Variation In Microbial Community Composition Research Articles

Can inoculation with living soil standardize microbial communities in soilless potting substrates?

Soilless plant cultivation substrates are commercially produced at large scales, but can show considerable variation in their performance in terms of sustaining plant growth and/or nutrition. This variation may be due to varying composition of microbial communities present in the substrates, mainly when composted organic materials are used as their components. Here we analyzed the portion of variability in composition of microbial (mainly the fungal) communities due to identity of substrate batches and compared it with variability due to the addition of a living soil (inoculation) or the presence of plant root system (i.e., the rhizosphere effect). The analysis was based on profiling total (DNA-based) and active (RNA-based) fungal and total (DNA-based) bacterial communities by using cultivation-independent molecular approaches. Contrary to expected effect of inoculation and rather limited variation across the substrate batches, identity of substrate batches in fact turned to explain the largest portion of biological variability, followed by the rhizosphere effect. The inoculation was completely ineffective as a factor affecting the indigenous microbial communities. These results indicate that the microbial communities in the soilless substrates are particularly resilient to plant- or inoculation-induced changes, but still highly variable between the individual production batches. Active fungal communities were dominated by yeasts recruiting either from Asco- or Basidiomycota. Due to phylogenetically and functionally similar but mutually exclusive dominants (Galactomyces and Candida) of the microbial communities in the different substrate batches without obvious correlation with their physico-chemical properties, we assume functional redundancy to play an important role in microbial community assembly within the substrates. Our results thus demonstrate as yet undescribed variation in microbial community composition with possible functional impact on plant performance in soilless substrates deserving further experimental attention.

Variations of Microbial Community Composition in Turfgrass Soil at Different Depth Using Illumine Sequencing

Variations of Microbial Community Composition in Turfgrass Soil at Different Depth Using Illumine Sequencing

Spatial and Temporal Dynamics of Pacific Oyster Hemolymph Microbiota across Multiple Scales.

Unveiling the factors and processes that shape the dynamics of host associated microbial communities (microbiota) under natural conditions is an important part of understanding and predicting an organism's response to a changing environment. The microbiota is shaped by host (i.e., genetic) factors as well as by the biotic and abiotic environment. Studying natural variation of microbial community composition in multiple host genetic backgrounds across spatial as well as temporal scales represents a means to untangle this complex interplay. Here, we combined a spatially-stratified with a longitudinal sampling scheme within differentiated host genetic backgrounds by reciprocally transplanting Pacific oysters between two sites in the Wadden Sea (Sylt and Texel). To further differentiate contingent site from host genetic effects, we repeatedly sampled the same individuals over a summer season to examine structure, diversity and dynamics of individual hemolymph microbiota following experimental removal of resident microbiota by antibiotic treatment. While a large proportion of microbiome variation could be attributed to immediate environmental conditions, we observed persistent effects of antibiotic treatment and translocation suggesting that hemolymph microbial community dynamics is subject to within-microbiome interactions and host population specific factors. In addition, the analysis of spatial variation revealed that the within-site microenvironmental heterogeneity resulted in high small-scale variability, as opposed to large-scale (between-site) stability. Similarly, considerable within-individual temporal variability was in contrast with the overall temporal stability at the site level. Overall, our longitudinal, spatially-stratified sampling design revealed that variation in hemolymph microbiota is strongly influenced by site and immediate environmental conditions, whereas internal microbiome dynamics and oyster-related factors add to their long-term stability. The combination of small and large scale resolution of spatial and temporal observations therefore represents a crucial but underused tool to study host-associated microbiome dynamics.

Polychlorinated biphenyls in agricultural soils from the Yangtze River Delta of China: Regional contamination characteristics, combined ecological effects and human health risks

The current contamination status of polychlorinated biphenyls (PCBs) was studied in the agricultural soils of the Yangtze River Delta (YRD), one of the largest economic zones in China. The concentrations of PCBs ranged from <0.1 to 130 ng/g dry weight. Higher concentrations of PCBs were observed in the 0–30 cm surface layers relative to the subsurface soils. A distinct spatial distribution was observed with a declining concentration gradient from the northwest to the southeast of the region. The composition of PCBs in the soils was consistent with the Chinese commercial PCB mixtures, but different from the compositions in global background soil. Local sources including large-scale use and disposal of PCB-containing products were the main potential sources to soil. The ecological effects and human health risks associated with combined persistent organic pollutants, including PCBs, organochlorine pesticides (OCPs), phthalate esters (PAEs) and polybrominated diphenyl ethers (PBDEs), were further estimated. The four toxic organic compounds and seven physicochemical parameters together could only explain 12.7% of the variation in microbial community composition, suggesting the soil ecosystem function was not strongly influenced by the combined pollution at low concentrations. However, the potential health risks to residents via multiple pathways were notably higher for PCBs than other chemicals. The potential risks were mainly derived from PCB-126, 81, and 169.

