Oribatid mite communities and foliar litter decomposition in canopy suspended soils and forest floor habitats of western redcedar forests, Vancouver Island, Canada

Litter decomposition and changes in oribatid mite community composition were studied for 2 years in litterbags collected from arboreal organic matter accumulations (canopy suspended soils) and forest floors associated with western redcedar trees on Vancouver Island, British Columbia. We tested the hypotheses that lower rates of mass loss, higher nutrient levels, and different patterns of oribatid mite richness and abundance in decomposing western redcedar litter would be observed in litterbags associated with canopy suspended soils compared to forest floors. Decomposition, measured by mass loss of cedar litter in litterbags, was not significantly different in canopy and forest floor habitats, although reduced in the canopy. Abundance and richness of oribatid mites inhabiting litterbags were significantly greater on the forest floor compared to the canopy suspended soils. Canopy suspended soils had higher levels of total nitrogen, available phosphorus and potassium than the forest floor, but moisture content was significantly lower in the suspended soils. Higher nutrient levels in the canopy system are attributed to differences in coarse woody debris input (but not foliar litter), combined with reduced nutrient uptake by roots and lower mobilisation rates of nutrients by detritivorous and fungivorous microarthropods. Moisture limitation in the canopy system possibly contributed to lower mass loss in litterbags, and lower abundance and richness of oribatid mites in litterbags placed on canopy suspended soils. Patterns of oribatid mite community composition were related to mite communities associated with the underlying substrate (forest floor or canopy suspended soil) which act as source pools for individuals colonising litterbags. Successional and seasonal trends in oribatid mite communities were confounded by moisture limitation at 24 months, particularly within the canopy habitat.     

Soil mesofauna dynamics,wheat residue decomposition and nitrogen mineralization in buried litterbags

The effect of soil microarthropods and enchytraeids on the decomposition of wheat straw in buried litterbags was studied by selective admission and exclusion. Litterbags with 20 m mesh size admitted nematodes, but excluded microarthropods, although temporarily. After 27 weeks of incubation part of these litterbags were colonized, probably through egg-deposition of mainly fungivorous Collembola and mites. When litterbags with a complete microarthropod community (1.5 mm mesh size) were compared to litterbags with strongly reduced microarthropod numbers (20 m mesh size), no differences between decomposition rates were found. However, in colonized 20-m mesh bags, we found reduced decomposition rates compared to the coarse mesh litterbags, probably due to overgrazing of the fungal population by large numbers of fungivorous microarthropods. These large numbers might be caused by the absence of predators. Extraction of microarthropods as well as enchytraeids and nematodes from the coarse mesh litterbags showed a distinct succession during decomposition. The decomposition process was dominated in the first phase by bacterivorous nematodes, nematophagous and bacterivorous mites, and in the later phase by fungivorous nematodes, fungivorous and omnivorous mites and Collembola, and predatory mites. This succession is indicative of a sequence from bacterial to fungal dominated decomposition of the buried organic matter. The results indicate that the decomposition rate is predator controlled.     

From forest to fen: Microarthropod abundance and litter decomposition in a southern Appalachian floodplain/fen complex

Our study compared decomposition and litter microarthropod abundance among five plant communities in a mountain floodplain/fen complex located in the southern Appalachian Mountains, USA. We found that the least disturbed plant communities, red maple in particular, have the quickest decomposition, the greatest number of litter microarthropods, the highest soil organic carbon, and the lowest soil pH. Positive correlations were shown between soil organic carbon and total microarthropods; negative correlations were found between soil pH and total microarthropods. No correlations were found between soil moisture and decomposition or total microarthropod numbers. We conclude that soil characteristics related to disturbance, rather than to the presence of a closed canopy, are the main influences on decomposition and litter microarthropods.     

