Uptake of an amino acid by ectomycorrhizal fungi in a boreal forest

We assessed the degree to which ectomycorrhizal fungi exploit organic nitrogen in situ. In an Alaskan boreal forest, we identified pairs of sporocarps from five taxa of ectomycorrhizal fungi. We added ¹³C-labeled alanine to the soil surrounding one sporocarp within each pair; the second served as an unlabeled control. Peak rates of ¹³C-respiration from alanine were higher in the labeled sporocarp plots than the controls, indicating that the ¹³C-alanine was detectably respired from the soil. “Reference” plots adjacent to the sporocarps served as an indication of background ¹³C-respiration rates released by the soil community as a whole. Ectomycorrhizal sporocarps displayed higher ¹³C-respiration rates than their reference plots. Thus, the sporocarps and associated mycorrhizal mycelium appeared to contribute significantly to the release of alanine-derived ¹³CO₂, confirming the hypothesis that ectomycorrhizal fungi may access soil amino acid pools under natural conditions.     

Species richness of ectomycorrhizal hyphal necromass increases soil CO2 efflux under laboratory conditions

The ectomycorrhizal mycelium is a large component of boreal and temperate forest soil microbial biomass and the resulting necromass is likely to be an important source of nutrients for saprotrophic microorganisms. Here we test the effects of species richness of ectomycorrhizal mycelial biomass on short-term CO₂ efflux by amending forest soil with necromass from 8 fungal species added separately and in mixtures of 2, 4 and 8 species. All additions of necromass rapidly increased soil CO₂ efflux compared to unamended controls but CO₂ efflux increased significantly with species richness. Efflux of CO₂ did not correlate with the carbon (C) or nitrogen (N) contents or the C:N ratio of the added necromass. The study demonstrates that species diversity of dead ectomycorrhizal fungal hyphae can have important consequences for soil CO₂ efflux, and suggests decomposition of hyphae is regulated by specific constituents of the nutrient pools in the necromass rather than the total quantities added.     

Can ectomycorrhizal fungi circumvent the nitrogen mineralization for plant nutrition in temperate forest ecosystems?

Nitrogen (N) limits plant growth in many forest ecosystems. The largest N pool in the plant-soil system is typically organic, contained primarily within the living plants and in the humus and litter layers of the soil. Understanding the pathways by which plants obtain N is a priority for clarifying N cycling processes in forest ecosystems. In this review, the interactions between saprotrophic microorganisms and ectomycorrhizal fungi in N nutrition with a focus on the ability of ectomycorrhizal fungi to circumvent N mineralization for the nutrition of plants in forest ecosystems will be discussed. Traditionally, it is believed that in order for plants to fulfill their N requirements, they primarily utilize ammonium (NH₄⁺) and nitrate (NO₃⁻). In temperate forest ecosystems, many woody plants form ectomycorrhizas which significantly improves phosphorus (P) and N acquisition by plants. Under laboratory conditions, ectomycorrhizal fungi have also been proven to be able to obtain N from organic sources such as protein. It was thus proposed that ectomycorrhizal fungi potentially circumvent the standard N cycle involving N mineralization by saprotrophic microorganisms. However, in many forest ecosystems the majority of the proteins in the forest floor form complexes with polyphenols. Direct access of N by ectomycorrhizal fungi from a polyphenol-protein complex may be limited. Ectomycorrhizal fungi may depend on saprotrophic microorganisms to liberate organic N sources from polyphenol complexes. Thus, interactions between saprotrophic microorganisms and ectomycorrhizal fungi are likely to be essential in the cycling of N within temperate forest ecosystems.     

