Characterisation of cold-tolerant fungi from a decomposing High Arctic moss

Given that changes in the patterns of vegetation and size of carbon (C) pools in the Arctic are likely to be profound by the end of this century, it is necessary to characterise the identity and ecological groupings, in terms of temperature response and C substrate utilisation, of saprotrophic (decomposer) fungi in organic matter in Arctic soils. Thus, the aims of this study were: (1) to identify the fungi isolated from standing-dead material of Schistidium apocarpum, as an example of a High Arctic moss, (2) to determine mycelial extension rates of these fungi at a range of temperatures (4, 10 and 25 °C), and (3) to characterise the functional potential, defined by C substrate utilisation at 6 °C, of fungal taxa.Fungi were isolated at 4 °C from standing-dead material of S. apocarpum from an area of polar semi-desert (79 degrees N), close to Ny-Ålesund, Svalbard, in the High Arctic. From a collection of 662 isolates, 43 pure cultures were identified by DNA extraction, amplification, and sequencing. Phoma sclerotioides, previously known as a temperate snow mould, was isolated most frequently. The ecology of fifteen fungal isolates was characterised in detail. In terms of temperature response, two groups were apparent, one of truly psychrophilic/psychrotrophic fungi and one of more mesophilic fungi which were generally less frequently isolated. In terms of carbon substrate utilisation in semi-defined solid media, most fungi could utilise a variety of carbon substrates (degradation of casein, cellulose and starch was widespread), except for tannic acid (degraded by only two of the five P. sclerotioides isolates and Scytalidium lignicola) and lignin and chitin (not decomposed by any isolate). The majority of isolates had been recorded previously from polar environments and/or as being able to survive at low temperatures.Fungi in tundra ecosystems, therefore, have significant potential to mineralise C at temperatures below 10 °C. A better understanding of the ecology of these fungi will allow us to improve predictions of C dynamics in arctic biomes in the future.     

Invertebrate grazing determines enzyme production by basidiomycete fungi

Extracellular enzymes produced by heterotrophic microorganisms in the soil are responsible for the decomposition of organic compounds. Basidiomycete fungi are the primary decomposer agents in temperate wooded ecosystems and contribute extensively to extracellular enzyme activity and nutrient mineralisation within soils. Growth and development of basidiomycete mycelia is influenced by soil-dwelling invertebrate grazers with potential implications for fungal activity and ecosystem functioning. The impacts of four invertebrate taxa belonging to Isopoda, Myriapoda, Collembola and Nematoda on the production of eight hydrolytic enzymes by four saprotrophic basidiomycetes (Phanerochaete velutina, Resinicium bicolor and two strains of Hypholoma fasciculare) were compared in a factorial microcosm study. Grazing generally increased enzyme production but invertebrates had species-specific impacts on enzyme activity. The magnitude of grazing influenced enzyme activity; macrofauna (woodlice and millipedes) induced the greatest responses. Enzymatic responses varied markedly between fungi. Grazing enhanced enzyme activity in the exploitative mycelial networks of P. velutina and H. fasciculare, while the opposite effects were observed in the explorative R. bicolor networks. The impacts of soil fauna on nutrient mineralisation depend on fungal community composition. β-glucosidase, cellobiohydrolase, N-acetylglucosaminidase, acid phosphatase and phosphodiesterase activities were affected most frequently by grazing and invertebrate activity, and thus had direct consequences for carbon, nitrogen and phosphorous cycling. The results indicate that invertebrate diversity and community composition may influence the spatial distribution and activity of extracellular enzymes with direct implications for nutrient mineralisation and trunover in woodland soils.     

Relationship between soil enzyme activities, nutrient cycling and soil fungal communities in a northern hardwood forest