Soil microbial community variation correlates most strongly with plant species identity, followed by soil chemistry, spatial location and plant genus.

Soil ecologists have debated the relative importance of dispersal limitation and ecological factors in determining the structure of soil microbial communities. Recent evidence suggests that 'everything is not everywhere', and that microbial communities are influenced by both dispersal limitation and ecological factors. However, we still do not understand the relative explanatory power of spatial and ecological factors, including plant species identity and even plant relatedness, for different fractions of the soil microbial community (i.e. bacterial and fungal communities). To ask whether factors such as plant species, soil chemistry, spatial location and plant relatedness influence rhizosphere community composition, we examined field-collected rhizosphere soil of seven congener pairs that occur at Bodega Bay Marine Reserve, CA, USA. We characterized differences in bacterial and fungal communities using terminal-restriction fragment length polymorphism. Plant species identity was the single best statistical predictor of both bacterial and fungal community composition in the root zone. Soil microbial community structure was also correlated with soil chemistry. The third best predictor of bacterial and fungal communities was spatial location, confirming that everything is not everywhere. Variation in microbial community composition was also related to combinations of spatial location, soil chemistry and plant relatedness, suggesting that these factors do not act independently. Plant relatedness explained less of the variation than plant species, soil chemistry, or spatial location. Despite some congeners occupying different habitats and being spatially distant, rhizosphere fungal communities of plant congeners were more similar than expected by chance. Bacterial communities from the same samples were only weakly similar between plant congeners. Thus, plant relatedness might influence soil fungal, more than soil bacterial, community composition.

Microbial biogeography of drinking water: patterns in phylogenetic diversity across space and time

In this study, we collected water from different locations in 32 drinking water distribution networks in the Netherlands and analysed the spatial and temporal variation in microbial community composition by high-throughput sequencing of 16S rRNA gene amplicons. We observed that microbial community compositions of raw source and processed water were very different for each distribution network sampled. In each network, major differences in community compositions were observed between raw and processed water, although community structures of processed water did not differ substantially from end-point tap water. End-point water samples within the same distribution network revealed very similar community structures. Network-specific communities were shown to be surprisingly stable in time. Biofilm communities sampled from domestic water metres varied distinctly between households and showed no resemblance to planktonic communities within the same distribution networks. Our findings demonstrate that high-throughput sequencing provides a powerful and sensitive tool to probe microbial community composition in drinking water distribution systems. Furthermore, this approach can be used to quantitatively compare the microbial communities to match end-point water samples to specific distribution networks. Insight in the ecology of drinking water distribution systems will facilitate the development of effective control strategies that will ensure safe and high-quality drinking water.

Emergence shapes the structure of the seed microbiota.

Seeds carry complex microbial communities, which may exert beneficial or deleterious effects on plant growth and plant health. To date, the composition of microbial communities associated with seeds has been explored mainly through culture-based diversity studies and therefore remains largely unknown. In this work, we analyzed the structures of the seed microbiotas of different plants from the family Brassicaceae and their dynamics during germination and emergence through sequencing of three molecular markers: the ITS1 region of the fungal internal transcribed spacer, the V4 region of 16S rRNA gene, and a species-specific bacterial marker based on a fragment of gyrB. Sequence analyses revealed important variations in microbial community composition between seed samples. Moreover, we found that emergence strongly influences the structure of the microbiota, with a marked reduction of bacterial and fungal diversity. This shift in the microbial community composition is mostly due to an increase in the relative abundance of some bacterial and fungal taxa possessing fast-growing abilities. Altogether, our results provide an estimation of the role of the seed as a source of inoculum for the seedling, which is crucial for practical applications in developing new strategies of inoculation for disease prevention.

