Variation in NH4 mineralization and microbial communities with stand age in lodgepole pine (Pinus contorta) forests, Yellowstone National Park (USA)

Soils and vegetation were analyzed in 20 lodgepole pine (Pinus contorta) forest stands, varying in age from 50 to 350 years, that had initiated following stand-replacing fire. Our goal was to determine how nitrogen availability (NH₄⁺-N) and microbial community composition varied with stand age-class and to determine whether differences could be explained by canopy, soil, or understory characteristics. Gross NH₄⁺ mineralization was measured using laboratory isotopic pool dilution, and microbial community composition was evaluated using microbial membrane lipids. The microbial community composition of stands in the 300-350 age class was distinct from stands in younger age classes. Microbial community composition among sites varied with pH, % organic matter, and phosphorus. Gross NH₄⁺ mineralization rates averaged 1.45±0.07 mg NH₄⁺ kg soil⁻¹ d⁻¹ while consumption averaged 1.37±0.20 mg NH₄⁺ kg soil⁻¹ d⁻¹, resulting in low net NH₄⁺ mineralization rates (0.08±0.18 mg NH₄⁺ kg soil⁻¹ d⁻¹), but rates were not significantly different with stand age-class at p<0.05. At p<0.10, net NH₄⁺ mineralization was significantly higher in the 300-350 age class compared to the 125-175 age class. None of the measured variables significantly explained NH₄⁺ consumption and net mineralization patterns. However, gross NH₄⁺ mineralization rates were best explained by information on microbial community structure (i.e. lipids). Variation among stands within a given age-classes was high, indicating that patterns of N cycling across landscapes reflect substantial heterogeneity among mature stands.     

Changes in chemical composition of Pinus sylvestris needle litter during decomposition along a European coniferous forest climatic transect

The objective of this investigation was to assess the changes in chemical composition (lignin, cellulose, hemicelluloses, non-structural compounds, N, and ash) of decomposing litter. Standard Pinus sylvestris needle litter, originating from southern Sweden, was incubated in litterbags at 15 sites selected from the Netherlands to south Spain. The changes in chemical composition of this litter were determined using near infrared reflectance spectroscopy. The hypothesis was that standard (chemically uniform) litter decomposing under a range of climates would show different dynamics of accumulation and loss of C-fractions, N, and ash, relative to mass loss. It was shown that, for a given mass-loss value (10, 20, 30, 40, or 50%), the proportion of lignin, cellulose, hemicelluloses, non-structural compounds, N, and ash in the decomposing pine needles differed between sites. Lignin concentration in the litter residue at 50% mass loss ranged from approximately 26 to 43%, cellulose from 19 to 27%, hemicelluloses from 7 to 11%, non-structural compounds from 19 to 25%, N from 0.7 to 1.3%, and ash content from 1.4 to 10.1%. Lignin concentrations showed the highest range of variation. Lignin concentrations during decomposition were positively related to moisture factors as significant correlations were found with actual evapotranspiration and were improved in multiple regressions by the mean annual precipitation or the water surplus. Cellulose was degraded further at sites with high precipitation whereas hemicellulose degradation was related to temperature. This leads to the conclusion that the remaining organic matter produced by standard litter decomposition within the studied climatic range of variations tended to be more recalcitrant under wet and warm climatic conditions than under cold or dry climate.     

The biochemical transformation of oak (Quercus robur) leaf litter consumed by the pill millipede (Glomeris marginata)

Soil macrofauna play an essential role in the initial comminution and degradation of organic matter entering the soil environment and yet the chemical effects of digestion on leaf litter are poorly understood at the molecular level. This study was undertaken to assess the selective chemical transformations that saprophagous soil invertebrates mediate in consumed leaf litter. A number of pill millipedes (Glomeris marginata) were fed oak leaves (Quercus robur) after which the biomolecular compositions (lipids and macromolecular components) of the leaves and millipede faeces were compared using a series of wet chemical techniques and subsequent analysis by gas chromatography (GC) and gas chromatography-mass spectrometry (GC/MS). It was found that the concentrations of short chain (<C₂₀) n-alkanoic acids, sterols and triacylglycerols reduced dramatically in the millipede faeces relative to the leaf litter. Hydrolysable carbohydrates and proteins both decreased in concentration in the faeces, whereas similar yields of phenolic components were observed for the cupric oxidation products of lignin, although the oxygenated functionalities were affected by passage through the millipede gut, yielding a more highly condensed state for lignin. This shows that the chemical composition of fresh organic matter entering the soil is directly controlled by invertebrates feeding upon the leaf litter and as such that they are key contributors to the early stages of diagenesis in terrestrial soils.     

