Forest Floor Microbial Communities Research Articles

Effects of Forest Gaps on Forest Floor Microbial Community Composition in Pinus tabulaeformis Forests in a Rocky Mountain Area, Beijing, China

Forest gaps induce environmental heterogeneity, but their effects on the local forest floor microbial communities are not fully understood. This research investigated the impact of forest gap positions on the forest floor microbial community composition to provide baseline information for projects to accelerate nutrient cycling and forest regeneration and enhance ecosystem services. A one-year-old forest gap and an area of 40–50 m2 in Pinus tabulaeformis plantations were selected in the Beijing mountainous area. Forest floor samples were collected from the following positions: gap center, gap border, and adjacent closed canopy. Our study demonstrated that gap positions significantly influenced the forest floor microbial community composition. The Gram-positive bacteria, Gram-negative bacteria, and total bacteria, as well as the fungi, were significantly greater in the forest gap center and gap border compared to those in the closed canopy, and the dissolved organic carbon, readily oxidized organic carbon, ammonia nitrogen, and nitrate nitrogen followed the same trend. Compared with those of the closed canopy, the Gram-positive bacteria, Gram-negative bacteria, total bacteria, and fungi in the gap center were markedly greater by 23%, 25%, 22%, and 24% and by 14%, 14%, 11%, and 16% in the gap border, respectively (p < 0.05). Redundancy analysis demonstrated that shifts in the litter microbial community composition were predominantly predicted by litter moisture and β-1,4-glucosidase. In addition, we discovered that the microbial community composition was greater in the undecomposed forest layer than that in the semi-decomposed layer. In summary, gap positions and forest floor layers have a significant impact on microbial community composition. Nevertheless, additional long-term investigations are needed. Our study provides a reference for the promotion of nutrient cycling to guide future ecological management.

Post-fire and harvest legacy on soil carbon and microbial communities in boreal forest soils

The boreal forest is currently facing multiple disturbances, which are increasing in extent, frequency and severity. Of particular concern in the western boreal forest of Canada are wildfires and harvesting, which, in the short-term, may impact the forest floor, a storehouse of organic matter and site of highest soil microbial activity. However, how these disturbances may individually and cumulatively affect soil properties during ecosystem recovery is not well documented. Here we compared the separate and compound (salvage-logging) effects of wildfire and harvesting in the mixedwood boreal forest of northern Alberta. Ten years following disturbance, carbon chemical composition of the harvested forest floors was comparable to the control plots as was indicated by solid-state nuclear magnetic resonance analysis. We report a legacy effect of fire in recovering forest floors, with the continued presence of black carbon. Further, as determined by phospholipid fatty acid analysis (PLFA), both wildfire and salvage-logging disturbance resulted in distinct microbial composition from the control and harvest treatments. A shift in organic matter composition indicative of fresh plant inputs was determined to be a key driver of community differences. Despite the presence of black carbon, and shallower depth, the fire-disturbed forest floor harbored the greatest microbial biomass. However, this greater microbial biomass was not present following salvage-logging of the fire disturbed stands. Instead, the salvage-logged stands had the shallowest forest floor, low microbial biomass, and differed the most from the control forest floors based on PLFA results. Taken together, these results indicate that, ten years after disturbance, the compound disturbance of salvage-logging had a greater impact on the recovery of forest floor microbial communities than harvest or wildfire alone.

