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Edaphic drought reduces phosphorus (P) diffusivity in soils and arbuscular mycorrhizal fungi (AMF) can compensate for this. We recently showed, along a high-resolution substrate moisture gradient, that AMF effectively deliver P to plants under drought from areas beyond the reach of roots. Here, we investigated how edaphic drought affected the active sites of P exchange between AMF and plants inside the roots using different histochemical stains. Inoculation of dwarf tomato plants (cv. MicroTom) with AMF (Rhizophagus irregularis) slightly increased water retention (decreased the water potential) in substrates, decreased root length, and increased shoot P concentrations compared to non-mycorrhizal plants. Despite their reduced root length, mycorrhizal tomato plants showed a surprisingly congruent physiological drought stress response with their non-mycorrhizal counterparts. However, the activity of alkaline phosphatase (ALP) detected in intraradical fungal structures decreased significantly as soil and xylem water potentials decreased. The decline of ALP activity with increasing drought intensity was accompanied by a reduction of elevated P concentration benefits in shoots of mycorrhizal plants compared to non-mycorrhizal plants. Therefore, we conclude that edaphic drought can restrict the nutritional advantages that plants receive from AMF at the root-fungal interface. This is one of the direct consequences of drought. • Short-term drought hardly showed an effect of colonization of tomato roots by AMF. • Yet, alkaline phosphatase activity in the intraradical hyphae strongly decreased. • Reductions in phosphatase activity correlated with declining shoot phosphorus. • Reduced P transfer from fungus to plant thus seems directly induced by drought. [ABSTRACT FROM AUTHOR]

Alternate wetting-drying (AWD) in rice cultivation controls soil redox conditions and consequently nutrient solubility. Under low redox potential, ferric iron reduction leads to bound phosphate (Fe(III)–P) dissolution, but lack of oxygen retards organic phosphorus (P org) mineralization. Microorganisms accelerate P org mineralization as the redox potential increases during drying, but it is not known which P source will be preferentially taken up by microorganisms and plants. Using double 32/33P labeling, we traced for the first time the P released from inorganic (32P) and organic (33P) sources into microbial biomass P (MBP), phospholipids, and plants under continuous flooding (CF) and AWD. The CF rapidly induced reducing conditions that increased Fe–P dissolution and thus P availability. The AWD had no preference for 32P and 33P incorporation into plants. However, the ratios of 32P in roots to 32P in MBP or to 32P in phospholipids in rooted soil were 4–9 times higher than the respective ratios of 33P. Moreover, the ratios of 33P in roots to 33P in MBP or to 33P in phospholipids were always <0.6. Thus, plants and microorganisms were more competitive for P from Fe–P and from P org , respectively. The AWD increased 32P and 33P incorporation into phospholipids in rooted soil by 53–56% as compared to CF. Also, the ratio of 32P in phospholipids to 32P in MBP was 1.4–2.2 times higher under AWD than under CF. Thus, AWD stimulated microorganisms to allocate more P for cell membrane synthesis as compared to CF. Plants were similarly effective to take up P from inorganic and organic sources. Increased root biomass stimulated arbuscular mycorrhizal symbiosis (represented by 16:1ω5c) and thus 33P recovery in roots under AWD. In turn, the acceleration of microbial P turnover and the microbe-specific preference for P org highlight the importance of arbuscular mycorrhiza for plant P uptake under fluctuating water conditions. [Display omitted] • Continuous flooding increased reductive dissolution of Fe–P and thus P availability. • Microbes allocated more P for cell membrane synthesis under alternate wetting-drying. • Microorganisms in a maintenance state allocated more P for cell compound synthesis. • Rice plants were similarly effective to take up P from inorganic and organic sources. [ABSTRACT FROM AUTHOR]

Abstract Nitrous oxide (N 2 O) is a potent greenhouse gas emitted at considerable rates from agricultural soils. A few recent studies have indicated the potential to reduce N 2 O emission rates in agricultural systems through management of soil microbiota, such as mycorrhizal fungi. Our limited understanding of how mycorrhiza influences N 2 O emission rates, however, forestalls such management practices. To address the knowledge gap regarding the mechanism of mycorrhizal modification of N 2 O formation and denitrification, we disentangle the direct effects of arbuscular mycorrhizal fungi (AMF) via hyphal growth on potential denitrification activity from those resulting from indirect AMF-induced changes in soil aggregation. We manipulated AMF status in a factorial experiment through adding or not AMF inoculum in the form of Rhizophagus irregularis and soil aggregation state through either crushing soil aggregates or using agricultural soil with intact soil aggregates. We then grew Zea mays (maize) in compartmentalized mesocosms for 4 months. At harvest, we observed approximately three-fold higher AMF root colonization and hyphal densities in the treatments where we had added Rhizophagus irregularis. Potential N 2 O activity rates and the denitrification ratio were higher in the undisturbed soil and in the mesocosms with a dense AMF hyphal network (direct effect of mycorrhiza). Further, AM-hyphae promoted soil aggregation while also altering potential denitrification activity (indirect effect of mycorrhiza). Overall, we present here a comprehensive case study on how mycorrhiza alters potential denitrification activity and related N 2 O activity from agricultural soils. Highlights • We disentangled direct and indirect effects of AM on denitrification. • AM hyphae (direct) promoted N 2 O activity. • Soil aggregation (indirect) also promoted N 2 O activity. • Mycorrhiza, here, stimulated potential N 2 O activity. [ABSTRACT FROM AUTHOR]

Abstract Intensive organic matter removal (OMR) associated with timber harvest has the potential to impart long-term alterations to soil biota and associated properties and processes; however, there is a lack of data to say if this trend persists at depth. This study investigated how OMR influences long-term stability of soil fungi to a depth of 1 m using a replicated experimental pine forest in the Gulf Coastal Plain, USA. Treatments included unharvested control stands as well as low- and high-OMR stands. Intensive OMR led to significant differences in community structure and the abundance of functional guilds in surficial soil. Saprophytic taxa increased while ectomycorrhizal (ECM) taxa decreased with intensive-OMR, which correlated strongly with increased surface temperature and reduced soil nitrogen. Ericoid mycorrhizae (ERM) also increased in intensive-OMR stands, which may indicate that following disturbance, ERM could outcompete ECM for colonization of subsequent seedlings. Overall, no differences were observed below 30 cm, except for alpha diversity, which was a function of high inter-replicate variability. Our results illustrate a distinct long-term structural and functional response of soil fungi to intensive-OMR in the upper portions of the soil profile, which could lead to altered stand productivity and ecosystem services. Highlights • Fungal community structure at 0–10 cm and 10–30 cm was altered by timber harvest. • Saprophytes increased and ectomycorrhizae decreased in intensively harvested stands. • Fungal communities at depth possessed high inter-replicate variability. [ABSTRACT FROM AUTHOR]

The presence of living roots can markedly change soil properties, microbial activities, and biogeochemical cycling in the rhizosphere compared to bulk soil. Such rhizosphere effect has been increasingly recognized to play a crucial role in carbon (C) and nitrogen (N) cycling in terrestrial ecosystems. Here, we investigated the rhizosphere effects of 12 tree species associated with two contrasting mycorrhizal types (arbuscular mycorrhizal, AM vs. ectomycorrhizal, ECM) and co-occurring in a temperate secondary forest in Northeast China. The adhering soil method was adopted to sample paired rhizosphere and bulk soils of field-grown trees in the peak growing season. Generally, we found positive rhizosphere effects on soil properties (ammonium and nitrate content, total C and N content, and C:N ratio), microbial abundances (bacteria, fungi, and actinomycetes), enzyme activities (C- and N-degradation enzymes), and C mineralization rate (C min), but minor rhizosphere effects on soil pH, phosphorus content and phosphatase activity, net N mineralization rate (N min), and specific C min and N min (per unit soil C or N). Because of large intraspecific and particularly interspecific variations, less than half of the rhizosphere effects were statistically significant (P < 0.05), and none of the rhizosphere effects differed significantly between the two mycorrhizal types (AM vs. ECM). Moreover, most soil variables varied greatly within and across species, and thus did not differ significantly between the two mycorrhizal types (AM vs. ECM). However, soil pH, nitrate content, microbial abundance, and C-degradation enzyme activity were notably lower under ECM species than under AM species. Collectively, these results show that rhizosphere effects varied greatly among the six species within both mycorrhizal groups, and such variations within mycorrhizal group were even larger than those between mycorrhizal group. Future work should include a larger number of tree species to increase statistical power, and combine phylogenic history and functional traits of plant species with the type and traits of mycorrhizal fungi to better understand how plant-microbe interactions influence biogeochemical cycling in the rhizosphere. Highlights • We quantified rhizosphere effects of 12 co-existing temperate tree species. • Six arbuscular mycorrhizal (AM) and six ectomycorrhizal (ECM) species were included. • Rhizosphere effects on soil properties, microbial abundances and activities were either positive or neutral. • Rhizosphere effects showed larger variations within mycorrhizal group than between mycorrhizal group. • Rhizosphere effects may be controlled by phylogeny and traits of plants and mycorrhizal fungi. [ABSTRACT FROM AUTHOR]