An ecosystem-scale radiocarbon tracer to test use of litter carbon by ectomycorrhizal fungi

The degree to which ectomycorrhizal fungi rely on decomposing litter as a carbon source in natural ecosystems is unknown. We used a radiocarbon (¹⁴C) tracer to test for uptake of litter carbon by ectomycorrhizal fungi as part of the Enriched Background Isotope Study (EBIS) in Oak Ridge Reservation, Tennessee. In EBIS, leaf litter from a highly ¹⁴C-labeled Quercus alba (white oak) forest was reciprocally transplanted with litter from a nearby low-labeled forest that had not been as strongly exposed to ¹⁴C. These litter transplants were conducted yearly. We measured Δ¹⁴C signatures of ectomycorrhizal fungi collected from each forest four months and 2.25 years after the first litter transplant. The ectomycorrhizas were associated with white oak trees. We found no significant differences in ¹⁴C signatures of ectomycorrhizal fungi exposed to low-labeled versus high-labeled litter, indicating that less than 2% of the carbon in ectomycorrhizal biomass originated from transplanted litter. In contrast, ectomycorrhizal Δ¹⁴C signatures from the high-labeled forest were 117-140‰ higher than those from the low-labeled forest. This pattern suggests that ectomycorrhizal fungi acquired most (or all) of their carbon from their host plants, probably via direct transfer of photosynthate through the roots.     

Suppression of other soil microorganisms by mycelium of arbuscular mycorrhizal fungi in root-free soil

The influence of mycelium of two arbuscular mycorrhizal (AM) fungi, Glomus intraradices and Glomus mosseae, on other soil microorganisms, was examined in root-free soil with and without organic substrate amendment in terms of cellulose. The AM fungi were grown in symbiosis with cucumber in a compartmented growth system, which allowed AM fungal external mycelium to grow into root-free compartments. The fungicide Benomyl was applied to the root-free compartments to create an alternative non-mycorrhizal control treatment. Whole cell biomarker fatty acids were employed to quantify different groups of soil microorganisms including the two AM fungi. Abundance of most microbial groups were reduced by external mycelium of both AM fungi, though differential effects on the microbial community composition were observed between the two AM fungi as revealed from principal component analysis. Inhibition of other soil microorganisms was more pronounced in root-free soil with mycelium of G. mosseae than with mycelium of G. intraradices. In general, cellulose increased the amount of biomarker fatty acids of most groups of soil microorganisms, but cellulose did not affect the influence of AM fungi on other soil microorganisms. Benomyl suppressed growth of the external mycelium of the two AM fungi and had limited non-target effects on other microbial groups. In conclusion, our results show differential effects of external mycelium of AM fungi on other soil microbial communities, though both AM fungi included in the study overall inhibited most microbial groups as examined using whole cell biomarker fatty acids.     

Influence of repeated prescribed burning on incorporation of C from cellulose by forest soil fungi as determined by RNA stable isotope probing

Repeated prescribed burning is frequently used as a forest management tool and can influence soil microbial diversity and activity. Soil fungi play key roles in carbon and nutrient cycling processes and soil fungal community structure has been shown to alter with increasing burning frequency. Such changes are accompanied by changes to soil carbon and nitrogen pools, however, we know little regarding how repeated prescribed burning alters functional diversity in soil fungal communities. We amended soil with ¹³C-cellulose and used RNA stable isotope probing to investigate the effect of biennial repeated prescribed burning over a 34-year period on cellulolytic soil fungi. Results indicated that repeated burning altered fungal community structure. Moreover, fungal community structure and diversity in ¹²C and ¹³C fractions from the unburned soil were not significantly different from each other, while those from the biennial burned soils differed from each other. The data indicate that fewer active fungi in the biennially burned soil incorporated ¹³C from the labelled cellulose and that repeated prescribed burning had a significant impact on the diversity of an important functional group of soil fungi (cellulolytic fungi) that are key drivers of forest soil decomposition and carbon cycling processes.     

Plant species influence microbial diversity and carbon allocation in the rhizosphere

Plant species effects on microbial communities are attributed to changes in microbial community composition and biomass, and may depend on plant species specific differences in the quality of resources (carbon) inputs. We examined the idea that plant-soil feedbacks can be explained by a chance effect, which is the probability of a highly productive or keystone plant species is present in the community and will influence the functions more than the number of species per se. A ¹³C pulse labelling technique was applied to three plant species and a species mixture in a greenhouse experiment to examine the carbon flow from plants to soil microbial communities. The ¹³C label was given as CO₂ to shoots of a legume (Lotus corniculatus), a forb (Plantago lanceolata), a grass (Holcus lanatus) and a mixture of the three species. Microbial phospholipid fatty acids (PLFA) was analysed in order to determine the biomass and composition of the soil microbial community. The incorporation of the stable isotope into soil microorganisms was determined through GC-IRMS analyses of the microbial PLFAs. Plant species identity did not influence the microbial biomass when determined as total carbon of microbial phospholipid fatty acids. However, the labelled carbon showed that the grass monoculture (H. lanatus) and the plant mixture allocated more ¹³C into bacteria and actinomycete biomass than the other plant species. H. lanatus monocultures had also the highest amounts of ¹³C allocated to AM-fungi and saprophytic fungi. The carbon allocation from plants to soil microorganisms in a plant species mixture can thus be explained by the presence of a highly productive species that influence soil functions.     