Soil fungi are highly diverse and act as the primary agents of nutrient cycling in forests. These fungal communities are often dominated by mycorrhizal fungi that form mutually beneficial relationships with plant roots and some mycorrhizal fungi produce extracellular and cell-bound enzymes that catalyze the hydrolysis of nitrogen (N)- and phosphorus (P)- containing compounds in soil organic matter. Here we investigated whether the community structure of different types of mycorrhizal fungi (arbuscular and ectomycorrhizal fungi) is correlated with soil chemistry and enzyme activity in a northern hardwood forest and whether these correlations change over the growing season. We quantified these relationships in an experimental paired plot study where white-tailed deer (access or excluded 4.5 yrs) treatment was crossed with garlic mustard (presence or removal 1 yr). We collected soil samples early and late in the growing season and analyzed them for soil chemistry, extracellular enzyme activity and molecular analysis of both arbuscular mycorrhizal (AM) and ectomycorrhizal/saprotrophic fungal communities using terminal restriction fragment length polymorphism (TRFLP). AM fungal communities did not change seasonally but were positively correlated with the activities of urease and leucine aminopeptidase (LAP), enzymes involved in N cycling. The density of garlic mustard was correlated with the presence of specific AM fungal species, while deer exclusion or access had no effect on either fungal community after 4.5 yrs. Ectomycorrhizal/saprotrophic fungal communities changed seasonally and were positively correlated with most soil enzymes, including enzymes involved in carbon (C), N and P cycling, but only during late summer sampling. Our results suggest that fine scale temporal and spatial changes in soil fungal communities may affect soil nutrient and carbon cycling. Although AM fungi are not generally considered capable of producing extracellular enzymes, the correlation between some AM taxa and the activity of N acquisition enzymes suggests that these fungi may play a role in forest understory N cycling.     

Invertebrates increase the sensitivity of non-labile soil carbon to climate change

The fate of global soil carbon stores in response to predicted climate change is a ‘hotly’ debated topic. Considerable uncertainties remain as to the temperature sensitivity of non-labile soil organic matter (SOM) to decomposition. Currently, models assume that organic matter decomposition is solely controlled by the interaction between climatic conditions and soil mineral characteristics. Consequently, little attention has been paid to adaptive responses of soil decomposer organisms to climate change and their impacts on the turnover of long-standing terrestrial carbon reservoirs. Using a radiocarbon approach we found that warming increased soil invertebrate populations (Enchytraeid worms) leading to a greater turnover of older soil carbon pools. The implication of this finding is that until soil physiology and biology are meaningfully represented in ecosystem carbon models, predictions will underestimate soil carbon turnover.     

Interactions between tree roots,mycorrhizas, a saprotrophic fungus and the decomposition of organic substrates in a microcosm

Summary A microcosm technique has been used to evaluate the effects of tree seedling root systems, their mycorrhizas and a saprotrophic basidiomycete fungus and their interactions in the decomposition of organic substrates. The component elements were added to the experimental system in a factorially designed experiment of increasing complexity. Roots and mycorrhizas significantly enhance the rate of decomposition of the substrates. The mycorrhizal fungus Suillus luteus was the most active decomposer of all substrates. This root-induced enhancement of decomposition was suppressed in the presence of the saprotroph, Mycena galopus. Plant growth was influenced by the substrate, in that the nitrogen-containing substrates, hide powder and chitin, promoted greater growth than the pure carbohydrate, cotton. Presence of the saprotroph, Mycena galopus significantly enhanced tree growth. The data are discussed in relation to previous studies on the influence of roots and their mycorrhizas on decomposition.     

Saprotrophic fungi transform organic phosphorus from spruce needle litter

Fungal decomposition of and phosphorus transformation from spruce litter needles (Picea abies) were simulated in systems containing litter needles inoculated with individual saprotrophic fungal strains and their mixtures. Fungal strains of Setulipes androsaceus (L.) Antonín, Chalara longipes (Preus) Cooke, Ceuthospora pinastri (Fr.) Höhn., Mollisia minutella (Sacc.) Rehm, Scleroconidioma sphagnicola Tsuneda, Currah & Thormann and an unknown strain NK11 were used as representatives of autochthonous mycoflora. Systems were incubated for 5.5 months in laboratory conditions. Fungal colonization in systems and competition among strains were assessed using the reisolation of fungi from individual needles. After incubation, needles were extracted with NaOH and extracts were analysed using ³¹P nuclear magnetic resonance spectroscopy (NMR). Needle decomposition was determined based on the decrease in C:N ratio. Systems inoculated with the basidiomycete S. androsaceus revealed substantial decrease in C:N ratio (from 25.8 to 11.3) while the effect of ascomycetes on the C:N ratio was negligible. We suppose that tested strains of saprotrophic ascomycetes did not participate substantially in litter decomposition, but were directly involved in phosphorus transformation and together with S. androsaceus could transform orthophosphate monoesters and diesters from spruce litter needles into diphosphates, polyphosphates and phosphonates. These transformations seem to be typical for saprotrophic fungi involved in litter needle decomposition, although the proportion of individual phosphorus forms differed among studied fungal strains. Phosphonate presence in needles after fungal inoculation is of special interest because no previous investigation recorded phosphonate synthesis and accumulation by fungi. Our results confirmed that the ³¹P NMR spectroscopy is an excellent instrumental method for studying transformations of soil organic phosphorus during plant litter decomposition. We suggest that polyphosphate production by S. androsaceus may contribute to the phosphorus cycle in forest ecosystems because this fungus is a frequent litter colonizer that substantially participates in decomposition.     