A possible role for saprotrophic microfungi in the N nutrition of ectomycorrhizal Pinus resinosa

We determined whether Pinus resinosa, selected ectomycorrhizal and saprotrophic microfungi have access to various organic nitrogen sources commonly found in the forest. Vector analysis demonstrated nitrogen limitation of the P. resinosa in the plantation from which most of the fungi were isolated, establishing this study's relevance. Nonmycorrhizal P. resinosa seedlings did not absorb significant N from amino acids. The ectomycorrhizal fungi, including Pisolithus tinctorius, Suillus intermedius and Tylopilus felleus, obtained substantial N from amino acids, a limited amount of N from glucosamine, and essentially no N from protein-tannin complex. In contrast, Penicillium and Trichoderma readily acquired N from protein-tannin and glucosamine. Thus, there was an increasing ability to obtain N from complex organic N sources from plant to ectomycorrhizal fungi to saprotrophic fungi. Furthermore, N mineralization from an organic N source by Penicillium depended on the C:N ratio. We conclude that acquisition of relatively simple organic N sources by P. resinosa is likely to be largely indirect via ectomycorrhizal fungi, and that more complex organic N sources may become accessible to ectomycorrhizal fungi (and thus possibly their host plants) following mineralization by saprotrophic fungi such as Penicillium or Trichoderma when C:N ratios are sufficiently low.     

Soil organic N dynamics and stand quality in Pinus radiata pinewoods of the temperate humid region

Soil organic-N dynamics, its controlling factors and its relationships with stand quality were studied in the 0-15 cm soil layer of 24 pinewoods with contrasting age, productivity and parent material (granite; acid schists), searching for N variables useful to predict stand growth and site quality. No significant differences were found between young and old stands for any of the N variables considered, nor two- or three-order interactions among stand age, site quality and parent material. The soil total-N content, which was correlated positively with the Al oxides content (a soil organic matter (SOM) stabilizing agent), did not vary significantly according to parent material, but it was lower (P≤0.02) in stands with high than with low site index (2.68±1.11 and 3.97±1.13 g N kg⁻¹ soil, respectively). The soil δ¹⁵N ranged from +3.5 to +6.5 δ, without significant differences among stand groups, and it was negatively correlated with water holding capacity, exchangeable bases, Al oxides and N content, suggesting that: (i) N losses by NO₃⁻ leaching are the most important controlling factor of δ¹⁵N in these temperate humid region soils; and (ii) soil N richness is related with limited N losses, which discriminate against ¹⁵N. At any incubation time, no significant differences were found in soil inorganic-N content among stand groups (7.78±4.57, 39.33±16.20 and 67.80±26.50 mg N kg⁻¹ soil at 0, 42 and 84 d, respectively). During the incubation, the relative importance of ammonification decreased and that of the nitrification increased. The net N mineralization rate (NNMR, in percentage of organic N) was significantly higher in granite than in schists soils at both 42 d (1.24±0.34 and 0.75±0.37%, respectively) and 84 d (2.18±0.56 and 1.53±0.66%, respectively). In high quality pinewoods, the NNMR at 42 and 84 d (1.16±0.45 and 2.12±0.79%, respectively) were significantly higher than in low quality stands (0.83±0.35 and 1.59±0.45%, respectively). This result, together with those on soil total-N and inorganic-N supply, suggests that soil N dynamics in low and high quality stands are different: in the former there is a bigger N pool with a slower turnover, whereas in the latter there is a smaller N pool with a faster turnover, both factors being nearly compensated, making the soil available N supply in both types of stand similar. After 42 and 84 d of incubation, the NNMR and the nitrification rates were higher in the coarse textured soils, likely due to the low physical and chemical protection of their SOM; both rates were positively correlated with available P, exchangeable K⁺ and CEC base saturation, suggesting strong relationships among the availabilities of the main plant nutrients, and they increased with SOM quality (low C-to-N ratio). The strong negative correlation of site index with soil total-N (r=−0.707; P≤0.005), and its positive correlations with NNMR after 42 and 84 d of incubation, suggested that site quality and potential productivity are closely related to soil organic-N dynamics. Half of the site index variation in the stands studied could be predicted with a cheap and easy analysis of soil N content, the prediction being slightly improved if soil δ¹⁵N is included and, more significantly, by including N mineralization measurements.     