Variable use of plant- and soil-derived carbon by microorganisms in agricultural soils

In this study we used compound specific ¹³C and ¹⁴C isotopic signatures to determine the degree to which recent plant material and older soil organic matter (SOM) served as carbon substrates for microorganisms in soils. We determined the degree to which plant-derived carbon was used as a substrate by comparison of the ¹³C content of microbial phospholipid fatty acids (PLFA) from soils of two sites that had undergone a vegetation change from C3 to C4 plants in the past 20-30 years. The importance of much older SOM as a substrate was determined by comparison of the radiocarbon content of PLFA from soils of two sites that had different ¹⁴C concentrations of SOM.The ¹³C shift in PLFA from the two sites that had experienced different vegetation history indicated that 40-90% of the PLFA carbon had been fixed since the vegetation change took place. Thus PLFA were more enriched in ¹³C from the new C4 vegetation than it was observed for bulk SOM indicating recent plant material as preferentially used substrate for soil microorganisms. The largest ¹³C shift of PLFA was observed in the soil that had high ¹⁴C concentrations of bulk SOM. These results reinforce that organic carbon in this soil for the most part cycles rapidly. The degree to which SOM is incorporated into microbial PLFA was determined by the difference in ¹⁴C concentration of PLFA derived from two soils one with high ¹⁴C concentrations of bulk SOM and one with low. These results showed that 0-40% of SOM carbon is used as substrate for soil microorganisms. Furthermore a different substrate usage was identified for different microorganisms. Gram-negative bacteria were found to prefer recent plant material as microbial carbon source while Gram-positive bacteria use substantial amounts of SOM carbon. This was indicated by ¹³C as well as ¹⁴C signatures of their PLFA. Our results find evidence to support ‘priming’ in that PLFA indicative of Gram-negative bacteria associated with roots contain both plant- and SOM-derived C. Most interestingly, we find PLFA indicative of archeobacteria (methanothrophs) that may indicate the use of other carbon sources than plant material and SOM to a substantial amount suggesting that inert or slow carbon pools are not essential to explain carbon dynamics in soil.     

Isotopic fractionation by saprotrophic microfungi: Effects of species, temperature and the age of colonies

Saprotrophic fungi represent an important resource for a number of fungivorous and omnivorous soil animals, but little is known about the patterns of isotopic fractionation by soil fungi. We grew five common species of saprotrophic microfungi in laboratory cultures on simple artificial substrate based on carbohydrates derived either from C3 or C4 plants. Fungal cultures were kept at 15, 20 or 25 °C. Isotopic composition of carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) in bulk fungal tissue was determined after 11, 21 and 32 days. The fractionation of carbon and nitrogen stable isotopes was species-specific, but generally did not differ in C3- and C4-based growth media. The Zygomycete Mucor plumbeus did not differ in δ¹³C from the carbon source used, though Ascomycetes (Alternaria alternata, Cladosporium cladosporioides, Trichoderma harzianum and Ulocladium botrytis) were depleted in heavy carbon relative to the carbon source by 0.5-0.9‰. Three species were significantly depleted in ¹⁵N relative to the sodium nitrate that was used as a single source of nitrogen. In all species, δ¹⁵N but not δ¹³C tended to increase with the age of fungal colonies. The effect of temperature on δ¹⁵N was weak and inconsistent in different species. In contrast, all fungi except T. harzianum accumulated more ¹³С at 25 °C than at 15 °C. The overall variation in the isotopic signatures of saprotrophic fungi growing in identical conditions reached 8‰ for δ¹⁵N and 2.5‰ for δ¹³C due to species-specific differences in the isotopic fractionation and the age of individual fungal colonies. This variation should be incorporated into the interpretation of the isotopic composition of fungivorous soil animals.     

