Relationship between soil microarthropod species diversity and plant growth does not change when the system is disturbed

Soil microarthropods influence vital ecosystem processes, such as decomposition and nutrient mineralisation. There is evidence, however, that proper functioning of ecosystems does not require the presence of all its constituent species, and therefore some species can be regarded as functionally redundant. It has been proposed that species redundancy can act as an insurance against unfavourable conditions, and that functionally redundant species may become important when a system has faced a disturbance (the “insurance hypothesis”).
We conducted a laboratory microcosm experiment with coniferous forest soil and a seedling of silver birch (Betula pendula). A gradient of microarthropod diversity (from one to tens of species of soil mites and Collembola) was created to the systems. We disturbed microcosms with drought to test whether systems with altering microarthropod species richness respond differently to perturbations. Primary production (birch biomass), uptake of nitrogen by the birch seedling, the system's ability to retain nutrients and the structure and biomass of the soil microbial community were analysed.
Primary production and nutrient uptake of the birch seedlings increased slightly with increasing microarthropod species richness but only at the species poor end of the diversity gradient. Loss of nutrients and the biomass and community structure of microbes were unaffected by the microarthropods. The effect of drought on the birch biomass production was independent of the species richness of microarthropods. During the disturbance the biomass of microarthropods declined in diverse systems but not in simple ones. These systems were, however, quite resilient; microarthropod communities recovered quickly after the disturbance. Our results suggest that soil microarthropod species are functionally redundant in respect to plant growth, and that the resistance of a system to and its recovery from a disturbance are only weakly related to the species richness of this fauna.     

Influence of decomposer food web structure and nitrogen availability on plant growth

We studied the sensitivity of soil microbial communities and ecosystem processes to variation in the vertical and horizontal structure of decomposer food web under nitrogen poor and N-enriched conditions. Microcosms with humus and litter layer of boreal forest floor, birch seedlings infected with mycorrhizal fungi, and decomposer food webs with differing trophic group and species composition of soil fauna were constructed. During the second growing period for the birch, we irrigated half of the microcosms with urea solution, and the other half with de-ionised water to create two levels of N concentration in the substrate. During the experiment night time respirations of the microcosms were measured, and the water leached through the microcosms was analysed for concentration of mineral N, and nematode numbers. The microcosms were destructively sampled after 37 weeks for plant biomass and N uptake, structure of soil animal and microbial community (indicated by PLFA profiles), and physical and chemical properties of the humus and litter materials. Predatory mites and nematodes had a negative influence on the biomass of their microbivorous and microbi-detritivorous prey, and microbi-detritivores affected the biomass and community structure of microbes (indicated by PLFA-analysis). Moreover, predatory mites and nematodes increased microbial biomass and changed the microbial community structure. The decomposer food web structure affected also N uptake and growth of plants. Microbi-detritivorous fauna had a positive effect, whereas predators of microbial and detritus feeding fauna exerted a negative influence on plant N uptake and biomass production. The impact of a trophic group on the microbes and plant was also strongly dependent on species composition within the group. Nitrogen addition magnified the influence of food web structure on microbial biomass and plant N uptake. We suggest that addition of urea-N to the soil modified the animal-microbe interaction by increasing microbial growth and altering community structure of microbes. The presence of microbi-detritivores and predators reduced loss of carbon from the microcosms, and the food web structure influenced also water holding capacity of the materials. The changes in plant growth, nutrient cycling, size of N and C pools, and in the physical properties of the soil emphasize the importance and diversity of indirect consequences of decomposer food web structure. This revised version was published online in June 2006 with corrections to the Cover Date.     

Habitat patchiness affects decomposition and faunal diversity: a microcosm experiment on forest floor

Environmental heterogeneity has been intensively studied, but little is known about relationships between habitat patchiness and soil processes. The aim of this study was to investigate (1) the impact of patchiness of the litter layer on the decomposer community and litter decomposition rate, and (2) whether the impact of soil fauna on the rates of processes differs in relation to patchiness. An experiment was carried out in microcosms with coniferous forest humus and four kinds of litter with different C:N ratios or stages of decomposition, either separately (i.e. in patches) or mixed with each other. Microarthropod species diversity was better maintained in the patchy systems. In the absence of soil fauna, community respiration was higher in the patchy microcosms, but in the presence of fauna the opposite pattern was observed. The contribution of soil fauna to the rate of decomposition was clearly greater in the mixed litter systems. Based on the results, a hypothesis is presented that in the patchy litter layer the soil fungi can create connections between different materials located some centimeters apart, thus enhancing decomposition, while in the mixed litter the scale of millimeters is more appropriate for the soil fauna, known to accelerate the process rates. Received: 10 November 1997 / Accepted: 20 April 1998     