Respiration and growth of a fungus, Mortierella isabellina, in response to grazing by Onychiurus armatus (Collembola)

Colonies of the fungus Mortierella isabellina (Oudem) were grazed by the collembolan Onychiurus armatus (Tullb.) for short periods interrupted by incubation without grazing in laboratory experiments. The grazed fungus had a lower respiration than ungrazed when Collembola were present on the mycelium. Fungal respiration was significantly increased with grazing when Collembola were periodically removed from the mycelium. Metal contamination of the substrate increased fungal respiration, regardless of grazing. Length of fungal mycelium was significantly greater in a natural soil in the presence of Collembola but reduced in a metal-polluted soil. Factors that may determine the impact of grazing on fungi are discussed.     

Exploitation of litter and soil by the train millipede Parafontaria laminata (Diplopoda: Xystodesmidae) in larch plantation forests in Japan

Swarms of the train millipede (Parafontaria laminata) are known to occur every 8 years; during the swarms, adults emerging from the soil have an extremely high density in natural and plantation forests in central Japan. Their influence on organic layer accumulation on forest floors was investigated under field conditions in plantation larch forests in central Japan, before and after the adult swarming period of 2000. In addition, the adult millipede feeding preference and the amount of food consumed were observed in the laboratory. The field density of adults ranged from 11 to 311 individuals m⁻² in October 2000; the highest biomass was 28.6±16.4 g dry wt m⁻². Reduction of the forest floor organic layer was density- dependent when the population consisted of 7th instar larvae; however, the adult population did not consume in a density-dependent manner. Higher levels of geophagy were observed in high-density treatments in the laboratory. We hypothesized that the geophagy of the millipedes was partly supported by the naturally high organic matter content of Andosol, and that soil consumption would sustain an extremely high biomass in a temperate conifer forest soil.     

The mycorrhizal fungus (Glomus intraradices) affects microbial activity in the rhizosphere of pea plants (Pisum sativum)

Pea plants were grown in γ-irradiated soil in pots with and without addition of the AM fungus Glomus intraradices at sufficient N and limiting P. Depending on the growth phase of the plant presence of AM had negative or positive effect on rhizosphere activity. Before flowering during nutrient acquisition AM decreased rhizosphere respiration and number of protozoa but did not affect bacterial number suggesting top-down regulation of bacterial number by protozoan grazing. In contrast, during flowering and pod formation AM stimulated rhizosphere respiration and the negative effect on protozoa decreased. AM also affected the composition of the rhizosphere bacterial community as revealed from DNA analysis (DGGE). With or without mycorrhiza, rhizosphere respiration was P-limited on very young roots, not nutrient limited at more mature roots and C-limited at withering. This suggests changes in the rhizosphere community during plant growth also supported by changes in the bacteria (DGGE).     

Relative saprophytic competence of Rhizobium japonicum strains in soils as determined by the quantitative fluorescent antibody technique (FA)

Experiments were conducted to estimate the saprophytic competence of four Rhizobium japonicum strains (110, 123, 532c, and 586) in a Rossmoyne silt loam (almost free of R. japonicum) and a Miamian silt loam (containing indigenous R. japonicum strains).All strains increased about 100-fold in sterilized Rossmoyne or Miamian soil during the first 2 weeks, and the population remained constant for 26 and 6 weeks respectively. Such results indicated that abiotic factors were not limiting Rhizobium populations in these soils under laboratory conditions. There was no inter-strain competition when strains were mixed and applied to either sterilized soil.All R. japonicum strains decreased about ten-fold but survived well in non-sterile Rossmoyne soil during 26 weeks incubation. But, all strains died at varying rates in non-sterile Miamian soil during a 6 week incubation. The most rapid decline occurred with strain 123, which decreased about 1000-fold during the first 5 days, while strain 586, decreased about 1000-fold during the first 10 days. The die-back of strain 123 was found to coincide with an increase in numbers of a parasitic microorganism which passed through a 0.45 μm pore-sized filter. A bacteriophage has been implicated, although Bdellovibrio cannot be ruled out completely. This parasitic microorganism is strain specific but does not lyse a field isolate of serogroup 123.     

