Species-specific impacts of collembola grazing on fungal foraging ecology

Fungi are primary agents of organic matter decomposition in forests. Although invertebrate grazing affects fungal biomass and morphology, the species-specific consequences of these interactions are little understood. Using three collembola species (Folsomia candida, Protaphorura armata, Proisotoma minuta) we employed a multi-trophic approach to investigate the individual effects of invertebrate grazing on four species of saprotrophic basidiomycete fungi growing in two species (one fungus: one collembola) soil microcosms. We studied these effects at three trophic levels: the rate of wood decay brought about by the fungi was assessed; fungal growth was characterized across multiple time points using a range of image analysis parameters (radial extension, hyphal coverage, fractal dimension); and collembola abundance was determined at the end of the experiment. Collembola species had different impacts both within and across fungal species; F. candida had the greatest effect on fungal mycelia whereas P. armata often had little impact. Fungal species varied in their resilience to grazing; all collembola species modified Phanerochaete velutina and Hypholoma fasciculare morphology, that of Resinicium bicolor was only markedly affected by F. candida, and effects on Phallus impudicus were negligible. In the case of H. fasciculare, these grazing effects translated into effects on the rate of fungus-induced wood decay: F. candida and P. armata, but not P. minuta, reduced wood decay rate compared to ungrazed controls. Rate of wood decay was unaffected in the other three fungal species. Changes in collembola population size were generally consistent across fungal species, with each species achieving a greater abundance on P. velutina than on H. fasciculare and P. impudicus. The collembola species did, however, respond differently to R. bicolor, with F. candida being more successful than P. armata. Our study suggests that a wide range of impacts can occur during fungus–collembola interactions, and that caution should be exercised when treating saprotrophic fungi and mycophagous collembola as uniform functional components.     

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.     

Collembolan grazing affects the growth strategy of the cord-forming fungus Hypholoma fasciculare

Mycelia of cord-forming fungi show remarkable patterns of reallocation of biomass and nutrients indicating an important role of these, often extensive, organisms in the spatial translocation of energy and nutrients in forest soils. Despite the rich tradition of interaction studies between soil microarthropods and fungi, the spatial implications of these interactions, due to the potential growth responses of the fungi and to the translocation of energy and nutrients within the mycelial network, have been largely ignored. In this paper we analyse fungal growth responses in two-dimensional model systems composed of compressed soil, the cord-forming fungus Hypholoma fasciculare and three fungivorous Collembolan species. We hypothesised that (i) the highly co-ordinated nature of cord-forming fungi would lead to growth responses following collembolan grazing, and that, (ii) such changes are dependent on grazing intensity, and (iii) changes are dependent on the species grazing. Mycelial extent and hyphal cover decreased with increasing grazing density; at highest grazing density also the fractal dimension of the mycelial border decreased, indicating a less branched foraging front due to the regression of fine hyphae and the development of mycelial cords. Effects differed greatly between collembolan species although they exerted comparable grazing pressure (the smaller species were added in larger numbers according to their allometric size-metabolic rate relationships): while grazing by Folsomia candida resulted in less mycelial extension and hyphal cover, these variables were not affected when Proisotoma minuta and Hypogastrura cf. tullbergi grazed. The effects of a species mix suggested an additive effect of the component species. This shows that fungal mycelia may suffer from damage caused by few but large collembolans, affecting extension as well as coverage of the mycelium, but that fungi may compensate for the biomass loss caused by more but slightly smaller collembolans. In about 20% of the model systems H. fasciculare switched from a growth pattern with a broad contiguous foraging front and uniform growth in all directions to a pattern with fast growing sectors while other sectors stopped growth completely. The switch occurred in grazed systems exclusively; thus we interpret this observation as a fugitive response and as a strategy for quickly escaping from places where grazing pressure is experienced.     