Perennial grass bioenergy cropping systems: Impacts on soil fauna and implications for soil carbon accrual

Perennial grass energy crop production is necessary for the successful and sustainable expansion of bioenergy in North America. Numerous environmental advantages are associated with perennial grass cropping systems, including their potential to promote soil carbon accrual. Despite growing research interest in the abiotic and biotic factors driving soil carbon cycling within perennial grass cropping systems, soil fauna remain a critical yet largely unexplored component of these ecosystems. By regulating microbial activity and organic matter decomposition dynamics, soil fauna influence soil carbon stability with potentially significant implications for soil carbon accrual. We begin by reviewing the diverse, predominantly indirect effects of soil fauna on soil carbon dynamics in the context of perennial grass cropping systems. Since the impacts of perennial grass energy crop production on soil fauna will mediate their potential contributions to soil carbon accrual, we then discuss how perennial grass energy crop traits, diversity, and management influence soil fauna community structure and activity. We assert that continued research into the interactions of soil fauna, microbes, and organic matter will be important for advancing our understanding of soil carbon dynamics in perennial grass cropping systems. Furthermore, explicit consideration of soil faunal effects on soil carbon can improve our ability to predict changes in soil carbon following perennial grass cropping system establishment. We conclude by addressing the major knowledge gaps that should be prioritized to better understand and model the complex connections between perennial grass bioenergy systems, soil fauna, and carbon accrual.     

土壤动物多样性及其生态功能

土壤无脊椎动物生物量通常小于土壤生物总生物量的10%,但它们种类丰富,取食行为及生活史策略多种多样,且土壤动物之间,土壤动物与微生物之间存在着复杂的相互作用关系。土壤动物的生态功能主要通过取食作用(trophic effect)和非取食作用(non-trophic effect)来实现。原生动物数量大、周转快,故原生动物本身的代谢活动(即取食作用)对碳氮矿化的贡献可以接近甚至超过细菌的贡献;然而大多数中小型土壤动物的本身代谢过程对碳氮矿化的贡献远低于土壤微生物,但它们可以通过取食作用来调节微生物进而影响碳氮的矿化。大型节肢动物中的蜘蛛和地表甲虫等捕食者经常活跃于地表,它们常常会通过级联效应对土壤生态系统产生重要的影响。蚯蚓、白蚁等大型土壤动物除可以通过取食作用以外,还可以通过非取食作用调控土壤微生物,进而显著影响土壤碳氮过程。土壤动物取食行为的多样性和复杂的非营养关系的存在造就了多维度的土壤食物网,给土壤动物的生态功能研究带来了巨大的挑战。介绍了土壤动物的多样性及主要的生态功能,并对研究的热点和前沿问题进行了探讨,以期引起关于土壤动物多样性及其生态功能的深入思考。     

Effects of soil decomposer invertebrates (protozoa and earthworms) on an above-ground phytophagous insect (cereal aphid) mediated through changes in the host plant

We investigated if the activity of soil invertebrates (protozoa and earthworms) affected the performance of barley and if effects propagated higher up the above-ground food chain into herbivores (cereal aphid, Sitobion avenae ). Barley plants were grown individually in microcosms containing defaunated soil and grass residues. Plants were grown in soil containing: a) no added fauna, b) protozoa, c) earthworms, or d) protozoa and earthworms. After 7 weeks growth at 20°C three adult cereal aphids were added to each plant on separate leaves. The aphids were allowed to grow and reproduce for another 2 weeks before the experiment was destructively sampled. Amounts of mineral N in the soil and leached from the microcosms were significantly reduced by the presence of soil animals. Correspondingly plant biomass and total plant N content were increased significantly by soil animals, protozoa in particular. The different mechanisms responsible for changes in nutrient turnover in presence of protozoa and earthworms are discussed. Aphid performance was strongly influenced by the presence of protozoa, but not by earthworms. In the presence of protozoa the numbers and biomass of adult and juvenile aphids were significantly increased. These effects are likely due to an increased N content in barley plants and consequently increased nitrogen availability to aphids. The results underline that the detritivore and herbivore systems are intimately linked.     