Differential response of ectomycorrhizal and saprotrophic fungal mycelium from coniferous forest soils to selected monoterpenes

The mycelia of saprotrophic (SP) and ectomycorrhizal (ECM) fungi occur throughout the upper soil horizons in coniferous forests and could therefore be exposed to high concentrations of monoterpenes occurring in the needle litter of some tree species.Monoterpenes are mycotoxic and could potentially affect fungi that are exposed to them in the litter layers. In order to investigate whether monoterpenes typical of coniferous litters could influence fungal communities, we analysed the monoterpene content of freshly fallen needles of Pinus sylvestris, Picea abies and Picea sitchensis. The most abundant monoterpenes were found to be α-pinene, β-pinene and 3-carene. We evaluated the effects of these three monoterpene vapours on the biomass production of 23 SP isolates and 16 ECM isolates. Overall, 75% of ECM isolates and 26% of SP isolates were significantly inhibited by at least one of the monoterpene treatments and both intra- and inter-specific variations in response were observed.Monoterpene concentrations are highest in surface litters. The differential effects on fungal taxa may influence the spatial and temporal distribution of fungal community composition, indirectly affecting decomposition and nutrient cycling, the fundamental ecosystem processes in which fungi have a key role in coniferous forest soils.     

Size matters: What have we learnt from microcosm studies of decomposer fungus–invertebrate interactions?

The ongoing research ‘boom’ in soil ecology has been advanced by a widespread use of laboratory experiments to investigate mechanisms that could not be unravelled with field observations alone. Interactions between soil fungi and invertebrates have received considerable attention due to their trophic and functional importance in belowground systems. Saprotrophic cord-forming basidiomycete fungi are major agents of primary decomposition in woodland ecosystems, where they are also an important source of nutrition for fungal-feeding soil invertebrates. A plethora of microcosm experiments, with their main benefit being that they enable most variables to be kept constant while just a few are manipulated, have provided detailed insights into the ecology of fungus–invertebrate interactions. This review identifies important trends from this body of work (including a meta-analysis of grazing effects on fungal growth and wood decomposition) and explores the extent to which these patterns are supported by the few related experiments conducted in more complex mesocosm and field systems. Grazing in microcosms reduced fungal growth and increased decomposition, but with interaction-specific magnitude, reflecting invertebrate feeding preferences for different fungi. Macro-invertebrates (woodlice and millipedes) had stronger effects than micro- (e.g. nematodes) and meso- (e.g. collembola) invertebrates. This greater grazing pressure generally increased enzyme activities beneath mycelia during interactions in which wood decay was increased. Top-down effects of fungal-feeding can be extrapolated to more complex systems, but only for macro-invertebrates, particularly woodlice. Soil enzyme activity was stimulated, in microcosms and more complex systems, by short-term or low intensity grazing, but reduced when large areas of mycelium were removed by high-intensity grazing. Effects of differential fungal palatability on invertebrate populations are evident in microcosm studies of collembola. These bottom-up effects can be extrapolated more broadly than top-down effects; fungal community dominance determined collembola abundance and diversity, in mesocosms, and woodlouse abundance in the field. Using, as a case study, a series of experiments conducted at a range of scales, mechanisms underlying potential climate change effects on grazing interactions and decomposition are also explored. Biotic effects on decomposer community functioning are heterogeneous, depending on fungal dominance and the density of key macro-invertebrate taxa.     

