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Summary Fungi are known to exert a significant influence over soil organic matter (SOM) turnover, however understanding of the effects of fungal community structure on SOM dynamics and its consequences for ecosystem fertility is fragmentary.Here we studied soil fungal guilds and SOM decomposition processes along a fertility gradient in a temperate mountain beech forest. High-throughput sequencing was used to investigate fungal communities. Carbon and nitrogen stocks, enzymatic activity and microbial respiration were measured.While ectomycorrhizal fungal abundance was not related to fertility, saprotrophic ascomycetes showed higher relative abundances under more fertile conditions. The activity of oxidising enzymes and respiration rates in mineral soil were related positively to fertility and saprotrophic fungi. In addition, organic layer carbon and nitrogen stocks were lower on the more fertile plots, although tree biomass and litter input were higher. Together, the results indicated a faster SOM turnover at the fertile end of the gradient.We suggest that there is a positive feedback mechanism between SOM turnover and fertility that is mediated by soil fungi to a significant extent. By underlining the importance of fungi for soil fertility and plant growth, these findings furthermore emphasise the dependency of carbon cycling on fungal communities below ground.

Land use change is one of the major causes of biodiversity loss, mostly due to habitat change and fragmentation. Belowground fungal diversity is very important in terrestrial ecosystems, however, the effect of land use change on soil fungal community is poorly understood. In this review, a total of 190 studies worldwide were analyzed. To monitor the effect of land use change, different fungal parameters such as richness, diversity, community composition, root colonization by arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi, spore density, ergosterol, and phospholipid fatty acid (PLFA) content and AM fungal glomalin related soil protein (GRSP) were studied. In general, results from analyzed studies often showed a negative response of fungal quantitative parameters after land use change from less-intensive site management to intensive site management. Land use change mostly showed significant shifts in fungal community composition. Considering land use change types, only 18 out of 91 land use change types were included in more than 10 studies, conversion of primary and secondary forest to various, more intensive land use was most often represented. All these 18 types of land use change influenced fungal community composition, however, the effects on quantitative parameters were mostly inconsistent. Current knowledge is not sufficient to conclude general land use impacts on soil fungi as the reviewed studies are fragmented and limited by the local context of land use change. Unification of the methodology, detailed descriptions of environmental factors, more reference sequences in public databases, and especially data on ecology and quantitative parameters of key fungal species would significantly improve the understanding of this issue.

We evaluated whether changes in fine root non-structural carbohydrate reserves of Fagus sylvatica and Pinus sylvestris trees influence potential enzymatic activities of their ectomycorrhizal symbionts from winter towards spring reactivation, and whether these changes influence potential soil enzymatic activities. We analyzed sugar and starch concentrations in the fine roots of Fagus sylvatica and Pinus sylvestris and potential activities of ß-glucosidase, ß-xylosidase, and cellobiohydrolase (as proxies for carbon-degrading enzymes) as well as leucine aminopeptidase and chitinase (as proxies for nitrogen-degrading enzymes) of their dominant ectomycorrhizal symbionts as well as in the soil. Sugar concentrations in the fine roots were significantly positively correlated with enzymatic activities of the ectomycorrhizal symbionts. In Pinus sylvestris, both carbon- and nitrogen-degrading enzyme activities showed significant positive correlations with fine root sugar concentrations. In Fagus sylvatica, fine root sugar concentrations were explicitly positively correlated with the activity of nitrogen-degrading enzymes. The chitinase activity in the soil was found to be strongly positively correlated with the enzymatic activity of the ectomycorrhizal symbionts as well as with fine root sugar concentrations. Fine root carbohydrate concentrations of Fagus sylvatica and Pinus sylvestris trees and enzymatic activities of their associated ectomycorrhizal fungi are connected. The specific nutrient demand of the tree species during spring reactivation may affect ectomycorrhizal enzymatic activity via carbon mobilization in the fine roots of Fagus sylvatica and Pinus sylvestris. Moreover, our results suggest that trees indirectly contribute to the degradation of fungal necromass by stimulating ectomycorrhizal chitinase activity in the soil.

Forests on steep slopes constitute a significant proportion of European mountain areas and are important as production and protection forests. This study describes the soil fungal community structure in a European beech-dominated mountain forest stands in the Northern Calcareous Alps and investigates how it is determined by season and soil properties. Samples were collected at high spatial resolution in an area of ca. 100 m × 700 m in May (spring) and August (summer). Illumina MiSeq high-throughput sequencing of the ITS2-region revealed distinct patterns for the soil fungal communities. In contrast to other studies from temperate European beech forest stands, Ascomycota dominated the highly diverse fungal community, while ectomycorrhizal fungi were of lower abundance. Russulaceae, which are often among the dominant ectomycorrhizal fungi associated with European beech, were absent from all samples. Potentially plant pathogenic fungi were more prevalent than previously reported. Only subtle seasonal differences were found between fungal communities in spring and summer. Especially, dominant saprotrophic taxa were largely unaffected by season, while slightly stronger effects were observed for ectomycorrhizal fungi. Soil characteristics like pH and organic carbon content, on the other hand, strongly shaped abundant taxa among the saprotrophic fungal community. Supplementary Information The online version contains supplementary material available at 10.1007/s00248-021-01736-5.

Belowground competition is an important structuring force in terrestrial plant communities. Uncertainties remain about the plasticity of functional root traits under competition, especially comparing interspecific vs. intraspecific situations. This study addresses the plasticity of fine root traits of competing Acer pseudoplatanus L. and Fagus sylvatica L. seedlings in nutrient-rich soil patches. Seedlings&rsquo, roots were grown in a competition chamber experiment in which root growth (biomass), morphological and architectural fine roots traits, and potential activities of four extracellular enzymes were analyzed. Competition chambers with one, two conspecific, or two allospecific roots were established, and fertilized to create a nutrient &lsquo, hotspot&rsquo, Interspecific competition significantly reduced fine root growth in Fagus only, while intraspecific competition had no significant effect on the fine root biomass of either species. Competition reduced root nitrogen concentration and specific root respiration of both species. Potential extracellular enzymatic activities of &beta, glucosidase (BG) and N-acetyl-glucosaminidase (NAG) were lower in ectomycorrhizal Fagus roots competing with Acer. Acer fine roots had greater diameter and tip densities under intraspecific competition. Fagus root traits were generally more plastic than those of Acer, but no differences in trait plasticity were found between competitive situations. Compared to Acer, Fagus roots possessed a greater plasticity of all studied traits but coarse root biomass. However, this high plasticity did not result in directed trait value changes under interspecific competition, but Fagus roots grew less and realized lower N concentrations in comparison to competing Acer roots. The plasticity of root traits of both species was thus found to be highly species- but not competitor-specific. By showing that both con- and allospecific roots had similar effects on target root growth and most trait values, our data sheds light on the paradigm that the intensity of intraspecific competition is greater than those of interspecific competition belowground.

Soil extracellular enzymes are crucial for belowground functioning and are sensitive to anthropogenic land use change. The potential effects of tree species on soil microbial and biochemical properties provide crucial feedbacks on mineralization, a key ecosystem function beneath the tree canopy. In the highlands of northern Ethiopia, remnants of the original Afromontane forests are largely restricted to church forests with indigenous tree species. However the impacts on potential soil enzymatic activity by conversion of those forests to monocultures for wood production is largely unknown. We investigated potential soil enzyme activities under four indigenous tree species and adjacent Eucalyptus globulus and Cupressus lusitanica plantations in Gelawdios, Amhara Regional State, Ethiopia. The potential activities of six enzymes associated with soil C, N and P cycling were measured following the fluorometrically labelled substrates techniques. All enzymes exhibited significantly higher activities in soils under the indigenous trees than the plantation species except, N-acetylglucosaminidase, that was the highest in Eucalyptus globulus soil due to the ectomycorrhizae, associated with the Eucalyptus root systems. Among the four indigenous species Apodytes dimidiata showed the lowest activitie for most of the enzymes. A stronger positive correlation was observed between enzyme activity and total N than with total C in the soil. Acid phosphatase had the highest activity followed by β-Glucosidase (482 and 167 nmol mg-1 microbial biomass respectively). The activities of leucine aminopeptidase, β-xylosidase, N-Acetylglucosaminidase and cellobiohydrolase in soils under indigenous trees ranged between 63-23 nmol mg-1 microbial biomass. The species specific effects of trees on soil enzyme activities indicate strong influence of tree traits on mineralization processes.

Tree lines form a transition ecotone from forest to tundra both at high elevation and high latitude and occur in a number of different forms. Nitrogen (N) deficiency is considered to be a factor involved in tree line formation, and also N dynamics are considered to differ between the trees and the ericaceous vegetation of the tundra. In the Austrian Alps at the tree line, N availability and N mineralization in soils of different vegetation types (Picea abies, Pinus mugo and Rhododendron ferrugineum) as well as total phenols were determined. Soil from under P. abies was taken from two different tree line forms, an island type and a diffuse type, as well as from P. abies growing at a lower elevation. N mineralization was measured in situ using a covered PVC tube incubation method and in a laboratory incubation under controlled conditions. Ion exchange resin capsules were installed at the interface of humus and mineral soil for estimating N in the soil solution. Net N mineralization showed a similar pattern for the vegetation types for both the in situ and laboratory incubation. The soil humus layer had the highest levels of N mineralization compared to the other soil layers. N mineralization rates were similar in P. abies and P. mugo at the tree line regardless of tree line form. Rates of N mineralization were lower under R. ferrugineum than the tree species, but this lower rate was not related to the occurrence of high levels of total phenols in the soil. Nitrogen deficiency was not evident in the island-type tree line, but was evident in the diffuse tree line type.

