Defoliation effects on enzyme activities of the ectomycorrhizal fungus Suillus granulatus in a Pinus contorta (lodgepole pine) stand in Yellowstone National Park

Extracellular Enzyme Activities and Carbon/Nitrogen Utilization in Mycorrhizal Fungi Isolated From Epiphytic and Terrestrial Orchids

Fungi employ extracellular enzymes to initiate the degradation of organic macromolecules into smaller units and to acquire the nutrients for their growth. As such, these enzymes represent important functional components in terrestrial ecosystems. While it is well-known that the regulation and efficiency of extracellular enzymes to degrade organic macromolecules and nutrient-acquisition patterns strongly differ between major fungal groups, less is known about variation in enzymatic activity and carbon/nitrogen preference in mycorrhizal fungi. In this research, we investigated variation in extracellular enzyme activities and carbon/nitrogen preferences in orchid mycorrhizal fungi (OMF). Previous research has shown that the mycorrhizal fungi associating with terrestrial orchids often differ from those associating with epiphytic orchids, but whether extracellular enzyme activities and carbon/nitrogen preference differ between growth forms remains largely unknown. To fill this gap, we compared the activities of five extracellular enzymes [cellulase, xylanase, lignin peroxidase, laccase, and superoxide dismutase (SOD)] between fungi isolated from epiphytic and terrestrial orchids. In total, 24 fungal strains belonging to Tulasnellaceae were investigated. Cellulase and xylanase activities were significantly higher in fungi isolated from terrestrial orchids (0.050 ± 0.006 U/ml and 0.531 ± 0.071 U/ml, respectively) than those from epiphytic orchids (0.043 ± 0.003 U/ml and 0.295 ± 0.067 U/ml, respectively), while SOD activity was significantly higher in OMF from epiphytic orchids (5.663 ± 0.164 U/ml) than those from terrestrial orchids (3.780 ± 0.180 U/ml). Carboxymethyl cellulose was more efficiently used by fungi from terrestrial orchids, while starch and arginine were more suitable for fungi from epiphytic orchids. Overall, the results of this study show that extracellular enzyme activities and to a lesser extent carbon/nitrogen preferences differ between fungi isolated from terrestrial and epiphytic orchids and may indicate functional differentiation and ecological adaptation of OMF to local growth conditions.

Phosphorus deficiencies invoke optimal allocation of exoenzymes by ectomycorrhizas

Ectomycorrhizal (EM) fungi can acquire phosphorus (P) through the production of extracellular hydrolytic enzymes (exoenzymes), but it is unclear as to the manner and extent native EM fungal communities respond to declining soil P availability. We examined the activity of six exoenzymes (xylosidase, N-acetyl glucosaminidase, β-glucosidase, acid phosphomonoesterase, acid phosphodiesterase [APD], laccase) from EM roots of Pseudotsuga menzesii across a soil podzolization gradient of coastal British Columbia. We found that APD activity increased fourfold in a curvilinear association with declining inorganic P. Exoenzyme activity was not related to organic P content, but at a finer resolution using ³¹P-NMR, there was a strong positive relationship between APD activity and the ratio of phosphodiesters to orthophosphate of surface organic horizons (forest floors). Substantial increases (two- to fivefold) in most exoenzymes were aligned with declining foliar P concentrations of P. menzesii, but responses were statistically better in relation to foliar nitrogen (N):P ratios. EM fungal species with consistently high production of key exoenzymes were exclusive to Podzol plots. Phosphorus deficiencies in relation to N limitations may provide the best predictor of exoenzyme investment, reflecting an optimal allocation strategy for EM fungi. Resource constraints contribute to species turnover and the assembly of distinct, well-adapted EM fungal communities.