Microbial growth rate measurements reveal that land-use abandonment promotes a fungal dominance of SOM decomposition in grazed Mediterranean ecosystems

The present study investigated the effects of land-use abandonment on the soil decomposer community of two grazed Mediterranean ecosystems (an annual grassland with scattered holm oaks and a low-density shrubland). To test the influence of grazing abandonment, a set of plots within each site were fenced and kept undisturbed during 4–5 years, during which above-ground plant community structure was monitored. After that, soil samples were collected from grazed and abandoned plots corresponding to the three different soil conditions: away from (“grass”) and below tree canopies (“oak”) within the annual grassland, and from the shrubland (“shrub”). Soil samples were split into two different layers (0–5 and 5–15 cm) and then analyzed for saprotrophic fungal (acetate into ergosterol incorporation) and bacterial (leucine incorporation) growth rates. Ergosterol content (as a fungal biomass estimator) and a standard set of soil chemistry variables were also measured. After 5 years of grazing exclusion, saprotrophic fungal growth rate clearly increased in both grass and oak surface layers whereas bacterial growth rate was not altered. This translated into significantly higher fungal-to-bacterial (F/B) growth rate ratios within the ungrazed plots. Similar trends were observed for the shrub soils after 4 years of exclusion. On the contrary, abandonment of grazing had negligible effects on the ergosterol content, as well as on the soil chemical variables (soil organic carbon, total N, C/N ratio, and pH), in all the three soil conditions assessed. These results indicated a shift toward a more fungal-dominated decomposer activity in soils following cessation of grazing and highlighted the sensitivity of the microbial growth rate parameters to changes associated with land use. Moreover, there were evidences of a faster fungal biomass turnover in the ungrazed plots, which would reflect an accelerated, though not bigger, fungal channel in soil organic matter mineralization.     

Spatial distribution of fungal communities in a coastal grassland soil

In grasslands, saprotrophic fungi, including basidiomycetes, are major decomposers of dead organic matter, although spatial distributions of their mycelial assemblages are little described. The aim of this study was to characterise the scale and distribution of saprotrophic fungal communities in a coastal grassland soil using terminal restriction fragment length polymorphism (T-RFLP).Soil fungi were sampled at Point Reyes, California, USA, by taking forty-five 26 mm diam. cores in a spatially defined manner. Within each sampled core, complete core sections at 1-2 cm and 14-15 cm depths were removed and sub-sampled for DNA extraction and amplification using the primer pairs ITS1F-FAM/ITS4 (general fungi) or ITS1F-FAM/ITS4B (basidiomycete-specific).Nonmetric Multidimensional Scaling showed that general fungal communities could be clearly separated by depth, although basidiomycete communities could not. There were no strong patterns of community similarity or dissimilarity for general or basidiomycete fungal communities at horizontal geographical distances from 25 cm to 96 m in the upper horizon. These results show considerable vertical, but little horizontal, variability in fungal community structure in a semi-natural grassland at the spatial scales measured here.     

No ‘home’ versus ‘away’ effects of decomposition found in a grassland-forest reciprocal litter transplant study

Plant litter often decomposes faster in the habitat from which it was derived (i.e. home) than when placed in foreign habitats (i.e. away), which has been called the home-field advantage (HFA) of litter decomposition. We tested whether the HFA of litter decomposition is driven by decomposer communities being specialized at decomposing litter in their home habitat, by reciprocally transplanting litter from grassland to early-successional forest. Unexpectedly, we found an overall disadvantage for at-home decomposition despite large differences in litter quality (lignin:N) between the two habitats. We found more evidence for habitat specialization among secondary decomposers (mites) than the primary decomposers (bacteria and fungi), suggesting that soil animals may be important in driving HFA patterns where they do exist. Grass litter decomposition in forest slowed down and became more fungal-based, while tree litter decomposition in grassland increased yet showed no shift to being bacterially-based, relative to ‘at home’ decomposition. This suggests a biological explanation for why a positive HFA was not observed. Our results highlight that both environmental context and soil biology can play an important and sometimes counter-intuitive role in modifying decomposition. A better understanding of the interaction between all three primary drivers of decomposition (the environment, litter quality and soil organisms) is necessary for reliable prediction of decomposition at global scales.     