Fine root turnover of trees is a major C input to soil. However, the quality of litter input is influenced by root morphological traits and tissue chemical composition. In this study, fine roots of ten tropical woody species were collected from an Afromontane forest in the northern highlands of Ethiopia. The fine roots were analysed for root morphological traits and tissue chemistry measured as proxy carbon fractionations. Based on stem increment, the 10 species were divided into faster- and slower-growing species. Faster-growing species exhibited higher specific root length (1362 cm g−1) than slower-growing species (923 cm g−1). Similarly specific root area was higher in faster-growing species (223 cm2 g−1) than in slower-growing species (167 cm2 g−1). Among the carbon fractions, the acid-insoluble fraction (AIF) was the highest (44–51%). The carbon content, AIF, and the lignocellulose index were higher for slower-growing species. Root tissue density was lower in faster-growing species (0.33 g cm−3) than slower-growing species (0.40 g cm−3) and showed a strong positive correlation with carbon content (r 2 = 0.84) and the AIF (r pearson = 0.93). The morphological traits of fine roots between faster- and slower-growing species reflect the ecological strategy they employ. Slower-growing species have a higher tissue density which may reflect a greater longevity.

Key message Over-yielding of stand biomass did not occur in a tree polyculture comprised of Betula pendula, Alnus glutinosa and Fagus sylvatica selected for contrasting traits. This was due to antagonistic interactions between the component species. Fine root dynamics and soil C stocks were unaffected by species mixture. Context Increasing CO2 fixation in tree biomass through afforestation and forest management actions has potential for cost-effective climate mitigation. The influences of tree mixture on biomass production and subsequent soil C accumulation in polyculture still remain uncertain. Aims We studied the polyculture of Alnus glutinosa (L.) Gaertn, Betula pendula Roth and Fagus sylvatica L. in a plantation forest to examine the effectiveness of species mixtures as a tool for increased biomass production and soil C accumulation. Methods Tree biomass was estimated by developing species-specific allometric models and 3 years tree measurement. Fine root biomass and production were estimated by root coring and root-mesh methods. The ‘relative yield of mixture’ approach was used to examine the mixture effect. Results In mixture, an additive effect was observed in A. glutinosa (13% increase in basal diameter relative to the monoculture); however, there was no overall effect of mixture on total standing biomass due to the suppression of F. sylvatica (2.75 g m−2 reduction in woody biomass). Fine root biomass production showed no mixture effect. The quantity and quality of soil C (top 0.5 m) was not affected by tree mixture. Conclusion We conclude that the contrasting growth responses of the A. glutinosa, B. pendula and F. sylvatica in polyculture resulted in no over-yielding of standing biomass despite the complementary traits of the component species.

Summary Intensifying forest disturbance regimes are likely to impact heavily on future carbon (C) budgets of forest ecosystems. Our understanding of how forest disturbance affects the sources of soil CO2 efflux (Fs) is however poor. This may lead to uncertainties over future C sink estimates of forest ecosystems and associated feedbacks to the atmosphere. We investigated the impact of forest windthrow on the heterotrophic and autotrophic sources of Fs, underlying biotic and abiotic drivers (i.e. plant community composition, soil organic matter (SOM) properties and soil microclimate), and consequences for soil organic carbon (SOC) stocks in situ along a disturbance chronosequence in the European Alps. This chronosequence facilitated the study of temporal changes in the above parameters between the third and sixth years after windthrow. Along the chronosequence, structural equation modelling revealed that soil temperature, soil moisture, SOM properties, and plant community composition explained 90% of the variation in Fs. While no direct interactions among plants and SOM properties could be determined, plants significantly affected soil microclimate. Windthrow had no obvious effect on Fs because reduced autotrophic soil respiration (Ra) was offset by a ˜60% increase in heterotrophic soil respiration (Rh), principally due to increased soil temperatures. Ra after windthrow was dominated by grasses and herbs rather than trees; however, a high abundance of ectomycorrhizal fungi suggests an important indirect tree contribution to post-windthrow Ra. SOC stocks significantly declined over the post-windthrow period. Our results show that Rh was by far the dominant source of Fs after forest windthrow. Since C losses from Rh and SOC stocks were in the same order of magnitude, this study demonstrates that post-windthrow declines in SOC stocks were mainly driven by a temperature-related increase in Rh. This article is protected by copyright. All rights reserved.

Afforestation changes soil chemical properties over several decades. In contrast, microbial community structure can be shifted within the first decade and so, the direct effects of tree species can be revealed. The aim of this study was to determine the alteration of soil microbial community composition 10 years after afforestation by trees with contrasting functional traits. The study was conducted at the BangorDIVERSE temperate forest experiment. Soil samples were collected under single, two and three species mixtures of alder and birch, beech and oak - early and secondary successional species, respectively, and contiguous agricultural field. Soil was analysed for total carbon (C) and nitrogen (N) contents, and microbial community structure (phospholipid fatty acids (PLFAs) analysis). The total PLFAs content (370–640 nmol g−1 soil) in forest plots increased for 30 to 110 % compared to the agricultural soil (290 nmol g−1 soil). In contrast, soil C, N and C/N ratios were altered over 10 years much less - increased only up to 20 % or even decreased (for beech forest). Afforestation increased bacterial PLFAs by 20–120 %, whereas it had stronger impact on the development of fungal communities (increased by 50–200 %). These effects were proved for all forests, but were more pronounced under the monocultures compared to mixtures. This indicates that species identity has a stronger effect than species diversity. Principal component analysis of PLFAs revealed that under mono and three species mixtures similar microbial communities were formed. In contrast, gram-positive PLFAs and actinomycete PLFAs contributed mainly to differentiation of two species mixtures from other forests. Thus, at the early afforestation stage: i) soil biological properties are altered more than chemical, and ii) tree species identity affects more than species amount on both processes.

This study evaluated the effect of ectomycorrhiza on the tolerance of Populus nigra cuttings to salinity. Half of the cuttings were inoculated with Scleroderma sp. and treated with 50 mM NaCl. Ectomycorrhizal inoculation significantly stimulated plant growth under non-stressed and salt-stressed conditions. Exposure to NaCl decreased leaf chlorophyll concentrations, however the decrease was less in mycorrhizal cuttings. Salt treatment significantly increased root respiration in the non-inoculated plants compared to the control. The respiration of mycorrhizal roots was not increased by the exposure to salt. Exposure to salt also significantly increased the activity of the extracellular enzymes β-glucosidase, N -acetyl-glucosaminidase and leucine-amino-peptidase on the surface of both inoculated and non-inoculated root tips compared to the respective unexposed treatments. The results of this study show that inoculation of cuttings of Populus nigra with ectomycorrhiza increases tolerance to salinity stress, determined in terms of an alleviated decrease in chlorophyll content, and unaltered root respiration, but that this increased tolerance was not due to improved N nutrition.

A changing climate in the future with more severe drought events will affect the conditions for forest growth and vitality. Most knowledge on tree species response to drought is based on monocultures, even though many of the forests in the world consist of mixed stands. We aimed to investigate how trees respond to summer drought when grown in a three species mixture. For two subsequent summers canopy throughfall, and subsequently soil water potential, was reduced using sub-canopy roofs in monocultures and mixtures of Betula pendula, Alnus glutinosa and Fagus sylvatica,. The overyielding of the mixed stand was not affected by the drought using either above or below ground production, standing fine root biomass or soil respiration as parameters. However, Alnus glutinosa was the most negatively affected when growing in monoculture, whereas this species was less affected when growing in mixture. In contrast, Betula pendula was most negatively affected when growing in mixture. Fagus sylvatica was least affected by the drought and maintained growth over the two years. A water demanding species as Alnus glutinosa can perform well in a mixture during drought and not be outcompeted. This is opposite to what is assumed in most models of forest responses to climate change.

International audience; AbstractKey messageK fertilization led to a significant increase in fine root biomass, fine root length, and root tip number in the mineral soil layer but does not affect the ecomycorrhizal community structure in the organic horizon.ContextPotassium (K) deficiency is common in Picea abies in the European Alps. Fertilization with other nutrients often influences fine root biomass and ectomycorrhizas, but less is known about the effects of K-fertilization.AimsThe aim of the investigation was to determine the effects of K-fertilization on stem growth and fine root biomass of Picea abies, as well as the influence on ectomycorrhizal community structure of the fine roots.MethodsEight years after a single fertilization of K-deficient Picea abies with 200 kg K ha−1, fine roots were collected from 7 control and 6 K-fertilized plots. Fine root biomass and morphology were determined. The identification of ectomycorrhizal taxa was determined by morphotyping and by amplification of the internal transcribed spacer region of the nuclear ribosomal DNA.ResultsK-fertilization did not affect the amount of fine root biomass and ectomycorrhizal community structure in the Oi + Oe and Oa layers but led to a significant increase in fine root biomass, fine root length, and root tip number in the mineral soil layer.ConclusionAn increase in growth due to K-fertilization leads to great exploration of the mineral soil by fine roots but does not affect the ectomycorrhizal community structure in the organic horizons.