The bacterial community inhabiting temperate deciduous forests is vertically stratified and undergoes seasonal dynamics

Bacterial communities living in forest soils contribute to the decomposition of organic matter and the recycling of nutrients in these ecosystems and form one of the most diverse habitats on Earth. Unfortunately, due to difficulty in culturing soil bacteria, the understanding of their ecology is still limited. In the case of temperate deciduous forests, soil microbial communities face large seasonal variations in environmental conditions, such as temperature or moisture. Moreover, the supply of nutrients also differs due to seasonal processes, such as the allocation of photosynthates into soil by the roots of primary producers or the seasonal input of fresh litter. The aim of this study was to reveal how the bacterial community responds to these seasonal processes in the litter and soil of a Quercus petraea forest. Bacterial communities from litter and from the organic and mineral horizons of soil were analyzed during the four seasons of the year by 16S rRNA gene pyrosequencing. The results revealed that the composition of the bacterial community is horizon specific. The litter horizon had a higher relative abundance of Proteobacteria and Bacteroidetes than soil, while the organic and mineral horizons had a higher abundance of Acidobacteria, Firmicutes and Actinobacteria than litter. Moreover, the bacterial community was significantly affected by seasonality in all horizons. Bacterial communities in the litter showed significant differences between the vegetation season (May and July) and the autumn and winter seasons (October, February). In mineral soil, bacterial community composition was specific in the summer, when it was significantly different from all other seasons, with a larger number of taxa described as rhizosphere and mycorrhizosphere inhabitants. The results indicate that litter decomposition is the main driver of bacterial community composition in litter horizon. In contrast to reports on fungal communities, bacterial community composition in mineral soil responds to the seasonal peaks of rhizodeposition in the summer.     

Effects of litter quality and microarthropods on N dynamics and retention of exogenous 15N in decomposing litter

Summary Surface additions of (¹⁵NH₄)₂SO₄ were used to measure the immobilization and subsequent movement of exogenous N added to two litter types of contrasting quality (Cornus florida and Quercus prinus). Litterbaskets were used to measure the litter mass loss and N dynamics and to follow the movement of the ¹⁵N label through litter, F layer, and soil pools. Half of the litterbaskets of each species were treated with naphthalene to reduce microarthropod densities. The faster decomposing C. florida litter maintained a higher excess atom % ¹⁵N, and a greater relative concentration of the labeled input (g ¹⁵N g^–1) than did Q. prinus litter. In both litter types the excess atom % ¹⁵N, relative concentration (g ¹⁵N g^–1), and absolute amount of label recovered in the litter declined over time. This occurred during a period of net accumulation of total litter N, implying simultaneous release of the initial input and immobilization of N from other sources. The concentration of ¹⁵N in the soil increased over time, while the F layer apparently acted as an intermediary in the transfer of ¹⁵N from litter to soil. Naphthalene effectively reduced microarthropod numbers in all horizons of the litterbaskets and significantly reduced the decay rates of Q. prinus, but not C. florida litter. Naphthalene did not appear to affect total N dynamics in the litter. However, with all horizons taken together, the naphthalene-treated litterbaskets retained more total ¹⁵N than the control litterbaskets. Naphthalene also changed the vertical distribution of ¹⁵N within litterbaskets, so that the litter retained less of the ¹⁵N-labeled input and the F layer and soil horizons retained more of the labeled input than in control litterbaskets. Our major conclusions are: (1) the N pool of decomposing litter is dynamic, with simultaneous N release and immobilization activating N turnover even during the net accumulation phase; (2) litter quality is an important determinant of immobilization and retention of exogenous N inputs and, therefore, turnover of the litter N pool; and (3) microarthropod activity can significantly affect the incorporation and retention of exogenous N inputs in decomposing litter, although these changes are apparently not reflected in net N accumulation or release during the 1st year of decomposition. However, the naphthalene may have affected microbially mediated N dynamics and this possibility needs to be considered in interpreting the results.     

The oribatid mite Scheloribates moestus (Acari: Oribatida) alters litter chemistry and nutrient cycling during decomposition