Carbon routes from decomposing plant residues and living roots into soil food webs assessed with 13C labelling

This field experiment investigated how C from fresh organic amendments and from a growing leek crop was allocated into different soil microbial and faunal groups in an arable field. A ¹³C-enriched red clover green manure was incorporated in one treatment, while the growing leek crop was pulse labelled with ¹³CO₂ in another. Incorporation of ¹³C into microbial fatty acids, micro- and macroarthropods, enchytraeids and earthworms was determined on several occasions during the growing season in order to determine whether different groups or species of microorganisms and fauna were specialised on either the decomposing green manure material or root-derived C. Compound-specific stable isotope ratio analysis showed fatty acid markers of actinomycetes and Gram-positive bacteria to be more strongly linked to C originating from the decomposing green manure material, whereas the marker for arbuscular mycorrhizal fungi was more linked to C from the growing leek crop. In contrast, several markers for Gram-negative bacteria were the most ¹³C-enriched and had incorporated more ¹³C than the other phospholipid fatty acids in both treatments, indicating a general dominance irrespective of C source. Most soil fauna seemed to derive their C directly or indirectly from the decomposing plant material, while C from the growing crop appeared to be of secondary importance in this agroecosystem.     

Spatial distribution of phosphatase activity associated with ectomycorrhizal plants is related to soil type

Several ectomycorrhizal fungi, including Hebeloma cylindrosporum, actively release large quantities of phosphatase enzymes into their growth medium. We fractionated the phosphatase activity of the ectomycorrhizal association between H. cylindrosporum and its host plant, Pinus pinaster, with the aim to quantify its spatial and temporal variation in response to contrasting soil phosphorus conditions. Seedlings were grown in mini-rhizoboxes and the phosphomonoesterase activity of rhizosphere soil, released by roots, surface-bound to roots or mycelium was determined spectrophotometrically with the p-nitrophenyl phosphate method or microscopically with the ELF-method as a function of culture time. We showed that acid phosphatase activity of the soil and the root increased with mycorrhizal association. We also observed that the phosphatase activity associated with ectomycorrhizal plants was related to soil type. All phosphatase fractions decreased over culture time, except the proportion of hyphae exhibiting phosphatase activity in the extramatrical mycelium, which increased over time. The specific fractions of phosphatase activity associated with the mycorrhizal plants were clearly related to the soil phosphorus type and content. Soils showed an increase in acid phosphomonoesterase activity with mycorrhizal association, supporting a role for this enzyme in the degradation of soil bound phosphorus. The gradually increasing proportion of hyphae in the extramatrical mycelium exhibiting alkaline phosphatase activity, particularly under low phosphorus conditions, indicates an induction of alkaline phosphatase activity by phosphorus limitation.     

Effects of inoculation with ectomycorrhizal fungi on microbial biomass and bacterial functional diversity in the rhizosphere of Pinus tabulaeformis seedlings

This study investigated the effects of inoculation with three individual ectomycorrhizal (ECM) fungal species on soil microbial biomass carbon and indigenous bacterial community functional diversity in the rhizosphere of Chinese pine (Pinus tabulaeformis Carr.) seedlings under field experimental conditions. The results showed that ECM fungal inoculation significantly increased the ectomycorrhizal colonization compared with non-inoculated seedlings. ECM fungal inoculations have higher soil microbial biomass carbon than that of control, ranging from 49.6 μg C g^?1 dry soil in control to 134.02 μg C g^?1 dry soil in treatment inoculated with Boletus luridus Schaeff ex Fr. Multivariate analyses (PCA) of BIOLOG data revealed that the application of ECM fungi significantly influenced bacterial functional diversity in the rhizosphere of P. tabulaeformis seedlings. The highest average well-color development (AWCD) and functional diversity indices were also observed in treatment inoculated with B. luridus. A wider range of sole carbon sources were utilized by the bacterial community in the rhizosphere of inoculated seedlings. The data gathered from this study provides important information for utilization of ECM fungi in forest restoration project in the Northwestern China. The present study will also significantly broaden our understanding of practical importance in the application of ECM fungal inoculum to promote soil microbial community diversity of soil.     