In forests in Mongolia, tree roots and ectomycorrhizal fungi must survive several months of soil freezing in winter. To investigate the ectomycorrhizal community after winter, we collected fine roots of Scots pine (Pinus sylvestris) and Siberian pine (Pinus sibirica) and associated soil from Nukht forest in the Bogd-Khan National Reserve, Mongolia. Soil samples were collected from frozen soil at the end of April 2016. We described the ectomycorrhizal community, and determined on ectomycorrhizal roots tips and in soils the potential activity of enzymes involved in the degradation of soil organic matter. In order to assess the temperature sensitivity of enzyme activity, potential soil enzyme activities were assayed at temperatures from 5 to 20 °C. We detected 24 different ectomycorrhizal morphotypes associated with Pinus sylvestris and Pinus sibirica, and 18 morphotypes were identified to taxa. The two Pinus species had dissimilar ectomycorrhizal communities, and only 2 ectomycorrhizal fungal taxa were common to both species. Most ectomycorrhizal taxa had measurable activity of at least one extracellular enzyme. A high contribution to the community extracellular enzyme activity was shown for both abundant and less abundant taxa. Among the eight tested soil enzymes, only the activity of leucine amino peptidase showed consistent higher Q10 values at 5–15 °C than at 10–20 °C, suggesting that the enzyme is adapted to colder temperatures. Total soil N was the strongest factor explaining differences in soil enzyme potential activity. A positive relationship was found between soil N and the soil potential enzyme activity of acid phosphatase. We suggest that viable ectomycorrhizas during winter provide an advantage to Pinus sibirica and Pinus sylvestris in acquiring nutrients as soil thaws in spring.

Ectomycorrhizal (EM) fungi are pivotal drivers of ecosystem functioning in temperate and boreal forests. They constitute an important pathway for plant-derived carbon into the soil and facilitate nitrogen and phosphorus acquisition. However, the mechanisms that drive ectomycorrhizal diversity and community composition are still subject to discussion. We investigated patterns in ectomycorrhizal diversity, community composition, and exploration types on root tips in Fagus sylvatica,Picea abies, and Pinus sylvestris stands across Europe. Host tree species is the most important factor shaping the ectomycorrhizal community as well as the distribution of exploration types. Moreover, abiotic factors such as soil properties, N deposition, temperature, and precipitation, were found to significantly influence EM diversity and community composition. A clear differentiation into functional traits by means of exploration types was shown for all ectomycorrhizal communities across the three analyzed tree species. Contact and short-distance exploration types were clearly significantly more abundant than cord- or rhizomorph-forming long-distance exploration types of EM fungi. Medium-distance exploration types were significantly lower in abundance than contact and short-distance types, however they were the most frequent EM taxa and constituted nearly half of the EM community. Furthermore, EM taxa exhibit distinct ecological ranges, and the type of soil exploration seemed to determine whether EM taxa have small or rather big environmental ranges.

In terrestrial environments, soil organic matter (SOM) is the largest organic carbon (C) pool. The quantity and quality of organic carbon in soils can be affected by vegetation through influencing the inputs and outputs of SOM. We examined how storage and quality of C in SOM were affected by vegetation under grass cover or single and a polyculture plot of Betula pendula , Alnus glutinosa and Fagus sylvatica . An acid hydrolysis approach was used to quantify three SOM fractions differing in biodegradability. Tree species identity and stand composition had no significant effect on the total amount of C stored in different SOM fractions to a depth of one meter. However, when examining individual SOM fractions in the upper layers of the soil profile, significantly more C was stored in the putatively more labile fractions 1 and 2 under F . sylvatica and A . glutinosa , respectively. In deeper soil layers, the highest storage of recalcitrant organic C was found under the tree polyculture. The vertical distribution of these three soil organic C pools was compared to C inputs via decomposed leaf litter. Our data indicated that in the tree species polyculture, combining litter inputs of multiple species can have a positive impact on the accumulation of acid resistant recalcitrant C in deep soil layers in 4 years. This C fraction has the greatest potential for long-term sequestration.

The tree root–mycorhizosphere plays a key role in resource uptake, but also in the adaptation of forests to changing environments. The adaptive foraging mechanisms of ectomycorrhizal (EcM) and fine roots of Picea abies, Pinus sylvestris and Betula pendula were evaluated along a gradient from temperate to subarctic boreal forest (38 sites between latitudes 48°N and 69°N) in Europe. Variables describing tree resource uptake structures and processes (absorptive fine root biomass and morphology, nitrogen (N) concentration in absorptive roots, extramatrical mycelium (EMM) biomass, community structure of root-associated EcM fungi, soil and rhizosphere bacteria) were used to analyse relationships between root system functional traits and climate, soil and stand characteristics. Absorptive fine root biomass per stand basal area increased significantly from temperate to boreal forests, coinciding with longer and thinner root tips with higher tissue density, smaller EMM biomass per root length and a shift in soil microbial community structure. The soil carbon (C) : N ratio was found to explain most of the variability in absorptive fine root and EMM biomass, root tissue density, N concentration and rhizosphere bacterial community structure. We suggest a concept of absorptive fine root foraging strategies involving both qualitative and quantitative changes in the root–mycorrhiza–bacteria continuum along climate and soil C : N gradients.

Elevated atmospheric CO 2 induced reductions in litter quality can adversely affect earthworms. However, this understanding is based on laboratory rather than field research and relates to single earthworm and tree species. Here earthworm populations were investigated under Alnus glutinosa , Betula pendula , and Fagus sylvatica in a Free Air Carbon dioxide Enrichment field experiment. Litters from this experiment were also fed to Lumbricus terrestris L. at two rates with live weight change and cast properties assessed. Elevated CO 2 (580 ppmv) reduced litter N (−12%) with a corresponding increase in C:N ratio, especially for A. glutinosa. In the field, elevated CO 2 caused a shift in overall population composition, mainly characterised by reduced anecic biomass (–25%); endogeic and epigeic species were less affected. CO 2 effects on total biomass were most pronounced for A. glutinosa (e.g. field total biomass −47% vs. −11% overall). Growth of L. terrestris was lower when fed elevated CO 2 litter (−18%), although increased inputs of A. glutinosa litter mitigated this effect. In mesocosms, fresh cast respiration was lower (−14%) for elevated CO 2 litter, an effect more pronounced for A. glutinosa (−24%). When normalised for C content, elevated CO 2 effects on cast respiration were again negative and most marked for A. glutinosa litter. Litter N concentration, and possibly ease of litter mineralisation were factors affecting litter resource quality Litter N and P concentrations varied with A. glutinosa > B. pendula > F. sylvatica ; F. sylvatica had the highest cellulose content. Field earthworm biomass was higher under A. glutinosa compared with B. pendula and F. sylvatica (+17 and +70%, respectively); live weight increased with A. glutinosa litter in the feeding trial almost three times more than for B. pendula , whereas it decreased for F. sylvatica. Cast respiration was highest for A. glutinosa , intermediate for B. pendula (ca. −36%) and lowest for F. sylvatica (ca. −78%). Earthworm responses to elevated CO 2 were complex, being characteristic of individual tree and earthworm species; responses were more adverse for trees with higher quality litter and for anecic earthworms.

International audience; ContextAnthropogenic activity has increased the level of atmospheric CO2, which is driving an increase of global temperatures and associated changes in precipitation patterns. At Northern latitudes, one of the likely consequences of global warming is increased precipitation and air humidity.AimsIn this work, the effects of both elevated atmospheric CO2 and increased air humidity on trees commonly growing in northern European forests were assessed.MethodsThe work was carried out under field conditions by using Free Air Carbon dioxide Enrichment (FACE) and Free Air Humidity Manipulation (FAHM) systems. Leaf litter fall was measured over 4 years (FACE) or 5 years (FAHM) to determine the effects of FACE and FAHM on leaf phenology.ResultsIncreasing air humidity delayed leaf litter fall in Betula pendula, but not in Populus tremula × tremuloides. Similarly, under elevated atmospheric CO2, leaf litter fall was delayed in B. pendula, but not in Alnus glutinosa. Increased CO2 appeared to interact with periods of low precipitation in summer and high ozone levels during these periods to effect leaf fall.ConclusionsThis work shows that increased CO2 and humidity delay leaf fall, but this effect is species-specific.