Plant roots contribute substantially to the formation of stable soil organic matter (SOM), and there is evidence that species differ in their contribution to SOM stabilization. However, it remains unclear what specific root traits contribute to the three SOM stabilization mechanisms: recalcitrance against decomposition, occlusion in soil aggregates and interaction with soil minerals and metals. This is likely because research is highly fragmented and hampered by disciplinary barriers. By reviewing both plant functional ecology and soil science literature, we identified 18 different traits: architectural, morphological, physiological, symbiotic and chemical root characteristics, influencing the three SOM stabilization mechanisms. We found that traits increasing root recalcitrance promote short term stabilization by slowing decomposition, but that traits reducing recalcitrance contribute to long term stabilization by reaction of microbial products with mineral surfaces. Root length density, mycorrhizal association and rhizodeposition contribute to microaggregation. These and other traits, such as hemicellulose, soluble compounds, and high root branching index, favor macroaggregation. For stabilization by minerals and metals, those root traits promoting higher microbial activity: root nitrogen, hemicellulose and soluble compound concentrations are fundamental, while polyphenols, and litter Al and Mn also contribute to complexification and stabilization. Root depth distribution is the most important trait to control root C storage and stabilization in the subsoil; once roots have reached deeper soil layers, other traits, such as rhizodeposition and root chemistry, influence interaction with minerals and metals. Both mycorrhizal presence and root suberin promote SOC stabilization by means of all three mechanisms, indicating that these are important targets for continued work. Surprisingly, morphological traits commonly measured, namely specific root length and root diameter, poorly relate to stabilization mechanisms. Alternative traits such as chemical composition of the different root orders, root apex characteristics, quantity and quality of rhizodeposits as well as mycorrhizal fungal traits, should be further investigated. For future research, this review highlights the need to evaluate root decomposition and root-C stabilization concomitantly over the long-term, considering simultaneously root litter quality, estimated by root traits, the microbial products and properties of the soil matrix. The information accrued in this review can be used to evaluate the potential of plant species and cultivars to promote SOM stabilization, based on their root traits. [ABSTRACT FROM AUTHOR]

Tree roots and mycorrhizal fungi form an important resource for soil microinvertebrates; however, there is little knowledge on the role of root feeders and microbial grazers on belowground carbon flux of forest trees. Using oak ( Quercus robur ) microcuttings (Clone DF159) inoculated with an ectomycorrhizal symbiont partner ( Piloderma croceum ), the present study investigated the impact of multitrophic interactions in the rhizosphere on the carbon flow to the soil food web in a pulse-chase labelling experiment. Changes in plant biomass, microbial communities (soil phospholipid fatty acids - PLFAs) and Collembola lipid pattern (total lipid fatty acids - TLFAs) as well as the 13 C incorporation into fatty acids were assessed at 2, 5 and 20 days post labelling of oak microcuttings. Microcosms were inoculated with Pratylenchus penetrans (root-feeding nematode), Protaphorura armata (fungal-feeding Collembola) or their combination. The interaction of P. penetrans and P. armata synergistically reduced oak biomass. Both soil animals independently impacted growth of microorganisms in the rhizosphere, P. penetrans inhibited whilst P. armata promoted bacterial biomass. Irrespective of treatment and time, Gram-positive bacteria derived most of the recent photoassimilated oak carbon, with amounts ranging between 80 and 92% of the total 13 C allocated in microbial PLFAs. Presence of either P. armata or P. penetrans enhanced the incorporation of plant carbon in bacteria, whereas collective effects of these two functional groups was scarce. Predominantly the impact of the root-feeding nematode on belowground carbon flow diminished with time. In sum, root feeders and detritivores played independent roles in carbon allocation patterns and community structure of microorganisms in the rhizosphere of oaks. [ABSTRACT FROM AUTHOR]

Little is known about how the adverse physical and chemical environment in sodic soils affects plant-mycorrhizal relationships. We investigated mycorrhizal colonisation and plant nutrient uptake of cotton plants under highly stressed (Exchangeable Na percentage (ESP) 21) and less stressed (ESP 7) conditions with two rates of applied P. The relative hyphal contribution to P uptake was quantified using dual isotope labelling techniques ( 32 P and 33 P). Root colonisation and P uptake of mycorrhizal cotton plants reduced by 16% and 20%, respectively, in highly sodic soil as compared to plants in low sodic soil, however, the relative proportion of P delivered via hyphal pathways ( 32 P from root-free hyphal compartment) was similar. Under high P conditions, the relative increase in the proportion of 33 P (root + hyphae compartment) taken up by inoculated plants was greater in the low sodic soil relative to the high sodic soil. Mycorrhization improved early seedling vigour, and nutrient uptake. Reduced colonisation and hyphal exploration of the soil, possibly due to the physical and chemical constraints imposed by highly-sodic soil, rather than poorer mycorrhizal function, may be responsible for limited early P uptake of cotton in highly-sodic soil. [ABSTRACT FROM AUTHOR]

• Feremycorrhiza means 'nearly mycorrhiza'. • Feremycorrhiza has mycorrhiza-like effects on plant growth and nutrition. • Austroboletus occidentalis has ectomycorrhizal traits and lineage. • A. occidentalis has limited saprotrophic capacity similar to ectomycorrhizal fungi. • A. occidentalis genome lacks invertase genes similar to ectomycorrhizal fungi. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizas (AM) are adapted to soils with higher pH, less organic matter and more available nitrogen than ectomycorrhizas (EM). We hypothesise that also climatic factors have a direct role in the relative success of the two types. We tested their colonisation rates after soil freezing. Soil was taken from forest and meadow sites to comprise propagules of both types of mycorrhizal fungi. The soil was sieved (6 mm) and mixed. Soil batches were exposed to +5 °C (control), −12, −25, −48 or −130 °C, time at target temperature was 6 h. Silver birch ( Betula pendula ), grey alder ( Alnus incana ) and white clover ( Trifolium repens ) were sown to these soils and grown in favourable conditions for 11 weeks. EM colonisation in birch and alder was similar in all treatments. Arbuscule formation in alder and clover was not affected, but vesicles, AM fungal hyphae and spores in roots were reduced with decreasing temperature. Soil soluble phosphorus (P) was increased by freezing as were also the foliar concentrations in birch. By contrast, clover had less foliar P in the lowest temperature treatments which may be due to less efficient functioning of AM. To conclude, the development and function of AM were impaired to some extent by very low temperatures, while EM were not affected. The low-temperature tolerance of EM may be another explanation to their predominance in cold climates. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal fungi (AMF) form symbioses with most plants, potentially improving their growth and nutrient assimilation activities. Cysteine (Cys) and methionine (Met) are nitrogen (N)- and sulphur (S)-containing amino acids. Compared with phosphate and N, limited attention has been paid to the role of AMF in low molecular weight organic S acquisition. To explore the uptake and relative contributions of organic and inorganic N and S to plants, and the role of AMF in S uptake, a pot study was conducted based on 14C, 35S, 13C, and 15N quad labelling (6-h labelling test after adding the labelled solution to the soil–plant system) using a mutant tomato genotype with highly decreased AMF symbiosis capacity. Tomato roots can uptake limited amounts of added Met and Cys (<1.77%) within 6 h, as indicated by the 14C and 13C labelling results, and most of them were utilised by soil microorganisms. After uptake for 6 h, 10.0–14.8% of N and 1.4–6.1% of S derived from added Cys and Met were utilised by plants, mainly in inorganic N and S forms derived from Cys and Met decomposition. Met and Cys could be important S sources (Met: 3.0–9.8%, Cys: 8.8–22.0%) for plants, however have negligible roles in N nutrition (∼1%), as most N uptake by plants was derived from soil inorganic N. The tomato uptake of inorganic S derived from Cys decomposition was much higher than that derived from Met, as higher ratios of S-Cys were released as SO 4 2− from microorganisms during the 6-h testing periods. Even with the artificial addition of AMF, most of the added Met and Cys were utilised by Gram-negative bacteria, as indicated by 13C-PLFA biomarkers. AMF reduced host plant uptake of organic N and S, but stimulated plant N uptake from Met and Cys, which was mainly inorganic N following mineralisation. AMF not only utilise organic carbon from host plants but also capture soil organic matter to satisfy their energy demands. In the spaces where both root and AMF occur, AMF colonisation decreased tomato 35S uptake from Cys, Met, and SO 4 2− by 24.6%, 20.6%, and 11.0% within the 6-h uptake period, respectively, when compared with that in the mutant genotype with reduced colonisation capacity; in contrast, AMF colonisation increased 35S-Cys uptake by 118.7% from areas that roots could not reach. Overall, AMF enhanced host plant N uptake but reduced organic N uptake under competition with plant roots for S in the rhizosphere, but stimulated plant S uptake by extraradical mycelia, based on this short-term pulse-chase test. • Limited Met and Cys was absorbed by tomato under competition with microorganisms. • Inorganic S uptake by tomato from Cys decomposition higher than that from Met. • Met and Cys could be key S sources for plants, with negligible roles in N nutrition. • AMF reduce plant organic N/S uptake but stimulate plant N uptake from Met and Cys. • AMF colonisation increased 35S-Cys uptake from areas roots could not reach. [ABSTRACT FROM AUTHOR]