It is widely accepted that microarthropods influence decomposition dynamics but we know relatively little about their effects on litter chemistry, extracellular enzyme activities, and other finer-scale decomposition processes. Further, few studies have investigated the role of individual microarthropod species in litter decomposition. The oribatid mite Scheloribates moestus Banks (Acari: Oribatida) is abundant in many U.S. ecosystems. We examined the potential effects of S. moestus on litter decomposition dynamics and chemical transformations, and whether these effects are influenced by variation in initial litter quality. We collected corn and oak litter from habitats with large populations of S. moestus and in microcosms with and without mites measured respiration rates, nitrogen availability, enzyme activities, and molecular-scale changes in litter chemistry. Mites stimulated extracellular enzyme activities, enhanced microbial respiration rates by 19% in corn litter and 17% in oak litter over 62 days, and increased water-extractable organic C and N. Mites decreased the relative abundance of polysaccharides in decomposing corn litter but had no effect on oak litter chemistry, suggesting that the effects of S. moestus on litter chemistry are constrained by initial litter quality. We also compared the chemistry of mite feces to unprocessed corn litter and found that feces had a higher relative abundance of polysaccharides and phenols and a lower relative abundance of lignin. Our study establishes that S. moestus substantially changes litter chemistry during decomposition, but specific effects vary with initial litter quality. These chemical transformations, coupled with other observed changes in decomposition rates and nutrient cycling, indicate that S. moestus could play a key role in soil C cycling dynamics.     

Impacts of herbivorous insects on decomposer communities during the early stages of primary succession in a semi-arid woodland

Changes in nutrient inputs due to aboveground herbivory may influence the litter and soil microbial community responsible for processes such as decomposition. The mesophyll-feeding scale insect (Matsucoccus acalyptus) found near Sunset Crater National Monument in northern Arizona, USA significantly increases piñon (Pinus edulis) needle litter nitrogen (N) and phosphorus (P) concentrations by 50%, as well as litter inputs to soil by 21%. Because increases in needle litter quality and quantity of this magnitude should affect the microbial communities responsible for decomposition, we tested the hypothesis that insect herbivory causes a shift in soil microbial and litter microarthropod function. Four major findings result from this research: (1) Despite increases in needle inputs due to herbivory, soil carbon (C) was 56% lower beneath scale-susceptible trees than beneath resistant trees; however, soil moisture, N, and pH were similar among treatments. (2) Microbial biomass was 80% lower in soils beneath scale-susceptible trees when compared to resistant trees in the dry season, while microbial enzyme activities were lower beneath susceptible trees in the wet season. (3) Bacterial community-level physiological profiles differed significantly between susceptible and resistant trees during the dry season but not during the wet season. (4) There was a 40% increase in Oribatida and 23% increase in Prostigmata in susceptible needle litter relative to resistant litter. Despite these changes, the magnitude of microbial biomass, activity, and community structure response to herbivory was lower than expected and appears to take a long time to develop. These results suggest that herbivores impact soils in subtle, but important ways; we suggest that while litter chemistry may strongly mediate soil fertility and microbial communities in mesic ecosystems, the influence is lower than expected in this primary succession xeric ecosystem where season mediates differences in microbial populations. Understanding how insect herbivores alter the distribution of susceptible and resistant trees and their associated decomposer communities in arid environments may lead to better prediction of how these ecosystems respond to climatic change.     

Impact of microarthropod biomass on the composition of the soil fauna community and ecosystem processes

The effect of soil organisms on ecosystem processes strongly depends on the composition of the overall community. Community composition however undergoes constant shifts due to pronounced spatio-temporal patterns in biomass and abundance of individual fauna groups. On this background the present experiment aimed to assess the potential impacts of shifts in the biomass of a dominant soil fauna group (microarthropods) on total community composition and on ecosystem processes mediated by fauna or microbes (e.g. decomposition, nitrogen mineralization).Microcosms, filled with spruce litter, hosted soil fauna communities that either contained ambient microarthropod biomass (control) or two elevated levels of microarthropod biomass (1.5 and 2 fold increase), while initial microbial biomass and that of other faunal groups remained unaltered. After an incubation period of 2 months, the biomass of microorganisms and fauna groups as well as ecosystem process variables were determined.The increase in microarthropod biomass at the investigated levels induced changes in the faunal community; mainly via negative or positive feeding interactions between microarthropods and the affected animal groups (Enchytraeidae, Nematoda). The abundance and activity of microorganisms at lower trophic levels however remained unaffected by these alterations; buffering the effect of shifts in the community structure on ecosystem processes.     