How browsing by red deer impacts on litter decomposition in a native regenerating woodland in the Highlands of Scotland

Herbivores can indirectly affect ecosystem productivity and processes such as nutrient cycling and decomposition by altering the quantity and quality of resource inputs into the decomposer subsystem. Here, we tested how browsing by red deer impacts on the decomposition of, and nutrient loss from, birch leaf litter (Betula pubescens), and tested whether effects of browsing on these measures were direct, via alteration of the quality of leaf litter, or indirect through long term impacts of deer browsing on soil biological properties. This was tested in a microcosm experiment using soil and litter taken from inside and outside three individual fenced exclosures located at Creag Meagaidh National Nature Reserve, Scotland. We found that litter of un-browsed trees decomposed faster than that from browsed trees, irrespective of whether soil was sourced from inside or outside exclosures. These findings suggest that effects of browsing on litter quality, rather than on soil biological properties, are the key determinant of enhanced decomposition in un-browsed areas of this ecosystem. Despite this, we found no consistent impact of browsing on litter C:N, a key indicator of litter quality; however, the rate of litter decomposition was linearly and negatively related to litter C:N when analysed across all the sites, indicating that this measure, in part, contributed to variation in rates of decomposition in this ecosystem. Our findings indicate that herbivores impact negatively on rates of decomposition in this ecosystem, ultimately retarding nutrient cycling rates, and that these effects are, in part, related to changes in litter quality.     

Effects of forest stand type on oribatid mite (Acari: Oribatida) assemblages in a southwestern Quebec forest

Despite the ubiquity of oribatid mites in soil and litter systems, and their importance in decomposition and nutrient cycling processes, little is known of the factors underlying the composition of their assemblages. Our objective was to address this by determining how oribatid assemblage composition changes by forest stand type. This work was done in and near a hardwood forest in southwestern Quebec, Canada. We sampled mites by collecting 1 L of litter and 170 cm³ of soil from four sites in each of four distinct habitat types: American beech stands, sugar maple stands, mixed deciduous stands and mixed conifer plantations. Samples were collected in July and September 2005, and June 2006, and over 6500 oribatid mites were collected and identified to species. Abundance and species richness differed between forest types: for abundance conifer>beech>maple>mixed deciduous while for species richness beech and conifer>maple>mixed deciduous. Ordination analyses revealed that conifer plantations and beech stands supported distinct assemblages, while there were some overlap in the assemblages found in maple stands and mixed deciduous stands. These data support the importance of aboveground plant communities in affecting the composition of oribatid assemblages even at local scales and provide insight into additional impacts that may be caused by shifts in plant species ranges due to global changes.     

Decomposition of beech leaves (Fagus sylvatica) and spruce needles (Picea abies) in pure and mixed stands of beech and spruce

The decomposition of spruce needles and beech leaves was investigated in a 30- and 120-yr-old beech, spruce and mixed (beech/spruce) forest using 1 mm mesh litterbags. The mass loss, content of C, N and water and microbial biomass, basal respiration and specific respiration of the litter materials were analyzed after exposure for 1.5, 3, 6, 9, 12, 18 and 24 months in the field. Decomposition of both types of litter was faster in beech than in spruce stands and after 24 months loss of C from litter materials was at a maximum in beech stands (>60%) and considerably less in the spruce and mixed stands (ca. 40%). Generally, spruce needles decomposed more rapidly than beech leaves, but the faster decay was not associated with higher N concentrations. Rather, N was accumulated more rapidly in beech leaves. Concomitantly, in beech stands microbial biomass of beech leaves exceeded that of spruce needles indicating that beech leaves consist of more favorable resources for microorganisms than spruce needles. Differences in decomposition between beech leaves and spruce needles were most pronounced in beech stands, intermediate in mixed stands and least pronounced in spruce stands. Decomposition, N content and microbial biomass in litter materials exposed in the 120-yr-old stand consistently exceeded that in the 30-yr-old stand indicating adverse conditions for litter decay in regrowing stands. Generally, mixed stands ranked intermediate between spruce and beech monocultures for most of the variables measured indicating that the adverse conditions for litter decay and microorganisms in spruce forest are effectively counteracted by admixture of beech to spruce monocultures. It is concluded that the accumulation of litter materials in spruce forests is not due to the recalcitrance of spruce needles to decay. Rather, adverse environmental conditions such as high polyphenol contents in the litter layer of spruce stands retard decomposition processes; spruce needles appear to be more sensitive to this retardation than beech leaves.     