Agriculture often leads to altered composition and reduced diversity of arbuscular mycorrhizal fungal (AMF) communities compared to natural grassland systems. However, the ecology of taxa that are lost in this transition has thus far not been well characterized. In this study we found that reduced or lost AMF taxa in farmlands were significantly stronger correlated with soil N:P ratio than a randomly sampled community; this indicates that taxa that prevail at high N:P ratio in grasslands are the ones most sensitive to agriculture. As a high N:P environment is also commonly argued to impose the highest AMF benefit to plants, the observation that those taxa are lost could indicate that agricultural fields are left with communities of reduced symbiotic quality. [ABSTRACT FROM AUTHOR]

Host-plants may rarely leave their ancestral niche and in which case they tend to be surrounded by phylogenetically distant neighbours. Phylogenetically isolated host-plants might share few mutualists with their neighbours and might suffer from a decrease in mutualist support. In addition host plants leaving their ancestral niche might face a deterioration of their abiotic and biotic environment and might hence need to invest more into mutualist partners. We tested whether phylogenetic isolation of hosts from neighbours decreases or increases abundance and activity of their mutualists and whether mutualist activity may help to compensate deterioration of the environment. We study oak-hosts and their ectomycorrhizal fungi mutualists established in the litter layer formed by the phylogenetically closely or distantly related neighbourhood. We find that oaks surrounded by phylogenetically distant neighbours show increased abundance and enzymatic activity of ectomycorrhizal fungi in the litter. Moreover, oaks surrounded by phylogenetically distant neighbours also show delayed budburst but ectomycorrhizal fungi activity partly compensates this negative effect of phylogenetic isolation. This suggests decreased nutrient availability in a phylogenetically distant litter partly compensated by increased litter-degradation by ectomycorrhizal fungi activity. Most observed effects of phylogenetic isolation cannot be explained by a change in baseline soil fertility (as reflected by nutritional status of fresh oak litter, or soil microbial biomass and activity) nor by simple reduction of percentages of oak neighbours, nor by the presence of gymnosperms. Our results show that colonizing new niche represented by the presence of distantly related neighbours may delay plant phenology but may be supported by mycorrhizal mutualists. Studies on other host-plant species are required to generalize our findings. [ABSTRACT FROM AUTHOR]

Tools for accurate measurements of microbial biomass/abundance are imperative in soil ecology. This perspective was prompted by a lack of consistency in how the neutral lipid fatty acid (NLFA) and phospholipid fatty acid (PLFA) 16:1ω5 are used to estimate arbuscular mycorrhizal (AM) fungal abundance, which we argue can lead to erroneous conclusions within studies and complicate among-study comparisons. We show that more than half of all studies published to date use PLFA 16:1ω5 to quantify AM fungal biomass without proper controls. This is problematic because AM fungi cannot be separated from the bacterial contribution to this fatty acid signature. We provide a set of recommendations for future research and specifically urge researchers to extract and analyze NLFA 16:1ω5 for more accurate and sensitive assessments of AM fungal biomass as this can be done without doubling effort. • Large number of studies have used PLFA 16:1ω5 as AM fungal indicator. • The use of PLFA 16:1ω5 without proper non-mycorrhizal controls is problematic as it can also reflect bacterial biomass. • We urge researchers to use NLFA 16:1ω5 to estimate AM fungal biomass as this can be done without doubling effort. [ABSTRACT FROM AUTHOR]

Abstract: Positive relationships between biodiversity and ecosystem functioning are reported in many studies. The underlying mechanisms are, however, only little understood, likely due to the focus on the aboveground system and neglecting species-specific traits. We used different clusters of tree species composition to investigate how tree species diversity and tree species identity affect belowground nematode communities. Since soil nematodes comprise different trophic groups and are strongly linked to the microbial community, results can provide insight on how soil food webs are structured. In addition, data on leaf litter and fine root biomass were included to account for different effects of aboveground and belowground resources. Clusters of three trees of one, two and three species were identified within a temperate deciduous forest. Target tree species were European beech (Fagus sylvatica), common ash (Fraxinus excelsior) and lime (Tilia cordata, T. platyphyllos) differing in physiology, leaf litter quality and type of mycorrhiza. Tree species identity strongly affected nematode trophic structure, whereas tree species diversity had no impact. Ash beneficially affected bacterial-feeding nematodes, whereas fungal feeders were suppressed, likely caused by ash litter increasing soil pH. Fostering of the bacterial food chain by ash additionally could be related to rhizodeposition gaining importance after disappearance of high quality ash litter in spring, highlighting seasonal shifts in root and leaf litter-derived resources. The negative effect of ash on fungal-feeding nematodes is suggested to be due to the lack of ectomycorrhizal fungi as ash roots only form arbuscular mycorrhiza. In contrast, beech benefited fungal feeders and detrimentally affected bacterial feeders due to more acidic soil conditions that increase the competitive strength of fungi. Lime tended to negatively impact total nematode density but positively influenced plant-feeding nematodes. Generally, beech and ash strongly but opposingly influenced the trophic structure of nematode communities suggesting that changes in tree species identity result in major shifts in the channeling of energy through decomposer food webs. The results indicate that the structure of soil food webs varies markedly with tree species and point to the importance of basal resources, i.e., leaf litter and rhizodeposits. This suggests bottom-up forces mediated by individual tree species to control major decomposition pathways rather than tree diversity. [Copyright &y& Elsevier]

Abstract: Arbuscular mycorrhizal fungi (AMF) have great potential as biocontrol organisms against the root–knot nematode Meloidogyne incognita which causes severe gall formation in plants, but knowledge about the underlying molecular mechanisms involved in the biocontrol of nematodes is scarce. In the present study, suppression subtractive hybridization (SSH) was used to investigate plant genes that are specifically up-regulated in tomato roots (Solanum lycopersicum cv. Marmande) pre-colonized by the AMF Glomus mosseae (BEG 12) and 12 days after soil inoculation with M. incognita juveniles. Nematode infection was significantly lower in the mycorrhizal roots as compared to the non-mycorrhizal roots, and identified genes were classified mainly in the categories of defense, signal transduction and protein synthesis and modification. The higher expression of a selection of defense-related plant genes specifically in the biocontrol interaction compared to in plants that were only mycorrhizal or only nematode-infected was confirmed, which pleads for the existence of mycorrhiza-induced priming of plant defense responses. In conclusion, by focusing on up-regulated gene expression in the biocontrol interaction between mycorrhizal tomato and M. incognita, new insights were found into the molecular mechanisms underlying the mycorrhiza-induced resistance against root–knot nematodes. In particular, the involvement of the phenylpropanoid pathway and reactive oxygen species (ROS) metabolism could explain the reduced root–knot nematode infection in mycorrhizal tomato roots, processes that have also been reported to play a pivotal role in plant resistance to nematodes. [Copyright &y& Elsevier]