Production of lignocellulose-degrading enzymes and degradation of leaf litter by saprotrophic basidiomycetes isolated from a Quercus petraea forest

Due to the production of lignocellulose-degrading enzymes, saprotrophic basidiomycetes can significantly contribute to the turnover of soil organic matter. The production of lignin- and polysaccharide-degrading enzymes and changes of the chemical composition of litter were studied with three isolates from a Quercus petraea forest. These isolates were capable of fresh litter degradation and were identified as Gymnopus sp., Hypholoma fasciculare and Rhodocollybia butyracea. Within 12 weeks of incubation, H. fasciculare decomposed 23%, R. butyracea 32% and Gymnopus sp. 38% of the substrate dry mass. All fungi produced laccase and Mn-peroxidase (MnP) and none of them produced lignin peroxidase or other Mn-independent peroxidases. There was a clear distinction in the enzyme production pattern between R. butyracea or H. fasciculare compared to Gymnopus sp. The two former species caused the fastest mass loss during the initial phase of litter degradation, accompanied by the temporary production of laccase (and MnP in H. fasciculare) and also high production of hydrolytic enzymes that later decreased. In contrast, Gymnopus sp. showed a stable rate of litter mass loss over the whole incubation period with a later onset of ligninolytic enzyme production and a longer lasting production of both lignin and cellulose-degrading enzymes. The activity of endo-cleaving polysaccharide hydrolases in this fungus was relatively low but it produced the most cellobiose hydrolase. All fungi decreased the C/N ratio of the litter from 24 to 15-19 and Gymnopus sp. also caused a substantial decrease in the lignin content. Analytical pyrolysis mass spectrometry of litter decomposed by this fungus showed changes in the litter composition similar to those caused by white-rot fungi during wood decay. These changes were less pronounced in the case of H. fasciculare and R. butyracea. All fungi also changed the mean masses of humic acid and fulvic acid fractions isolated from degraded litter. The humic acid fraction after degradation by all three fungi contained more lignin and less carbohydrates. Compared to the decomposition by saprotrophic basidiomycetes, litter degradation in situ on the site of fungal isolation resulted in the relative enrichment of lignin and differences in lignin composition revealed by analytical pyrolysis. It can most probably be explained by the participation of non-basidiomycetous fungi and bacteria during natural litter decomposition.     

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.     

Allelopathic effects of artemisinin on ectomycorrhizal fungal isolates in vitro

The anti-malarial drug artemisinin is extracted from the leaves of Artemisia annua L. The release of artemisinin into forest soils could produce a potential risk for forest ecosystems, including effects on ectomycorrhizal fungal nutrient uptake, in areas where commercial and continual cultivation of the medicinal plant A. annua L. is practiced. Therefore, growth, proton and oxalate efflux, and nutrient uptake (nitrogen, phosphorus and potassium) of three isolates of Suillus luteus (S. luteus 1, S. luteus 13, and S. luteus 11) and of one isolate of Suillus subluteus (S. subluteus 12) were compared in culture solutions with different nominal artemisinin concentrations. The results showed that artemisinin inhibited significantly the growth of all studied fungi. With 25 mg artemisinin L⁻¹ added, fungal biomass was decreased by 78.6% (S. luteus 1), 96.7% (S. luteus 13), 77.8% (S. luteus 11) and 86.8% (S. subluteus 12) compared with the control (without artemisinin). This could explain, at least in part, why ectomycorrhizal fungal sporocarps in forests are consistently not found near cultivated A. annua L. fields. The amount of proton efflux by the fungal isolates also decreased as nominal artemisinin concentrations increased, indicating the limited ability of ectomycorrhizal fungi to mobilize nutrients from soil minerals. However, nominal artemisinin significantly increased the rate of fungal oxalate efflux, suggesting membrane damage and the abnormal opening of anion channels on hyphae cell membranes. Nominal artemisinin also decreased the uptake of nitrogen, phosphorus and potassium by the fungal isolates, which may not benefit from the nutrient uptake by ectomycorrhizae. Therefore, artemisinin released from large A. annua L. plantations may inhibit ectomycorrhizal fungal growth, nutrition and functions in forest ecosystems in Southwest China.     

Fungal growth and effects of different wood decomposing fungi on the indigenous bacterial community of polluted and unpolluted soils

Thirty-two different basidiomycete isolates were inoculated separately into contaminated soil and the soil colonization ability was assessed visually. Large differences in the colonization ability and growth patterns were found between the different fungi. Phospholipid fatty acids (PLFAs) were extracted from the soils of the seven isolates with the best colonizing ability. All PLFAs that were not found in pure cultures of the seven fungi were considered as bacterial PLFAs. The bacterial PLFA data were subjected to principal component analysis (PCA) to indicate changes in the indigenous bacterial community. The experiment was repeated in a sandy agricultural soil. The bacterial PLFA patterns were altered when fungi were inoculated into soil, irrespective of whether it was polluted or not. In particular the PLFA cy19:0, indicative of Gram-negative bacteria, was higher in fungal-inoculated soil than in uninoculated controls. The PLFA patterns for each fungal treatment were distributed more or less similarly in the PCA plots of both contaminated and sandy agricultural soil. Soil inoculated with Antrodia vaillantii, Hypholoma fasciculare or Recinicium bicolor was considerably different from the control along PC 1. Soil inoculated with Phanerochaete chrysosporium was characterized by different values along PC 2 compared with the other fungal soils.     

