腐病株、健株根域的丛枝菌根真菌(arbuscular mycorrhizal fungi,AMF)群落多样性。【方法】使用AMF 18S SSU rRNA基因特异引物AMV4.5NF/AMDGR对正茬及连作2年和3年的软腐病株、健株魔芋根系和根际土壤DNA扩增建库,通过高通量测序和生物信息学分析探究魔芋软腐病与其根域AMF群落多样性的关系。【结果】魔芋根系具有明显的AMF菌丝、泡囊和丛枝等结构。在相同连作年限条件下,健株根系AMF总侵染率、侵染强度和孢子密度均显著高于病株(P<0.05);在不同连作年限条件下,病株根系AMF总侵染率和侵染强度随连作年限延长而降低。从所有样品中共鉴定到9属53种AMF,其中有49个已知种和4个新种。球囊霉属(Glomus)和类球囊霉属(Claroideoglomus)是AMF群落的优势属,其AMF种分别占总AMF种数的41.5%和26.4%;丰度最高的Paraglomus sp.VTX00308是所有样品的共有种。连作、软腐病及二者的交互作用显著影响根系AMF群落的Shannon指数和Simpson指数及根际土壤AMF的Chao1指数(P<0.05)。通过丰度差异分析发现6个在连作软腐病发生后丰度差异显著的AMF种(P<0.05);NMDS分析表明,不同连作年限的魔芋软腐病株与健株之间的根域AMF菌种组成、相对丰度和群落结构存在差异。相关性分析表明,软腐病发病率和病情指数与魔芋根系和根际土壤AMF的Shannon指数、根系AMF的Chao1和Simpson指数以及AMF总侵染率、侵染强度和孢子密度极显著负相关(P<0.01)。【结论】比对健株,连作魔芋软腐病株根际土壤AMF孢子密度以及根系AMF侵染率、种数和多样性均降低,其群落结构显著改变。     

Response of the invasive Centaurea maculosa and two native grasses to N-pulses

Nitrogen is often a limiting resource on semi-arid grasslands. During the growing season, N is often only available during short-term pulses associated with wetting events. The Eurasian forb Centaurea maculosa Lam. has invaded millions of hectares of semi-arid grasslands in western North America. C. maculosa's success could be attributed to greater use of N-pulses, or more efficient use of N supplied in those pulses compared with native grasses. In a glasshouse, C. maculosa and two native grasses, the caespitose Pseudoroegneria spicata [Scribn. and Smith] A. Love and the rhizomatous Pascopyrum smithii [Rybd.] A. Love, were established in mixed- and monoculture combinations, and then conditioned to weekly N-pulses of 8, 24, or 72 h for 8 weeks. These pulse durations are typical on semi-arid grasslands. At the end of the 8 weeks, plants were exposed to ¹⁵N-labeled nitrate (¹⁵NO₃ ^–) for 8 h and harvested 16 h later to compare short-term root uptake of ¹⁵NO₃ ^–. C. maculosa did not have greater enrichment (atom % ¹⁵N), rate of ¹⁵N-uptake (mol g^–1 h^–1), or ¹⁵N acquired (relative to ¹⁵N applied) than the grasses. C. maculosa's ¹⁵N-uptake per unit mass was relatively consistent across pulse durations, whereas ¹⁵N-uptake was lower at the longer pulse durations for the grasses. In general, C. maculosa acquired more of the applied ¹⁵N than P. spicata but less than P. smithii. ¹⁵N acquired was often influenced by the neighbour's identity. Regarding growth responses, C. maculosa produced more total biomass than the grasses, except for P. smithii plants growing with C. maculosa conditioned to 72 h pulses of N. Root mass ratios varied depending on the neighbor. Overall, C. maculosa used nitrogen less efficiently than the grasses. C. maculosa's success as an invasive species cannot be explained wholly by a greater response to N-pulses or more efficient use of N-pulses compared with native grasses with which it competes.     