Limited evidence for short-term succession of microarthropods during early phases of surface litter decomposition

Succession of microarthropods during the decomposition of organic matter is an important concept in soil biology. However, few studies have tested whether the pattern of microarthropod colonisation during decomposition is independent of season. We investigated the pattern of colonisation and dominance of microarthropods on decomposing organic matter placed at two different times. Litterbags containing canola leaf or stem material were placed on the soil surface of a Western Australian agricultural field in July and September 1999. They were collected weekly to fortnightly until November. A final set of bags was collected in May 2000. Mass loss and nutrient contents (C, N, Ca, K, P and S) were measured at each sample time. Microarthropods were sorted to order and the mites to species level. Nematode abundance was determined at each sample time. Mass loss of the leaf and stem material was similar between the two placement times (33% and 15% ash-free dry mass lost over 33 days from leaf and stem material, respectively), although the dynamics of nutrient loss for some elements was different between the two placements. However, over the dry summer, material placed in September continued to lose nutrients whilst there was little additional loss from the material placed in July. A similar pattern of dominance of microarthropod and mite species was found on the leaf and stem material placed in July with the succession of dominant animals more rapid on the stem material. Nematode abundance appeared to increase as populations of microarthropods declined over time. Populations of microarthropods on the material placed in the September samples never achieved similar levels of abundance to that of the July samples, and the dominant fauna groups were dissimilar to those in the July samples for the same degree of decomposition. Our data indicate that the early phase of surface litter decomposition is not a successional process in terms of the microarthropod community irrespective of season and, that abiotic factors are more likely to be determining nutrient loss from organic matter within particular seasons.     

A comparison of microarthropod assemblages with emphasis on oribatid mites in canopy suspended soils and forest floors associated with ancient western redcedar trees

Microarthropod abundance, oribatid mite species richness and community composition were assessed in the high canopy (ca. 35 m) of an ancient temperate rainforest and compared with microarthropod communities of the forest floor. Microarthropods were extracted from 72 core samples of suspended soils and 72 core samples from forest floors associated with six western redcedar trees in the Walbran Valley on the southwest coast of Vancouver Island, Canada. Total microarthropod abundances, mesostigmatid and astigmatid mites, Collembola and other microarthropod abundances were significantly greater in forest floors compared to canopy habitats. Oribatid and prostigmatid mite abundance were not significantly different between habitats. The relative abundances of all microarthropod groups considered in this study differed significantly between habitats. Eighty-eight species of oribatid mites were identified from the study area. Eighteen of the 53 species observed in suspended soils were unique to the canopy. Cluster analysis indicates that the arboreal oribatid mite community is distinct and not a taxonomic subset of the forest floor assemblage, however, canopy oribatid mite communities are more heterogeneous in species composition than in the forest floor.     

The influence of plant litter diversity on decomposer abundance and diversity

Although there has been much recent interest in the effect of litter mixing on decomposition processes, much remains unknown about how litter mixing and diversity affects the abundance and diversity of decomposer organisms. We conducted a litter mixing experiment using litterbags in a New Zealand rainforest, in which treatments consisted of litter monocultures of each of 8 forest canopy and understory plant species, as well as mixtures of 2, 4 and 8 species. We found litter mixing to have little effect on net decomposition rates after either 279 or 658 days, and for each species decomposition rates in mixture treatments were the same as in monoculture. Litter species identity had important effects on litter microfauna, mesofauna and macrofauna, with different litter types promoting different subsets of the fauna. Litter mixing had few effects on densities of mesofauna and macrofauna, but did have some important effects on components of the microfauna, notably microbe-feeding and predatory nematodes. At day 279, litter mixing also consistently reduced the ratio of bacterial-feeding to microbe-feeding (bacterial-feeding+fungal-feeding) nematodes, pointing to mixing causing a significant switch from the bacterial-based to the fungal-based energy channel. Litter mixing sometimes influenced the community composition and diversity of nematodes and macrofauna, but effects of litter mixing on diversity were not necessarily positive, and were much weaker than effects of litter species identity on diversity. We conclude that litter mixing effects on the abundance and diversity of decomposer biota, when they occur, are likely to be of secondary and generally minor significance when compared to the effects of litter species identity and composition.     