Interactions between bacteria and ectomycorrhizal fungi

Interactions between eight ectomycorrhizal fungi and eight bacteria were tested on five laboratory media and in the rhizoplane of Pinus radiata. Depression of growth of the fungi by the bacteria in laboratory media was dependent on the medium and bore little relation to effects in the rhizoplane. In the rhizoplane, different bacteria could depress, have no effect or even stimulate growth of mycorrhizal fungi. Competition and antagonism are suggested as mechanisms for depression of the fungi. Some bacteria gave protection against the depressive effects of other bacteria. Considerable differences occurred between ectomycorrhizal fungi in their colonization of the rhizoplane in the absence of bacteria and also in their presence. The common mycorrhizal fungi Rhizopogon luteolus and Thelephora terrestris generally colonized roots well but the strain of Pisolithus tinctorius studied colonized poorly. Direct microscopy showed the percentage cover of the root by microorganisms was usually only 10–20%.It is proposed that interactions of ectomycorrhizal fungi with soil organisms are important in determining the successful introduction and persistence of inoculated ectomycorrhizal fungi. Fungi should be selected for compatibility with a wide range of soil microflora as well as efficiency in plant stimulation.     

Microbial response to exudates in the rhizosphere of young beech trees (Fagus sylvatica L.) after dormancy

Plants act as an important link between atmosphere and soil: CO2 is transformed into carbohydrates by photosynthesis. These assimilates are distributed within the plant and translocated via roots into the rhizosphere and soil microorganisms. In this study, 3 year old European beech trees (Fagus sylvatica L.) were exposed after the chilling period to an enriched ¹³C–CO₂ atmosphere (δ¹³C = 60‰ – 80‰) at the time point when leaves development started. Temporal dynamics of assimilated carbon distribution in different plant parts, as well as into dissolved organic carbon and microbial communities in the rhizosphere and bulk soil have been investigated for a 20 days period. Photosynthetically fixed carbon could be traced into plant tissue, dissolved organic carbon and total microbial biomass, where it was utilized by different microbial communities. Due to carbon allocation into the rhizosphere, nutrient stress decreased; exudates were preferentially used by Gram-negative bacteria and (mycorrhizal) fungi, resulting in an enhanced growth. Other microorganisms, like Gram-positive bacteria and mainly micro eucaryotes benefited from the exudates via food web development. Overall our results indicate a fast turnover of exudates and the development of initial food web structures. Additionally a transport of assimilated carbon into bulk soil by (mycrorhizal) fungi was observed.     

Compartmentalization of the soil animal food web as indicated by dual analysis of stable isotope ratios (N/N and C/C)

The soil animal food web has become a focus of recent ecological research but trophic relationships still remain enigmatic for many taxa. Analysis of stable isotope ratios of N and C provides a powerful tool for disentangling food web structure. In this study, animals, roots, soil and litter material from a temperate deciduous forest were analysed. The combined measurement of δ¹⁵N and δ¹³C provided insights into the compartmentalization of the soil animal food web. Leaf litter feeders were separated from animals relying mainly on recent belowground carbon resources and from animals feeding on older carbon. The trophic pathway of leaf litter-feeding species appears to be a dead end, presumably because leaf litter feeders (mainly diplopods and oribatid mites) are unavailable to predators due to large size and/or strong sclerotization. Endogeic earthworms that rely on older carbon also appear to exist in predator-free space. The data suggest that the largest trophic compartment constitutes of ectomycorrhizal feeders and their predators. Additionally, there is a smaller trophic compartment consisting of predators likely feeding on enchytraeids and potentially nematodes.     