Fertilization and Tree Species Influence on Stable Aggregates in Forest Soil

Background and objectives: aggregation and structure play key roles in the water-holding capacity and stability of soils and are important for the physical protection and storage of soil carbon (C). Forest soils are an important sink of ecosystem C, though the capacity to store C may be disrupted by the elevated atmospheric deposition of nitrogen (N) and sulfur (S) compounds by dispersion of soil aggregates via acidification or altered microbial activity. Furthermore, dominant tree species and the lability of litter they produce can influence aggregation processes. Materials and methods: we measured water-stable aggregate size distribution and aggregate-associated organic matter (OM) content in soils from two watersheds and beneath four hardwood species at the USDA Forest Service Fernow Experimental Forest in West Virginia, USA, where one watershed has received (NH4)2SO4 fertilizer since 1989 and one is a reference/control of similar stand age. Bulk soil OM, pH, and permanganate oxidizable carbon (POXC) were also measured. Research highlights: fertilized soil exhibited decreased macro-aggregate formation and a greater proportion of smaller micro-aggregates or unassociated clay minerals, particularly in the B-horizon. This shift in aggregation to soil more dominated by the smallest (<53 µm) fraction is associated with both acidification (soil pH) and increased microbially processed C (POXC) in fertilized soil. Intra-aggregate OM was also depleted in the fertilized soil (52% less OM in the 53–2000 µm fractions), most strongly in subsurface B-horizon soil. We also document that tree species can influence soil aggregation, as soil beneath species with more labile litter contained more OM in the micro-aggregate size class (<250 µm), especially in the fertilized watershed, while species with more recalcitrant litter promoted more OM in the macro-aggregate size classes (500–2000 µm) in the reference watershed. Conclusions: long-term fertilization, and likely historic atmospheric deposition, of forest soils has weakened macro-aggregation formation, with implications for soil stability, hydrology, and storage of belowground C.

Non-structural carbohydrate concentrations of Fagus sylvatica and Pinus sylvestris fine roots are linked to ectomycorrhizal enzymatic activity during spring reactivation

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

Rapid carbon turnover beneath shrub and tree vegetation is associated with low soil carbon stocks at a subarctic treeline

Climate warming at high northern latitudes has caused substantial increases in plant productivity of tundra vegetation and an expansion of the range of deciduous shrub species. However significant the increase in carbon (C) contained within above-ground shrub biomass, it is modest in comparison with the amount of C stored in the soil in tundra ecosystems. Here, we use a 'space-for-time' approach to test the hypothesis that a shift from lower-productivity tundra heath to higher-productivity deciduous shrub vegetation in the sub-Arctic may lead to a loss of soil C that out-weighs the increase in above-ground shrub biomass. We further hypothesize that a shift from ericoid to ectomycorrhizal systems coincident with this vegetation change provides a mechanism for the loss of soil C. We sampled soil C stocks, soil surface CO2 flux rates and fungal growth rates along replicated natural transitions from birch forest (Betula pubescens), through deciduous shrub tundra (Betula nana) to tundra heaths (Empetrum nigrum) near Abisko, Swedish Lapland. We demonstrate that organic horizon soil organic C (SOCorg ) is significantly lower at shrub (2.98 ± 0.48 kg m(-2) ) and forest (2.04 ± 0.25 kg m(-2) ) plots than at heath plots (7.03 ± 0.79 kg m(-2) ). Shrub vegetation had the highest respiration rates, suggesting that despite higher rates of C assimilation, C turnover was also very high and less C is sequestered in the ecosystem. Growth rates of fungal hyphae increased across the transition from heath to shrub, suggesting that the action of ectomycorrhizal symbionts in the scavenging of organically bound nutrients is an important pathway by which soil C is made available to microbial degradation. The expansion of deciduous shrubs onto potentially vulnerable arctic soils with large stores of C could therefore represent a significant positive feedback to the climate system.