The stoichiometric signature of high‐frequency fire in forest floor food webs

AbstractFire regimes are shifting under climate change. Decadal‐scale shifts in fire regime can disrupt the biogeochemical cycling of carbon (C), nitrogen (N), and phosphorus (P) within forest ecosystems, but the full extent of these disruptions is unknown. It is also unclear whether these disruptions have consequences for the ecological characteristics (e.g., biomass, abundance, and composition) of microbial and invertebrate communities, which together comprise the majority of terrestrial biodiversity and underpin many ecosystem processes. The theoretical framework of ecological stoichiometry has great potential in this context, but it has rarely been used to develop an integrated understanding of the biogeochemical and ecological effects of altered fire regime across trophic levels. Using one of the world's longest‐running fire experiments, located in Queensland, Australia, we carried out a comprehensive investigation into the stoichiometric consequences of a decadal‐scale divergence in prescribed fire frequency and their links to coinciding changes in various ecological characteristics of forest floor microbial and invertebrate communities. Compared to long‐term fire exclusion, forty‐three years of biennial burning led to significantly N‐depleted and/or P‐enriched stoichiometry in soil, leaf litter, leaf litter–associated microbial biomass, and certain groups of invertebrates, although total invertebrate community stoichiometry was not affected. Microbial biomass was 42% lower in biennially burned soils. Invertebrate community composition differed between fire regime treatments on some sampling dates, but fire regime did not have consistent effects on invertebrate biomass or abundance. Microbial biomass and the abundances of some invertebrate taxa were depressed at particularly low and/or high resource N:P, consistent with a coupling of these variables to the stoichiometric effects of decadal‐scale fire regime. Litter transplants likewise indicated that some invertebrate abundances were sensitive to litter properties over 12 months. Together, our results indicate that long‐term changes in fire regime can decouple the within‐ecosystem cycling of N and P, with N and P cycling growing more and less conservative, respectively, under high‐frequency fire in a way that propagates throughout forest floor food webs. Our study provides new insights into the coupled biogeochemical and ecological responses of forest ecosystems to novel fire regimes and establishes a basis for a stoichiometric framework for fire ecology.

Quercus species control nutrients dynamics by determining the composition and activity of the forest floor fungal community

In forest ecosystems, microbial community structure and activity play an important role in C and nutrient dynamics and are strongly influenced by litter composition. Moreover, litter decomposition rates also depend on microbial activity. Fungi are a key component of the microbial community as they produce extracellular hydrolytic and oxidative enzymes that degrade organic polymers and mobilize inorganic nutrients. In this study, we examined how litter composition affects microbial activity and fungal community structure by comparing decomposing litter derived from sympatric Quercus castanea (Qc) and Quercus deserticola (Qd) in a deciduous forest in Mexico. Microbial N and P biomass were higher in the Qd litter than in the Qc litter. The differences in N and P were seasonal and were not observed at the end of the rainy season (October). The specific enzyme activity was higher in the Qc than in the Qd litter, mainly at the onset of senescence (May). At this time, phenol oxidase and β-glucosidase activities were significantly higher in the Qc litter than in the Qd litter; thereby the nutrient acquisition strategies of the microbial community differ in the litter derived from each Quercus species. Internal transcribed spacer sequence analysis of the fungal community indicated higher richness (24 operational taxonomic units: OTUs) in the Qc litter than in the Qd litter (18 OTUs). Six taxonomic orders of microbes were common to both types of litter, and the phylum Basidiomycota was most abundant in the Qc litter. The lignin and tannin contents were highest in the Qc litter, which also contained diverse fungal taxa associated with POX production. The findings of the present study suggest that the Quercus species control the organic nutrient mineralization by determining the composition and activity of the forest floor microbial community.

Understory Plant Community Composition Is Associated with Fine-Scale Above- and Below-Ground Resource Heterogeneity in Mature Lodgepole Pine (Pinus contorta) Forests.

Understory plant communities play critical ecological roles in forest ecosystems. Both above- and below-ground ecosystem properties and processes influence these communities but relatively little is known about such effects at fine (i.e., one to several meters within-stand) scales, particularly for forests in which the canopy is dominated by a single species. An improved understanding of these effects is critical for understanding how understory biodiversity is regulated in such forests and for anticipating impacts of changing disturbance regimes. Our primary objective was to examine the patterns of fine-scale variation in understory plant communities and their relationships to above- and below-ground resource and environmental heterogeneity within mature lodgepole pine forests. We assessed composition and diversity of understory vegetation in relation to heterogeneity of both the above-ground (canopy tree density, canopy and tall shrub basal area and cover, downed wood biomass, litter cover) and below-ground (soil nutrient availability, decomposition, forest floor thickness, pH, and phospholipid fatty acids (PLFAs) and multiple carbon-source substrate-induced respiration (MSIR) of the forest floor microbial community) environment. There was notable variation in fine-scale plant community composition; cluster and indicator species analyses of the 24 most commonly occurring understory species distinguished four assemblages, one for which a pioneer forb species had the highest cover levels, and three others that were characterized by different bryophyte species having the highest cover. Constrained ordination (distance-based redundancy analysis) showed that two above-ground (mean tree diameter, litter cover) and eight below-ground (forest floor pH, plant available boron, microbial community composition and function as indicated by MSIR and PLFAs) properties were associated with variation in understory plant community composition. These results provide novel insights into the important ecological associations between understory plant community composition and heterogeneity in ecosystem properties and processes within forests dominated by a single canopy species.