Impact of changes in rainfall amounts predicted by climate-change models on decomposition in a deciduous forest

Climate-change models predict a more intense hydrological cycle, with both increased and decreased amounts of rainfall in areas covered with temperate deciduous forests. These changes could alter rates of litter decomposition, with consequences for rates of nutrient cycling in the forest ecosystem. To examine impacts of predicted changes in precipitation on the rate of decay of canopy leaves, we placed litterbags in replicated, fenced 14 m² low-rainfall and high-rainfall plots located under individual rainout shelters. Unfenced, open plots served as an ambient treatment. Litter in the high-rainfall and ambient plots decayed 50% and 78% faster, respectively, than litter in the low-rainfall plots. Litter in the ambient plots disappeared 20% faster than in the high-rainfall treatment, perhaps via greater leaching during heavy rainfall events. Ambient rainfall during the experiment was similar in total amount to the high-rainfall treatment, but was more variable in intensity and timing. We used litterbags of different mesh sizes to examine whether changes in rainfall might alter the impacts of major categories of the fauna on litter decay. However, we found no consistent evidence that excluding arthropods of different sizes affected litter decay rate within any of the three rainfall treatments. This research reveals that changes in rainfall predicted to occur with global climate change will likely strongly alter rates of litter decay in deciduous forests.     

Patterns of microarthropod abundance in oak-hickory forest ecosystems in relation to prescribed fire and landscape position

We examined patterns of microarthropod abundance in oak-hickory (Quercus-Carya) forest ecosystems in southern Ohio (USA) in relation to landscape position and fire frequency. Abundances of various suborders of Acari and Collembola were determined in samples taken in June 1999 in three forested watersheds, one that had been burned annually for four consecutive springs (1996-1999), one that had been burned periodically (1996 and 1999), and an unburned control. Microarthropod abundance was significantly lower in the annually burned watershed than the periodically burned and control watersheds. Since both the periodically burned and annually burned watersheds were burned in April 1999, the lower microarthropod abundance in the annually burned watershed was not simply an immediate effect of burning. At the landscape scale, the abundance of oribatid mites was greater in xeric than intermediate or mesic landscape positions. Within any single watershed, there was no significant linear relationship between litter mass and microarthropod abundance. However, when all three watersheds were combined, there was a significant, positive relationship between litter mass and microarthropod abundance, mainly due to the annually burned watershed where there was very low litter mass and low microarthropod abundance. Both fire frequency and landscape position have significant effects on microarthropod abundance; however, those effects cannot be robustly predicted based solely on forest floor litter mass differences.     

Altered precipitation seasonality impacts the dominant fungal but rare bacterial taxa in subtropical forest soils

How soil microbial communities respond to precipitation seasonality change remains poorly understood, particularly for warm-humid forest ecosystems experiencing clear dry-wet cycles. We conducted a field precipitation manipulation experiment in a subtropical forest to explore the impacts of reducing dry-season rainfall but increasing wet-season rainfall on soil microbial community composition and enzyme activities. A 67% reduction of throughfall during the dry season decreased soil water content (SWC) by 17–24% (P < 0.05), while the addition of water during the wet season had limited impacts on SWC. The seasonal precipitation redistribution had no significant effect on the microbial biomass and enzyme activities, as well as on the community composition measured with phospholipid fatty acids (PLFAs). However, the amplicon sequencing revealed differentiated impacts on bacterial and fungal communities. The dry-season throughfall reduction increased the relative abundance of rare bacterial phyla (Gemmatimonadetes, Armatimonadetes, and Baoacteriodetes) that together accounted for only 1.5% of the total bacterial abundance by 15.8, 40, and 24% (P < 0.05), respectively. This treatment also altered the relative abundance of the two dominant fungal phyla (Basidiomycota and Ascomycota) that together accounted for 72.4% of the total fungal abundance. It increased the relative abundance of Basidiomycota by 27.4% while reduced that of Ascomycota by 32.6% (P < 0.05). Our results indicate that changes in precipitation seasonality can affect soil microbial community composition at lower taxon levels. The lack of community-level responses may be ascribed to the compositional adjustment among taxonomic groups and the confounding effects of other soil physicochemical variables such as temperature and substrate availability.     