Eragrostis tef (teff) is a (sub-)tropical cereal crop and a staple food in Eastern Africa. As soil erosion has become increasingly worse in the Ethiopian highlands, we test the hypotheses that 1) eroded soils possess low arbuscular mycorrhizal fungi (AMF) spore densities, 2) teff growth is limited by low endophytic fungal root colonization rates and 3) colonization rates and spore densities are additionally reduced by fertilization. A pot experiment was set up to study the effect of cropland soil inoculation using pristine fungal communities (from adjacent forests) or fertilization. AMF spore densities in soil with and without teff and root colonization by AMF and dark septate endophytes (DSE) were related to straw and grain yields. AMF and DSE colonization rates were higher after inoculation, which provides evidence that a low spore density limits teff root colonization in eroded soils. However, teff yields were significantly increased after fertilisation but not inoculation. N-P fertilization further lowered root colonization rates and spore density. We conclude that forest soils serve as a refugium for soil biota in the degraded landscape of the Ethiopian highlands. As both increased AMF and DSE increase the stress resistance of plants, their inoculation potential should be considered when developing sustainable management methods for teff.

Mycorrhizal fungi are essential for ecosystem functioning. They are the most important pathway of plant-derived C into the soil and facilitate most of the N and P uptake. In boreal and temperate forest ecosystems, the subgroup of ectomycorrhizas is of special significance, and ectomycorrhizal symbiosis is obligatory for a number of tree species. These fungal assemblages can be highly diverse, which is often contrasting with the relatively low number of tree species. This is also true for small monoculture stands, where several studies showed dozens of different ectomycorrhizal species. The elemental mechanisms that drive ectomycorrhizal community composition and structure, especially on larger scales, are still subject of debate. In particular, the role and function of most of the rare species remain largely unknown. In this chapter, we investigate the structural traits of ectomycorrhizal communities of European beech (Fagus sylvatica), one of the most important tree species in temperate deciduous forest ecosystems. We therefore compiled data from ten different beech dominated sites across a European gradient to seek for ubiquitously valid statements regarding the species composition and community structure of ectomycorrhizae. A total of 205 ectomycorrhizal species were detected, of which 45% and 35% could be identified to the species and genus level, respectively. The majority of them are site-specific and found in very low abundances. A few species with narrow host range such as Lactarius subdulcis, Laccaria amethystina or Lactarius pallidus show low abundances on many sites. Three species, namely Hebeloma sinapizans, Lactarius salmonicolor and Elaphomyces aculeatus, appeared to be very dominant on one respective site. Cenococcum geophilum and Russula ochroleuca, both considered multi-host species, occur at all or rather several sites with relatively high abundances. Species ranking-abundance curves revealed a universal distribution pattern, with few dominant ectomycorrhizal species and a long tail of rare species. This shape is regardless of stand age and management practice, though no statistically significant correlations with species richness and the latter mentioned parameters could be found. By looking at several community structures, we were able to add another dimension of rarity and found most of the species not only being rare at a particular site, but also rare in terms of single appearance across several analysed stands. However, a vast part of ectomycorrhizal fungal species still remains unidentified. This circumstance further impedes our understanding concerning the occurrence of rare species. Yet, understanding their functional behaviour is of particular significance, as they could pose a key factor in ecosystem functioning, especially under changing climatic conditions.

Global warming will most likely lead to increased drought stress in forest trees. We wanted to describe the adaptive responses of fine roots and fungal hyphae, at different soil depths, in a Norway spruce stand to long-term drought stress induced by precipitation exclusion over two growing seasons. We used soil cores, minirhizotrons and nylon meshes to estimate growth, biomass and distribution of fine roots and fungal hyphae at different soil depths. In control plots fine roots proliferated in upper soil layers, whereas in drought plots there was no fine root growth in upper soil layers and roots mostly occupied deeper soil layers. Fungal hyphae followed the same pattern as fine roots, with the highest biomass in deeper soil layers in drought plots. We conclude that both fine roots and fungal hyphae respond to long-term drought stress by growing into deeper soil layers.

This study aimed to use callus cultures of Acacia trees as indicator for the validity of employing these trees in the phytoremediation programs that were used to manage the pollution with uranium in soil. Various concentrations of uranium (0.0, 50,100, 200, 500 and 1000mg.l -1 ) in the form of uranyl nitrate were added to MS medium that used for the initiation and growth of Acacia callus in the presence of 0.5 mg/l of each NAA and BA. For comparison, two species of Acacia were used in this study, A. albida and A. nilotica. Fresh and dry weight of callus was determined along three growth periods (30, 60 and 90 days). levels of uranium in callus cells were determined using TXRF analysis. The Results showed that fresh and dry weight of callus increased with the increase of uranium concentrations except that grown on MS with 1000 mg.l -1 of uranium in which inhibition in both fresh and dry weight of callus after three months of growth was recorded .Analysis of the cellular content of uranium using TXRF revealed that the maximum level obtained when callus grown on MS with 500mg.l -1 .

Through increases in net primary production (NPP), elevated CO2 is hypothesized to increase the amount of plant litter entering the soil. The fate of this extra carbon on the forest floor or in mineral soil is currently not clear. Moreover, increased rates of NPP can be maintained only if forests can escape nitrogen limitation. In a Free atmospheric CO2 Enrichment (FACE) experiment near Bangor, Wales, 4 ambient and 4 elevated [CO2] plots were planted with patches of Betula pendula, Alnus glutinosa and Fagus sylvatica on a former arable field. After 4 years, biomass averaged for the 3 species was 5497 (se 270) g m−2 in ambient and 6450 (se 130) g m−2 in elevated [CO2] plots, a significant increase of 17% (P = 0.018). During that time, only a shallow L forest floor litter layer had formed due to intensive bioturbation. Total soil C and N contents increased irrespective of treatment and species as a result of afforestation. We could not detect an additional C sink in the soil, nor were soil C stabilization processes affected by elevated [CO2]. We observed a decrease of leaf N content in Betula and Alnus under elevated [CO2], while the soil C/N ratio decreased regardless of CO2 treatment. The ratio of N taken up from the soil and by N2-fixation in Alnus was not affected by elevated [CO2]. We infer that increased nitrogen use efficiency is the mechanism by which increased NPP is sustained under elevated [CO2] at this site.

Plants with ericoid mycorrhizal and ectomycorrhizal associations coexist at the tree line and in many boreal forests. Both ericoid mycorrhizal and ectomycorrhizal roots are known to produce extracellular enzymes, but ericoid mycorrhizal fungi have been demonstrated in vitro to have higher enzyme activities. On hair roots of four ericoid mycorrhizal species (Rhododendron ferrugineum, Vaccinium vitis-idaea, Vaccinium myrtillus, Calluna vulgaris) and on ectomycorrhizal and non-mycorrhizal root tips of Picea abies growing at the tree line (1700 m) in the Austrian Alps, potential activities of eight extracellular root enzymes were estimated. Our results show that the activities of all the different extracellular root enzymes were generally similar among the ericaceous plant species. The mean laccase enzyme activity of ectomycorrhizal root tips of Picea abies was significantly higher than that of both the hair roots and fine roots of the ericaceous species. Leucine-amino-peptidase activity on hair roots was significantly higher than on non-mycorrhizal fine roots for the ericaceous vegetation. However, the mean activity of &beta, glucuronidase of the ericaceous species was significantly higher in fine roots compared to the hair roots. Generally extracellular root enzyme activity is not higher on ericaceous roots compared to ectomycorrhizas of Picea abies.

Use of composts for habitat restoration offers advantages in terms of efficient use of resources. Chemical amendment of compost to reduce its pH and P availability was investigated in order to improve suitability for use in reclamation of blocky quarry waste to acidic grassy heathland. The effect of these amendments was observed on competition between two grass species: Agrostis capillaris and Festuca ovina. A factorial, pot-scale greenhouse experiment was set up using two composts (one a mixture of green waste and catering waste, and the other a mixture of green waste and sewage sludge). In addition, two soils were collected from upland acidic grassland to provide a natural comparison. S0 was applied to reduce soil pH, and Fe(OH)3 from a coal waste treatment plant was applied to counteract the expected increase in P availability due to acidification of the composts by S0. Addition of S0 significantly reduced soil solution pH and addition of Fe(OH)3 significantly reduced soil solution P concentration. In one compost S0 reduced the biomass of F. ovina while increasing that of A. capillaris, whereas Fe(OH)3 had no significant effect on the biomass of either species. Although S0 and Fe(OH)3 did adjust the chemical properties of the soil solution, Fe(OH)3 did not bind P strongly enough to make it unavailable to plants. Further work is required, however, the use of chemically amended composts provides a sustainable sink for organic wastes and we conclude from this study that they have great potential for large-scale restoration of blocky waste tips.

Fine roots (

Anthropogenic activities during the last century have increased levels of atmospheric CO2. Forest net primary productivity increases in response to elevated CO2, altering the quantity and quality of carbon supplied to the rhizosphere. Ectomycorrhizal fungi form obligate symbiotic associations with the fine roots of trees that mediate improved scavenging for nutrients in exchange for a carbohydrate supply. Understanding how the community structure of ectomycorrhizal fungi is altered by climate change is important to further our understanding of ecosystem function. Betula pendula and Fagus sylvatica were grown in an elevated CO2 atmosphere delivered using free air carbon dioxide enrichment (FACE) under field conditions in the U.K., and Picea abies was grown under elevated CO2 in glass domes in the Czech Republic. We used morphotyping and sequencing of the internal transcribed spacer region of the fungal ribosomal operon to study ectomycorrhizal community structure. Under FACE, un-colonised roots tips increased in abundance for Fagus sylvatica, and during 2006, sporocarp biomass of Peziza badia significantly increased. In domes, ectomycorrhizal community composition shifted from short-distance and smooth medium-distance to contact exploration types. Supply and competition for carbon belowground can influence ectomycorrhizal community structure with the potential to alter ecosystem function.