Effects of a simulated hurricane disturbance on forest floor microbial communities

Forest floor microbial communities play a critical role in the processes of decomposition and nutrient cycling. The impact of cultivation, contamination, fire, and land management on soil microbial communities have been studied but there are few studies of microbial responses to the effects of tropical storms. The Canopy Trimming Experiment was executed in the Luquillo Experimental Forest of Puerto Rico to decouple two prominent effects of a hurricane—canopy opening and debris deposition on the forest floor—on forest biota and processes. We studied the independent and interactive hurricane effects of canopy openness and debris deposition on the relative abundance and diversity of microorganisms in soil and leaf litter using ester link fatty acids methyl esters (EL-FAME) analysis, and terminal restriction fragment length polymorphism (TRFLP) profile. Non-metric multi-dimensional scaling analysis of soil FAME showed soil microbial community composition was significantly different between pre- and post-hurricane periods including in the unmanipulated plots and among blocks, but there was no significant separation among treatments. This shows that there are strong spatiotemporal dynamics in the structure of soil microbial communities which masked hurricane effects (canopy opening and deposition of green debris). The degree of difference among treatments decreased with time in soil which suggests that our study may have started too late after the manipulations and therefore missed the effects of canopy opening and debris addition. This reflects the resilience of the soil microbial communities. The richness of soil bacterial TRF’s however, showed a significant positive response to added debris. Neither fungal nor bacterial NMDS clusters for leaf microbial communities showed significant grouping by treatment, time or litter cohorts. Significant differences were observed through time for fungal diversity in green leaves and for both bacterial and fungal diversity in senesced leaves. Senesced leaves microbial succession apparently stopped when both the canopy and debris were removed, and there was a suggestive trimming by time interaction which reflects the susceptibility of the leaf litter microbial community. Our findings contribute to the understanding of how microbial community structures can be affected by hurricane disturbances and forestry management practices that remove canopy and debris from the forest floor, and shows the need to analyze the microbial community immediately after the disturbance. Short-term changes in microbial communities due to forest disturbances can have significant implications for litter decomposition, soil organic matter accumulation, nutrient cycling, and food web dynamics in tropical forests. All of these factors should be taken into consideration when selecting the appropriate forest management practice.

Linkages between the forest floor microbial community and resource heterogeneity within mature lodgepole pine forests

Below-ground microbial communities are influenced by a wide variety of above and below-ground abiotic and biotic factors. Thus, variation in properties, such as vegetation composition and cover, litter cover, soil pH, and nutrient availability likely influence their structure and composition. Examining microbial community variation within forests dominated by just a single tree species provides insights into the factors, other than dominant tree species, that influence the structure and function of below-ground microbial communities at the within-stand scale. In this study we examined fine scale (within-stand) patterns in microbial communities within mature lodgepole pine forests, and their relationships with abiotic and biotic factors. Specifically, the existence of fine-scale microbial structure (using phospholipid fatty acids – PLFAs) and function (using respiration of multiple carbon substrates – MSIR) community types; which PLFAs and carbon substrates were indicators of these, and which above- and below-ground properties and processes were related to these community types. At 108 sampling points in 12 0.48-ha plots abundance of understory plants, canopy tree size and cover, downed wood biomass, forest floor thickness, litter cover, pH, decomposition rate, available soil nutrients, microbial PLFAs and MSIR were assessed. Cluster and indicator species analysis revealed four fine-scale structural (PLFA) and four functional (MSIR) microbial community types. The biotic and abiotic variables measured had low explanatory power for describing the MSIR communities but high explanatory power for describing the PLFA communities. The main factors contributing to the separation of the structural (PLFA) microbial community types were understory plant species. Our findings suggest that the spatial partitioning of understory plant species and their rhizosphere resources (e.g., root exudates, nutrients) creates heterogeneity that influences the patterns of variation in below-ground microbial communities, in particular the microbial structure.