Abstract: In some cases amending soil with biochar improves fertility, although the exact mechanisms through which biochar alters soil processes are not well understood. In other cases, however, biochar amendment can have no effect on plant growth, or can have negative effects. When crop benefits occur, simultaneous amendment with biochar and mineral nutrients causes results that are not additive, suggesting that biochar may be capable of improving the efficiency of nutrient uptake by plants, but the mechanisms of this synergy remain unknown. One possible mechanism that has not been fully explored is alterations to the plant–mycorrhizal fungus mutualism, a relationship that occurs in most land plants. In a 4 week greenhouse experiment, we investigated possible effects of the presence of biochar, mycorrhizal fungi, and nitrogen fertilizer on sorghum seedling growth. Results indicated that the combined treatment of biochar, mycorrhizal fungi, and high nitrogen decreased aboveground plant biomass by 42% relative to the mycorrhizae and high nitrogen treatment, while simultaneously promoting mycorrhizal root colonization. This is evidence for an induced parasitism of the mycorrhizal fungus in the presence of nitrogen and biochar within the 4 week timescale of our experiments. Using x-ray photoelectron spectroscopy, we found evidence of increased surface oxidation on biochar particles over the 4 weeks of our trial, consistent with sorption of labile, plant derived dissolved organic matter or char oxidation, either via biotic or abiotic processes. Biochar in soils with mycorrhizae but without sufficient nitrogen showed more surface oxidation than other treatment combinations, and showed a significantly greater fraction of surface carbon present in carbonyl (–Conalities. Our results suggest that soil nitrogen acts as a switch controlling the ability of char to influence the mycorrhizal symbiosis and, in turn, the degree to which the fungi oxidize the char surface. [Copyright &y& Elsevier]

Abstract: Mycorrhiza-induced resistance has been observed against a broad range of mainly soil-borne pathogens, including plant-parasitic nematodes, but the modes of action involved remain unclear. In this study the role of mycorrhiza-induced resistance was investigated during the pre-infectional phase of nematode host finding and penetration. Banana plants were colonized by Glomus mosseae or Glomus intraradices, two arbuscular mycorrhizal fungi. The plant-parasitic nematode Radopholus similis was inoculated after establishment of the mycorrhizal colonization. Nematode attraction and penetration were assessed within a 12-day period. In root exudate experiments, root exudates collected from both control and mycorrhizal plants were added both to control and mycorrhizal plants to assess their direct impact on the nematode penetration. In an in vitro chemotaxis bio-assay, the chemotactic behavior of R. similis was determined towards isolated root exudates of control and mycorrhizal plants. The penetration experiments clearly showed lower nematode penetration in mycorrhizal plants and the important contribution of differential root exudation by mycorrhizal plants was demonstrated in the exudate experiments as well as in the in vitro chemotaxis bio-assay, with the largest impact on juveniles. The root exudate experiments and in vitro chemotaxis bio-assay point towards a reduced attraction of the nematodes to the mycorrhizal plant roots. The results demonstrate that a water-soluble compound in mycorrhizal root exudates is at least partially responsible for the mycorrhiza-induced resistance at the pre-infectional level of R. similis infection. [Copyright &y& Elsevier]

Abstract: Low molecular mass organic acids (LMMOAs) and hydroxamate siderophores (HS) are molecules secreted by microbes and have previously been found in soil solution and in cultures. Mycorrhizal fungi are suggested to be involved in the nutrient uptake processes of trees and weathering of minerals. In this study soil samples taken from the O and E horizons of a podzol were extracted with 10 mM potassium phosphate buffer at pH 7.2. Variable parameters included addition of methanol to the extraction buffer and the use of ultrasonication or rotary shaking during extraction. LMMOAs and HS content of the soil extracts were determined. Analysis of soil extracts were carried out by liquid chromatography mass spectrometry (LC–MS) and the extraction results compared to results for soil solution samples obtained by centrifugation of the soils sampled. The extraction yields were significantly increased by addition of methanol to the extraction buffer, especially for the O horizon samples. Rotary shaking of the samples for 90 min gave slightly higher yields than ultrasonication for 15 min but the reduction in extraction time makes ultrasonication an attractive option. Of the HSs determined, ferricrocin was found in all samples. Optimal extraction conditions showed citric acid and isocitric acid to be the most abundant organic acids in the O and E horizons, respectively. [Copyright &y& Elsevier]

Abstract: This review focuses on literature pertaining to the interactions of soil yeasts with biotic and abiotic factors in their environment. Soil yeasts not only affect microbial and plant growth, but may also play a role in soil aggregate formation and maintenance of soil structure. Serving as a nutrient source for bacterial, faunal and protistan predators, soil yeasts contribute to essential ecological processes such as the mineralization of organic material and dissipation of carbon and energy through the soil ecosystem. Some soil yeasts may also play a role in both the nitrogen and sulphur cycles and have the ability to solubilize insoluble phosphates making it more readily available for plants. Recently, the potential of soil yeasts as plant growth promoters and soil conditioners has been studied with the goal of using them in the field of sustainable agriculture. [ABSTRACT FROM AUTHOR]

Abstract: Host trees can modify their soil abiotic conditions through their leaf fall quality which in turn may influence the ectomycorrhizal (ECM) fungal community composition. We investigated this indirect interaction using a causal modelling approach. We identified ECM fungi on the roots of two coexisting oak species growing in two forests in southern Spain –Quercus suber (evergreen) and Quercus canariensis (winter deciduous)-using a PCR-based molecular method. We also analysed the leaf fall, litter and soil sampled beneath the tree canopies to determine the concentrations of key nutrients. The total mycorrhizal pool was comprised of 69 operational taxonomic units (OTUs). Tomentella and Russula were the most species-rich, frequent and abundant genera. ECM fungi with epigeous and resupinate fruiting bodies were found in 60% and 34% of the identified mycorrhizas, respectively. The calcium content of litter, which was significantly higher beneath the winter-deciduous oak species due to differences in leaf fall quality, was the most important variable for explaining ECM species distribution. The evaluation of alternative causal models by the d-sep method revealed that only those considering indirect leaf fall-mediated host effects statistically matched the observed covariation patterns between host, environment (litter, topsoil, subsoil) and fungal community variables. [Copyright &y& Elsevier]

Abstract: Soil tillage practices affect the soil microbial community in various ways, with possible consequences for nitrogen (N) losses, plant growth and soil organic carbon (C) sequestration. As microbes affect soil organic matter (SOM) dynamics largely through their activity, their impact may not be deduced from biomass measurements alone. Moreover, residual microbial tissue is thought to facilitate SOM stabilization, and to provide a long term integrated measure of effects on the microorganisms. In this study, we therefore compared the effect of reduced (RT) and conventional tillage (CT) on the biomass, growth rate and residues of the major microbial decomposer groups fungi and bacteria. Soil samples were collected at two depths (0–5 cm and 5–20 cm) from plots in an Irish winter wheat field that were exposed to either conventional or shallow non-inversion tillage for 7 growing seasons. Total soil fungal and bacterial biomasses were estimated using epifluorescence microscopy. To separate between biomass of saprophytic fungi and arbuscular mycorrhizae, samples were analyzed for ergosterol and phospholipid fatty acid (PLFA) biomarkers. Growth rates of saprophytic fungi were determined by [14C]acetate-in-ergosterol incorporation, whereas bacterial growth rates were determined by the incorporation of 3H-leucine in bacterial proteins. Finally, soil contents of fungal and bacterial residues were estimated by quantifying microbial derived amino sugars. Reduced tillage increased the total biomass of both bacteria and fungi in the 0–5 cm soil layer to a similar extent. Both ergosterol and PLFA analyses indicated that RT increased biomass of saprophytic fungi in the 0–5 cm soil layer. In contrast, RT increased the biomass of arbuscular mycorrhizae as well as its contribution to the total fungal biomass across the whole plough layer. Growth rates of both saprotrophic fungi and bacteria on the other hand were not affected by soil tillage, possibly indicating a decreased turnover rate of soil microbial biomass under RT. Moreover, RT did not affect the proportion of microbial residues that were derived from fungi. In summary, our results suggest that RT can promote soil C storage without increasing the role of saprophytic fungi in SOM dynamics relative to that of bacteria. [Copyright &y& Elsevier]

Abstract: The effects of an arbuscular mycorrhizal (AM) fungus (Glomus etunicatum) on atrazine dissipation, soil phosphatase and dehydrogenase activities and soil microbial community structure were investigated. A compartmented side-arm (‘cross-pot’) system was used for plant cultivation. Maize was cultivated in the main root compartment and atrazine-contaminated soil was added to the side-arms and between them 650 or 37μm nylon mesh was inserted which allowed mycorrhizal roots or extraradical mycelium to access atrazine in soil in the side-arms. Mycorrhizal roots and extraradical mycelium increased the degradation of atrazine in soil and modified the soil enzyme activities and total soil phospholipid fatty acids (PLFAs). Atrazine declined more and there was greater stimulation of phosphatase and dehydrogenase activities and total PLFAs in soil in the extraradical mycelium compartment than in the mycorrhizal root compartment when the atrazine addition rate to soil was 5.0mg kg−1. Mycelium had a more important influence than mycorrhizal roots on atrazine degradation. However, when the atrazine addition rate was 50.0mg kg−1, atrazine declined more in the mycorrhizal root compartment than in the extraradical mycelium compartment, perhaps due to inhibition of bacterial activity and higher toxicity to AM mycelium by atrazine at higher concentration. Soil PLFA profiles indicated that the AM fungus exerted a pronounced effect on soil microbial community structure. [Copyright &y& Elsevier]