Differences in the activities of eight enzymes from ten soil fungi and their possible influences on the surface structure, functional groups, and element composition of soil colloids

How soil fungi function in soil carbon and nutrient cycling is not well understood by using fungal enzymatic differences and their interactions with soil colloids. Eight extracellular enzymes, EEAs (chitinase, carboxymethyl cellulase, β-glucosidase, protease, acid phosphatase, polyphenol oxidase, laccase, and guaiacol oxidase) secreted by ten fungi were compared, and then the fungi that showed low and high enzymatic activity were co-cultured with soil colloids for the purpose of finding fungi-soil interactions. Some fungi (Gomphidius rutilus, Russula integra, Pholiota adiposa, and Geastrum mammosum) secreted 3-4 enzymes with weak activities, while others (Cyathus striatus, Suillus granulate, Phallus impudicus, Collybia dryophila, Agaricus sylvicola, and Lactarius deliciosus) could secret over 5 enzymes with high activities. The differences in these fungi contributed to the alterations of functional groups (stretching bands of O-H, N-H, C-H, C = O, COO- decreased by 11-60%, while P = O, C-O stretching, O-H bending and Si-O-Si stretching increased 9-22%), surface appearance (disappearance of adhesive organic materials), and elemental compositions (11-49% decreases in C1s) in soil colloids. Moreover, more evident changes were generally in high enzymatic fungi (C. striatus) compared with low enzymatic fungi (G. rutilus). Our findings indicate that inter-fungi differences in EEA types and activities might be responsible for physical and chemical changes in soil colloids (the most active component of soil matrix), highlighting the important roles of soil fungi in soil nutrient cycling and functional maintenance.

Ectomycorrhizal Cortinarius species participate in enzymatic oxidation of humus in northern forest ecosystems

In northern forests, belowground sequestration of nitrogen (N) in complex organic pools restricts nutrient availability to plants. Oxidative extracellular enzymes produced by ectomycorrhizal fungi may aid plant N acquisition by providing access to N in macromolecular complexes. We test the hypotheses that ectomycorrhizal Cortinarius species produce Mn-dependent peroxidases, and that the activity of these enzymes declines at elevated concentrations of inorganic N. In a boreal pine forest and a sub-arctic birch forest, Cortinarius DNA was assessed by 454-sequencing of ITS amplicons and related to Mn-peroxidase activity in humus samples with- and without previous N amendment. Transcription of Cortinarius Mn-peroxidase genes was investigated in field samples. Phylogenetic analyses of Cortinarius peroxidase amplicons and genome sequences were performed. We found a significant co-localization of high peroxidase activity and DNA from Cortinarius species. Peroxidase activity was reduced by high ammonium concentrations. Amplification of mRNA sequences indicated transcription of Cortinarius Mn-peroxidase genes under field conditions. The Cortinarius glaucopus genome encodes 11 peroxidases - a number comparable to many white-rot wood decomposers. These results support the hypothesis that some ectomycorrhizal fungi--Cortinarius species in particular--may play an important role in decomposition of complex organic matter, linked to their mobilization of organically bound N.

Testing the link between community structure and function for ectomycorrhizal fungi involved in a global tripartite symbiosis

Alnus trees associate with ectomycorrhizal (ECM) fungi and nitrogen-fixing Frankia bacteria and, although their ECM fungal communities are uncommonly host specific and species poor, it is unclear whether the functioning of Alnus ECM fungal symbionts differs from that of other ECM hosts. We used exoenzyme root tip assays and molecular identification to test whether ECM fungi on Alnus rubra differed in their ability to access organic phosphorus (P) and nitrogen (N) when compared with ECM fungi on the non-Frankia host Pseudotsuga menziesii. At the community level, potential acid phosphatase (AP) activity of ECM fungal root tips from A. rubra was significantly higher than that from P. menziesii, whereas potential leucine aminopeptidase (LA) activity was significantly lower for A. rubra root tips at one of the two sites. At the individual species level, there was no clear relationship between ECM fungal relative root tip abundance and relative AP or LA enzyme activities on either host. Our results are consistent with the hypothesis that ECM fungal communities associated with Alnus trees have enhanced organic P acquisition abilities relative to non-Frankia ECM hosts. This shift, in combination with the chemical conditions present in Alnus forest soils, may drive the atypical structure of Alnus ECM fungal communities.