Spatiotemporal Variations in Microbial Communities in a Landfill Leachate Plume

AbstractWe examined the spatiotemporal changes of microbial communities in relation to hydrochemistry variation over time and space in an aquifer polluted by landfill leachate (Banisveld, The Netherlands). Sampling in 1998, 1999, and 2004 at the same time of the year revealed that the center of the pollution plume was hydrochemically rather stable, but its upper fringe moved to the surface over time, especially at distances greater than 40 m away from the landfill. Complex and spatiotemporal heterogeneous bacterial and eukaryotic communities were resolved using denaturing gradient gel electrophoresis (DGGE) of 16S and 18S rRNA gene fragments. Large fluctuations were noted in the eukaryotic communities associated with strongly polluted and cleaner groundwater. The bacterial communities in strongly polluted samples were different from those in cleaner groundwater in 1998 and 1999, but no longer in 2004. The temporal variation in microbial communities was greater than the spatial variation: the 1998 bacteria communities in strongly polluted groundwater were more related to each other than to those recovered in 1999 and 2004. During the three sampling periods, the bacterial communities were more stable close to the landfill than at larger distances from the landfill. Overall, pollution appears to have only a minor influence on microbial communities. The considerable spatiotemporal variation in microbial community composition may contribute to better biodegradation of pollutants. Designing management strategies for natural attenuation of aquifer pollution will benefit from further long‐term, high‐density monitoring of changes in microbial communities, their diversity and physiological properties, in relation to changes in hydrochemistry.

Microbially enhanced dissolution and reductive dechlorination of PCE by a mixed culture: Model validation and sensitivity analysis

Reductive dechlorination catalyzed by organohalide-respiring bacteria is often considered for remediation of non-aqueous phase liquid (NAPL) source zones due to cost savings, ease of implementation, regulatory acceptance, and sustainability. Despite knowledge of the key dechlorinators, an understanding of the processes and factors that control NAPL dissolution rates and detoxification (i.e., ethene formation) is lacking. A recent column study demonstrated a 5-fold cumulative enhancement in tetrachloroethene (PCE) dissolution and ethene formation (Amos et al., 2009). Spatial and temporal monitoring of key geochemical and microbial (i.e., Geobacter lovleyi and Dehalococcoides mccartyi strains) parameters in the column generated a data set used herein as the basis for refinement and testing of a multiphase, compositional transport model. The refined model is capable of simulating the reactive transport of multiple chemical constituents produced and consumed by organohalide-respiring bacteria and accounts for substrate limitations and competitive inhibition. Parameter estimation techniques were used to optimize the values of sensitive microbial kinetic parameters, including maximum utilization rates, biomass yield coefficients, and endogenous decay rates. Comparison and calibration of model simulations with the experimental data demonstrate that the model is able to accurately reproduce measured effluent concentrations, while delineating trends in dechlorinator growth and reductive dechlorination kinetics along the column. Sensitivity analyses performed on the optimized model parameters indicate that the rates of PCE and cis-1,2-dichloroethene (cis-DCE) transformation and Dehalococcoides growth govern bioenhanced dissolution, as long as electron donor (i.e., hydrogen flux) is not limiting. Dissolution enhancements were shown to be independent of cis-DCE accumulation; however, accumulation of cis-DCE, as well as column length and flow rate (i.e., column residence time), strongly influenced the extent of reductive dechlorination. When cis-DCE inhibition was neglected, the model over-predicted ethene production ten-fold, while reductions in residence time (i.e., a two-fold decrease in column length or two-fold increase in flow rate) resulted in a more than 70% decline in ethene production. These results suggest that spatial and temporal variations in microbial community composition and activity must be understood to model, predict, and manage bioenhanced NAPL dissolution.

Spatial Variations in Microbial Community Composition in Surface Seawater from the Ultra-Oligotrophic Center to Rim of the South Pacific Gyre

Surface seawater in the South Pacific Gyre (SPG) is one of the cleanest oceanic environments on earth, and the photosynthetic primary production is extremely low. Despite the ecological significance of the largest aquatic desert on our planet, microbial community composition in the ultra-oligotrophic seawater remain largely unknown. In this study, we collected surface seawater along a southern transect of the SPG during the Integrated Ocean Drilling Program (IODP) Expedition 329. Samples from four distinct sites (Sites U1368, U1369, U1370 and U1371) were examined, representing ∼5400 kilometers of transect line from the gyre heart to the edge area. Real-time PCR analysis showed 16S rRNA gene abundance in the gyre seawater, ranging from 5.96×105 to 2.55×106 copies ml−1 for Bacteria and 1.17×103 to 1.90×104 copies ml−1 for Archaea. The results obtained by statistic analyses of 16S rRNA gene clone libraries revealed the community composition in the southern SPG area: diversity richness estimators in the gyre center (Sites U1368 & U1369) are generally lower than those at sites in the gyre edge (Sites U1370 & U1371) and their community structures are clearly distinguishable. Phylogenetic analysis showed the predominance of Proteobacteria (especially Alphaproteobacteria) and Cyanobacteria in bacterial 16S rRNA gene clone libraries, whereas phylotypes of Betaproteobacteria were only detected in the central gyre. Archaeal 16S rRNA genes in the clone libraries were predominated by the sequences of Marine Group II within the Euryarchaeota, and the Crenarchaeota sequences were rarely detected, which is consistent with the real-time PCR data (only 9.9 to 22.1 copies ml−1). We also performed cultivation of heterotrophic microbes onboard, resulting in 18.9% of phylogenetically distinct bacterial isolates at least at the species level. Our results suggest that the distribution and diversity of microbial communities in the SPG surface seawater are closely related to the ultra-oligotrophic oceanographic features in the Pacific Ocean.