Linking above-ground and below-ground effects in autotrophic microcosms: effects of shading and defoliation on plant and soil properties

Although factors affecting plant growth and plant carbon/nutrient balance – e.g., light availability and defoliation by herbivores – may also propagate changes in below‐ground food webs, few studies have aimed at linking the above‐ground and below‐ground effects. We established a 29‐week laboratory experiment (～one growing season) using autotrophic microcosms to study the effects of light and defoliation on plant growth, plant carbon/nutrient balance, soil inorganic N content, and microbial activity and biomass in soil. Each microcosm contained three substrate layers – mineral soil, humus and plant litter – and one Nothofagus solandri var. cliffortioides seedling. The experiment constituted of the presence or absence of two treatments in a full factorial design: shading (50% decrease in light) and artificial defoliation (approximately 50% decrease in leaf area in the beginning of the growing season). At the end of the experiment a range of above‐ground and below‐ground properties were measured. The shading treatment reduced root and shoot mass, root/shoot ratio and leaf production of the seedlings, while the defoliation treatment significantly decreased leaf mass only. Leaf C and N content were not affected by either treatment. Shading increased NO ₃–N concentration and decreased microbial biomass in humus, while defoliation did not significantly affect inorganic N or microbes in humus. The results show that plant responses to above‐ground treatments have effects which propagate below ground, and that rather straightforward mechanisms may link above‐ground and below‐ground effects. The shading treatment, which reduced overall seedling growth and thus below‐ground N use and C allocation, also led to changes in humus N content and microbial biomass, whereas defoliation, which did not affect overall growth, did not influence these below‐ground properties. The study also shows the carbon/nutrient balance of N. solandri var. cliffortioides seedlings to be highly invariant to both shading and defoliation.     

Growth Response of Young Birch Trees (Betula pendulaRoth.) After Four and a Half Years of CO2Exposure

A field experiment consisting of 18 birch trees grown in opentop chambers in ambient and elevated CO₂concentrations was setup with the aim of testing whether the positive growth responseobserved in many short-term studies is maintained after severalgrowing seasons. We present the results of growth and biomassafter 4.5 years of CO₂exposure, one of the longest studies sofar on deciduous tree species. We found that elevated CO₂ledto a 58% increase in biomass at the end of the experiment. However,estimation of stem mass during the growing season showed thatelevated CO₂did not affect relative growth rate during the fourthgrowing season, and therefore, that the large accumulation ofbiomass was the result of an early effect on relative growthrate in previous years. Trees grown in elevated CO₂investedmore carbon into fine roots and had relatively less leaf areathan trees grown in ambient CO₂. In contrast with previous studies,acceleration of growth did not involve a significant declinein nutrient concentrations of any plant tissue. It is likelythat increased fine root density assisted the trees in meetingtheir nutrient demands. Changes in the species composition ofthe ectomycorrhizal fungi associated with the trees grown inelevated CO₂in favour of late successional species supportsthe hypothesis of an acceleration of the ontogeny of the treesin elevated CO₂.Copyright 1997 Annals of Botany Company Betula pendula; silver birch; elevated CO₂; growth; biomass allocation; ectomycorrhizas; tissue composition; nutrients; leaf morphology; specific leaf area; stomatal density; shoot structure     

Initial effects of experimental warming on above- and belowground components of net ecosystem CO2 exchange in arctic tundra

The Arctic contains extensive soil carbon reserves that could provide a substantial positive feedback to atmospheric CO₂ concentrations and global warming. Evaluation of this hypothesis requires a mechanistic understanding of the in situ responses of individual components of tundra net ecosystem CO₂ exchange (NEE) to warming. In this study, we measured NEE, total ecosystem respiration and respiration from below ground in experimentally warmed plots within Alaskan acidic tussock tundra. Soil warming of 2-4°C during a single growing season caused strong increases in total ecosystem respiration and belowground respiration from moss-dominated inter-tussock areas, and similar trends from sedge-dominated tussocks. Consequently, the overall effect of the manipulation was to substantially enhance net ecosystem carbon loss during mid-summer. Components of vascular plant biomass were closely correlated with total ecosystem respiration and belowground respiration in control plots of both microsites, but not in warmed plots. By contrast, in the warmed inter-tussock areas, belowground respiration was most closely correlated with organic-layer depth. Warming in tussock areas was associated with increased leaf nutrient pools, indicating enhanced rates of soil nutrient mineralisation. Together, these results suggest that warming enhanced net ecosystem CO₂ efflux primarily by stimulating decomposition of soil organic matter, rather than by increasing plant-associated respiration. Our short-term experiment provides field evidence to support previous growth chamber and modelling studies indicating that arctic soil C reserves are relatively sensitive to warming and could supply an initial positive feedback to rising atmospheric CO₂ concentrations/changing climate.     