Changes to the N cycle following bark beetle outbreaks in two contrasting conifer forest types

Outbreaks of Dendroctonus beetles are causing extensive mortality in conifer forests throughout North America. However, nitrogen (N) cycling impacts among forest types are not well known. We quantified beetle-induced changes in forest structure, soil temperature, and N cycling in Douglas-fir (Pseudotsuga menziesii) forests of Greater Yellowstone (WY, USA), and compared them to published lodgepole pine (Pinus contorta var. latifolia) data. Five undisturbed stands were compared to five beetle-killed stands (4-5 years post-outbreak). We hypothesized greater N cycling responses in Douglas-fir due to higher overall N stocks. Undisturbed Douglas-fir stands had greater litter N pools, soil N, and net N mineralization than lodgepole pine. Several responses to disturbance were similar between forest types, including a pulse of N-enriched litter, doubling of soil N availability, 30-50 % increase in understory cover, and 20 % increase in foliar N concentration of unattacked trees. However, the response of some ecosystem properties notably varied by host forest type. Soil temperature was unaffected in Douglas-fir, but lowered in lodgepole pine. Fresh foliar %N was uncorrelated with net N mineralization in Douglas-fir, but positively correlated in lodgepole pine. Though soil ammonium and nitrate, net N mineralization, and net nitrification all doubled, they remained low in both forest types (<6 μg N g soil(-1) NH(4) (+)or NO(3) (-); <25 μg N g soil(-1) year(-1) net N mineralization; <8 μg N g soil(-1) year(-1) net nitrification). Results suggest that beetle disturbance affected litter and soil N cycling similarly in each forest type, despite substantial differences in pre-disturbance biogeochemistry. In contrast, soil temperature and soil N-foliar N linkages differed between host forest types. This result suggests that disturbance type may be a better predictor of litter and soil N responses than forest type due to similar disturbance mechanisms and disturbance legacies across both host-beetle systems.

Growth response of drought-stressed Pinus sylvestris seedlings to single- and multi-species inoculation with ectomycorrhizal fungi

Many trees species form symbiotic associations with ectomycorrhizal (ECM) fungi, which improve nutrient and water acquisition of their host. Until now it is unclear whether the species richness of ECM fungi is beneficial for tree seedling performance, be it during moist conditions or drought. We performed a pot experiment using Pinus sylvestris seedlings inoculated with four selected ECM fungi (Cenococcum geophilum, Paxillus involutus, Rhizopogon roseolus and Suillus granulatus) to investigate (i) whether these four ECM fungi, in monoculture or in species mixtures, affect growth of P. sylvestris seedlings, and (ii) whether this effect can be attributed to species number per se or to species identity. Two different watering regimes (moist vs. dry) were applied to examine the context-dependency of the results. Additionally, we assessed the activity of eight extracellular enzymes in the root tips. Shoot growth was enhanced in the presence of S. granulatus, but not by any other ECM fungal species. The positive effect of S. granulatus on shoot growth was more pronounced under moist (threefold increase) than under dry conditions (twofold increase), indicating that the investigated ECM fungi did not provide additional support during drought stress. The activity of secreted extracellular enzymes was higher in S. granulatus than in any other species. In conclusion, our findings suggest that ECM fungal species composition may affect seedling performance in terms of aboveground biomass.

Belowground carbon allocation by trees drives seasonal patterns of extracellular enzyme activities by altering microbial community composition in a beech forest soil

*Plant seasonal cycles alter carbon (C) and nitrogen (N) availability for soil microbes, which may affect microbial community composition and thus feed back on microbial decomposition of soil organic material and plant N availability. The temporal dynamics of these plant-soil interactions are, however, unclear. *Here, we experimentally manipulated the C and N availability in a beech forest through N fertilization or tree girdling and conducted a detailed analysis of the seasonal pattern of microbial community composition and decomposition processes over 2 yr. *We found a strong relationship between microbial community composition and enzyme activities over the seasonal course. Phenoloxidase and peroxidase activities were highest during late summer, whereas cellulase and protease peaked in late autumn. Girdling, and thus loss of mycorrhiza, resulted in an increase in soil organic matter-degrading enzymes and a decrease in cellulase and protease activity. *Temporal changes in enzyme activities suggest a switch of the main substrate for decomposition between summer (soil organic matter) and autumn (plant litter). Our results indicate that ectomycorrhizal fungi are possibly involved in autumn cellulase and protease activity. Our study shows that, through belowground C allocation, trees significantly alter soil microbial communities, which may affect seasonal patterns of decomposition processes.