Seasonal distribution of Xenylla brevispina (Collembola) in the canopy and soil habitat of a Cryptomeria japonica plantation

We investigated the life cycle and habitat use of an arboreal collembolan species, Xenylla brevispina, in the canopy and soil of a conifer (Cryptomeria japonica D. Don) plantation. The adaptive significance of migration between arboreal and soil habitats in the maintenance of its population in relation to the vertical structure of the forest is discussed. We sampled dead branches with foliage in the canopy (canopy litter) and on the forest-floor (soil litter). X. brevispina had one generation a year throughout the 3 years of the study. The mean densities of X. brevispina were similar in the canopy litter (0.06 to 14.57 g⁻¹ dry weight) and the soil litter (0.44 to 18.99 g⁻¹ dry weight). Seasonal patterns of density and relative abundance indicate that individuals of X. brevispina in the canopy were closely associated with those in the soil. These results suggest that vertical migration between the canopy and the soil might be a strategy allowing X. brevispina to be a predominant collembolan species in this forest.     

Relationships between soil–litter interface enzyme activities and decomposition in <Emphasis Type="Italic">Pinus massoniana</Emphasis> plantations in China

Purpose

Enzyme activities in decomposing litter are directly related to the rate of litter mass loss and have been widely accepted as indicators of changes in belowground processes. Studies of variation in enzyme activities of soil–litter interface and its effects on decomposition are lacking. Evaluating enzyme activities in this layer is important to better understand energy flow and nutrient cycling in forest ecosystems.

Materials and methods

Litter decomposition and the seasonal dynamics of soil–litter enzyme activities were investigated in situ in 20- (younger) and 46-year-old (older) Pinus massoniana stands for 540 days from August 2010 to March 2012 by litterbag method. We measured potential activities of invertase, cellulase, urease, polyphenol oxidase, and peroxidase in litter and the upper mineral soils, and evaluated their relationships with the main environment factors.

Results and discussion

Remaining litter mass was 57.6 % of the initial weights in the younger stands and 61.3 % in the older stands after 540-day decomposition. Levels of enzyme activity were higher in the litter layer than in the soil layer. Soil temperature, litter moisture, and litter nitrogen (N) concentration were the most important factors affecting the enzyme activities. The enzyme activity showed significantly seasonal dynamics in association with the seasonal variations in temperature, water, and decomposition stages. Remaining litter dry mass was found to be significantly linearly correlated with enzyme activities (except for litter peroxidase), which indicates an important role of enzyme activity in the litter decomposition process.

Conclusions

Our results indicated the important effects of biotic (litter N) and abiotic factors (soil temperature and litter moisture) on soil–litter interface enzyme activities. Overall significant linear relationship between remaining dry mass and enzyme activities highlighted the important role of enzyme activity in affecting litter decomposition processes, which will further influence nutrient cycling in forest ecosystems. Our results contributed to the better understanding of the mechanistic link between upper soil–litter extracellular enzyme production and litter decomposition in forest ecosystems.

     

内盖夫荒漠中小型土壤节肢动物群落的时空变化

Desert ecosystems are characterized by sparse vegetation that affects both abiotic parameters and soil biota along the soil profile.This study was conducted in 2010–2011 in a loess plain in the northern Negev Desert highlands, Israel, to test two main hypotheses:1) the abundance and diversity of microarthropods would vary seasonally in the top 30-cm soil layer, but would be relatively stable at soil depths between 30 and 50 cm and 2) soil microarthropods would be more abundant in soils under shrubs with large litter accumulations than under shrubs with less litter or bare soil. Soil samples were collected each season from the 0–50 cm profile at10-cm intervals under the canopies of Hammada scoparia and Zygophyllum dumosum and from the bare interspaces between them.Soil moisture and soil organic carbon in the top 30-cm layers varied seasonally, but there was little variation in the soil layers deeper than 30 cm. Soil mites were most abundant in the top 30-cm soil layer in autumn and winter, with the highest number of families found in winter. There were no differences in soil microarthropod abundance attributable to the presence or absence of shrubs of either species. The microarthropod communities of the microhabitats studied consisted of Acari, Psocoptera, and Collembola. The Acari were mostly identified to the family level and were dominated by Oribatida(55%) and Prostigmata(41%) in all seasons and microhabitats, while the psocopterans were most abundant in summer. These results are opposite to those obtained in other studies in similar xeric environments. Moreover, our findings were not in line with our hypothesis that a better microhabitat played a major role in microarthropod community composition, diversity, and density.     