Interaction of increasing atmospheric carbon dioxide and soil nitrogen on the carbon balance of tundra microcosms

Summary Natural cores of vegetation and soils of arctic tundra were collected in frozen condition in winter near Barrow, Alaska (71°20N). These cores were used as microcosms in a phytotron experiment to measure the interactions, if any, between increasing atmospheric CO₂ concentration and fertilization by ammonium nitrate on net ecosystem CO₂ exchange and net yield of tundra vegetation. Increased soil N significantly enhanced net ecosystem CO₂ uptake. The effect of increased CO₂ concentration had little or no effect on mean net ecosystem carbon balance of the tundra microcosms. Added N significantly increased leaf area and phytomass of vascular plants in the microcosms while increased atmospheric CO₂ had no effect on these parameters. We conclude that atmospheric CO₂ is not now limiting net ecosystem production in the tundra and that its direct effects will be slight even at double the present concentration. the most probable effects of carbon dioxide in the coastal tundra will be through its indirect effects on temperature, water table, peat decomposition, and the availability of soil nutrients.     

Scale-specific correlations between habitat heterogeneity and soil fauna diversity along a landscape structure gradient

Habitat heterogeneity contributes to the maintenance of diversity, but the extent that landscape-scale rather than local-scale heterogeneity influences the diversity of soil invertebrates—species with small range sizes—is less clear. Using a Scottish habitat heterogeneity gradient we correlated Collembola and lumbricid worm species richness and abundance with different elements (forest cover, habitat richness and patchiness) and qualities (plant species richness, soil variables) of habitat heterogeneity, at landscape (1 km²) and local (up to 200 m²) scales. Soil fauna assemblages showed considerable turnover in species composition along this habitat heterogeneity gradient. Soil fauna species richness and turnover was greatest in landscapes that were a mosaic of habitats. Soil fauna diversity was hump-shaped along a gradient of forest cover, peaking where there was a mixture of forest and open habitats in the landscape. Landscape-scale habitat richness was positively correlated with lumbricid diversity, while Collembola and lumbricid abundances were negatively and positively related to landscape spatial patchiness. Furthermore, soil fauna diversity was positively correlated with plant diversity, which in turn peaked in the sites that were a mosaic of forest and open habitat patches. There was less evidence that local-scale habitat variables (habitat richness, tree cover, plant species richness, litter cover, soil pH, depth of organic horizon) affected soil fauna diversity: Collembola diversity was independent of all these measures, while lumbricid diversity positively and negatively correlated with vascular plant species richness and tree canopy density. Landscape-scale habitat heterogeneity affects soil diversity regardless of taxon, while the influence of habitat heterogeneity at local scales is dependent on taxon identity, and hence ecological traits, e.g. body size. Landscape-scale habitat heterogeneity by providing different niches and refuges, together with passive dispersal and population patch dynamics, positively contributes to soil faunal diversity. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