Forest floor properties across sharp compositional boundaries separating trembling aspen and jack pine stands in the southern boreal forest

Jack pine and trembling aspen are two early-seral boreal tree species with contrasting nutrient cycling strategies. Both species may form adjacent mono-specific stands separated by sharp compositional boundaries. We hypothesized that such boundaries result in wider functional ecotones. Spatial transitions in humus forms, forest floor chemistry and microbial communities were assessed across 32 m long transects set perpendicular to sharp compositional boundaries separating four jack pine and aspen stands. Split moving window analysis (SMWA) and moving window regression analysis (MWRA) were used to locate functional boundaries and ecotones. We found a gradual transition from moder (aspen) to mor (jack pine) humus spanning 16 m across the compositional boundary. An abrupt increase in forest floor water content at 3 m within jack pine stands was possibly due to aspen roots foraging for water beyond the boundary. The functional boundary and associated ecotone for forest floor pH, C:N ratio, Mg and ammonification were skewed toward jack pine stands, likely the result of aspen leaf dispersal. Low nitrification rates throughout jack pine stands and up to 11 m into aspen stands suggested that jack pine roots might extend far within aspen stands and produce metabolites that suppress nitrification. SMWA performed on the multivariate dataset of microbial fatty acids (FAs) revealed three distinct forest floor microbial communities that were skewed toward jack pine stands. Pine-type communities were associated to fungal FAs, pine-type and transition-type communities to non-fungal eukaryotic FAs, and aspen-type communities to bacterial FAs. Taken collectively, our data delimit a 24 m wide functional ecotone straddling sharp compositional boundaries separating trembling aspen and jack pine stands. We conclude that the functional diversity of boreal landscapes, where adjacent mono-specific stands are prevalent, is related to the patchiness of the landscape.

Effects of tree species and N additions on forest floor microbial communities and extracellular enzyme activities

Forest nitrogen (N) retention and soil carbon (C) storage are influenced by tree species and their associated soil microbial communities. As global change factors alter forest composition, predicting long-term C and N dynamics will require understanding microbial community structure and function at the tree species level. Because atmospheric N deposition is increasing N inputs to forested ecosystems across the globe, including the northeastern US, it is also important to understand how microbial communities respond to added N. While prior studies have examined these topics in mixed-species stands, we focused on the responses of different tree species and their associated microbial communities within a single forest type – a northern hardwood forest in the Catskills Mountains, NY. Based on prior studies, we hypothesized that N additions would stimulate extracellular enzyme activities in relatively labile litters, but suppress oxidative enzyme activities in recalcitrant litters, and tested for independent tree species effects within this context. During the 2007 growing season (May–June), we measured enzyme activities and microbial community composition (using phospholipid fatty acid analysis - PLFA) of the forest floor in single-species plots dominated by sugar maple ( Acer saccharum ), yellow birch ( Betula alleghaniensis ), red oak ( Quercus rubra ), American beech ( Fagus grandifolia ) and eastern hemlock ( Tsuga canadensis ), species whose litters range from relatively labile to recalcitrant. Half the plots were fertilized with N by adding NH 4 NO 3 (50 kg ha −1 y −1 ) from 1997 to 2009. Non-metric multidimensional scaling (NMS) and multi-response permutation procedures (MRPP) were used to examine microbial community structure and relationship to enzyme activities. We found that in response to N additions, both microbial community composition and enzyme activities changed; however the strength of the changes were tree species-specific and the direction of these changes was and not readily predictable from prior studies conducted in mixed-species stands. For example, in contrast to other studies, we found that N additions caused a significant overall increase in fungal biomass that was strongest for yellow birch (24% increase) and weakest for sugar maple (1% increase). Contrary to our initial hypotheses and current conceptual models, N additions reduced hydrolytic enzyme activities in hemlock plots and reduced oxidative enzyme activity in birch plots, a species with relatively labile litter. These responses suggest that our understanding of the interactions between microbial community composition, enzyme activity, substrate chemistry, and nutrient availability as influenced by tree species composition is incomplete. NMS ordination showed that patterns in microbial community structure (PLFA) and function (enzyme activity) were more strongly influenced by tree species than by fertilization, and only partially agreed with the structure–function relationships found in other studies. This finding suggests that tree species-specific responses are likely to be important in determining the structure and function of northeastern hardwood forests in the future. Enhanced understanding of microbial responses to added N in single and mixed-species substrates with varying amounts of lignin and phenols may be needed for accurate predictions of future soil C and N dynamics.