Effect of Rainfall-Induced Soil Geochemistry Dynamics on Grassland Soil Microbial Communities

In Mediterranean-type grassland ecosystems, the timing of rainfall events controls biogeochemical cycles, as well as the phenology and productivity of plants and animals. Here, we investigate the effect of short-term (days) soil environmental conditions on microbial community structure and composition during a natural wetting and drying cycle. Soil samples were collected from a meadow in Northern California at four time points after the first two rainfall events of the rainy season. We used 16S rRNA microarrays (PhyloChip) to track changes in bacterial and archaeal community composition. Microbial communities at time points 1 and 3 were significantly different than communities at time points 2 and 4. Based on ordination analysis, the available carbon, soil moisture, and temperature explained most of the variation in community structure. For the first time, a complementary and more comprehensive approach using linear regression and generalized logical networks were used to identify linear and nonlinear associations among environmental variables and with the relative abundance of subfamilies. Changes in soil moisture and available carbon were correlated with the relative abundance of many phyla. Only the phylum Actinobacteria showed a lineage-specific relationship to soil moisture but not to carbon or nitrogen. The results indicate that the use of a high taxonomic rank in correlations with nutritional indicators might obscure divergent subfamily-level responses to environmental parameters. An important implication of this research is that there is short-term variation in microbial community composition driven in part by rainfall fluctuation that may not be evident in long-term studies with coarser time resolution.

Changes in root-associated microbial communities are determined by species-specific plant growth responses to stress and disturbance

The diversity changes in the rhizosphere microbial communities of four grass species submitted to a stress (drought) and/or a disturbance (defoliation) were examined in a full factorial common garden experiment. The diversity of bacterial and fungal communities was determined using Single Strand Conformation Polymorphism (SSCP) and multivariate analyses of variance were performed to test for the effect of experimental factors on microbial beta diversity, i.e. the variation of microbial community composition among pots. Plant species identity contributed the most to microbial beta diversity with 22% of the total variance explained and this effect was stronger for fungi than for bacteria. The effect of defoliation was significant for fungal communities, and more pronounced for plant species that exhibited an increased root biomass following defoliation. Drought had a higher effect on bacterial beta diversity, and this was particularly noticeable for plant species that were the least able to tolerate water shortage. Our results showed that both plant species identity and characteristics of plant growth response captured the plant-mediated effects on microbial diversity. They illustrate the biotic cascade between plants responding to a changing environment and the dynamics of rhizosphere microbial communities.

Impacts of Inter- and Intralaboratory Variations on the Reproducibility of Microbial Community Analyses

With the advent of molecular biological techniques, especially next-generation sequencing and metagenomics, the number of microbial biogeography studies is rapidly increasing. However, these studies involve the synthesis of data generated by different laboratories using different protocols, chemicals, etc., all with inherent biases. The aim of this study was to assess inter- as well as intralaboratory variations in microbial community composition when standardized protocols are applied to a single soil sample. Aliquots from a homogenized soil sample from a rice field in Italy were sent to five participating laboratories. DNA was extracted by two investigators per laboratory using an identical protocol. All DNA samples were sent to one laboratory to perform DNA quantification, quantitative PCR (QPCR), and microarray and denaturing gradient gel electrophoresis (DGGE) analyses of methanotrophic communities. Yields, as well as purity of DNA, were significantly different between laboratories but in some cases also between investigators within the same laboratory. The differences in yield and quality of the extracted DNA were reflected in QPCR, microarray, and DGGE analysis results. Diversity indices (Shannon-Wiener, evenness, and richness) differed significantly between laboratories. The observed differences have implications for every project in which microbial communities are compared in different habitats, even if assessed within the same laboratory. To be able to make sensible comparisons leading to valid conclusions, intralaboratory variation should be assessed. Standardization of DNA extraction protocols and possible use of internal standards in interlaboratory comparisons may help in rendering a "quantifiable" bias.