Abstract: Rice plants (Oryza sativa L.) were grown in microcosms containing soil with a diverse bacterial community (control) and inoculated either with an axenic arbuscular mycorrhizal fungus (Glomus intraradices) or an axenic inoculum of protozoan grazers of bacteria (Acanthamoeba castellanii), or both, in a factorial design. Amoebae and mycorrhiza affected the root architecture of rice in opposite directions, with mycorrhiza reducing and protozoa increasing early root growth. Rice biomass did not increase in presence of mycorrhiza (×1.08), but strongly increased in presence of Acanthamoebae (×1.29). The positive effects of amoebae were always reduced when plants were also infected with mycorrhiza. Microbial biomass increased (×1.4) and microbial growth was less limited by phosphorus in presence of mycorrhiza. However, plant phosphorus uptake did not increase, rather, plant concentrations of carbon and nutrients decreased in presence of mycorrhiza, suggesting a sequestration of resources during the establishment of a mycorrhizal network. Amoebae strongly interacted with, and partly compensated for, the effects of mycorrhiza, demonstrating that interactions between AM fungi and the microbial food web in the rhizosphere significantly feed back on early plant performance. [Copyright &y& Elsevier]

Abstract: Current knowledge concerning ‘decomposer’ Basidiomycota in Arctic and Antarctic ecosystems is based on two sources: (a) collections and surveys of basidiomata, which have resulted in high-quality catalogues of species, although much of the species’ distribution and ecology are tentative and (b) isolations from soils and plant litter which typically result in a “low incidence of basidiomycetes” [Dowding, P., Widden, P., 1974. Some relations between fungi and their environment in tundra regions. In: Holding, A.J., Heal, O.W., MacLean Jr., S.F., Flanagan, P.W. (Eds.), Soil Organisms and Decomposition in Tundra. Tundra Biome Steering Committee, Stockholm, Sweden, pp. 123–150], probably because of selectivity in isolation methods. In the few molecular studies carried out in Arctic and Antarctic soils to date, basidiomycetes, particularly yeasts, have been found. These techniques should give better estimates of the order of magnitude of fungal species richness in Arctic and Antarctic soils, although caution should be used concerning primer choice and amplification conditions. From collections in Arctic regions, species of basidiomycetes appear to be circumpolar in distribution with restricted endemism. Using culture-independent methods, it should be possible to test whether selected Arctic or Antarctic species are truly cosmopolitan, circumpolar, endemic, or are cryptic phylogenetic species. Particularly in Arctic ecosystems, potential ‘decomposer’ fungi in soils and roots may be from phylogenetically diverse taxa, and currently it is unclear whether ‘decomposer’ basidiomycetes are the fungi undertaking the majority of organic matter decomposition in Arctic and Antarctic ecosystems. For example, in some recent studies, wood decomposition in cold Arctic and Antarctic sites appears to proceed via ‘soft rot’ by anamorphic ascomycetes (e.g. Cadophora species), rather than by ‘white rot’ or ‘brown rot’ basidiomycete species. Additionally, it appears basidiomycetes and ascomycetes as ericoid and ectomycorrhizal fungi have the potential to be involved directly in decomposition. Given that profound changes are likely to occur in patterns of vegetation (Arctic and Antarctic) and size of soil carbon (C) pools (particularly in the Arctic) by the end of this century, it is necessary to know more about which species of ‘decomposer’ basidiomycetes are present and to try to define their potentially pivotal roles in ecosystem C (and N) cycling. One solution to characterise further the identity and roles of these fungi in a logical way, is to standardise methods of detection and ‘function’ at networks of sites, including along latitudinal gradients. Results of functional tests should be related to community structure, at least for ‘key’ species. [Copyright &y& Elsevier]

Abstract: Many studies have shown that the total abundance of hyphae in the soil covaries seasonally with soil moisture. We investigated the extent to which soil hyphal abundance varies as a function of depth and moisture availability within the soil profile during the dry season, and determined whether soil moisture compensation via hydraulic lift (HL) buffers rhizosphere fungi from the effects of severe soil drying. We measured soil water potential, isotopic composition of soil water and total hyphal length in a California coast live oak stand and adjacent grassland at the beginning and end of the 5-month summer drought period. Throughout the summer, oaks maintained predawn water potential values (−0.4±0.1MPa) that were significantly above those recorded in the 0–200cm soil depth interval, strongly suggesting root access to groundwater. Direct evaporation of soil water was much more intense and affected deeper layers of the profile in the grassland compared to the oak stand, as indicated by extremely negative water potential values and very enriched isotopic composition of soil water near the surface. Significantly higher soil water potential and less isotopically enriched soil water at 15–40cm depth in the oak stand were consistent with oak root exudation of isotopically depleted groundwater or deep soil water not exposed to evaporation. Hyphal length in the soil profile declined markedly during the summer drought period in the grassland, particularly in upper layers (41–75% decrease at 0–40cm depth), indicating rapid turnover of the arbuscular mycorrhizae (AMF) dominated hyphal carbon pool after grass senescence. By contrast, soil hyphal length in the ectomycorrhizal (EM)/AM oak stand remained remarkably constant throughout the summer drought period, with the only exception of the topsoil layer exposed to direct evaporation (49% decrease at 5cm depth). The sustained exudation of water from roots to soil through HL may have buffered rhizosphere hyphae against the negative effects of extreme soil desiccation in the oak stand. These data suggest that HL by deep-rooted trees may influence the biogeochemical cycling of carbon and nutrients in seasonally dry ecosystems through effects on rhizosphere fungi. [Copyright &y& Elsevier]

Abstract: Legumes may respond to non-rhizobial inoculants such as arbuscular mycorrhizal (AM) fungi either through an effect on plant growth or, in addition, through an effect on the function of the legume–Rhizobium symbiosis. We have examined the literature where the application of 15N isotope dilution methodology permits the effect of indigenous AM and AM inoculants to be quantitatively separated into plant-growth-mediated and biological N2 fixation (BNF)-mediated components. These studies clearly demonstrate the beneficial effects that both indigenous and inoculated AM have on legume growth, N uptake and the proportional dependence of the legume on atmospheric N2. While the published data allow an assessment of various biological, edaphic and environmental factors that affect the response of various legumes to AM inoculation, they also highlight the paucity of quantitative field data and the lack of understanding of the interaction of legume genotype with AM species with respect to legume symbiotic performance. [Copyright &y& Elsevier]

Abstract: Planting trees to stabilize metalliferous mine tailings is a widely used form of land reclamation although substantial soil amendment is invariably required, both to improve the physico-chemical status of the tailings and to ameliorate toxicity prior to planting. Here, we report a glasshouse study of the combined effects of burrowing earthworms (Pheretima guillelmi) and arbuscular mycorrhizal fungi (Glomus spp., AMF) on establishment of a naturally invasive, woody, nitrogen-fixing legume, Leucaena leucocephala, on topsoil-amended Pb/Zn mine tailings. AMF provided the most effective preliminary inoculant, improving N, P and K uptake, but earthworms had more influence improving N nutrition. In most cases, the combined effects of AMF and earthworms were additive and proved to be beneficial to plant growth, plant nutrition and for protection against uptake of toxic metals. AMF influenced metal uptake more than earthworms, but together they reduced mobility of Pb and Zn in soil by as much as 25%. Some minor but significant negative interactions were also evident; for example, earthworms enhanced soil microbial activity but inhibited the beneficial effects of AMF on N2-fixation. We argue that increased attention to ecological interactions in soil could reduce costs and improve the efficacy of restoring a vegetation cover to land impacted by contaminated spoils. [Copyright &y& Elsevier]

Glomalin concentrations of extra-radical arbuscular mycorrhizal (AM) hyphae were estimated by deploying hyphal in-growth cores containing glomalin-free sand in field soils in a tropical forest and in pot cultures. In field soils, glomalin was 0.044±0.013 μg m-1 hyphae. In pot cultures glomalin concentrations were lower (range 0.0068–0.036 μg m-1), and varied significantly among species. Using this technique, preliminary estimates of extraradical AM hyphal production on Inceptisols were 1.91 Mg ha-1yr-1 and on Oxisol were 1.47 Mg ha-1 yr-1, but they could range between 0.9–5.7 Mg ha-1 yr-1. These rates of hyphal production are approximately 10% (range 5–33%) of estimated above ground primary production of the forest. [Copyright &y& Elsevier]