Microbial community structure and function: The effect of silvicultural burning and topographic variability in northern Alberta

Forest floor chemistry and microbial communities can be influenced by forest land management, such as harvesting and prescribed burning. Here, we used phospholipid fatty acid (PLFA) and multiple carbon-source substrate-induced respiration (MSIR) analyses to characterize microbial communities of deciduous, mixedwood and coniferous boreal forest floors with different silvicultural treatments. The sites were stem-only harvested with 10% retention, and silvicultural treatments consisting of slash being evenly distributed on the site and then burned, or not burned. The burned sites exhibited lower microbial biomass and greater NO 3 − concentrations than the unburned sites. However, burning appeared to have no effect on forest floor microbial community structure or function. On the other hand, during drier months (August sampling), the composition of forest floor microbial communities appeared to be strongly influenced by topographic position rather than stand related differences. Harvested sites located at higher elevations had similar microbial communities, regardless of the overstory composition, while coniferous and mixedwood sites located at lower elevations had similarly structured microbial communities that were distinct from deciduous sites. Differences in microclimatic conditions of the forest floor between higher elevation sites and lower elevations sites may select for some microbial groups over others. Indicator analysis found a strong association of a fungal PLFA biomarker (20:1ω9c), with sites at higher elevation, while a biomarker for actinomycetes (10Me19:0) was strongly associated with deciduous sites at lower elevation. Structural differences in microbial communities observed between sites at higher and lower elevations appear to be linked to seasonal patterns in moisture, as previous studies in this region found no apparent effect of elevation during times of higher monthly precipitation.

The microbial communities of aspen and spruce forest floors are resistant to changes in litter inputs and microclimate

Previous studies have shown that forest floors from stands dominated by trembling aspen (ASPEN; Populus tremuloides Michx.) tend to support a greater microbial biomass with a different microbial community structure than forest floors from stands dominated by white spruce (SPRUCE; Picea glauca (Moench) Voss). A reciprocal transfer experiment, in concert with coarse and fine mesh bags that allowed or excluded fine root in-growth, was used to examine how the composition of these forest floor microbial communities respond to changes in belowground inputs from fine roots, aboveground inputs (e.g. from litter and through-fall) and soil microclimatic conditions over 1 year. Neither the microbial biomass nor the microbial community structure (assessed using phospholipid fatty acid analyses and substrate-induced respiration techniques) of forest floors of ASPEN or SPRUCE origin were altered by reciprocal transfer to SPRUCE or ASPEN stands, with or without fine root inputs. Despite the lack of changes in microbial community structure, the stand type during incubation had a strong effect on forest floor moisture content and concentrations of nitrate, while mesh size had a significant effect on forest floor pH and the abundance of mesofauna. Thus, changes in microbial community structure did not co-occur with changes in other characteristics of these forest floors. The resistance of the forest floor microbial communities to change may be a function of the high C contents of these soils. Further treatment effects may have been detected if the study had been extended beyond 1 year. Reciprocal transfer studies using coarse and fine mesh bags allow transferred soils to respond to fluctuations in microclimate, organic inputs and soil biota and, therefore, hold considerable promise for studies examining the influence of disturbances on soil properties.