Differences in amplification efficiency of standard curves in quantitative real-time PCR assays and consequences for gene quantification in environmental samples

High and comparable efficiency values are the key for reliable quantification of target genes from environmental samples using real-time PCR. Therefore it was the aim of this study to investigate if PCR amplification efficiencies of plasmid DNA used for the calculation of standard curves (i) remain constant along a logarithmic scale of dilutions and (ii) if these values are comparable to those of DNA extracted from environmental samples. It could be shown that comparable efficiency values within the standards cannot be achieved using log scale serial dilutions and a comparison of gene copy numbers from DNA extracted from environmental samples and standard DNA extracted from plasmids is only possible in a very small interval.

Slowed decomposition is biotically mediated in an ectomycorrhizal, tropical rain forest

Bacteria and fungi drive the cycling of plant litter in forests, but little is known about their role in tropical rain forest nutrient cycling, despite the high rates of litter decay observed in these ecosystems. However, litter decay rates are not uniform across tropical rain forests. For example, decomposition can differ dramatically over small spatial scales between low-diversity, monodominant rain forests, and species-rich, mixed forests. Because the climatic patterns and soil parent material are identical in co-occurring mixed and monodominant forests, differences in forest floor accumulation, litter production, and decomposition between these forests may be biotically mediated. To test this hypothesis, we conducted field and laboratory studies in a monodominant rain forest in which the ectomycorrhizal tree Dicymbe corymbosa forms >80% of the canopy, and a diverse, mixed forest dominated by arbuscular mycorrhizal trees. After 2 years, decomposition was significantly slower in the monodominant forest (P < 0.001), but litter production was significantly greater in the mixed forest (P < 0.001). In the laboratory, we found microbial community biomass was greater in the mixed forest (P = 0.02), and the composition of fungal communities was distinct between the two rain forest types (P = 0.001). Sequencing of fungal rDNA revealed a significantly lower richness of saprotrophic fungi in the monodominant forest (19 species) relative to the species-rich forest (84 species); moreover, only 4% percent of fungal sequences occurred in both forests. These results show that nutrient cycling patterns in tropical forests can vary dramatically over small spatial scales, and that changes in microbial community structure likely drive the observed differences in decomposition.

Ectomycorrhizal fungi and their enzymes in soils: is there enough evidence for their role as facultative soil saprotrophs?

Although ectomycorrhizal (ECM) fungi are generally regarded as dependent upon the supply of carbon from their plant hosts, some recent papers have postulated a role for these fungi in the saprotrophic acquisition of carbon from soil. This theory was mainly based on the increase in enzymatic activity during periods of low photosynthate supply from tree hosts and emergence of the theory has led to a question about the overall influence of saprotrophy by ECM fungi on soil carbon turnover. However, I argue here that there is still not enough evidence to confirm this proposed function. My argument is based on inference from several lines of observation and concern over several aspects of the past studies. First, ECM fungi mainly inhabit deeper soil horizons, in which the availability of carbon compounds with positive energetic value is low. Second, the ability of ECM fungi to produce ligninolytic enzymes and cellulases is much weaker than that of saprotrophic basidiomycetes. This is most apparent in the low copy abundance of corresponding genes in the sequenced genomes of ECM species Laccaria bicolor and Amanita bisporigenes compared to the saprotrophic species Galerina marginata. I offer alternative hypotheses to explain the past observations of increased enzyme activity during starvation periods. These include, the induction of autolytic processes in ECM fungal mycelia or an attack on the host tissues to support escape from a dying root and to allow for a search for new hosts.