Nitrogen Transformations and Microbial Communities in Soil Aggregates from Three Tillage Systems

Quantifying N transformation processes in soil aggregates is relevant since microbial communities central to the N cycle may differ among aggregate size fractions. Our objective was to test the hypothesis that variations in microbial community composition of aggregate size fractions influence N transformation rates of soil from three long‐term (22‐yr) tillage systems (no‐till, chisel plow, and moldboard plow). Aggregate size fractions (2–4, 0.5–1, and &lt;0.25 mm) were obtained by dry sieving. Nitrogen transformation rates were estimated by analysis of 15N pool dilution data with the FLUAZ model, and microbial community composition by phospholipid fatty acid (PLFA) profiles. Aggregate size fraction and tillage system had significant (P &lt; 0.01) effects on total and microbial biomass C and N, gross N mineralization rate (GNMR), gross nitrification rate (GNR), and gross N immobilization rate (GIR). No‐till soils and the 0.5‐ to 1.0‐mm aggregate size fraction had the highest N transformation rates. Net N mineralization rates were greater for no‐till than for tilled soils. Multiple response permutation analysis of PLFA data revealed that microbial community composition did not differ with aggregate size fraction. Stepwise regression analysis indicated that microbial community composition (nonmetric multidimensional scaling Axis 1) accounted for 89% of the variation in GIR, soil C and N concentrations accounted for 88% of the variation in GNMR, and microbial biomass C concentration accounted for 81% of the variation in GNR. These results indicate that greater N transformation rates in no‐till than tilled soil were due primarily to increased microbial biomass (i.e., microbial population size) rather than altered microbial community composition.

Increases in soil respiration following labile carbon additions linked to rapid shifts in soil microbial community composition

Organic matter decomposition and soil CO2 efflux are both mediated by soil microorganisms, but the potential effects of temporal variations in microbial community composition are not considered in most analytical models of these two important processes. However, inconsistent relationships between rates of heterotrophic soil respiration and abiotic factors, including temperature and moisture, suggest that microbial community composition may be an important regulator of soil organic matter (SOM) decomposition and CO2 efflux. We performed a short-term (12-h) laboratory incubation experiment using tropical rain forest soil amended with either water (as a control) or dissolved organic matter (DOM) leached from native plant litter, and analyzed the effects of the treatments on soil respiration and microbial community composition. The latter was determined by constructing clone libraries of small-subunit ribosomal RNA genes (SSU rRNA) extracted from the soil at the end of the incubation experiment. In contrast to the subtle effects of adding water alone, additions of DOM caused a rapid and large increase in soil CO2 flux. DOM-stimulated CO2 fluxes also coincided with profound shifts in the abundance of certain members of the soil microbial community. Our results suggest that natural DOM inputs may drive high rates of soil respiration by stimulating an opportunistic subset of the soil bacterial community, particularly members of the Gammaproteobacteria and Firmicutes groups. Our experiment indicates that variations in microbial community composition may influence SOM decomposition and soil respiration rates, and emphasizes the need for in situ studies of how natural variations in microbial community composition regulate soil biogeochemical processes.

Variation in Microbial Community Composition and Culturability in the Rhizosphere of Leucanthemopsis alpina (L.) Heywood and Adjacent Bare Soil Along an Alpine Chronosequence