Effects of heavy metals on the litter consumption by the earthworm Lumbricus rubellus in field soils

Aim of this study was to determine effects of heavy metals on litter consumption by the earthworm Lumbricus rubellus in National Park the “Brabantsche Biesbosch”, the Netherlands. Adult L. rubellus were collected from 12 polluted and from one unpolluted field site. Earthworms collected at the unpolluted site were kept in their native soil and in soil from each of the 12 Biesbosch sites. Earthworms collected in the Biesbosch were kept in their native soils. Non-polluted poplar (Populus sp.) litter was offered as a food source and litter consumption and earthworm biomass were determined after 54 days. Cd, Cu and Zn concentrations were determined in soil, pore water and 0.01 M CaCl₂ extracts of the soil and in earthworms. In spite of low available metal concentrations in the polluted soils, Cd, Cu and Zn concentrations in L. rubellus were increased. The litter consumption rate per biomass was positively related to internal Cd and Zn concentrations of earthworms collected from the Biesbosch and kept in native soil. A possible explanation is an increased demand for energy, needed for the regulation and detoxification of heavy metals. Litter consumption per biomass of earthworms from the reference site and kept in the polluted Biesbosch soils, was not related to any of the determined soil characteristics and metal concentrations.     

Soil ecosystem engineering by the train millipede Parafontaria laminata in a Japanese larch forest

Periodic swarming by adult train millipedes Parafontaria laminata (Attems, 1909) occurs in central Japan on an 8-year cycle, and the emergence of new adults is highly predictable. Millipede biomass reaches a maximum and feeding habits change upon the emergence of adults. Larvae are geophagous while adults feed on both litter and soil. We hypothesized that the shift in the developmental stages of P. laminata influenced the carbon dynamics in the soil and conducted a field mesocosm experiment in a larch plantation forest over 2 years (1999 and 2000) using three developmental stages: sixth- and seventh-instar larvae and adults. By experimentally manipulating millipede density at four levels, we obtained the following results: larvae were geophagous, while adults consumed both litter and soil (mixed-feeding) and consequently showed stronger density effects on litter decomposition rates than larvae; adult activities in the high-density treatment increased soil microbial biomass but not at low adult densities or at the larval stages; and adults increased the carbon accumulation in soil layers especially at high densities due to their mixed-feeding on litter and soil. We determined that due to synchronized postembryonic development with high densities and changes in feeding habits, the train millipede periodically sequestered carbon in this forest.     

A multi-scale analysis of species-environment relationships: rare moths in a pitch pine-scrub oak (Pinus rigida-Quercus ilicifolia) community

Community level analyses of species-environment relationships can provide critical information for conservation planners. A comprehensive analysis of the factors that influence species distributions should include variables measured at multiple scales because species-environment relationships are known to vary with the scale of observation. The pitch pine-scrub oak (Pinus rigida-Quercus ilicifolia) communities, or pine barrens, of the northeastern USA are severely threatened by development and fire suppression. They also provide critical habitat for many species of rare moths. We used partial canonical correspondence analysis to assess the relative effects of three levels of environmental variables (plot, patch, and landscape) on the distribution and abundance of 10 species of rare moths in a pine barrens community in southeastern Massachusetts, USA. We also used a set of spatial variables to quantify and partial out the effects of spatial autocorrelation of species composition among sampling locations. All three levels of environmental factors combined, independent of spatial factors, accounted for virtually half (48.4%) of the total variation in the moth community. Sequential partitioning of the variance explained by each level of environmental factors indicated that landscape level factors explained more than twice as much variance as plot and patch level factors. Another environmental model that included only landscape level variables explained 53% of the total variation in the moth community. Patch density and percentage of the landscape comprised of open and sparse canopy, scrub oak habitats were the most significant variables. These results suggest that the presence of scrub oak habitat within relatively large, heterogeneous landscape mosaics may be more important for the maintenance of many rare pine barrens associated moth populations than plot or patch level characteristics.     

A multi-scale analysis of species-environment relationships: breeding birds in a pitch pine-scrub oak (Pinus rigida-Quercus ilicifolia) community