Forest floor microbial communities in relation to stand composition and timber harvesting in northern Alberta

With the growing interest in silvicultural techniques that more closely emulate natural disturbance regimes, there is a need to better understand how partial harvesting affects the soil microbial community in stands with varying ecological characteristics, e.g., tree species composition. Four and a half and 5.5 years post-harvest, we used phospholipid fatty acid (PLFA) and substrate-induced respiration (SIR) analyses to compare the microbial biomass and microbial community structure of forest floors from stands dominated by white spruce ( Picea glauca; SPRUCE) or by trembling aspen ( Populus tremuloides; ASPEN) and from mixed-species (MIXED) stands in northern Alberta, Canada, that had been clearcut, partial-cut with 20% retention, partial-cut with 50% retention or left uncut (controls). PLFA and SIR analyses revealed that ASPEN forest floors supported a larger microbial biomass with a very different community structure than MIXED or SPRUCE forest floors. The microbial community structure of these soils appeared to be strongly affected by the presence of white spruce and the composition of the understory vegetation. There were no effects of timber harvesting detected within or across stand types on any of the variables measured, with the exception of the PLFA 16:1ω5, which was relatively more abundant in the clearcuts and 50% retention treatments than in the uncut controls, perhaps in response to an increased forest floor pH and grass cover in the disturbed areas. The resilience to timber harvesting of the forest floors from these stands may be the result of efforts to minimize soil disturbance during harvesting and to allow vegetation to regenerate naturally. From the perspective of the forest floor microbial community, partial harvesting does not appear to have any benefit over clearcut harvesting at these boreal forest sites.

Forest-floor chemical properties are altered by clear-cutting in boreal mixedwood forest stands dominated by trembling aspen and white spruce

Alterations in the chemical properties of the forest floor following clear-cut harvesting may have implications for forest productivity in boreal stands. We used proximate analysis, carbon-13 (13C) isotopic determination, and cross-polarization, magic-angle spinning (CPMAS) 13C nuclear magnetic resonance (NMR) spectroscopy to examine differences in the characteristics of the forest floors from uncut stands and clear-cut stands dominated by white spruce (Picea glauca (Moench) Voss; SPRUCE) and trembling aspen (Populus tremuloides Michx.; ASPEN) in northern Alberta. Proximate analysis revealed no difference in the chemical properties of forest floors from clear-cut and uncut stands in either stand type, but the acid-insoluble residue of forest floors from clear-cut ASPEN stands was enriched in 13C compared with those from uncut ASPEN stands. CPMAS 13C NMR spectroscopy revealed that forest floors from clearcuts were enriched in total aromatic C, particularly in ASPEN stands, and depleted in phenolic C, particularly in SPRUCE stands. These patterns indicate that forest floors from the clearcuts have become more humified, which may reflect stand-type differences in the amount of labile C available to the forest-floor microbial community and reductions in above- and below-ground inputs to the forest floor following clear-cutting in both stand types. Changes in the chemical properties of forest floors from clear-cut SPRUCE and ASPEN stands could exacerbate C limitation in these soils and alter patterns of nutrient cycling.

Microbial communities in forest floors under four tree species in coastal British Columbia

We compare forest floor microbial communities in pure plots of four tree species ( Thuja plicata, Tsuga heterophylla, Pseudotsuga menziesii, and Picea sitchensis) replicated at three sites on Vancouver Island. Microbial communities were characterised through community level physiological profiles (CLPP), and profiling of phospholipid fatty acids (PLFA). Microbial communities from cedar forest floors had higher potential C utilisation than the other species. The F layer of the forest floor under cedar contained significantly higher bacterial biomass (PLFA) than the F layer under the other three tree species. There were differences in microbial communities among the three sites: Upper Klanawa had the highest bacterial biomass and potential C utilisation; this site also had the highest N availability in the forest floors. Forest floor H layers under hemlock and Douglas-fir contained greater biomass of Gram positive, Gram negative bacteria and actinomycetes than F layers based on PLFA, and H layers under spruce contained greater biomass of Gram negative bacteria than F layers. There were no significant differences in bacterial biomass between forest floor layers under cedar. Fungal biomass displayed opposite trends to bacteria and actinomycetes, being lowest in cedar forest floors, and highest in the F layer and at the site with lowest N availability. There were also differences in community composition among species and sites, with cedar forest floors having a much lower fungal:bacterial ratio than spruce, hemlock and Douglas-fir. The least fertile Sarita Lake site had a much greater fungal:bacterial ratio than the more fertile San Juan and Upper Klanawa sites. Forest floor layer had the greatest effect on microbial community structure and potential function, followed by site, and tree species. The similarity in trends among measures of N availability and microbial communities is further evidence that these techniques provide information on microbial communities that is relevant to N cycling processes in the forest floor.