Accurate estimates of mycelial exudation in time and space are crucial for the assessment of ectomycorrhizal involvement in biogeochemical processes. Knowledge of exudation from mycelia of ectomycorrhizal fungi is still limited, especially for fungi in symbiosis with a host. Pinus sylvestris seedlings colonized by Hebeloma crustuliniforme were grown in aseptic multicompartment dishes. This novel system enabled identification of exudates originating only from extramatrical mycelium. At harvest, hyphal density and numbers were estimated using microscopic imaging. A fractal geometric approach was adopted for calculation of exudation rates. The main compounds identified were oxalate and ferricrocin. The exudation rate for oxalate was 19 +/- 3 fmol per hyphal tip h(-1) (mean +/- standard error of the mean) or 488 +/- 95 fmol hyphal mm(-2) h(-1). Ferricrocin rates were approx. 10 000 times lower. The fractal dimension (D) of the mycelia was 1.4 +/- 0.1, suggesting an explorative growth. Potassium nutrition was a significant regulatory factor for ferricrocin but not oxalate. The results suggest that hyphal exudation may alter the chemical conditions of soil microsites and affect mineral dissolution. Calculations also indicated that oxalate exudation may be a significant carbon sink.

Calcareous soils are frequently characterized by the low bioavailability of plant nutrients. Consequently, many vascular plant species are unable to successfully colonize calcareous sites and the floristic composition of calcareous and acid silicate soils has been shown to differ markedly. The root exudation of oxalate and citrate has been suggested to play a pivotal role in same nutrient acquisition mechanisms operating in calcareous soils. The aim of this study was therefore to investigate the nutrient extraction efficiency of three individual organic acids commonly identified in root exudates, i.e. citric, malic and oxalic acid. Our results clearly demonstrate the context dependent nature of nutrient release by organic acids. The degree of P extraction was highly dependent on which organic acid was added, their concentration and pH, and their contact time with the soil. P is generally more efficiently extracted by organic acids at a high pH and follows the series oxalate > citrate > malate. The opposite relationship between pH and extraction efficiency was apparent for most other cations examined (e.g. Zn, Fe), which are more efficiently extracted by organic acids at low pH. A serious constraint to the ecological importance of organic acid exudation in response to P deficiency is, however, their very low P mobilization efficiency. For every mol of soil P mobilized, 1000 mol of organic acid has to be added. It can, however, be speculated that in a calcareous soil with extremely low P concentrations it is still beneficial to the plants to exude organic acids in spite of the seemingly high costs in terms of carbon.

Spatial coarse root distribution of the in situ grown species Strychnos cocculoides Bak., Strychnos spinosa Lam. (Loganiaceae), Vangueria infausta Burch. (Rubiaceae) and Grewia flava DC. (Tiliaceae) was investigated. The woody roots provide the scaffolding for fine roots, and thus underpin potential competition with fine roots of other species. We developed a method for quantitative description of spatial patterns of coarse roots and correlated fine root distribution with the spatial arrangement of the coarse root systems. In order to estimate different exploration and exploitation strategies, we used the spatial distribution of structural roots within the individual soil volumes (ISV) of each root system, and compared the results with other published parameters. We defined a new parameter, “generalized efficiency of exploitation” GEE(φ), unifying different notions from the literature. Among all the investigated species and dependent on the considered parameter, either Vangueria infausta or Grewia flava had the highest mean values of exploration, hence they could clearly be separated from both Strychnos species. For the exploitation indices, no significant differences were found. However, the generalized efficiency of exploitation GEE(φ) was again different between the species. For correlations between coarse and fine roots, the restriction to the ISV was useful to increase the strength of correlations.

Organic acids may play a key role in rhizosphere and pedogenic processes. The effects of young trees and ectomycorrhizas on the soil solution concentrations of low molecular weight organic acids (LMWOAs) were studied in soil columns (E horizon) in the presence or absence of Pinus sylvestris and Picea abies with or without three ectomycorrhizal fungi. Several LMWOAs were identified at concentrations ranging from

Fine root distribution and turnover were investigated in ca. 40-year-old pure Norway spruce (Picea abies Karst.) stands in Germany, growing on four sites that differed in soil acidity (Ebergotzen < Barbis < Fichtelgebirge = Harz). The density of fine root biomass and necromass in different soil horizons differed among the sites. At one of the most acidic sites (Harz), fine root density in the humus layer was more than twice that at the least acidic site (Ebergotzen). At the two most acidic sites, Fichtelgebirge and Harz, the ratio of biomass to necromass was significantly lower than at Ebergotzen and Barbis, particularly in the subsoil layer. In each stand, clear vertical gradients in fine root length density and root tip density were observed. Most of the roots and the root tips were in the humus layer and in the first mineral soil horizon (0-10 cm). There was a significantly different decrease in specific root length (cm gDM (-1)) and specific root tip density (root tips gDM (-1)) in the more acidified stands Fichtelgebirge and Harz compared with Ebergotzen and Barbis. Fine root production estimated by ingrowth cores and a net method was approximately twice as high in the more acidic stands Fichtelgebirge and Harz compared with Ebergotzen and Barbis. Rates of living fine root biomass turnover were higher at the Fichtelgebirge and Harz sites than at the Ebergotzen site. Rates of necromass turnover were similar at all sites. The results suggest that the accumulation of necromass was not due to a slower disappearance at the more acid sites, but to earlier root death. Roots contributed 46% to root + needle litter and 32% to root + total aboveground litter at the Harz site in 1997.

A fast growing high density Populus plantation located in central Italy was exposed to elevated carbon dioxide for a period of three years. An elevated CO2 treatment (550 ppm), of 200 ppm over ambient (350 ppm) was provided using a FACE technique. Standing root biomass, fine root turnover and mycorrhizal colonization of the following Populus species was examined: Populus alba L., Populus nigra L., Populus x euramericana Dode (Guinier). Elevated CO2 increased belowground allocation of biomass in all three species examined, standing root biomass increased by 47–76% as a result of FACE treatment. Similarly, fine root biomass present in the soil increased by 35–84%. The FACE treatment resulted in 55% faster fine root turnover in P. alba and a 27% increase in turnover of roots of P. nigra and P. x euramericana. P. alba and P. nigra invested more root biomass into deeper soil horizon under elevated CO2. Response of the mycorrhizal community to elevated CO2 was more varied, the rate of infection increased only in P. alba for both ectomycorrhizal (EM) and arbuscular mycorrhizas (AM). The roots of P. nigra showed greater infection only by AM and the colonization of the root system of P. x euramericana was not affected by FACE treatment. The results suggest that elevated atmospheric CO2 conditions induce greater belowground biomass investment, which could lead to accumulation of assimilated C in the soil profile. This may have implications for C sequestration and must be taken into account when considering long-term C storage in the soil.

The Himalayas are predicted to experience more than 3 times the mean global rise in temperature, as well as erratic rainfall patterns and an increased likelihood of total monsoon failures. While many ecosystem manipulation experiments aiming at understanding the effects of altered precipitation, temperature, and carbon dioxide are conducted globally, such experiments are rare in Asia, particularly in the Himalayas. To fill this gap, we simulated late onset of monsoon precipitation, as well as total monsoon failure, in a multiyear drought stress experiment in Bhutan. Two treatments, 100% throughfall exclusion and ambient control plots, were applied to 725 m2 plots (25 m × 29 m), each with 2 replicates in a hemlock-dominated (Tsuga dumosa) and oak-dominated (Quercus lanata and Quercus griffithii) ecosystem at 3260 and 2460 m elevations, respectively. Roof application reduced the volumetric soil water content in the upper (0–20 cm) soil layer by ∼ 20% in coniferous and ∼ 31% in broadleaved forest; the deeper soil layers were less affected. We demonstrate that large-scale throughfall-exclusion experiments can be successfully conducted even in a remote Bhutan Himalayan setting. The experiences gathered could be utilized for future long-term ecological monitoring studies in the Himalayan region.

(1) Elevated atmospheric CO2 (eCO2) may affect organic inputs to woodland soils with potential consequences for C dynamics and associated aggregation; (2) The Bangor Free Air Concentration Enrichment experiment compared ambient (330 ppmv) and elevated (550 ppmv) CO2 regimes over four growing seasons (2005–2008) under Alnus glutinosa, Betula pendula and Fagus sylvatica. Litter from the experiment (autumn 2008) and Lumbricus terrestris were added to mesocosm soils. Microbial properties and aggregate stability were investigated in soil and earthworm casts. Soils taken from the field experiment in spring 2009 were also investigated; (3) eCO2 litter had lower N and higher C:N ratios. F. sylvatica and B. pendula litter had lower N and P than A. glutinosa; F. sylvatica had higher cellulose. In mesocosms, eCO2 litter decreased respiration, mineralization constant (respired C:total organic C) and soluble carbon in soil but not earthworm casts; microbial‐C and fungal hyphal length differed by species (A. glutinosa = B. pendula > F. sylvatica) not CO2 regime. eCO2 increased respiration in field aggregates but increased stability only under F. sylvatica; (4) Lower litter quality under eCO2 may restrict its initial decomposition, affecting C stabilization in aggregates. Later resistant materials may support microbial activity and increase aggregate stability. In woodland, C and soil aggregation dynamics may alter under eCO2, but outcomes may be influenced by tree species and earthworm activity.