Analyses of species-environment relationships are critically needed to guide conservation of many declining species. Comprehensive investigations of these relationships must incorporate environmental variables at multiple spatial scales since species responses to the environment vary with the scale of observation. We used partial canonical correspondence analysis to associate a bird assemblage in a threatened pitch pine-scrub oak (Pinus rigida-Quercus ilicifolia) community in Plymouth County, Massachusetts, USA, with plot, patch, and landscape level variables. The first level of analysis partitioned the amount of variance in the bird community explained by plot, patch, and landscape factors from that explained by spatial autocorrelation. The second level partitioned the amount of variance explained by plot, patch, and landscape factors alone and in combination. All three levels of environmental variables together accounted for 43% of the variance in the species data and only 5% of the variance was explainable by spatial factors alone. Landscape level factors accounted for a slightly larger amount of the explained variation (12%) than plot (11%) or patch (8%) level factors. We also examined the cumulative fit of each species to the plot, patch, and landscape partial models. Nine species of regional and/or national conservation concern had distributions that fit one model considerably better than the others. The Great Crested Flycatcher (Myiarchus crinitus) and Black-and-white Warbler (Mniotilta varia) were predominantly associated with plot level factors; the Whip-poor-will (Caprimulgus vociferus) was predominantly associated with patch level factors; and the Purple Finch (Carpodacus purpureus), Gray Catbird (Dumetella carolinensis), Scarlet Tanager (Piranga olivacea), Ovenbird (Seiurus aurocapillus), Brown Thrasher (Toxostoma rufum), and Eastern Kingbird (Tyrannus tyrannus) were predominantly associated with landscape level factors. This study suggests that focusing conservation efforts at the landscape level would provide the most effective protection for the largest number of sensitive species.     

Foraging pits of the short-beaked echidna (Tachyglossus aculeatus) as small-scale patches in a semi-arid Australian box woodland

Many animals create disturbances on the soil surface while constructing habitat and resting sites, or foraging for food. This soil disturbance, which is sometimes known as biopedturbation, is a major contributor to landscape patchiness in arid and semi-arid environments. In the semi-arid woodlands of eastern Australia, the Short-Beaked Echidna (Tachyglossus aculeatus) creates a mosaic of foraging pits close to the canopies of large trees. The effects of pits on physical, chemical and biological properties of soils were compared at seven sites, each with two levels of disturbance (foraging pit vs. surface) and two canopy locations (under the canopy, in the open) associated with two tree species (Eucalyptus intertexta, Alectryon oleifolius). Foraging pits trapped twice the mass of litter compared with adjacent non-pit surfaces, and there was more litter under the tree canopies than in the open. Pits contained more bark and leaf material, and larger pits tended to trap more litter. Soil electrical conductivity levels were lower in the pits, and although there were greater concentrations of soil nutrients under the canopy, the concentrations of total carbon, nitrogen and sulphur were lower in the pits compared with the surface. Changes in litter mass did not explain differences in soil carbon, nitrogen or phosphorus. Soil in the pit was moister and more porous, and surface temperatures below the litter in the pits about 2 °C lower than at the surface. Respiration was about 30% greater in the pits, and both the early (sorptivity) and late (steady-state infiltration) stages of infiltration were significantly greater in the pits. Soil micro-arthropods were more abundant in the pits, which supported a different complement of taxa, but a similar diversity, to non-pit surfaces. Our results indicate that echidna foraging pits act as substantial resource traps. Given their extensive distribution in semi-arid woodlands, and their marked influence on soil biogeochemistry, echidnas should be seen as important ecosystem engineers in woodland critical for the maintenance of small-scale patchiness and, therefore, the efficient functioning of arid and semi-arid ecosystems.     

Microbial biomass and activity in pine litter in the presence of Tomocerus minor (Insecta,Collembola)

Summary Laboratory microcosms were used to study microbial populations and biomasses developing in fragmented litter of Pinus nigra Arnold var. nigra (A. et G.). Direct observations (fungal standing crop and fluorescein-stainable mycelia), litter enzyme analyses (cellulase and dehydrogenase), and measurements by physiological methods (microbial CO₂ production and total microbial, fungal, and bacterial viable biomasses) were made at 3-week intervals for 15 weeks. Most variables showed great changes during this period, which were ascribed to a rise in litter moisture content during the initial phase of the experiment, and to substrate depletion towards its final phase. The addition of the collembolan Tomocerus minor (Lubbock) for 1 week enhanced cellulase activities by 4%. When the animals were introduced after 6 weeks, the fungal standing crop was enhanced, and the percentage of fluorescein-stainable mycelia was reduced. Dehydrogenase activity was increased by grazing when the microbial population had been established for 9 weeks or longer. Eucaryotic and procaryotic substrate-induced respiration were positively correlated, which was explained by partial segregation of resources for the two groups. Litter cellulase and dehydrogenase activity showed correlations by other techniques, indicating their suitability as parameters for microbial activity in general, and for the collembolan grazing impact on microbial activity in particular.