Arbuscular mycorrhizal (AM) development in different soil types, and the influence of AM fungal hyphae on their original soil were investigated. Plantago lanceolata, which can grow in soils of a very wide pH range, was grown in two closely related limestone soils and an acid soil from rock habitats. Plants were colonised by the indigenous AM fungal community. The use of compartmented systems allowed us to compare soil with and without mycorrhizal hyphae. Root colonisation of P. lanceolata was markedly higher in the limestone soils (30–60%) than in the acid soil (5–20%), both in the original habitat and in the experimental study. Growth of extraradical AM fungal hyphae was detected in the limestone soils, but not in the acid soil, using the signature fatty acid 16:1ω5 as biomass indicator. Analysis of signature fatty acids demonstrated an increased microbial biomass in the presence of AM fungal hyphae as judged for example from an increased amount of NLFA 16:0 with 30 nmol g−1 in one of the limestone soils. Bacterial activity, but not soil phosphatase activity, was increased by around 25% in the presence of mycorrhizal hyphae in the first harvest of limestone soils. AM fungal hyphae can thus stimulate microorganisms. However, no effect of AM hyphae was observed on the soil pH or organic matter content in the limestone soils and the available P was not depleted. [Copyright &y& Elsevier]

The interaction between the salt marsh plant Spartina patens, arbuscular mycorrhizal fungi (AMF) and bacteria in salt marsh sediment was examined in a long-term arbuscular mycorrhizas (AM) suppression study by applying the systemic fungicide benomyl to field-collected sediment cores with and without S. patens plants. Microbial populations were sampled four times corresponding to major plant phenological stages (dormancy, vegetative growth, reproduction, and senescence) previously linked to changes in microbial populations under field conditions. Benomyl-treatment of soil cores significantly suppressed AM colonization on S. patens, keeping values relatively consistent throughout the growing season (11.5%) whereas plants in non-treated cores experienced seasonal increases and declines in AM colonization (26.6% during vegetative growth to 11.5% during dormancy). Soil physicochemical parameters were not affected by benomyl application. In unvegetated cores, no benomyl- or seasonal effects were displayed by cell numbers and specific biomass of DAPI-stained organisms, members of the domain bacteria and here especially members of the α-, β-, γ- and δ-subdivisions of proteobacteria that were the most abundant bacterial groups. In vegetated cores, the microbial community as well as specific bacterial populations were at least twice as large in terms of number and biomass than in samples from unvegetated cores with significant seasonal changes for DAPI-stained cells, for members of the domain bacteria and for members of the α- and γ-subdivisions of proteobacteria. In benomyl-treated cores, the population of γ-subdivision of proteobacteria was significantly smaller than in non-treated cores, and a positive association was found between this bacterial group and root length colonized by AM suggesting that AM-suppression can affect populations of specific soil bacterial populations in salt marsh sediment. Benomyl-treatment had no effect on the diversity of N-fixing bacteria as evidenced by PCR-RFLP analysis, but seasonal changes were noted in vegetated cores with populations during active plant growth substantially different from populations during dormancy and senescence. [Copyright &y& Elsevier]

Electroshocking has been used to manipulate earthworm populations in agroecosystems. However, data on the effects of electroshocking on vegetation are lacking. Here we report on a field experiment with the aim to validate electroshocking as a means of manipulating earthworms without otherwise affecting the soil–plant system. We showed that there was no effect of electroshocking on canopy CO2 exchange, root respiration or mycorrhizal fungal abundance and vitality (i.e. the proportion of mycorrhizal fungal structure which was alive). The potential for using electroshocking to manipulate earthworm populations in the field and test ecological hypotheses relating to the role of soil biodiversity in soil processes is discussed. [Copyright &y& Elsevier]

The biocontrol bacterium Burkholderia cepacia is known to suppress a broad range of root pathogenic fungi, while its impact on other beneficial non-target organisms such as arbuscular mycorrhizal (AM) fungi is unknown. Direct interactions between five B. cepacia strains and the AM fungus, Glomus intraradices (BEG87) were studied in root-free soil compartments separated from a rooting compartment by a fine nylon-mesh. B. cepacia had no effect on AM fungal biomass and energy reserves measured using the signature fatty acid 16:1ω5 from phospholipid fatty acids (PLFAs) and neutral lipid fatty acids (NLFAs), respectively. Hyphal P transport was also unaffected by the biocontrol bacterium, which either stimulated, reduced or had no effect on length of the external mycelium of G. intraradices. The cyclic PLFAs cy17:0 and cy19:0 were suggested to be useful markers for estimation of biomass of B. cepacia. The presence of mycelium of G. intraradices reduced the biomass of three out of five B. cepacia strains as indicated by a reduction in PLFAs cy17:0 and cy19:0, while other bacterial PLFAs were unaffected by mycelium of G. intraradices. On the other hand, two out of five B. cepacia strains reduced the amount of branched PLFAs suggesting a reduction in the population of Gram-positive bacteria in these cases. In conclusion, the B. cepacia seems to have no impact on neither mycorrhiza formation nor on the functioning of the AM fungus G. intraradices in terms of P transport, whereas our results suggest that mycorrhiza might have adverse effects on B. cepacia. [Copyright &y& Elsevier]

We studied the influence of olive mill dry residue (DOR) on growth of soybean (Glycine max) and lettuce (Lactuca sativa) colonized with the arbuscular fungi Glomus mosseae or G. deserticola. The DOR has 6 mg g−1 of soluble phenolic compounds and the application of 2.5 g kg−1 of DOR (15 mg kg−1 of phenolic content) to soil decreased the growth of plants colonized with the AM fungi. The application to the soil of 10 g kg−1 of DOR (60 mg kg−1 of phenolic content) decreased the dry weight of nonarbuscular mycorrhizal (AM) plants. The application of DOR decreased the percentage of AM colonization of plants except those were inoculated with G. mosseae or G. deserticola 4 weeks before the application of DOR. No effect of the residue on the colonization of these previously AM inoculated plants was observed. DOR had no effect neither on the fungal succinate dehydrogenase activity nor on the most probable number of AM propagules in all treated soils. The obtained results showed than AM fungi increased the phytotoxicity of DOR in lettuce and soybean. [Copyright &y& Elsevier]

The movement of carbon from plants into their natural communities of arbuscular-mycorrhizal (AM) fungi was investigated. Mesh-bound cores, which allowed in-growth of AM mycelium to be controlled but excluded roots, were inserted into turf monoliths removed from an upland grassland and were exposed to 14CO2. Flux of 14C-labelled carbon from plants to hyphae of AM fungi for 70 h post-labelling was measured by (a) trapping CO2 released from soil cores containing AM hyphae linked to the plants compared to cores from which AM hyphal connections to the plant roots had been severed, and (b) quantification of the total amount of 14C in the cores. Release of 14CO2 from the cores colonised by active AM mycelium was highest for 0–28 h from the onset of labelling and declined rapidly thereafter. The amount of 14C allocated into mycorrhizal mycelium 0–70 h after labelling accounted for 3.4% of the 14C initially fixed by the plants. The results confirm the rapidity of photosynthate allocation to AM mycelium and demonstrate the importance of the short-term dynamics of C fluxes in undisturbed grasslands. [Copyright &y& Elsevier]

A field experiment was undertaken to evaluate the effect of mycorrhizal inoculation with Glomus intraradices and added composted residue on the establishment of Pistacia lentiscus L. seedlings in a semiarid area. Composted residue greatly increased macronutrient (NPK) content, soil microbial activity and enzymatic activities, and decreased soil bulk density. There was a significant correlation between soil bulk density and both enzyme activities and labile C fractions (water-soluble C and water-soluble carbohydrates), which are also related to soil microbial activity. The most suitable methodology for revegetating with P. lentiscus seedlings was addition of composted residue to soil in conjunction with a mycorrhizal inoculation pretreatment of seedlings in a nursery, to increase available P uptake from composted residue. One year after planting, such a combined treatment had increased the plant height of P. lentiscus seedlings by 106% with respect to the control. [Copyright &y& Elsevier]