Within branched root systems, a distinct heterogeneity of traits exists. Knowledge about the ecophysiology of different root types is critical to understand root system functioning. Classification schemes have to match functional root types as closely as possible to be used for sampling and modeling. Among ecophysiological root traits, respiration is of particular importance, consuming a great amount of carbon allocated. Root architecture differs between the four deciduous tree seedlings. However, two types of terminal root segments (i.e. first and second orders), white colored and brown colored, can be distinguished in all four species but vary in frequency, their morphology differing widely from each other and higher coarse root orders. Root respiration is related to diameter and tissue density. The use of extended root ordering (i.e. order and color) explains the variance of respiration two times as well as root diameter or root order classes alone. White terminal roots respire significantly more than brown ones; both possess respiration rates that are greater than those of higher orders in regard to dry weight and lower in regard to surface area. The correlation of root tissue density to respiration will allow us to use this continuous parameter (or easier to determine dry matter content) to model the respiration within woody root systems without having to determine nitrogen contents. In addition, this study evidenced that extended root orders are better suited than root diameter classes to picture the differences between root functional types. Together with information on root order class frequencies, these data allow us to calculate realistic, species-specific respiration rates of root branches.

The purpose of the research topic “Ecophysiology of root systems-environment interactions” was to shed light on belowground processes—in an effort to further enhance its understanding, but also to increase the awareness of the research community and funding bodies toward this utmost important part of plants and ecosystems. Why is it important to increase our understanding and awareness? The new challenge of global climate change is driven by an increase in atmospheric CO2 levels, a factor which in itself affects plant and root growth, but is also expected to increase the intensity of climatic and edaphic extremes. In addition, other stresses such as salinization and heavy metal contamination of soils are either increasing or continue to persist world-wide. To secure crop yield and soil quality, and to understand the functioning, and thus the resilience and resistance of pristine ecosystems under changing environmental conditions, an increased understanding of acclimation and adaptation processes is imperative. In the past, plant sciences' research has focused predominantly on parameters above ground—resulting in disproportional less knowledge regarding root systems and the way root system functioning is affected by both internal and external factors. Similarly, soil scientists have often preferred studying bulk soil over the rhizosphere and as a consequence root-driven soil processes are still far less studied. Because it is “better to light a candle than curse the darkness” (Herron et al., 2013), this research topic puts research on root and rhizosphere into the spotlight. Increased net photosynthesis and decreased shoot nitrogen and water use under elevated CO2 can alter source–sink relations of plant organs. In this research topic, Easlon and Bloom (2013) emphasize the important role of root-shoot signaling for plant acclimation to increasing CO2 levels. Addressing water availability, Carminati (2013) shows that fast and almost immediate rewetting after soil drought took place in the rhizosphere of distal maize root segments while the rhizosphere of higher root orders possessed slower rewetting. The difference in the speed of rewetting for different root orders, may possibly be an adaptation strategy to drought periods, increasing the water uptake by young root segments and hydraulically disconnecting the older ones. Two comprehensive reviews shed light on the influence and interplay of heavy metals on/with root systems, and influence of root traits on whole plant stress resistance. In one review, Fahr et al. (2013) address the wide range of tolerance mechanism of roots against lead exposure, while in another review Brunner and Sperisen (2013) focus on the current understanding of aluminium exclusion and tolerance mechanisms in woody plants. The response of roots to abiotic stress can be modified by some root-associated bacteria and fungi. This is expressed as modification of root morphology and whole plant ecophysiology, which often enhances plant growth under stress (Alavi et al., 2013; Vacheron et al., 2013). Increasing knowledge of bacterial nutrition in the rhizosphere will further increase the understanding of the role of certain bacteria as plant-growth promoters (Lopez-Guerrero et al., 2013). Similarly, mycorrhizal symbionts are very important for soil exploration; in this context, Lang et al. (2013) report on spatial structuring of ectomycorrhizal assemblages within beech root systems. While earthworms often improve soil structure and nutrient availability, Arnone and Zaller (2014) report decreasing grass root length densities under increasing earthworm densities, with yet unknown consequences for nutrient foraging. Besides abiotic stress, root-microbe and root-fauna interactions, plant-plant interactions below ground are common and can influence plant performance considerably. For example, Bolte et al. (2013) show that beech fine roots are facilitated in the presence of spruce roots—possibly by lowering the competitive pressure (for resources) compared to intraspecific competition. While information on the mechanisms of belowground neighbor perception is rare, Schmid et al. (2013) outline that Arabidopsis roots perceive neighboring roots or their associated microorganisms by a mechanism that involves the induction of pathogenesis-related proteins. Their findings reveal that belowground neighbor detection may occur independently of resource depletion, possibly allowing roots to anticipate future competition. In the past years an increasing awareness has developed that understanding root traits will help to understand plant functioning. Since resources are acquired by the root system, breeding for crops with root traits/phenotypes increasing water and nutrient acquisition should increase yields on infertile soils and under a range of other abiotic stresses such as drought (Comas et al., 2013; White et al., 2013). In doing so, a better understanding of how root and root system traits interact to affect soil resource acquisition is needed (York et al., 2013). In woody plants, our understanding of species- or even variety-specific root trait plasticity under variable environments and the importance of specific traits such as deep rooting (Laclau et al., 2013; Maeght et al., 2013) is even more scarce than in crop plants. A primary reason for this difference in understanding being the challenges caused by more complex, difficult-to-access, perennial root systems. Fortunately, several studies of this research topic shed light on the variability/plasticity of (some) fine root traits of several tree species with ontogeny, and/or under different environmental or management conditions. For example, Noguchi et al. (2013) showed that N-fertilization had a more pronounced effect on Cryptomeria japonica root morphology than on root biomass. Studying ectomycorrhizal short roots, Ostonen et al. (2013) found that morphological root traits of late-successional spruce are as plastic as that of pioneer silver birch, and that differences between root traits of the two species was less under more temperate conditions compared to more boreal conditions. The work of Tobner et al. (2013) evidenced that the responses to ontogeny or soil conditions are species but also trait dependent. Hajek et al. (2013) found distinct intraspecific variation in most root traits among seven Poplar demes. As highlighted by Fort (2013), their results challenge the existence of well-defined species-specific trait values, but rather highlight the existence of pronounced within-species trait diversity linked to genetic differentiation. While manuscripts in this research topic address a plethora of root traits, increasing efforts are also required to understand root system branching in situ and in silica (Bodner et al., 2013) and to develop meaningful classification approaches for functional units within root systems. Bodner and colleagues showed that statistical classification methods can integrate knowledge on morphological traits obtained with different methods and at various scales. Currently morphology seems to be the most promising basis for classification approaches due to its wide use. However, the lack of consensus about fine root classification (and a clear nomenclature), and the importance of specific traits constrains the development of a unified framework toward a “root economics spectrum” as was achieved for both leaves (see e.g., Poorter et al., 2014 and references therein) and wood. In addition, more suitable methods are needed allowing advanced root research; here Danjon et al. (2013) describe a modeling approach to estimating root loss during tree root system sampling, and Faget et al. (2013) introduce a combined root fluorescence and planar Optode technique which allows to distinguish between different plant species grown in natural soil and to measure the impact of root (exudates) on the soil environment. Using light-emitting soil microbial “biosensors,” Herron et al. (2013) were able to determine hotspots of microbial growth along the growing root. The growth of these microbial hotspots was supported by high carbon availability. In addition to articles on basic research which often accentuate the uncertainties in the field of root research, some articles already outline the application of knowledge of root traits in applied plant production systems. The study by Kerbiriou et al. (2013) provides first evidence that “robustness” and head growth rates of lettuce cultivars are related to the size of the root system. Terzaghi et al. (2013) investigated C and N concentrations in Fagus sylvatica fine roots in relation to different stand characteristics resulting from conversion of coppiced forests to high forests. The fine-root C:N ratio was higher in coppiced than in converted stands and showed an inverse relationship with fine-root turnover rate, illustrating a significant change of fine-root status under different management practices—likely influencing e.g., the C sequestration potential of stands. Chairungsee et al. (2013) revealed a significant negative correlation between fine root dynamics and production in rubber plantations. Latex harvesting might disturb carbon dynamics in the whole tree, far beyond the trunk; the results emphasize the impact of root systems on the carbon budget and thus yield of tree crops plantations. When taken as a whole, the 28 contributions to this research topic cover many, although by no means all, aspects of root and rhizosphere research. The number of articles collected within a relatively short period of time, and other recently published special issues addressing root-environmental interactions (e.g., Annals of Botany, 2012, 2013; New Phytologist, 2013), demonstrate that the awareness about root and rhizosphere research and its applicability is rising.