Olfactory cues associated with fungal grazing intensity and secondary metabolite pathway modulate Collembola foraging behaviour

Fungal secondary compounds play an important role for springtail food choice and fitness. Little is known, however, on the role of olfactory cues for Collembola foraging behaviour and whether Collembola can olfactorily perceive volatiles associated with fungal secondary metabolite pathways. We investigated the ability of three species of Collembola (Folsomia candida, Heteromurus nitidus and Supraphorura furcifera) to use olfactory cues for discriminating between fungi of different phylogenetic affiliation (Aspergillus nidulans, Cladosporium cladosporioides, Ascomycota; Laccaria bicolor, Basidiomycota) and toxicity using fungal strains of A. nidulans with reduced secondary metabolite production. Furthermore, we studied if olfactory cues from hyphae injured by grazing affect the foraging behaviour of Collembola. We hypothesized that (i) Collembola are able to olfactorily perceive and respond to fungal species/strains with different secondary metabolite pathways and that (ii) Collembola are able to sense fungal mycelia injured by grazing. Each of the Collembola species studied preferentially oriented their foraging towards fungal strains of A. nidulans with suppressed secondary metabolites, and in particular towards the mutant where the global regulator for secondary metabolites (ΔlaeA) has been silenced. Two of the three Collembola species (H. nitidus and S. furcifera) sensed olfactory cues of previously grazed fungi and moved towards ungrazed fungi, however, the response of S. furcifera was restricted to fungi extensively exposed to grazing (5 days) suggesting that the response varies between Collembola species.Overall, the results support the first and second hypothesis indicating that Collembola (1) are able to olfactorily differentiate fungi of different toxicity and move towards more palatable fungi, and (2) showed a lesser preference for fungi previously exposed to grazing.     

Introduced slugs and indigenous caterpillars as facilitators of carbon and nutrient mineralisation on a sub-Antarctic island

Indigenous soil macroinvertebrates (moth larvae, weevil larvae, earthworms) are cardinal agents of nutrient release from litter on sub-Antarctic Marion Island (47°S, 38°′E). Their populations are threatened through predation by introduced house mice, which do not prey on an introduced slug Deroceras panormitanum. A microcosm study was carried out to explore whether slugs affect rates of carbon and inorganic nutrient mineralisation from plant litter differently to an indigenous caterpillar (larva of a flightless moth Pringelophaga marioni). Caterpillars stimulated N, Ca, Mg and K mineralisation from plant litter two to five times more than slugs did, whereas the two invertebrate types stimulated C and P mineralisation to the same degree. Consequently, ratios of C:N and N:P released from the litter were different for slugs and caterpillars. Such differences might affect peat nutrient quality and ultimately the peat accumulation-decomposition balance, an important driver of ecological succession. This suggests that slugs cannot simply replace caterpillars without consequences for ecosystem structure and functioning on the island.     

Soil enzyme activity in an old-growth northern hardwood forest: Interactions between soil environment,ectomycorrhizal fungi and plant distribution

Plants and soil microbes produce extracellular enzymes (EE) that catalyze the hydrolysis of nitrogen (N) and phosphorus (P) containing compounds in soil and other enzymes involved in degradation of lignin and cellulose. We explored whether soil enzyme activity involved in carbon (C), N and P cycling were correlated with plant distribution, soil chemical conditions and the identity of fungi colonizing tree roots in an old growth forest remnant. Terminal restriction fragment length polymorphism (TRFLP) was used to determine the presence of root fungi and standard fluorometric analysis was used to determine soil enzyme activities. Soil enzymes were consistently positively correlated with soil C and N, but not CN ratio. Soil P was also correlated with enzyme activity during both June and September sampling. We saw no significant relationships between herbaceous plant cover and enzyme activity in June, but there were significant positive correlations between α-glucosidase and herbaceous plant coverage in September. We also found that some enzymes were significantly correlated with the identity of fungi colonizing tree roots separated from the soil cores. Chitinase and β-glucosidase were positively correlated with the genera Russula and Piloderma while chitinase was negatively correlated with Amanita and Entoloma. In addition, phosphatase was positively correlated with Russula, Meliniomyces and Solenopezia. Our results suggest that enzyme activity in old growth forest soils are affected by a variety of environmental factors, and that herbaceous plants and some root fungi may be associated with sites of elevated or decreased decomposition potential and nutrient cycling.     