半干旱草原大中型土壤动物在畜粪分解中的作用

土壤动物是陆地生态系统的重要组分, 在有机质分解过程中具有重要作用。目前有关土壤动物在生态系统分解中的作用研究主要聚焦于植物凋落物的分解, 而对动物粪便分解的研究稀少。本研究在内蒙古典型草原设置了马粪和牛粪分解原位实验, 使用不同孔径的金属隔离网排除不同体型大小的土壤动物, 通过测定大中型土壤动物对畜粪分解过程中质量损失、碳氮含量和微生物呼吸以及土壤养分动态变化的影响, 解析其在分解中的作用。设置5个处理, 即CK, 仅土壤, 无粪; T0, 粪添加+0.425 mm隔离网(排除了粪居型和掘洞型粪金龟和中型土壤动物); T1, 粪添加+1 mm隔离网(排除了粪居型和掘洞型粪金龟); T2, 粪添加+2 mm隔离网(排除了掘洞型粪金龟); T3, 仅粪添加(不排除土壤动物)。结果表明: (1)在畜粪分解60天内, 土壤动物对畜粪的干质量损失没有显著的促进作用(P > 0.05); 相反, 在畜粪分解360天, 不隔离土壤动物处理(T3)显著地提高了牛粪干质量损失(P < 0.05), 而降低了马粪干质量损失(P < 0.05)。(2)在畜粪分解的60天内, 畜粪中碳和氮含量下降速度在有土壤动物存在的情况下(T3)快于隔离土壤动物(T0和T1)。(3)两种畜粪添加增加了土壤微生物的呼吸, 且这种增加趋势在实验的第15天和第30天在土壤动物存在时(T3)最明显。(4)与对照(CK)相比, 马粪添加处理提高了土壤速效氮、有机碳的含量和土壤含水量, 且这种增加趋势在排除掘洞型粪金龟(T2)和不排除土壤动物(T3)条件下表现更显著(P < 0.05), 而牛粪添加处理没有明显改变这些指标(P > 0.05)。研究表明, 分解初期粪金龟的取食和活动会改变畜粪的理化性质, 进而影响分解后期土壤生物在畜粪分解中的作用。     

萘对川西亚高山森林土壤呼吸、养分和酶活性的影响

萘作为土壤动物化学抑制剂已在土壤动物生态功能的研究中广泛使用,但其非目标效应使其应用仍存在很大的不确定性.为了了解在亚高山森林土壤应用萘抑制土壤动物群落是否存在非目标效应,以青藏高原东缘的川西亚高山森林土壤为研究对象,采用微缩试验研究了萘对土壤呼吸速率、养分含量和酶活性的短期影响.结果表明: 萘处理显著抑制了培养0～10 d的土壤呼吸速率,随后(24～52 d)表现出明显的促进作用.萘处理显著影响了土壤铵态氮和硝态氮含量的动态变化,萘处理铵态氮和硝态氮含量分别以培养的3和17 d最高,对照则以培养的45 d和结束时的52 d最高.萘处理土壤可溶性碳含量在培养3 d急剧增加后迅速降低,对照则略有升高后降低,而萘处理和对照的可溶性氮含量均表现为先升高后降低.萘处理和对照的土壤酶活性均具相似的动态规律,两者的脲酶、硝酸还原酶和亚硝酸还原酶活性分别在培养45、38和10 d至最高.萘处理和采样时间的交互作用显著影响了土壤呼吸速率,以及土壤铵态氮、硝态氮和可溶性氮含量,但对可溶性碳含量、蔗糖酶、硝酸还原酶和亚硝酸还原酶活性的影响不显著.萘作为驱虫剂的非目标效应可能在短期内对川西亚高山森林土壤的氮循环过程产生强烈的影响.     

Response of winter chemical defense in Alaska paper birch and green alder to manipulation of plant carbon/nutrient balance

Summary Plant carbon/nutrient balance has been implicated as an important factor in plant defensive chemistry and palatability to herbivores. We tested this hypothesis by fertilizing juvenile growth form Alaska paper birch and green alder with N, P and N-plus-P in a balanced 2x2 factorial experiment. Additionally, we shaded unfertilized plants of both species. Fertilization with N and N-plus-P increased growth of Alaska paper birch, reduced the concentration of papyriferic acid in internodes and increased the palatability of birch twigs to snowshoe hares. Shading decreased birch growth, decreased the concentration of papyriferic acid in internodes and increased twig palatability. These results indicate that the defensive chemistry and palatability of winter-dormant juvenile Alaska paper birch are sensitive to soil fertility and shade. Conversely the defensive chemistry and palatability of green alder twigs to snowshoe hares were not significantly affected by soil fertility or shade. The greater sensitivity of Alaska paper birch defensive chemistry and palatability to snowshoe hares in comparison to green alder is in agreement with the hypothesis that early successional woody plants that are adapted to high resource availability are more plastic in their chemical responses to the physical environment than are species from less favorable environments.     