We compared the size, culturability, diversity, and dominant species similarity of the bacterial communities of Leucanthemopsis alpina (L.) Heywood rhizosphere and adjacent bare soil (interspace) along a chronosequence of soil development time (5, 50, and 70 years) in the forefield of the Dammaglacier (Switzerland). We found no evidence that the size of the bacterial community was significantly affected by either soil age or the presence of L. alpina. In contrast, the proportion of the bacterial community that could be cultured on nonselective agars, and which was taken as an indication of the proportion of r-selected populations, was significantly higher in the 50- and 70-year-old soils than in the 5-year-old soil, and was also significantly higher in the rhizosphere of L. alpina at all time points. RDA indicated significant correlations between the increased culturability of the bacterial community over time and increasing concentrations of labile N, and between the increased culturability in the rhizosphere and increased concentrations of labile C and N. HaeIII-amplified ribosomal DNA (rDNA) restriction analysis of a library of 120 clones of 16S rDNA revealed 85 distinct phylotypes. Hurlbert's probability of interspecific encounter (PIE) values derived from this library ranged from 0.95 to 1.0, indicating a very high genetic diversity. There was no significant difference in the PIE values of rhizosphere and interspace communities. Detrended correspondence analysis (DCA) of 16S ribosomal RNA (rRNA) denaturing gradient gel electrophoresis (DGGE) community profiles clearly distinguished the rhizosphere from the interspace community in the 5-year-old soils and also clearly distinguished between these communities and the rhizosphere and interspace communities of the 50- and 70-year-old soils. However, 16S rRNA DGGE revealed little difference between rhizosphere and interspace communities in the 50- and 70-year-old soils. The relative similarity of the 16S rRNA profiles strongly reflected labile carbon and nitrogen availability. Overall, our results suggest that improved C and N availability in the rhizosphere of L. alpina increases the size of r-selected bacterial species populations, but that the influence of L. alpina depends on soil age, being maximal in the youngest soils and minimal in the oldest. The reduced influence of L. alpina in the older soils may reflect a feedback between improved nutrient availability and reduced rhizodeposition.

Variations in microbial community composition through two soil depth profiles

Soil profiles are often many meters deep, but with the majority of studies in soil microbiology focusing exclusively on the soil surface, we know very little about the nature of the microbial communities inhabiting the deeper soil horizons. We used phospholipid fatty acid (PLFA) analysis to examine the vertical distribution of specific microbial groups and to identify the patterns of microbial abundance and community-level diversity within the soil profile. Samples were collected from the soil surface down to 2 m in depth from two unsaturated Mollisol profiles located near Santa Barbara, CA, USA. While the densities of microorganisms were generally one to two orders of magnitude lower in the deeper horizons of both profiles than at the soil surface, approximately 35% of the total quantity of microbial biomass found in the top 2 m of soil is found below a depth of 25 cm. Principal components analysis of the PLFA signatures indicates that the composition of the soil microbial communities changes significantly with soil depth. The differentiation of microbial communities within the two profiles coincides with an overall decline in microbial diversity. The number of individual PLFAs detected in soil samples decreased by about a third from the soil surface down to 2 m. The ratios of cyclopropyl/monoenoic precursors and total saturated/total monounsaturated fatty acids increased with soil depth, suggesting that the microbes inhabiting the deeper soil horizons are more carbon limited than surface-dwelling microbes. Using PLFAs as biomarkers, we show that Gram-positive bacteria and actinomycetes tended to increase in proportional abundance with increasing soil depth, while the abundances of Gram-negative bacteria, fungi, and protozoa were highest at the soil surface and substantially lower in the subsurface. The vertical distribution of these specific microbial groups can largely be attributed to the decline in carbon availability with soil depth.

Response of the chitinolytic microbial community to chitin amendments of dune soils

The dynamics of culturable chitin-degrading microorganisms were studied during a 16-week incubation of chitin-amended coastal dune soils that differed in acidity. Soil samples were incubated at normal (5% w/w) and high (15% w/w) moisture levels. More than half of the added chitin was decomposed within 4 weeks of incubation in most soils. This rapid degradation was most likely due to fast-growing chitinolytic fungi (mainly Mortierella spp. and Fusarium spp.) at both moisture levels, as dense hyphal networks of these fungi were observed during the first 4 weeks of incubation. Chitin N mineralization was inhibited by cycloheximide, and fast-growing fungal isolates were capable of rapid chitin decomposition in sterile sand, further suggesting that these fungi play an important role in initial chitin degradation. The strong increase in fast-growing fungi in chitin-amended dune soils was only detected by direct observation. Plate counts and microscopic quantification of stained hyphae failed to reveal such an increase. During the first part of the incubation, numbers of unicellular chitinolytic bacteria also increased, but their contribution to chitin degradation was indicated to be of minor importance. During prolonged incubation, colony forming units (CFU) of chitinolytic streptomycetes and/or slow-growing fungi increased strongly in several soils, especially at the 5% moisture level. Hence, the general trend observed was a succession from fast-growing fungi and unicellular bacteria to actinomycetes and slow-growing fungi. Yet, the composition of chitinolytic CFU over time differed strongly between chitin-amended dune soils, and also between the two moisture levels. These differences could not be attributed to pH, organic matter or initial microbial composition. The possible consequence of such unpredictable variation in microbial community composition for the use of chitin-amendments as a biocontrol measure is discussed.