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.     

Spatial distribution of arbuscular mycorrhizal fungi, glomalin and soil enzymes under the canopy of Astragalus adsurgens Pall. in the Mu Us sandland, China

To measure and manage plant growth in arid and semi-arid sandlands, improved understanding of the spatial patterns of desert soil resources and the role of arbuscular mycorrhizal (AM) fungi is needed. Spatial patterns of AM fungi, glomalin and soil enzyme activities were investigated in five plots located in the Mu Us sandland, northwestern China. Soils to 50 cm depth in the rhizosphere of Astragalus adsurgens Pall. were sampled. The study demonstrated that A. adsurgens Pall. could form strong symbiotic relationships with AM fungi. Arbuscular mycorrhizal fungal status and distributions were significantly different among the five studied plots. Correlation coefficient analysis demonstrated that spore density was significantly and positively correlated with soil organic carbon (SOC), soil acid phosphatase and to two Bradford-reactive soil protein (BRSP) fractions (P < 0.01). Colonization of arbuscules and vesicles were positively correlated with protease activity. The BRSP fractions were also significantly and positively correlated to edaphic factors (e.g. SOC, available nitrogen, and Olsen phosphorus) and soil enzymes (e.g. soil urease and acid phosphatase). The means of total BRSP and easily extractable BRSP were 0.95 mg g⁻¹ and 0.5 mg g⁻¹ in all data, respectively. The levels of BRSP in the desert soil were little lower than those in native and arable soils, but the ratios of BRSP to SOC were much higher than farmland soils. The results of this study support the conclusion that glomalin could be an appropriate index related to the level of soil fertility, especially in desert soil. Moreover, AM fungal colonizations and glomalin might be useful to monitor desertification and soil degradation.     

Slow turnover and production of fungal hyphae during a Californian dry season

We used minirhizotrons to examine the production and turnover of fungal hyphae in situ during the dry season in a Californian grassland. Hyphae were produced relatively slowly throughout the season at rates that did not vary significantly over time, indicating that a portion of the fungal community was active even when soils were very dry. In addition, fungi displayed relatively long residence times, with half of the hyphae remaining in the soil for at least 145 days. Together, these results suggest that a contingent of active fungi may be capable of performing nutrient transformations when plants are otherwise dormant, while relatively long-lasting hyphae may immobilize nutrients for several months before turning over.     

Collembola grazing on arbuscular mycorrhiza fungi modulates nutrient allocation in plants

Arbuscular mycorrhiza (AM) mycelia networks are important for nutrient allocation in many plants, but fungivorous soil invertebrates such as Collembola can modulate the symbiosis by grazing on the extra-radical mycelium (ERM). This study employs a dual biomarker approach with stable isotopes and fatty acids to disentangle trophic interactions of Collembola in a plant-fungal soil system with maize (Zea mays) and the AM fungus Glomus mosseae. To separate ERM and root mediated effects, root (RC) and hyphal compartments (HC) were used, and the latter was spiked with labeled ¹⁵N substrate. The euedaphic Collembola species Protaphorura fimata was introduced as the fungal and root grazer. Generally, the presence of Collembola in RC fostered biomass and phosphorous uptake in roots colonized with AM. Nitrogen transport from HC to RC was not altered, indicating that Collembola did not disrupt the ERM network via grazing. Collembola–fungus interactions fostered AM hyphal proliferation in HC, whereas in RC it induced a change from fungal senescence with build-up of storage reserves, to an active foraging phase. A distinct diet switch by Collembola between HC and RC indicated different ERM palatability meditated by the presence or absence of the host plant. Overall, Collembola grazing increased ERM nutrient sequestration, particularly phosphorus, and in turn plant performance. Collembola modified fungal phenology, favoring fungal colonization over reproductive phases. These trophic interactions were strongly determined by fungal life stage, with the establishment of a functional mycorrhiza as a crucial factor.     