To measure P transfers, two small pots, placed 1 cm apart and each containing one leek (Allium porrum L.) plant, were embedded within a larger sand volume. Mesh windows on the facing pot sides allowed the penetration of hyphae but not roots. The effect of earthworms (Aporrectodea caliginosa Sav.) in the hyphal compartment and of arbuscular mycorrhizal fungi (AMF) on plant growth and 32P transfer between plants was investigated. In addition, the effect of mechanical disruption of the hyphal compartment on mycorrhizal plants was studied. AMF did not affect plant growth, although infection levels were high. Earthworms generally increased shoot and root growth; this effect did not appear to be related to location of earthworm activity. 32P transfer from the dying root system of labelled ‘donor’ plants to unlabelled ‘receiver’ plants was increased where earthworm activity was evident between pots, whereas mechanical disruption eliminated the effect of AMF on 32P transfer between mycorrhizal plants. Magnitude of transfer was only related to AMF colonisation levels in the donor plant in the absence of earthworms or mechanical disruption. These results suggest that movement of 32P into the hyphal compartment via the donor AMF mycelium was more important in influencing transfer magnitude than direct transfer via AMF hyphal interconnections between the two plants. Uptake of 32P by the receiver AMF hyphae appears to have been increased by earthworm activity in the hyphal compartment; it is proposed that earthworm-mediated mobilisation of 32P, partly contained within the donor AMF mycelium, led to enhanced 32P availability in the hyphal compartment. [Copyright &y& Elsevier]

This study examined the effect of nitrogen fertilization (0, 4 or 8 mmol N added kg−1 soil) on plant growth, nitrate reductase activity (NR) (EC 1.6.6.1) and protein content in Medicago sativa. Plants were inoculated with two strains of Sinorhizobium meliloti [the wild type (WT) strain GR4 and its genetically modified (GM) derivative GR4(pCK3)]. In combination with these inoculations, the arbuscular mycorrhizal (AM) fungus, Glomus mosseae, was compared to a non-mycorrhizal control supplemented with phosphate. The effects of AM on plant growth were greatest when no nitrogen was added to the soil. Nitrogen fertilization reduced these effects according to the S. meliloti strain involved. Growth responses of mycorrhizal plants coinoculated with the GM strain were affected less negatively than those of mycorrhizal plants associated with the WT strain. These results were not related to differential colonization by AM. Increasing nitrogen concentrations reduced mycorrhizal infection and nodule formation was drastically inhibited in mycorrhizal plants by the addition of 4 and 8 mmol N kg−1 soil. Nevertheless, AM symbiosis increased nodule formation in the absence of N fertilization. In non-mycorrhizal plants, however, N fertilizer application did not significantly affect shoot and root growth or nodule formation. Although the P content was higher in P-fertilized, non-mycorrhizal plants than in mycorrhizal ones AM colonization significantly improved P use efficiency. The N content and use efficiency were also highest in mycorrhizal plants. However, the most relevant result regarding NR activity was the varying distributions in mycorrhizal plant shoots and roots as affected by the particular Sinorhizobium strain. The root portion was enhanced by the GM strain, while the WT strain increased the shoot portion. This change in the distribution pattern of root and shoot NR activities was unaffected by N concentrations in the soil. Protein content was substantially higher in mycorrhizal plants. The protein distribution pattern in shoot and root was also highly influenced by mycorrhizal colonization, which enhanced the root portion. Increased nitrogen supply lowered the protein content in both shoot and root tissues. This effect was greatest in AM colonized plants. These results suggest that high N fertilization levels are detrimental to mycorrhizal legume plants. However, under stress conditions (e.g. high N levels) mycorrhizal legumes nodulated by the GM Sinorhizobium strain displayed a physiological response which was better than those nodulated by the WT strain. [Copyright &y& Elsevier]

Possible interactions between mycorrhiza, atmospheric CO2, free-living soil microorganisms and protozoa were investigated in pot experimental systems. Pea plants (Pisum sativum L. cv. Solara) were grown under ambient (360 μl l−1) or elevated (700 μl l−1) atmospheric CO2 concentration with or without the presence of the arbuscular mycorrhizal (AM) fungus Glomus caledonium. It was hypothesised that (1) the populations of free-living soil protozoa would increase as a response to elevated CO2, (2) the effect of elevated CO2 on protozoa would be moderated by the presence of mycorrhiza and (3) the presence of arbuscular mycorrhiza would affect soil protozoan numbers regardless of atmospheric CO2. After 3 weeks growth there was no difference in bacterial numbers (direct counts) in soil, but the number of free-living bacterial-feeding protozoa was significantly higher under elevated CO2 and was significantly reduced in the mycorrhizal treatments. These effects on protozoa disappeared after 5 and 9 weeks. Neither mycorrhiza nor CO2 concentration had any substantial effect on the microbial community structure as evaluated by phospholipid fatty acid analysis. The increased protozoan populations under elevated CO2 suggest increased bacterial production, whereas the lower populations in response to presence of mycorrhiza suggest a depressing effect on bacterial production by AM colonisation. [Copyright &y& Elsevier]

Young pitch pine seedlings are frequently subjected to prescribed burns in the New Jersey pine barrens as part of the management to eliminate fuel load in these forests, thereby reducing the risk of wildfire. The burns are relatively cool and release nutrients without killing the vegetation. Not all nutrients mineralized are immediately available to the plants, however. To determine factors that may contribute to nutrient release and to link this to availability of nutrients to plants following a fire in the pine barrens, a greenhouse study was conducted. The experimental design was a 3×2×2×3 factorial with three types of litter, high and low watering regime, burned and unburned litter, and three sample dates. Nitrogen, phosphorus, and potassium (through a rubidium analog) uptake rates were measured using natural isotope or radioisotope root bioassays. Though in the short time the experiment ran, neither an overall burned litter nor a litter quality effect was seen, high watering made P less available and K more available. N and K were the most available immediately following application of treatments, whereas P was not available until after 6 weeks following treatment application. Net mineralization of nitrogen, phosphorus, and potassium occurred in burned litter, whereas net immobilization of N and P occurred in highly watered, unburned litter. Since watering and timing were most influential in making nutrients available to seedlings following application of treatments in this greenhouse study, these factors may also be important in making nutrients available to mature trees in the New Jersey pine barrens. [Copyright &y& Elsevier]

Recent studies on crop rotation have suggested that the cultivation of mycorrhizal host crops increases the arbuscular mycorrhizal (AM) colonization of succeeding crops. A pot experiment was conducted to determine whether the positive effect of AM host cropping on the growth of succeeding maize is mainly due to the multiplication of indigenous AM fungi. Maize plants were grown in soils after mustard (non-host) cropping without AM fungal (AMF) inoculum (MDNI); with inoculum from the soil after sunflower cropping (MDISF); with sterilized inoculum (MDSI); and in the soil after sunflower (host) cropping without inoculum (SFNI). The growth of maize after mustard cropping (MDNI) was inferior to that after sunflower cropping (SFNI). The AMF inoculum from the soil after sunflower cropping (MDISF) improved the growth and AM colonization of maize, and shoot weight was increased from 17 to 49% of that in the SFNI treatment. However, the sterilized inoculum (MDSI) did not show similar effects. Similar AMF species to those increased by sunflower cropping were dominant in SFNI-treated or MDISF-treated soils following maize cropping, also indicating that the AM colonization of maize was improved by multiplied AM fungi through sunflower cropping. These results suggest that the effects of preceding crops on maize growth are at least partly due to differences in AMF density caused by various preceding crops. [Copyright &y& Elsevier]

In a field microcosm experiment, we tested the hypothesis that adult collembola, depending on their density, may enhance or decrease mycorrhizal development and growth of maize (Zea mays L.) or red fescue (Festuca rubra L.). Collembola decreased mycorrhizal spore number in the soil. A significant decrease in the spore number, compared to the treatment without collembola, was recorded and ranged from 0.2 to 1.6 individuals g−1 soil in both experiments. Highest AM fungi colonization was found at 0.2 and 0.4 collembola g−1 soil density in the maize and red fescue treatments, respectively. This finding may explain why about 0.1 individuals g−1 soil was reported as optimum for plant growth by some authors. [Copyright &y& Elsevier]