Roots from beech (Fagus sylvatica L.) and ash (Fraxinus excelsior L.) differentially affect soil microorganisms and carbon dynamics

Knowledge about the influence of living roots on decomposition processes in soil is scarce but is needed to understand carbon dynamics in soil. We investigated the effect of dominant deciduous tree species of the Central European forest vegetation, European beech (Fagus sylvatica L.) and European ash (Fraxinus excelsior L.), on soil biota and carbon dynamics differentiating between root- and leaf litter-mediated effects. The influence of beech and ash seedlings on carbon and nitrogen flow was investigated using leaf litter enriched in ¹³C and ¹⁵N in double split-root rhizotrons planted with beech and ash seedlings as well as a mixture of both tree species and a control without plants. Stable isotope and compound-specific fatty acid analysis (¹³C-PLFA) were used to follow the incorporation of stable isotopes into microorganisms, soil animals and plants. Further, the bacterial community composition was analyzed using pyrosequencing of 16S rRNA gene amplicons. Although beech root biomass was significantly lower than that of ash only beech significantly decreased soil carbon and nitrogen concentrations after 475 days of incubation. In addition, beech significantly decreased microbial carbon use efficiency as indicated by higher specific respiration. Low soil pH probably increased specific respiration of bacteria suggesting that rhizodeposits of beech roots induced increased microbial respiration and therefore carbon loss from soil. Compared to beech δ¹³C and δ¹⁵N signatures of gamasid mites in ash rhizotrons were significantly higher indicating higher amounts of litter-derived carbon and nitrogen to reach higher trophic levels. Similar δ¹³C signatures of bacteria and fine roots indicate that mainly bacteria incorporated root-derived carbon in beech rhizotrons. The results suggest that beech and ash differentially impact soil processes with beech more strongly affecting the belowground system via root exudates and associated changes in rhizosphere microorganisms and carbon dynamics than ash.     

Invertebrate grazing affects nitrogen partitioning in the saprotrophic fungus Phanerochaete velutina

The heterogeneity of nutrients in forest soils is governed by many biotic and abiotic factors. The significance of nutrient patchiness in determining soil processes remains poorly understood. Some saprotrophic basidiomycete fungi influence nutrient heterogeneity by forming large mycelial networks that enable translocation of nutrients between colonized patches of dead organic matter. The effect of mycophagous soil fauna on these networks and subsequent nutrient redistribution has, however, been little studied. We used a soil microcosm system to investigate the potential effects of a mycophagous collembola, Protaphorura armata, on nutrient transfer within, and nutrient loss from, the mycelium of a saprotrophic basidiomycete fungus, Phanerochaete velutina. A ¹⁵N label, added to central mycelium, was used to track nitrogen movement within the microcosms across 32 days. Although collembola grazing had little impact on δ¹⁵N values, it did alter the partitioning of ¹⁵N between different regions of mycelia. Less ¹⁵N was transferred to new mycelial growth in grazed systems than in ungrazed systems, presumably because collembola reduced fungal growth rate and altered mycelial morphology. Surprisingly, collembola grazing did not increase the mineralization of N from mycelium into the bulk soil. Overall, our results suggest that mycophagous soil fauna can alter nutrient flux and partitioning within fungal mycelium; this has the potential to affect the dynamics and spatial heterogeneity of forest floor nutrients.     

Rice rhizodeposition and its utilization by microbial groups depends on N fertilization

Rhizodeposits have received considerable attention, as they play an important role in the regulation of soil carbon (C) sequestration and global C cycling and represent an important C and energy source for soil microorganisms. However, the utilization of rhizodeposits by microbial groups, their role in the turnover of soil organic matter (SOM) pools in rice paddies, and the effects of nitrogen (N) fertilization on rhizodeposition are nearly unknown. Rice (Oryza sativa L.) plants were grown in soil at five N fertilization rates (0, 10, 20, 40, or 60 mg N kg^?1 soil) and continuously labeled in a ¹³CO₂ atmosphere for 18 days during tillering. The utilization of root-derived C by microbial groups was assessed by ¹³C incorporation into phospholipid fatty acids. Rice shoot and root biomass strongly increased with N fertilization. Rhizodeposition increased with N fertilization, whereas the total ¹³C incorporation into microorganisms, as indicated by the percentage of ¹³C recovered in microbial biomass, decreased. The contribution of root-derived ¹³C to SOM formation increased with root biomass. The ratio of ¹³C in soil pools (SOM and microbial biomass) to ¹³C in roots decreased with N fertilization showing less incorporation and faster turnover with N. The ¹³C incorporation into fungi (18:2ω6,9c and 18:1ω9c), arbuscular mycorrhizal fungi (16:1ω5c), and actinomycetes (10Me 16:0 and 10Me 18:0) increased with N fertilization, whereas the ¹³C incorporation into gram-positive (i14:0, i15:0, a15:0, i16:0, i17:0, and a17:0) and gram-negative (16:1ω7c, 18:1ω7c, cy17:0, and cy19:0) bacteria decreased with N fertilization. Thus, the uptake and microbial processing of root-derived C was affected by N availability in soil. Compared with the unfertilized soil, the contribution of rhizodeposits to SOM and microorganisms increased at low to intermediate N fertilization rates but decreased at the maximum N input. We conclude that belowground C allocation and rhizodeposition by rice, microbial utilization of rhizodeposited C, and its stabilization within SOM pools are strongly affected by N availability: N fertilization adequate to the plant demand increases C incorporation in all these polls, but excessive N fertilization has negative effects not only on environmental pollution but also on C sequestration in soil.     