Abundance, production and stabilization of microbial biomass under conventional and reduced tillage

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

‘Decomposer’ Basidiomycota in Arctic and Antarctic ecosystems

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

Low importance for a fungal based food web in arable soils under mineral and organic fertilization indicated by Collembola grazers

We investigated the Collembola community at an arable field where mineral and organic fertilizers have been applied at low and high rates for 27 years. As food resources for Collembola, the soil microbial community was analyzed using phospholipid fatty acids (PLFAs). A special focus was put on AM fungi, which were estimated by the marker 16:1ω5 in PLFA (viable hyphae) and neutral lipid fatty acid (NLFA – storage fat in spores) fractions. Additionally, whole cellular lipids in crop plant tissues and manure were assessed. Greater Collembola species richness occurred in plots where mineral fertilizer was added. In contrast, soil microbial biomass including AM fungal hyphae increased with addition of organic fertilizer, while the amount of AM fungal spores and biomass of saprotrophic fungi were not affected by fertilizer type. The lipid pattern in wheat roots was altered by fertilizer type, application rate and their interaction, indicating different rhizosphere communities. In sum, the availability and composition of food resources for Collembola changed considerably due to farm management practice. The major diet of three dominant Collembola species, Isotoma viridis, Willemia anophthalma and Polyacanthella schäffer was determined by lipid profiling. Multivariate analysis demonstrated species specific lipid patterns, suggesting greater importance of species than management practice on the diet choice. Nevertheless, feeding strategy was affected by fertilizer type and availability of resources, as trophic biomarker fatty acids indicated feeding on wheat roots (and to some extent saprotrophic fungi) with mineral and a shift to soil organic matter (litter, detritus) with organic fertilization. Although AM fungi dominated the soil fungal community, the AMF marker 16:1ω5 was not detected in Collembola lipids, indicating that these were not consumed. The very low amount of saprotrophic fungi in the soil and the fact that Collembola as major fungal grazers did not feed on AM fungi indicates that the fungal energy channel in the investigated arable field is of little importance to the faunal food web.     

Do oribatid mites (Acari: Oribatida) show a higher preference for ubiquitous vs. specialized saprotrophic fungi from pine litter?

Previous studies of oribatid mite feeding preferences for different saprotrophic fungi were limited to ubiquitous fungal species, whereas saprophytes specialized to decompose particular substrates have been neglected. We examined the preference of seven oribatid mite species (Adoristes ovatus, Eniochthonius minutissimus, Eueremaeus silvestris, Nothrus silvestris, Oppiella subpectinata, Porobelba spinosa and Spatiodamaeus verticillipes) for nine autochthonous saprotrophic fungi from Scots pine litter (Pinus sylvestris). Among the fungal species offered were specific coniferous litter colonizers (Allantophomopsis lycopodina, Ceuthospora pinastri, Hormonema dematioides, Scleroconidioma sphagnicola, Verticicladium trifidum, Marasmius androsaceus and Sympodiella acicola) and two ubiquitous species (Cladosporium herbarum and Oidiodendron griseum). The fungi were inoculated on fragments of pine needles and offered simultaneously and separately to the mites. Our main hypothesis, that oribatid mites (usually occurring in more than one type of ecosystems) would prefer the ubiquitous fungal species rather than those specific to pine litter, was supported only partly. The ubiquitous C. herbarum was highly preferred by all studied mites, but most of them preferred one or more of the specialized fungi with similar intensity. The basidiomycete M. androsaceus along with sterile needles were consistently avoided by all mites in all experiments. Our results do not support the hypothesis, that the “true” fungivorous oribatid mites in traditional sense are more selective fungal feeders than are the “unspecialized” panphytophagous ones. We observed no gradation in preference of fungi for oribatid mites as a group, but rather a discontinuous and dynamic mosaic with particular mites preferring particular fungal species. This heterogeneous mosaic shapes the feeding niches occupied by particular oribatid mite species and probably reduces competition for food source among numerous species coexisting in a given habitat and time.     