Forest floor microbial community response to tree species and fertilization of regenerating coniferous forests

We studied the effect of tree species and fertilization on the forest floor microbial community of 15-year-old regenerating forests. We sampled F and H forest floor layers of plots planted to Thuja plicata (Donn ex D. Don.) or Tsuga heterophylla (Raf.) Sarg. on N-poor and N-rich sites, with and without fertilizer treatments. Microbial community composition was assessed using phospholipid fatty acid analysis and by enumerating populations of culturable bacteria and fungi. Potential microbial functioning was assessed using community-level physiological profiling. There was little differentiation of community-level physiological profiles of F and H layers and few differences among the treatments. Total microbial biomass was greater in the F than H layer, and the two layers had distinct phospholipid fatty acid profiles. Site effects were detected mainly in the residual H layer, and tree species effects were seen mainly in the F layer, which has developed since harvesting. The effect of fertilization depended on site and tree species, with very little response in cedar plots, and the greatest effects in hemlock plots, coinciding with the greater growth response of hemlock. These results indicate that differences in plant growth rates, rather than direct effects of fertilization, influenced the microbial communities.

Inferred effects of cloud deposition on forest floor nutrient cycling and microbial properties along a short elevation gradient

Cloud water deposition often increases with elevation, and it is widely accepted that this cloud water increases acid loading to upland forest ecosystems. A study was undertaken in south-eastern Québec to determine if a 250 m elevation gradient (i.e. 420–665 m), along a uniform sugar-maple stand on the slope of Mount Orford, corresponded to a pH gradient in the forest floor and to predictable changes in soil nutrient availability and microbial properties. Precipitation data from a nearby study, and a photographic survey, provided presumptive evidence that this elevation gradient corresponded to a strong gradient in cloud water deposition. Forest floor temperature did not differ significantly across elevations. Forest floor moisture content was significantly higher, whereas pH and exchangeable Ca and Mg were significantly lower, at the higher elevations. Average seasonal net nitrification rates, determined by long-term laboratory incubations, did not differ significantly across elevations, whereas average seasonal net ammonification rates were significantly higher at higher elevations. Basal respiration rates and microbial biomass did not differ significantly across elevations, but metabolic quotient was significantly higher at higher elevations indicating possible environmental stress on forest floor microbial communities due to cloud water deposition. Anaerobic N mineralisation rates were significantly higher at higher elevations suggesting that N-limited microbial communities frequently exposed to cloud cover can be important short-term sinks for atmospheric N, thereby contributing to increase the active-N fraction of forest floors. We conclude that, where no significant changes in vegetation or temperature occur, elevation gradients can still be used to understand the spatial variability of nutrient cycles and microbial properties.

MICROBIAL ENZYME SHIFTS EXPLAIN LITTER DECAY RESPONSES TO SIMULATED NITROGEN DEPOSITION

Some natural ecosystems near industrialized and agricultural areas receive atmospheric nitrogen inputs that are an order of magnitude greater than those presumed for preindustrial times. Because nitrogen (N) often limits microbial growth on dead vegetation, increased N input can be expected to affect the ecosystem process of decomposition. We found that extracellular enzyme responses of a forest-floor microbial community to chronically applied aqueous NH4NO3 can explain both increased and decreased litter decomposition rates caused by added N. Microbes responded to N by increasing cellulase activity in decaying leaf litter of flowering dogwood, red maple, and red oak, but in high-lignin oak litter, the activity of lignin-degrading phenol oxidase declined substantially. We believe this is the first report of reduced ligninolytic enzyme activity caused by chronic N addition in an ecosystem. This result provides evidence that ligninolytic enzyme suppression can be an important mechanism explaining decreased ...