To investigate whether Cd induces common plant defense pathways or unspecific necrosis, the temporal sequence of physiological reactions, including hydrogen peroxide (H2O2) production, changes in ascorbate-glutathione-related antioxidant systems, secondary metabolism (peroxidases, phenolics, and lignification), and developmental changes, was characterized in roots of hydroponically grown Scots pine (Pinus sylvestris) seedlings. Cd (50 μm, 6 h) initially increased superoxide dismutase, inhibited the systems involved in H2O2 removal (glutathione/glutathione reductase, catalase [CAT], and ascorbate peroxidase [APX]), and caused H2O2accumulation. Elongation of the roots was completely inhibited within 12 h. After 24 h, glutathione reductase activities recovered to control levels; APX and CAT were stimulated by factors of 5.5 and 1.5. Cell death was increased. After 48 h, nonspecific peroxidases and lignification were increased, and APX and CAT activities were decreased. Histochemical analysis showed that soluble phenolics accumulated in the cytosol of Cd-treated roots but lignification was confined to newly formed protoxylem elements, which were found in the region of the root tip that normally constitutes the elongation zone. Roots exposed to 5 μm Cd showed less pronounced responses and only a small decrease in the elongation rate. These results suggest that in cells challenged by Cd at concentrations exceeding the detoxification capacity, H2O2 accumulated because of an imbalance of redox systems. This, in turn, may have triggered the developmental program leading to xylogenesis. In conclusion, Cd did not cause necrotic injury in root tips but appeared to expedite differentiation, thus leading to accelerated aging.

• Translocation is shown of phosphorus, nitrogen, potassium and magnesium to a P-deficient host from ectomycorrhizal fungal hyphae. • Mycorrhizal (with Paxillus involutus) and nonmycorrhizal P-deficient spruce (P. abies) seedlings were grown in a two-compartment sand-culture system. Hyphal translocation of nutrients from the inner compartment (penetrated only by hyphae) to the host was measured using mass balance (for N, P and K) or stable isotope (15N and 25Mg) methods. • Addition of P to the hyphal compartment strongly stimulated hyphal growth, and this also increased both seedling P status and growth. Hyphae translocated nonlimiting elements in addition to P, contributing 52, 17, 5 and 3–4%, respectively, to total P, N, K or Mg plant uptake. The potential role of the ectomycorrhizal mycelium in K acquisition was demonstrated. Translocation to mycorrhizal seedings of N, K and Mg was strongly reduced when hyphal P-fluxes ceased; this translocation of nonlimiting nutrients depended on simultaneous translocation of P. • The ectomycorrhizal mycelium has an active role in P acquisition from sources not available to roots. Nutrient fluxes within fungal hyphae are interdependent and strong coupling of N, K and Mg fluxes with long-distance P translocation in the mycorrhizal mycelium occurs.

Increasing evidence suggests that forest soils in central and northern Europe as well as in North America have been significantly acidified by acid deposition during the last decades. The present investigation was undertaken to examine the effect of soil acidity on rooting patterns of 40-year-old Norway spruce trees by comparing fine and coarse roots among four stands which differed in soil acidity and Mg (and Ca) nutrition. The coarse root systems of four to five 40-year-old Norway spruce trees per stand were manually excavated. The sum of cross sectional area (CSA) at 60 cm soil depth and below of all vertical coarse roots, as a measure of vertical rooting intensity, was strongly reduced with increasing subsoil acidity of the stands. This pattern was confirmed when 5 additional acidic sites were included in the analysis. Fine root biomass in the mineral soil estimated by repeated soil coring was strongly reduced in the heavily acidified stands, but increased in the humic layer. Using ingrowth cores and a screen technique, we showed that the higher root biomass in the humic layer of the more acidic stands was a result of higher root production. Thus, reduced fine root biomass and coarse root CSA in deeper soil layers coincided with increased root growth in the humic layer. Root mineral analysis showed Ca/Al ratios decreased with decreasing base saturation in the deeper mineral soil (20–40 cm). In the top mineral soil, only minor differences were observed among stands. In general, low Ca/Al ratios coincided with low fine root biomass. Calcium/aluminum ratios determined in cortical cell walls using X-ray microanalysis showed a similar pattern as Ca/Al ratios based on analysis of whole fine roots, although the amplitude of changes among the stands was much greater. Aluminum concentrations and Ca/Al ratios in cortical cell walls were at levels found to inhibit root growth of spruce seedlings in laboratory experiments. The data support the idea that Al (or Ca/Al ratios) and acid deposition-induced Mg (and possibly Ca) deficiency are important factors influencing root growth and distribution in acidic forest soils. Changes in carbon partitioning within the root system may contribute to a reduction in deep root growth.

International audience; Coarse root systems of four different fruit tree species from southern Africa were completely excavated and semi-automatically digitized. Spatial distributions of root length were determined from the digitally-reconstructed branching systems. Furthermore, the fractal characteristic of the coarse root systems was shown by determining the box-counting dimensions. These quantitative methods revealed architectural differences between the species, probably due to different ecophysiological strategies. For fine root samples, which were taken before digging out the whole systems, fractal analysis of the planar projections showed no significant inter-species differences. Methodologically, the study underlines the usefulness of digital 3-D reconstruction in root research.; Structures et dimensions fractales des systèmes racinaires provenant de quatre espèces d'arbres fruitiers de Botswana. Des systèmes de grosses racines, provenant de quatre espèces différentes d'arbres fruitiers d'Afrique du Sud, ont été complètement déterrés et digitalisés semi-automatiquement. Les distributions spatiales des longueurs de racines ont été calculées à partir des maquettes informatiques reconstituées. En outre, le caractère fractal des systèmes de grosses racines a été prouvé par une détermination de dimensions utilisant la méthode du comptage de boites. Ces méthodes quantitatives révèlent des différences architecturales entre les espèces, résultant probablement de différentes stratégies écophysiologiques. Pour les échantillons de racines fines, obtenus avant l'excavation des systèmes complets, l'analyse fractale des projections planes n'a pas montré de différences significatives entre les espèces. Concernant la méthode, l'étude fait apparaître l'apport de la reconstruction digitale 3-D dans le domaine de la recherche sur les appareils racinaires.

Metal toxicity (Al and heavy metals) is a major constraint affecting root growth in a number of natural or managed ecosystems. Fine roots of the majority of plant species are associated with mycorrhizal fungi, which may modify the sensitivity of roots to metal stress. In this review, we summarise the available evidence demonstrating beneficial effects of ectomycorrhizas in alleviation of metal toxicity in forest tree seedlings. We identify experimental shortcomings of past research (e.g. the use of shoot metal concentrations as a measure of metal uptake, use of microanalytical techniques biased by element redistribution) that may confound major conclusions drawn from these experiments. Although there is no doubt that in many cases ectomycorrhizal fungi indeed ameliorate metal stress in their host plants, the mechanism(s) involved remain(s) unclear. The role of metal sorption on fungal tissues thought to reduce metal exposure of the host plant is critically reviewed. As direct evidence (both under artificial and soil conditions) supporting a unique role of fungal immobilisation of metals is lacking so far, there is an urgent need to also test alternative tolerance mechanisms such as the release of metal chelating substances, or nutritional and hormonal effects mediated by mycorrhizal fungi.

Although it is well established that ectomycorrhizas improve the mineral nutrition of forest trees, there has been little evidence that they mediate uptake of divalent cations such as Mg. We grew nonmycorrhizal seedlings and seedlings mycorrhizal with Paxillus involutus Batsch in a sand culture system with two compartments separated by a 45-μm Nylon mesh. Hyphae, but not roots, can penetrate this net. Labeling the compartment only accessible to hyphae with 25Mg showed that hyphae of the ectomycorrhizal fungus Paxillus involutus transported Mg to their host plant. No label was found in nonmycorrhizal control plants. Our data support the idea that ectomycorrhizas are important for the Mg nutrition of forest trees.