The soil invertebrate contribution to nitrogen mineralisation differs between soils under organic and conventional dairy management

Organic management aims to promote soil biological activity. To test whether organic management stimulates soil biological activity, invertebrates (macrofauna, mesofauna and microfauna) were collected from four paired commercial organically and conventionally managed dairy farms on different soil types (Allophanic, Pallic, Recent and flooded Recent). Food webs were constructed and rates of invertebrate-mediated N mineralisation calculated. The organic dairy operations used fewer nutrient inputs and had lower stocking rates than their paired conventional farms. This translated into lower calculated pasture production and less available plant litter entering the soil food web. Despite the lower plant litter inputs into the organic system, earthworm biomass was higher (particularly in the Recent and flooded Recent soils), suggesting that under conventional management the physical condition of the soil, as influenced by stock treading pressures, was more important for invertebrate activity and their influence on N mineralisation than was food supply. Nitrogen mineralisation was higher in organic systems, with earthworms contributing the most (24–98 kg N/ha/year). As the physical loading on the soil increased under conventional management, the ability of the soil to provide soil services (i.e. N mineralisation and litter decomposition) became compromised. Organic management on four soils stimulated biological activity by reducing the treading pressure on the soil and highlights the need to consider the influence of management practices on the faunal environment (food availability and physical condition) to understand the impacts of organic management and the role of fauna in N mineralisation.     

Fungal and bacterial enzyme activities in Alnus nepalensis D. Don

Fungal and bacterial numbers and their enzymes activities in terms of enzymes, namely cellulase, amylase and invertase were estimated in two forest stands of alder (open and closed forest). The fungal and bacterial population numbers were higher in the closed forest than in the open ones. Consequently, the different enzyme activities were also greater in the closed forest. A correlation coefficient was calculated between fungal population numbers, bacterial population numbers, moisture content, pH, total nitrogen, weight loss, cellulose and total sugars and the enzymes activities. Invertase activity showed a positive correlation (P < 0.05) with litter soluble sugars and total nitrogen but negatively with weight loss. Amylase and cellulase activities were correlated significantly with fungi and bacteria and moisture content of litter. Cellulase also correlated significantly but negatively with the cellulose  (P < 0.01). The results of the investigation indicated that changes in forest canopy has an effect on fungal and bacterial population numbers and microbial enzymes activities.     

Ectomycorrhizal and Saprotrophic Fungal Communities Vary Across mm-Scale Soil Microsites Differing in Phosphatase Activity

To understand nutrient cycling in soils, soil processes and microorganisms need be better characterized. To determine whether specific trophic groups of fungi are associated with soil enzyme activity, we used soil imprinting to guide mm-scale sampling from microsites with high and low phosphatase activities in birch/Douglas-fir stands. Study 1 involved sampling one root window per site at 12 sites of different ages (stands); study 2 was conducted at one of the stem-exclusion stands, at which 5 root windows had been installed. Total fungal and ectomycorrhizal (EM) fungal terminal-restriction fragment length polymorphism (TRFLP) fingerprints differed between high-and low-phosphatase activity microsites at 8 of 12 root windows across 12 sites. Where differences were detected, fewer EM fungi were detected in high-than low-phosphatase activity microsites. Using 5 root windows at one site, next-generation sequencing detected similar fungal communities across microsites, but the ratio of saprotrophic to EM fungal reads was higher in high-phosphatase activity microsites in the two windows that had low EM fungal richness. In windows with differences in fungal communities, both studies indicated that EM fungi were less successful than saprotrophic fungi in colonizing fine-scale, organic matter-rich microsites. Fine-scale sampling linked with in situ detection of enzyme activity revealed relationships between soil fungal communities and phosphatase activity that could not be observed at the scales employed by conventional approaches, thereby contributing to the understanding of fine-scale phosphorus cycling in forest soils.     