Fungi mediate long term sequestration of carbon and nitrogen in soil through their priming effect

It is increasingly recognized that soil microbes have the ability to decompose old recalcitrant soil organic matter (SOM) by using fresh carbon as a source of energy, a phenomena called priming effect (PE). However, efforts to determine the consequences of this PE for soil carbon and nitrogen dynamics are in their early stage. Moreover, little is known about the microbial populations involved. Here we explore the consequences of PE for SOM dynamics and mineral nitrogen availability in a soil incubation experiment (161 days), combining the supply of dual-labeled (¹³C and ¹⁴C) cellulose and mineral nutrients. The microbial groups involved in PE were investigated using molecular fingerprinting techniques (FAMEs and B- and F-ARISA). We show that mean residence time of SOM pool controlled by the PE decreased from 3130 years in the subsoil, where the availability of fresh carbon is very low, to 17-39 years in the surface layer. This result suggests that the decomposition of this recalcitrant soil C pool is strictly dependent on the presence of fresh C and is not an energetically viable mean of accessing C for soil microbes. We also suggest that fungi are the predominant actors of cellulose decomposition and induced PE and they adjust their degradation activity to nutrient availability. The predominant role of fungi can be explained by their ability to grow as mycelium which allows them to explore soil space and mine large reserve of SOM. Finally, our results support the existence of a bank mechanism that regulates nutrient and carbon sequestration in soil: PE is low when nutrient availability is high, allowing sequestration of nutrients and carbon; in contrast, microbes release nutrients from SOM when nutrient availability is low. This bank mechanism may help to synchronize the availability of soluble nutrients to plant requirement and contribute to long-term SOM accumulation in ecosystems.     

Alleviation of P limitation makes tree roots competitive for N against microbes in a N-saturated conifer forest: A test through P fertilization and 15N labelling

Chronic N deposition to forests may induce N saturation and stand decline, leading to reduced ecosystem N retention capacity, triggered by a shift from N limitation of trees to limitation by another nutrient. We conducted a ¹⁵N soil labelling experiment in non-fertilized and P-fertilized plots at two elevations in an N-saturated Mediterranean-fir (Abies pinsapo) forest in southern Spain which shows P limitation symptoms. Root-exclusion was applied to identify the relative contributions of roots (plus mycorrhizal fungi) uptake, and heterotrophic immobilization by free-living microbes, to N retention. Overall ¹⁵N recovery from the litter, 0–15-cm soil and root-uptake components was c.a. 35% higher in P-fertilized than in non-fertilized plots at both elevations. In non-fertilized plots, soil was the biggest sink for added ¹⁵N. Phosphorus fertilization increased the competitive ability of tree roots for soil N resulting in equal importance of the autotrophic (roots plus associated mycorhizal fungi) and heterotrophic (free-living microbes) components with respect to total ¹⁵N recovery in P-fertilized plots. Phosphorus addition increased litter and soil N immobilization only if roots had been excluded. By combining in situ fertilization, root-exclusion and isotope labelling we have demonstrated that reduced N retention capacity and dominance of soil microbial over plant immobilization in a N-saturated forest results from a shift from N to P limitation of trees, while alleviation of P limitation makes tree roots and associated mycorrhizal fungi competitive for N against free soil microorganisms.