CO2 efflux from a Mediterranean semi-arid forest soil. II. Effects of soil fauna and surface stoniness

Many forest soils in the Mediterranean basin areshallow and contain high amounts of gravel in theorganic layers. Recent studies on soil organic matteraccumulation have shown high amounts of organic matteroccurring mainly in soils with high levels ofstoniness at the soil surface. The gravel layer mayaffect the microclimatic conditions of the soilsurface and probably the distribution and activity ofsoil fauna.In order to quantify the combined effects soil fauna(epigeic macrofauna and earthworms) and stoniness onthe release of soil CO₂, we performed a threefactor field experiment by using a series ofreconstructed soil profiles. Factors 1 and 2 consistedof the exclusion/presence of soil epigeic macrofaunaand earthworms, and factor 3 of the presence/absenceof a gravel layer intermingled with the H horizon. Weincubated ¹⁴C straw in the H horizon and carriedout three 40 mm rainfall simulations.Soil respiration primarily depended on the season. Theeffects of soil fauna were generally small and did notcoincide with periods of high faunal activity. Thelargest effects of both earthworms and soil epigeicfauna were found after wetting the soil in summer. Theeffects of the earthworms were concentrated in themineral soil while the effects of the epigeic faunawere concentrated in the H horizon and mainly arosetowards the end of the experiment. This suggests thatthe effects of epigeic fauna may have beenunderestimated due to the length of the experiment.The gravel layer increased the effect of faunaprobably by creating more favorable microclimaticconditions. The accumulation of organic matter insoils with high levels of stoniness cannot beexplained by the effect of gravel on soil microclimatenor by its effect on the activity of soil fauna.     

Soil fauna diversity increases CO2 but suppresses N2O emissions from soil

Soil faunal activity can be a major control of greenhouse gas (GHG) emissions from soil. Effects of single faunal species, genera or families have been investigated, but it is unknown how soil fauna diversity may influence emissions of both carbon dioxide (CO₂, end product of decomposition of organic matter) and nitrous oxide (N₂O, an intermediate product of N transformation processes, in particular denitrification). Here, we studied how CO₂ and N₂O emissions are affected by species and species mixtures of up to eight species of detritivorous/fungivorous soil fauna from four different taxonomic groups (earthworms, potworms, mites, springtails) using a microcosm set‐up. We found that higher species richness and increased functional dissimilarity of species mixtures led to increased faunal‐induced CO₂ emission (up to 10%), but decreased N₂O emission (up to 62%). Large ecosystem engineers such as earthworms were key drivers of both CO₂ and N₂O emissions. Interestingly, increased biodiversity of other soil fauna in the presence of earthworms decreased faunal‐induced N₂O emission despite enhanced C cycling. We conclude that higher soil fauna functional diversity enhanced the intensity of belowground processes, leading to more complete litter decomposition and increased CO₂ emission, but concurrently also resulting in more complete denitrification and reduced N₂O emission. Our results suggest that increased soil fauna species diversity has the potential to mitigate emissions of N₂O from soil ecosystems. Given the loss of soil biodiversity in managed soils, our findings call for adoption of management practices that enhance soil biodiversity and stimulate a functionally diverse faunal community to reduce N₂O emissions from managed soils.     

Trophic interactions in soils as they affect energy and nutrient dynamics. IV. Flows of metabolic and biomass carbon

Flows of biomass and respiratory carbon were studied in a series of propylene-oxide sterilized soil microcosms. One-half of the microcosms received three pulsed additions of 200 ppm glucose-carbon to mimic rhizosphere carbon inputs. Biotic variables were: bacteria (Pseudomonas) alone, or amoebae (Acanthamoeba) and nematodes (Mesodiplogaster) singly, or both combined in the presence of bacteria.Over the 24-day experiment, respiration was significantly higher in the microcosms containing the bacterial grazers. Biomass accumulation by amoebae was significantly higher than that by nematodes. The nematodes respired up to 30-fold more CO₂ per unit biomass than did amoebae. Similar amounts of carbon flowed into both respiratory and biomass carbon in microcosms with fauna, compared with the bacteria-alone microcosms. However, partitioning of available carbon by the microfauna varied considerably, with little biomass production and relatively more CO₂-C produced in the nematode-containing microcosms. The amoebae, in contrast, allocated more carbon to tissue production (about 40% assimilation efficiency) and correspondingly less to CO₂.     