Soil microbial abundance and community structure vary with altitude and season in the coniferous forests,China

Purpose

Soil microbes control the bioelement cycles and energy transformation in forest ecosystems, and are sensitive to environmental change. As yet, the effects of altitude and season on soil microbes remain unknown. A 560 m vertical transitional zone was selected along an altitude gradient from 3023, 3298 and 3582 m, to determine the potential effects of seasonal freeze-thaw on soil microbial community.

Materials and methods

Soil samples were collected from the three elevations in the growing season (GS), onset of freezing period (FP), deeply frozen period (FPD), thawing period (TP), and later thawing period (TPL), respectively. Real-time qPCR and polymerase chain reaction denaturing gradient gel electrophoresis (PCR-DGGE) were used to measure the abundance and structure of soil microbial community.

Results and discussion

The bacterial, archaeal, and fungal ribosomal DNA (rDNA) copy numbers decreased from GS to freezing stage (FP and FPD) and then increased in thawing stage (TP and TPL). Similarly, the diversity of microbial community varied with seasonal freeze-thaw processes. The diversity index (H) of the bacterial and archaeal communities decreased from GS to FP and then increased to TPL. The fungal community H index increased in the freezing process.

Conclusions

Our results suggested that abundance and structure of soil microbial community in the Tibetan coniferous forests varied by season and bacterial and archaeal communities respond more promptly to seasonal freeze-thaw processes relative to fungal community. This may have important implications for carbon and nutrient cycles in alpine forest ecosystems. Accordingly, future warming-induced changes in seasonal freeze-thaw patterns would affect soil nutrient cycles via altering soil microbial properties.

     

Influence of drought and litter age on Collembola communities

A field experiment was carried out to study the impact of drought and litter quality on the structure and performance of collembolan communities. The hypothesis was tested that changes in substrate humidity and resource quality significantly influence decomposition processes via alterations in soil faunal community structure. Litterbags (1000 μm mesh size) containing either freshly fallen or aged spruce litter were placed on the floor of a German spruce forest for one year. The bags were exposed to either ambient conditions (control) or drought (covered with roofs). Drought-induced changes in biological parameters were associated with a strong reduction in decomposition rates. Moreover, drought stress decreased Collembola abundance and species richness. The influence of drought on some microbiological parameters strongly depended on the litter age. A comparison of the two litter treatments revealed major effects of litter age on microbiological and physico-chemical parameters, but no effects on Collembola abundance and species richness. A detailed analysis of the collembolan community structure showed that certain species are highly adapted to specific characteristics of the substrate and thus rapidly respond to changes in microhabitat conditions.     

Effects of repeated drought on soil microarthropod communities in the northern Chihuahuan Desert

 Soil microarthropods were sampled in plots centered on creosotebushes (Larrea tridentata) and in plots centered on mesquite (Prosopis glandulosa) coppice dunes. Nine plots in each area were covered by rain-out shelters with greenhouse plastic roofs which excluded natural rainfall and nine plots received natural rainfall. There were differences in the abundance of several mite taxa in soils from the mesquite coppice dune plots. Some taxa (Stigmaeidae, Nanorchestidae, and Entomobryidae) occurred in significantly lower numbers in the soils of the drought plots. Other taxa (Tarsonemidae and Cunaxidae) were more abundant in the drought plots in the mesquite coppice dunes. There were no significant differences in the abundance of any of the dominant taxa of soil microarthropods in the drought and control plots centered on creosotebush. In the creosotebush habitat, there were significantly fewer Prostigmata in the plots exposed to drought. In an area with both creosotebush and mesquite, there were no significant differences in microarthropod population responses to drought and in recovery from drought. The differences in responses of soil microarthropods to drought in creosotebush and mesquite habitats are attributed to the differences in soil stability, litter accumulations, and microclimate associated with the shrubs. Received: 29 December 1997