Al and Pb tolerance of different ectomycorrhizal fungi was tested in Petri dishes divided into three compartments. One compartment contained nutrient solution supplemented with Al or Pb but without P to avoid precipitation of Al or Pb phosphates. Phosphate was supplied in the agar compartment while a third compartment served as control containing nutrient solution without P. The tested ectomycorrhizal fungal species and isolates varied greatly in their Al and Pb tolerance. A more than 50% growth reduction was observed at 2000 μM Al in Paxillus involutus MAI but already at 200 μM in P. involutus NAU and P. involutus 533. In contrast, growth of Pisolithus tinctorius 956 appeared to be stimulated by Al. Al tolerance therefore decreased in the following order: P. tinctorius 956 > P. involutus MAI > P. involutus NAU, P. involutus 533. To test the effect of agar on Al tolerance, P. involutus MAI was grown with agar in all three compartments. In presence of agar, even 2000 μM Al had no effect on growth. Lead at concentrations of 1 and 10 μM Pb had no significant effect on fungal growth. A more than 50% decrease of mycelial surface area occurred at 500 μM Pb in P. involutus MAI and P. involutus 533 and at 100 μM Pb in P. involutus NAU and P. tinctorius 956. Pb tolerance therefore decreased in the following order: P. involutus MAI, P. involutus 533 > P. involutus NAU, P. tinctorius 956. Divided Petri plates appear to be a valuable tool to test metal tolerance of fungi as complexation of the metals by agar or precipitation with P are avoided. The results indicate that some ectomycorrhizal fungi may be more sensitive to Al and Pb than previously reported. Aluminium- und Bleitoleranz bei Ektomykorrhizapilzen Die Al- und Pb-Toleranz von Ektomykorrhizapilzen wurde auf dreigeteilten Petrischalen untersucht. Das erste Kompartiment enthielt die Nahrlosung mit Al oder Pb aber ohne P. Dadurch wurde die Bildung von Al- und Pb-Phosphaten verhindert. Die Pilze wurden uber das Agarkompartiment mit P versorgt. Das dritte Kompartiment diente als Kontrolle und enthielt die gleiche Nahrlosung aber ohne Al oder Pb. Die untersuchten Ektomykorrhizapilze unterschieden sich stark hinsichtlich ihrer Al- und Pb-Toleranz. Das Wachstum von P. tinctorius 956 wurde durch 800 μM Al stimuliert und war bei 2000 μM Al nicht beeintrachtigt. Eine mehr als 50%ige Reduktion des Wachstums erfolgte bei P. involutus MAI bei 2000 μM Al wahrend sie bei P. involutus 533 und P. involutus NAU bereits bei 200 μM Al auftrat. Die Reihenfolge der Al-Toleranz lautet daher: P. tinctorius 956 > P. involutus MAI > P. involutus NAU, P. involutus 533. Den starken Einflus von Agar als Komplexbildner auf die Al-Toleranz zeigte ein weiteres Experiment mit P. involutus MAI, bei dem alle drei Kompartimente Agar enthielten. Im Gegensatz zu den vorherigen Ergebnissen in Flussigkultur wurde das Wachstum von P. involutus MAI in Anwesenheit von Agar auch durch 2000 μM Al nicht beeintrachtigt. Das Pilzwachstum wurde durch 1 oder 10 μM Pb nicht reduziert. Wahrend das Wachstum von P. involutus MAI und P. involutus 533 erst bei 500 μM Pb um 50% reduziert wurde, erfolgte eine ahnlich starke Reduktion bei P. involutus NAU und P. tinctorius 956 bereits bei 100 μM Pb. Die Reihenfolge der Pb-Toleranz lautet daher: P. involutus MAI, P. involutus 533 > P. involutus NAU, P. tinctorius 956. Die dreigeteilten Petrischalen sind zur Untersuchung der Metalltoleranz von Ektomykorrhizapilzen geeignet, da eine Verringerung der Metalltoxizitat durch die Bildung von Al- und Pb-Phosphaten oder Komplexierung durch Agar vermieden wird. Die Ergebnisse zeigen, das manche Ektomykorrhizapilze eine hohere Al und Pb Empfindlichkeit aufweisen als bisher angenommen.

Seedlings of sugar beet (Beta vulgaris) were grown in nutrient solutions containing a range of Cu and Zn concentrations. Based on measurements of shoot and root length and dry weight, copper was found to be already toxic at 10 μM, while Zn became toxic at 100 μM. At Cu and Zn levels found to induce a similar level of growth inhibiton, the influence of increasing the supply of K, Ca and Mg was investigated. Increasing the concentration of both Ca and Mg in the nutrient solution attenuated the degree of inhibition of root growth by Zn, but not Cu. Potassium did not affect the toxicity of either Cu or Zn. An increase in Ca decreased the level of both Cu and Zn in roots. Magnesium ameliorated the toxicity effects of Zn without effecting the Zn concentration in the roots. Treatment with Zn significantly decreased the concentration of Mg in the roots. An increased supply of Mg lowered the percentage decrease in root Mg concentration due to Zn toxicity. The maintenance of an adequate Mg level in the roots may be critical to prevent Zn induced inhibition of root growth. Basische Kationen vermindern toxische Wirkungen von Zink, nicht aber von Kupfer in Beta vulgaris Samlinge der Zuckerrube (Beta vulgaris) wurden in Nahrlosungs-kultur angezogen und mit Cu und Zn behandelt. Wahrend Cu bereits in einer Konzentration von 10 μM das Wurzel- und Sproswachstum hemmte, wirkte Zn erst in einer Konzentration von 100 μM toxisch. Wurde bei dieser Cu- oder Zn-Konzentration das Ca- oder Mg-Angebot in der Nahrlosung erhoht, reduzierte dies die toxische Wirkung von Zn, nicht aber die von Cu. Kalium hatte weder auf die toxische Wirkung von Cu noch von Zn einen Einflus. Ein erhohtes Ca-Angebot verminderte sowohl die Cu als auch die Zn-Konzentration in den Wurzeln. Im Gegensatz dazu reduzierte Mg zwar die Wirkung von Zn, hatte aber keinen Einflus auf die Zn-Konzentration in der Wurzel. Die Zn-Behandlung hemmte signifikant die Mg-Aufnahme in die Wurzeln. Durch eine Erhohung der Mg-Konzentration in der Nahrlosung konnte diese Hemmung erheblich vermindert werden. Eine adaquate Mg-Versorgung der Wurzeln scheint eine entscheidende Rolle bei der Vermeidung einer Zn-induzierten Hemmung des Wurzelwachstums zu spielen.

In this work, a sand culture system for ecto- or arbuscular mycorrhizal plants was developed. Nutrients were added into the sand at a constant rate using solutions with similar concentrations of nutrients as those found in forest soil solutions. Plants grew well in the system and inoculated plants developed abundant ecto- or arbuscular mycorrhizas. To test the suitability of the culture system for studies on metal toxicity, aluminium was added to the nutrient solutions of non-mycorrhizal and mycorrhizal spruce seedlings. Measurments of labile Al in the solution draining from the sand revealed that Al was mainly present as phytotoxic monomeric Al. In addition, concentrations of Ca2+, Mg2+ and SO42— ions which have the potential to alleviate Al toxicity, were similar in the draining as in the nutrient solutions. After 10 weeks of exposure to 400 μM Al, taproot growth as well as Ca and Mg uptake of the seedlings were impaired by Al. Mycorrhizal colonization had no effect on the degree of inhibition of these processes. We conclude that this culture system is suitable for investigations on effects of Al and other toxic metals on mycorrhizal tree seedlings. Sandkultur fur mykorrhizierte Pflanzen In der vorliegenden Arbeit wurde ein Sandkultursystem zur Anzucht von Pflanzen mit Ekto- oder arbuskularer Mykorrhiza entwickelt. Nahrstoffe wurden dem Sand mit konstanter Rate in Form einer Nahrlosung zugefuhrt, deren Nahrstoffkonzentrationen weitgehend denen in sauren Waldboden entsprach. Die Pflanzen zeigten gutes Wachstum und inokulierte Pflanzen entwickelten zahlreiche Ekto- oder arbuskulare Mykorrhizen. Um die Eignung des Kultursystems fur Untersuchungen zur Wirkung toxischer Metalle zu testen, wurden nichtmykorrhizierte und mykorrhizierte Fichtenkeimlinge mit Aluminium behandelt. Messungen von labilem Al in der Dranagelosung zeigten, das der uberwiegende Teil des Al in phytotoxischer monomerer Form vorlag. Ca2+-, Mg2+- und SO42—-Ionen, die potentiell die Al-Wirkungen reduzieren konnen, wurden in annahernd gleicher Konzentration in der Dranage- wie in der Nahrlosung gemessen. Nach 10wochiger Behandlung mit 400 μM Al zeigten die Keimlinge verminderte Ca- und Mg-Gehalte in den Nadeln und Wurzeln sowie ein reduziertes Wachstum der Hauptwurzel. Die Mykorrhiza hatte keinen Einflus auf den Grad der Hemmung dieser Prozesse. Die Daten zeigten, das das Kultursystem geeignet ist, Effekte von Al und anderen toxischen Metallen auf mykorrhizierte Baumkeimlinge zu untersuchen.

The contribution of the extramatrical mycelium to N and P nutrition of mycorrhizal Norway spruce (Picea abies (L.) Karst.) was investigated. Seedlings either inoculated with Paxillus involutus (Batsch) Fr. or non-mycorrhizal were grown in a two compartment sand culture system where hyphae were separated from roots by a 45 μm nylon net. Nutrient solution of the hyphal compartment contained either 1.8 mm NH4+ and 0.18 mm H2PO4− or no N and P. Aluminium added to the hyphal compartment as a tracer of mass flow was not detected in the plant compartment, indicating that measurements of N and P transfer by the mycelium were not biased by solute movement across the nylon net.The addition of N and P to the hyphal compartment markedly increased dry weight, N and P concentration and N and P content of mycorrhizal plants. Calculating uptake from the difference in input and output of nutrient in solution confirmed a hyphal contribution of 73% and 76% to total N and P uptake, respectively. Hyphal growth was increased at the site of nutrient solution input.