Population- and ecosystem-level effects of predation on microbial-feeding nematodes

We studied the role of nematode predation in the functioning of detrital food webs assembled in microcosms. The microcosms contained defaunated humus and litter materials, a diverse microbial community with bacteria, fungi and protozoa, and a birch (Betula pendula) seedling infected with mycorrhizal fungi. Different levels of top-down control upon microbivorous nematodes were set up by assembling food webs either without predators, or in combinations with a specialist and a non-specialist predatory mite (Mesostigmata). The nematode community was composed of either (1) three species of bacterivorous, or (2) three species of fungivorous nematodes or (3) both groups together. After two growing periods for the birch (38 weeks), the microcosms were destructively sampled for animal and microbial biomasses, concentration of mineral N in the soil, plant biomass and plant N concentration. The specialist predator reduced biomasses of both bacterial- and fungal-feeding nematodes by more than 50%, whereas the non-specialist predator weakly increased the biomass of fungivorous nematodes. Thus, under high predation pressure, the biomass of microbivores changed as predicted by trophic dynamic models assuming strong top-down control and uniformly behaving trophic levels. Despite this, microbial biomass was unaffected by the predators. However, microbial respiration increased slightly in the presence of predators. Assuming that microbial respiration correlates with microbial productivity, the increase in microbial respiration indicates a cascading productivity regulation. The composition of the microbivore community had only a minor effect on the outcome of the top-down control on microbes. The >50% reduction in nematode biomass and respiration coincided with <16% increase in microbial respiration and did not affect microbial biomass. Presence of the specialist predator slightly reduced soil NH⁺ ₄ concentration in communities with fungivore nematodes but plant growth and N uptake remained unchanged. Thus, the structure of the community only weakly controlled nutrient mineralisation. Received: 18 May 1998 / Accepted: 3 May 1999     

北京山区3种暖温带森林生态系统未来碳平衡的模拟与分析

使用LPJ-GUESS植被动态模型, 在北京山区研究了未来100a以辽东栎 (Quercus liaotungensis) 为优势种的落叶阔叶林、以白桦 (Betula platyphylla) 为主的阔叶林和油松 (Pinus tabulaeformis) 为优势种的针阔混交林的碳变化, 定量分析了生态系统净初级生产力 (NPP) 、土壤异养呼吸 (Rh) 、净生态系统碳交换 (NEE) 和碳生物量 (Carbon bio-mass) 对两种未来气候情景 (SRES A2和B2) 以及相应大气CO2浓度变化情景的响应特征。结果表明:1) 未来100a两种气候情景下3种森林生态系统的NPP和Rh均增加, 并且A2情景下增加的程度更大;2) 由于3种生态系统树种组成的不同, 未来气候情景下各自NPP和Rh增加的比例不同, 导致三者NEE的变化也相异:100a后辽东栎林由碳汇转变为弱碳源, 白桦林仍保持为碳汇但功能减弱, 油松林成为一个更大的碳汇;3) 3种森林生态系统的碳生物量在未来气候情景下均增大, 21世纪末与20世纪末相比:辽东栎林在A2情景下碳生物量增加的比例为27.6%, 大于B2情景下的19.3%;白桦林和油松林在B2情景下碳生物量增加的比例分别为34.2%和52.2%, 大于A2情景下的30.8%和28.4%。     

Role of soil animals in C and N mineralisation

Addition of single species of soil animals to animal-free microcosms often increases total heterotrophic respiration, but sometimes additions of microarthropods have been reported not to increase or even decrease CO₂ evolution rates. Most studies indicate that addition of soil animals increases net N mineralisation. In a study with F/H layer materials from a spruce stand in central Sweden kept at two temperatures (5 and 15°C) and three moisture levels (15, 30 and 60% of WHC), addition of a mixed fauna of soil arthropods, mainly microarthropods, could not be shown to change the CO₂ evolution rates in comparison with materials where arthropods were absent. However, addition of the arthropods significantly increased net N mineralisation for each of the temperature and moisture combinations. The increase due to the arthropods was dependent on soil temperature but not on soil moisture. Because the total net N mineralisation decreased with decreasing soil moisture, the soil arthropods had a much larger relative effect on net N mineralisation under dry than under moist conditions. It is concluded that soil arthropods are important in maintaining net N mineralisation under dry conditions when the microflora is largely inactive. The microbial/faunal release of mineral N is discussed in relation to the CN of the substrate.