Low levels of nitrogen addition stimulate decomposition by boreal forest fungi

Climate warming and associated increases in nutrient mineralization may increase the availability of soil nitrogen (N) in high latitude ecosystems, such as boreal forests. These changes in N availability could feed back to affect the decomposition of litter and organic matter by soil microbes. Since fungi are important decomposers in boreal forest ecosystems, we conducted a 69-day incubation study to examine N constraints on fungal decomposition of organic substrates common in boreal ecosystems, including cellulose, lignin, spruce wood, spruce needle litter, and moss litter. We added 0, 20, or 200 μg N to vials containing 200 mg substrate in factorial combination with five fungal species isolated from boreal soil, including an Ascomycete, a Zygomycete, and three Basidiomycetes. We hypothesized that N addition would increase CO₂ mineralization from the substrates, particularly those with low N concentrations. In addition we predicted that Basidiomycetes would be more effective decomposers than the other fungi, but would respond weakly or negatively to N additions. In support of the first hypothesis, cumulative CO₂ mineralization increased from 635 ± 117 to 806 + 108 μg C across all fungal species and substrates in response to 20 μg added N; however, there was no significant increase at the highest level of N addition. The positive effect of N addition was only significant on cellulose and wood substrates which contained very little N. We also observed clear differences in the substrate preferences of the fungal species. The Zygomycete mineralized little CO₂ from any of the substrates, while the Basidiomycetes mineralized all of the substrates except spruce needles. However, the Ascomycete (Penicillium) was surprisingly efficient at mineralizing spruce wood and was the only species that substantially mineralized spruce litter. The activities of β-glucosidase and N-acetyl-glucosaminidase were strongly correlated with cumulative respiration (r = 0.78 and 0.74, respectively), and Penicillium was particularly effective at producing these enzymes. On moss litter, the different fungal species produced enzymes that targeted different chemical components. Overall, our results suggest that fungal species specialize on different organic substrates, and only respond to N addition on low N substrates, such as wood. Furthermore, the response to N addition is non-linear, with the greatest substrate mineralization at intermediate N levels.     

Manipulation of the soil pore and microbial community structure in soil mesocosm incubation studies

Soil pore structure exerts a profound influence on distribution of moisture, O₂ and micro-organisms, thereby potentially controlling organic matter (OM) decomposition in soils. Although pore space is the habitat for soil micro-organisms and the actual location of soil biochemical processes, to date, very few studies looked into this relation mainly because of practical constraints. New experimental designs need to be developed which allow specific investigations of the relation between soil pore network structure, the microbial community and OM decomposition. We therefore subjected a sandy loam soil to a number of artificial manipulations namely i) compaction, ii) artificial change in particle size distribution, iii) addition of different substrates and iv) change in soil pH to manipulate soil pore structure and the decomposer community for use in lab incubation set-ups. Moisture retention data showed that compaction and artificial change in particle size distribution decreased volumes of large (9–300 μm) and small (<0.2 and 3–9 μm) pore size classes, respectively. PLFA signature analysis showed that acidification promoted fungi, while an effect of application of either sawdust or grass on the decomposer community was smaller. Acidification significantly reduced C mineralization and microbial biomass C. Surprisingly, the largest shift in microbial community (with promotion of fungi and protozoa relative to bacteria) over all treatments was observed in the treatments with artificially changed particle size distribution. We conclude that it is possible to ‘tailor’ soil pore structure and the decomposer community in soil mesocosm incubation experiments by such manipulations. However, non-targeted effects on microbial community structure, microbial biomass and gross C mineralization seem unavoidable.     

Temporal and functional pattern of secreted enzyme activities in an ectomycorrhizal community

The ectomycorrhizal community of an oak forest has been monitored monthly throughout fifteen months. Eight enzymatic activities secreted by the ectomycorrhizal root tips and involved in the mobilization of nutrients from soil organic matter have been measured using microplate assays, resulting in potential activity patterns of individual fungal species. Both the species structure of the community and the specific activity level of each individual species changed with the season and soil horizon. This versatility may be an adaptative response of the ectomycorrhizal fungal community to a highly variable environment. The results also suggest that some ectomycorrhizal fungi behave as occasional saprobes and contribute to the decomposition of soil organic matter and nutrient cycling together with true saprotrophic fungi.     

Decomposition of lignin in wheat straw in a sand-dune grassland

The degradation of organic macromolecules, including lignin, in plant-derived soil organic matter, is important to the global carbon cycle. In grasslands, saprotrophic (decomposer) fungi are major decomposers of such organic material. The aim of this study was to characterise lignin degradation, particularly with respect to lignin oxidation typical of white-rot basidiomycete fungi. Lignin breakdown products, analysed by gas chromatography–mass spectrometry (GC–MS) with TMAH thermochemolysis, in initial wheat (Triticum aestivum var. Swatham) straw samples were compared with those in samples which had been buried as a “model” resource for 46 months in a sand-dune grassland at Ainsdale National Nature Reserve, Lancashire, UK.Our results showed that lignin oxidation occurred in the straw over the 46 month period, as there were general increases in the [Ac/Al]_S and [Ac/Al]_G ratios and a clear decrease in the [S/G] ratio. These data provide tentative support for the theory that white-rot basidiomycete fungi are involved in the degradation of lignin in grasslands.