Alkali-extractable soil organic matter: An important factor affecting the mycelial growth of ectomycorrhizal fungi

Significant effects of two fractions of alkali-extractable soil organic matter (AEOM) extracted from three different soils (agricultural soil, soil from deciduous forest, soil from spruce monoculture) on mycelial growth of 17 isolates of ectomycorrhizal fungi were observed. Growth of Lactarius deterrimus, Meliniomyces bicolor and one of the isolates of Leccinum aurantiacum was significantly stimulated by acid-insoluble fraction extracted from all three soils. The stimulatory effects were frequent but inhibition of mycelial growth of some isolates was also observed. The fungal response to the presence of the organic extract in the nutrient medium was isolate-specific rather than species-specific. Organic matter extracted from different source soils affected differently the mycelial growth, the largest number of stimulatory effects being observed in an experiment where the extract was richest in trace elements Zn and Cu. At the same time, the observed stimulatory effects were not attributable to increased concentrations of trace elements in the nutrient medium. The results indicate that soil may be used as a source of extractable organic fractions which, when used as a cultivation medium additive, may significantly improve the growth of responsive fungal isolates. Under natural conditions, AEOM (traditionally designated humic substances) represent a potential factor affecting the composition of cenosis of ectomycorrhizal fungi in soil.     

Physical extent,frequency, and intensity of phosphatase activity varies on soil profiles across a Douglas-fir chronosequence

In forest soils, the availability of phosphate is largely dependent on phosphatase activity. We used soil imprinting to compare in situ activity and fine-scale distribution of phosphatase on soil profiles located across forest chronosequences of four age classes young (5–6 yrs), canopy closure (24–30 yrs), stem exclusion (61–71 yrs), and older (90–103 yrs) of mixed Douglas-fir/paper birch stands regenerated after fire or clearcutting in southern interior British Columbia, Canada. Chromatography paper treated with a mixture of substrate and colorimetric reagent was applied directly to vertical soil surfaces, accessed through root windows. Stands older than 61 years had both the highest level of in situ phosphatase activity and larger, more intense regions of activity. Bray-extractable phosphorus was negatively related to imprintable phosphatase activity. We compared the changes in phosphatase activity with differences in the ectomycorrhizal fungal (EMF) community that had been documented previously in the same stands. Of 84 ectomycorrhizal fungi found on roots in at least two of the stand-age classes, eight taxa were positively correlated and one taxon (Rhizopogon vinicolor/vesiculosus) negatively correlated with high phosphatase activity. The frequency of three taxa appeared to be positively correlated with larger areas of activity on the soil profiles. By using an imprinting approach, this study was able to demonstrate, for the first time, that in situ phosphatase activity and physical attributes of that activity (i.e., number, size, and relative rates of each area of activity) were related to concentrations of soil nutrients and with the frequency of individual ectomycorrhizal fungi.     

Litter, warming and plants affect respiration and allocation of soil microbial and plant C, N and P in arctic mesocosms

In a mesocosm experiment, we studied decomposition rates as CO₂ efflux and changes in plant mass, nutrient accumulation and soil pools of nitrogen (N) and phosphorus (P), in soils from a sub-arctic heath. The soil was incubated at 10 °C and 12 °C, with or without leaf litter and with or without plants present. The purpose of the experiment was to analyse decomposition and nutrient transformations under simulated, realistic conditions in a future warmer Arctic.Both temperature enhancement and litter addition increased respiration rates. Temperature enhancement and surprisingly also litter addition decreased microbial biomass carbon (C) content, resulting in a pronounced increase of specific respiration. Microbial P content increased progressively with temperature enhancement and litter addition, concomitant with increasing P mineralisation, whereas microbial N increased only in the litter treatment, at the same time as net N mineralisation decreased. In contrast, microbial biomass N decreased as temperature increased, resulting in a high mobilisation of inorganic N.Plant responses were closely coupled to the balance of microbial mineralisation and immobilisation. Plant growth and N accumulation was low after litter addition because of high N immobilisation in microbes and low net mineralisation, resulting in plant N limitation. Growth increased in the temperature-enhanced treatments, but was eventually limited by low supply of P, reflected in a low plant P concentration and high N-to-P ratio. Hence, the different microbial responses caused plant N limitation after litter addition and P limitation after temperature enhancement. Although microbial processes determined the main responses in plants, the plants themselves influenced nutrient turnover. With plants present, P mobilisation to the plant plus soil inorganic pools increased significantly, and N mobilisation non-significantly, when litter was added. This was presumably due to increased mineralisation in the rhizosphere, or because the nutrients in addition to being immobilised by microbes also could be absorbed by plants. This suggests that the common method of measuring nutrient mineralisation in soils incubated without plants may underestimate the rates of nutrient mobilisation, which probably contributes to a commonly observed discrepancy of measured lower rates of net nutrient mineralisation than uptake rates in arctic soils.