Determination of root biomasses of three species grown in a mixture using stable isotopes of carbon and nitrogen

A method is evaluated that employs variation in stable C and N isotopes from fractionations in C and N acquisition and growth to predict root biomasses of three plant species in mixtures. Celtis laevigata Willd. (C₃), Prosopis glandulosa Torr. (C₃, legume) and Schizachyrium scoparium (Michx.) Nash (C₄), or Gossypium hirsutum L. (C₃), Glycine max (L.) Merr. (C₃ legume), and Sorghum bicolor (L.) Moench (C₄) were grown together in separate, three-species combinations. Surface roots (0–10 cm depth) of each species from each of the two combinations were mixed in various proportions, and the relative abundances of ¹⁵N and ¹⁴N and ¹³C and ¹²C in prepared mixtures, surface roots of single species, and roots extracted from the 80-cm soil profile in which each species combination was grown were analyzed by mass spectrometry. An algebraic determination which employed the δ ¹³C, % ¹⁵N, and C and N concentrations of root subsamples of individual species accounted for more than 95% of the variance in biomass of each species in prepared mixtures with G. max, G. hirsutum, and S. bicolor. A similar analysis demonstrated species-specific differences in rooting patterns. Root biomasses of the C₄ monocots in each combination, S. scoparium and S. bicolor, were concentrated in the upper 20 cm of soil, while those of G. hirsutum and the woody P. glandulosa were largest in lower soil strata. Analyses of stable C and N isotopes can effectively be used to distinguish roots of species which differ in ratios of ¹⁵N to ¹⁴N and ¹³C to ¹²C and thus to study belowground competition between or rooting patterns of associated species with different C and N isotope signatures. The method evaluated can be extended to quantify aboveground and belowground biomasses of component species in mixtures with isotopes of other elements or element concentrations that differ consistently among plants of interest.     

Community interactions between the filamentous alga Cladophora glomerata (L.) Kuetzing,its epiphytes,and epiphyte grazers

Summary Interactions between epiphytes, epiphyte grazers and the filamentous green alga Cladophora glomerata (L.) Kuetzing were explored with smaples from rivers in Montana. Extracts of C. glomerata lowered photosynthetic rates of Nitzschia fonticola Grunow (an epiphytic diatom). Nutrient enrichment showed that C. glomerata from the Madison River was N deficient and its epiphytes were P deficient on 2 dates and N deficient on one date, while no nutrient deficiencies were detected in samples from 3 other rivers; this implies there was little nutrient competition between the epiphytes and C. glomerata. Epiphytes lowered drag on C. glomerata tufts and current velocity inside the tufts, apparently by decreasing the effective surface area. Lower drag may decrease detachment, but lowering current velocity from 8 to 0 cm s^-1 resulted in a 100 % decrease in photosynthesis. Light absorption by epiphyte pigments may lower photosynthetic rate of C. glomerata when irradiance is below 200–500 E m^-2 s^-1, and protect against photoinhibition above this irradiance range. Invertebrate grazers (predominantly Baetis tricaudatus Dodds, Trycorythodes minutus Traver and Brachycentrus occidentalis Banks) at high densities removed 75% of epiphytes and B. occidentalis grazed on C. glomerata. Invertebrates regenerated a mean of 0.16 mol NH _inf4 ^sup+ individual^-1 d-1 which could have enhanced growth of downstream C. glomerata. Competition and grazing were not the only interactions in the C. glomerata community, positive (mutualistic) interactions were also important.     

Allocating nitrogen away from a herbivore: a novel compensatory response to root herbivory

Centaurea maculosa, an invasive North American plant species, shows a high degree of tolerance to the root-boring biocontrol herbivore, Agapeta zoegana. For example, infested individuals of C. maculosa often exhibit more rigorous growth and reproduction compared with their non-infested counterparts. Compensatory responses to aboveground herbivores often involve increases in leaf area and/or photosynthetic capacity, but considerably less is known about root system compensatory responses to belowground herbivory. We used a ¹⁵N labeling approach to evaluate whether compensatory adjustments in N acquisition via changes in root morphology and/or physiological uptake capacity could explain the ability of C. maculosa to tolerate root herbivory. Root herbivory reduced whole plant N uptake by more than 30% and root uptake capacity by about 50%. Despite a marked reduction in N procurement, herbivory did not affect total biomass or shoot N status. Infested plants maintained shoot N status by shifting more of the acquired N from the root to the shoot. To our knowledge, shifting N allocation away from a root herbivore has not been reported and provides a plausible mechanism for the host plant to overcome an otherwise devastating effect of a root herbivore-induced N deficit.