The identity and diversity of arbuscular mycorrhizal (AM) fungal symbionts strongly affect the functioning of mycorrhiza, but little is still known about the functional relevance of the individual taxa abundances within AM fungal communities. We hypothesized that proportions of AM fungal taxa influence mycorrhizal benefits to the host, and that a community with spontaneously established ratios of AM fungal species is more beneficial than communities with artificially manipulated ratios. Medic (Medicago truncatula) was inoculated with synthetic AM fungal communities composed of five fungal species in different proportions, and a 'functionally optimized' community generated by previous co-cultivation of the five AM fungi. The composition of the communities was monitored along with the host plant responses to mycorrhiza in three sequential harvests. Most of the artificial AM fungal communities differed compositionally and functionally from the presumably 'functionally optimized' community, which indeed promoted plant growth to the greatest extent. The higher ability to promote plant growth was partly explained by higher intraradical fungal biomass, but functional differences between the communities were also related to the abundances of certain AM fungal species. Thus, the experiment demonstrated functional relevance of species' abundances within AM fungal communities. The observed 'functional optimization' of the AM fungal community is discussed in context with the host plant's and AM fungal species' traits. • Species ratios in mycorrhizal fungal communities influence mycorrhiza functioning. • A spontaneously established mycorrhizal community induced the highest plant benefits. • Manipulation of the fungal species abundances mostly decreased plant benefits. • Fungal species traits explain the differences in the functioning of the communities. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal fungi (AMF) greatly facilitate uptake of phosphorus (P) by most plant species. Whether or how this changes if water becomes limiting remained little explored. Medicago truncatula plants were grown in previously sterilized cultivation substrate and inoculated or not with AMF isolate Rhizophagus irregularis 'PH5' (M or NM, respectively) in two-compartment rhizoboxes. The compartments were separated or not with root-excluding meshes. A 15-step soil moisture gradient (maintained over 5 weeks) spanned generous water supply through strong water deficiency. Two radioisotopes (33P and 32P) were injected into the plant and distant compartments, respectively, to distinguish cumulative and immediate-term plant P uptake from either of the compartments at each moisture level. M plants were larger than NM plants at high and medium moisture levels and accumulated markedly more P throughout the whole gradient. The immediate-term P uptake did not differ between M and NM plants at high moisture, but at medium and low moistures, M plants acquired significantly more 33P than NM plants, even after accounting for differences in root biomass. Increased immediate-term P uptake by M plants grown under medium and low substrate moistures was probably due to complementary effect of direct hyphal uptake and indirect alterations of substrate hydraulic properties by AMF. This research illustrates that AMF remain functional part of plants even under severe drought stress and thus they should be considered when plant functioning under water deficiency is to be fully understood. • Medicago truncatula plants were inoculated or not with Rhizophagus irregularis. • Mycorrhizal phosphorus uptake was quantified under 15-step moisture gradient. • Radioisotopes enabled to distinguish cumulative and immediate-term phosphorus uptake. • Mycorrhiza increased cumulative phosphorus uptake throughout the whole gradient. • Mycorrhiza increased immediate-term phosphorus uptake at medium and low moistures. [ABSTRACT FROM AUTHOR]

Abstract: The aqueous biphasic system (ABS) method using PEG and ammonium sulphate improved the recovery of spores from tropical soils as compared with the sucrose gradient method. The ABS was not affected by low temperatures. The optimal recovery of spores was achieved when 100% PEG and 50% ammonium sulphate concentration and PEG8000 instead of PEG4000 were used. Under these conditions an optimal interface formation which facilitates the localization and recovery of spores was obtained. Both phases of the ABS are aqueous and did not affect either the viability of spores or the level of root colonization reached by these spores. Soil composition did not affect the number of spores obtained by the ABS method either. [Copyright &y& Elsevier]

Arbuscular mycorrhizal (AM) plants and fungi associate with lower soil organic matter, higher pH, lower phosphorus and higher nitrogen than ectomycorrhizal (EM) ones. However, soil conditions correlate with climatic factors, and we suggest that temperature and humidity have also direct roles in the success of mycorrhiza types. The hypothesis here is that EM perform better at low temperatures than AM, and AM resist drought better than EM. Narrowleaf cottonwood (Populus angustifolia E. James) forms both AM and EM. We grew seedlings in soil at 14, 20 and 26 °C in factorial combinations with adequate watering and a cyclic mild drought for 4 and 7 weeks. As hypothesized, the percent of EM root tips was largest at 14 °C, while the proportional root length with AM was largest at the two higher temperatures. However, unlike expectations, drought increased EM formation slightly, while the AM colonization was lower in the dry treatment. Plant growth was reduced more by low temperature than drought. Root branching was more prominent at low temperature and root length and mass growth at higher temperatures. Soil nutrient availability did not provide a direct explanation to the results, as both soluble soil N and P were the same in 14 and 20 °C, while the change in mycorrhiza colonization took place between these temperatures. Differences in root morphology (root branching vs length) may affect the proportions of the mycorrhiza types at different temperature regimes. The most likely explanation to the differential colonization is that temperature affects AM and EM fungi in a different way. In nature, temperature and humidity regimes are tightly correlated, and temperature as such may be a stronger determinant for the success of mycorrhiza types than has been previously considered. The poorer performance of AM in low-temperature and drought conditions may reflect stress avoidance rather than stress tolerance by AM fungi. • Arbuscular mycorrhiza (AM) may resist drought better than ectomycorrhiza (EM). • EM may resist low temperature better than AM. • Drought reduced AM formation but not EM in dual-mycorrhizal Populus angustifolia. • More EM formed at 14 than at 20 and 26 °C, and more AM at 20 and 26 than at 14 °C. • Temperature may be a more important determinant for mycorrhiza than thought before. [ABSTRACT FROM AUTHOR]

Phytate is considered a poorly available plant P source but proved to be useful for particular soil bacteria strains. In soil-free conditions, it has been shown that bacteria locked up the mineralized phosphorus from phytate whereas bacterial grazers like nematodes were able to deliver P to plants. Here, we aimed to determine if the interactions between phytate-mineralizing bacteria, bacterial grazer nematodes, and mycorrhizal fungi could increase plant P acquisition from phytate in high P-adsorbing soils. Pinus pinaster was grown in a Cambisol supplemented with phytate. Plants, whether associated or not associated with the ectomycorrhizal fungus Hebeloma cylindrosporum, were either inoculated or not inoculated with the phytase-releasing bacteria Bacillus subtilis and the bacterial-feeding nematode Rhabditis sp. After 100 days, the dual inoculation of bacteria and nematodes significantly increased net plant P accumulation. We observed that, on average, mycorrhizal plants accumulated more P in their shoots than non-mycorrhizal plants. However, the highest plant P acquisition efficiency was found when the three soil organisms were present in the P. pinaster rhizosphere. We conclude that, in a highly inorganic P-fixing soil, plant P acquisition from phytate strongly depends on the grazing of phytate-mineralizing bacteria. Our results confirm the importance of the soil microbial loop to improve plant P nutrition from phytate, which should be considered a route to improve the utilization of this source of poorly available P by plants. • In contrast to trees and ectomycorrhizal fungi, bacteria can mineralize phytate. • We used a high P-sorbing cambisol amended with phytate to study plant P acquisition. • We studied bacteria-grazer nematodes-ectomycorrhizal fungi interactions on plant P. • The ectomycorrhizal fungus and bacteria increased plant P use from phytate. • Non mycorrhizal plants depend on bacterial grazing to get P from soil phytate. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal fungi (AMF) represent an important route for plant carbon (C) inputs into the soil. Nonetheless, the C input via AMF as well as its impact on soil organic matter (SOM) stabilization and C sequestration remains largely unknown. A mycorrhizal wild type progenitor (MYC) and its mycorrhiza defective mutant (reduced mycorrhizal colonization: rmc) of tomato were continuously labeled with 13CO 2 to trace root C inputs into the soil and quantify rhizosphere priming effects (RPE) as affected by AMF symbiosis and N fertilization. Mycorrhizal abundance and 13C incorporation into shoots, roots, soil and CO 2 were measured at 8, 12 and 16 weeks after transplanting. AMF symbiosis decreased the relative C allocation (% of total assimilated C) to roots, in turn increased the net rhizodeposition. Positive RPE was recorded for both MYC and rmc plants, ranging from 16–71% and 25–101% of the unplanted control, respectively. Although net rhizodeposition was higher for MYC than rmc plants 16 weeks after transplanting, the RPE was comparatively lower. This indicated a higher potential for C sequestration by plants colonized with AMF (MYC) because the reduced nutrient availability restricts the activity of free-living decomposers. Although N fertilization decreased the relative C allocation to roots, rhizosphere and bulk soil, it had no effect on the absolute amount of rhizodeposition to the soil. The RPE and N-cycling enzyme activities decreased by N fertilization 8 and 12 weeks after transplanting, suggesting a lower microbial N demand from SOM mining. The positive relationship between enzyme activities involved in C cycling, microbial biomass C and SOM decomposition underlines the microbial activation hypothesis, which explains the RPE. We therefore concluded that AMF symbiosis and N fertilization increase C sequestration in soil not only by increasing root C inputs, but also by lowering native SOM decomposition and RPE. • Arbuscular mycorrhizal fungi (AMF) reduced rhizosphere priming effect (RPE). • N fertilization lowered the magnitude of RPE by decreasing enzymes. • The net rhizodeposition induced by AMF was dependent on N availability. [ABSTRACT FROM AUTHOR]