Dynamics and stratification of functional groups of micro- and mesoarthropods in the organic layer of a Scots pine forest

This paper addresses the abundance, biomass and microstratification of functional groups of micro- and mesoarthropods inhabiting the organic layers of a Scots pine forest (Pinus sylvestris L.). An experiment using stratified litterbags, containing organic material of four degradation stages, i.e., freshly fallen litter, litter, fragmented litter and humus, was performed over a period of 2.5 years. Statistical data analysis revealed that each organic layer had a different, characteristic species composition that changed with time following successive degradation stages. Species of Acari, Araneae and Collembola were assigned to different functional groups based on taxonomy, microstratification, food type or feeding mode. The abundance and biomass carbon of functional groups were dependent on the organic layer and most functional groups showed a particular preference for one of the upper organic layers. Temporal and spatial differences in density and biomass carbon of functional groups could partly be related to fluctuations in the soil climate, although effects of trophic interactions could not be ruled out. A general decline in abundance and biomass, especially in populations of fungal feeders, during the last year of the study could not be explained by a reduction in litterbag volume, changed litter chemistry or soil climate, but was attributed to an indirect effect of a remarkable increase in soil coverage by wavy hair grass, Deschampsia flexuosa (L.). The analysis demonstrated that species diversity, microhabitat specification, soil fauna succession, and degradation stages of organic material are interrelated. The results obtained indicate that both the chemistry of organic matter and decomposition rates have an important effect on trophic relationships and community structure. Received: 26 June 1997     

Crop resistance traits modify the effects of an aboveground herbivore,brown planthopper,on soil microbial biomass and nematode community via changes to plant performance

Plant-mediated effects of aboveground herbivory on the belowground ecosystem are well documented, but less attention has been paid to agro-ecosystems and in particular how crop cultivars with different traits (i.e. resistance to pests) shape such interactions. A fully factorial experiment was conducted using four rice cultivars with different insect-resistance, with and without the aboveground herbivore Nilaparvata lugens (brown planthopper), and to test two hypotheses (1) aboveground herbivory affects the soil microbial biomass and nematode community by altering plant performance and soil resource availability and (2) herbivory effects will depend on cultivar resistance traits. Our results suggested that cultivar resistance mediated both herbivory intensity and herbivore effects on plant performance. N. lugens decreased the availability of soil resources (soluble sugars, amino acids, organic acids, dissolved organic carbon and nitrogen), microbial biomass and percentages of bacterivores when feeding on a susceptible cultivar but increased them in a resistant cultivar. However, total nematode abundance and the percentage of plant-parasitic nematodes responded in the opposite way, increasing under a susceptible cultivar and decreasing under a resistant cultivar. The development of plant-parasites under resistant cultivars before aboveground herbivory might contribute to their resistance traits. Our findings provide evidence that N. lugens significantly reversed the pattern of soil resource availability, microbial biomass and nematode community structure (abundance and trophic composition) across cultivars with distinct resistance. In the presence of aboveground pests, the agronomic use of resistant rice cultivars could also control populations of plant-parasites and promote soil resource availability, further extended to higher trophic level of soil food web.     

Linking above-ground and below-ground interactions: how plant responses to foliar herbivory influence soil organisms

Studies of the effects of above-ground herbivory on soil organisms and decomposer food webs, as well as the processes that they regulate, have largely concentrated on the effects of non-living inputs into the soil, such as dung, urine, body parts and litter. However, there is an increasing body of information which points to the importance of plant physiological responses to herbivory in regulating soil organisms and therefore, implicitly, key soil processes such as decomposition and nutrient mineralisation. In this review we identify the mechanisms by which foliar herbivory may indirectly affect the soil biota and associated below-ground processes through affecting plants, so as to better understand the nature of interactions which exist between above-ground and below-ground biota. We consider two broad pathways by which above-ground foliar herbivory may affect soil biotic communities. The first of these occurs through herbivore effects on patterns of root exudation and carbon allocation. These effects manifest themselves either as short-term changes in plant C allocation and root exudation or as long-term changes in root biomass and morphology. Evidence suggests that these mechanisms positively influence the size and activity of the soil biotic community and may alter the supply of nutrients in the rhizosphere for plant uptake and regrowth. The second of these involves herbivores influencing soil organisms through altering the quality of input of plant litter. Possible mechanisms by which this occurs are through herbivory enhancing nitrogen contents of root litter, through herbivory affecting production of secondary metabolites and concentrations of nutrients in foliage and thus in leaf litter and through selective foliar feeding causing shifts in plant community structure and thus the nature of litter input to the soil. While the effects of herbivory on soil organisms via plant responses may be extremely important, the directions of these effects are often unpredictable because several mechanisms are often involved and because of the inherently complex nature of soil food-web interactions; this creates obvious difficulties in developing general principles about how herbivory affects soil food-webs. Finally, it is apparent that very little is understood on how responses of soil organisms to herbivory affect those ecosystem-level processes regulated by the soil food-web (e.g. decomposition, nutrient mineralisation) and that such information is essential in developing a balanced understanding about how herbivory affects ecosystem function.     

Significance of litter layer in enhancing mesofaunal abundance and microbial biomass nitrogen in sweet corn-white clover living mulch systems

Living mulch is a type of sustainable farming system that consists of cover crops planted either before or with a main crop; a living mulch is maintained as a living ground cover throughout the growing season of the main crop. Microbial biomass and abundance of mesofauna (microarthropods and enchytraeids) are important soil biological parameters in relation to soil function, plant productivity, and nutrient cycling; however, the effects of living mulch on these parameters are not fully understood. In this study we examined the effects of living mulch treatment with nitrogen fertilizer (0, 40, 160, or 200?kg?ha^?1) on the abundance of soil microarthropods (Oribatida, Mesostigmata, Prostigmata, and Collembola) and the effects of living mulch treatment on the dynamics of the soil biota (mesofauna, microarthropods, enchytraeids, and microbial biomass nitrogen) from spring to autumn. Our results showed that living mulch treatment significantly (p?p?相似文献   

Legacies of plant litter on carbon and nitrogen dynamics and the role of the soil community

There is increasing awareness of the importance of ecological legacies in contemporary ecosystem processes. Decomposition is regulated by a set of interacting hierarchically organized factors. As spatial and temporal scales decrease, decomposition is largely dependent on the quality of resources and the decomposer community, but whether and how these factors manifest via historical legacy effects is not well understood. We tested whether the history of plant litter inputs had short-term legacy effects on contemporary litter and soil organic matter carbon (C) and nitrogen (N) mineralization. Using a field/laboratory microcosm approach, we exposed soils to two litters of contrasting chemistry and, after adding fresh substrates, we monitored C and N dynamics. In a parallel experiment, we manipulated the soil community to reduce litter-history impacts on its composition and size to investigate whether the soil community could be an important contributor to legacy effects We found strong short-term litter legacy effects on contemporary litter and soil N mineralization, the duration of which was dependent on the contemporary substrate for decomposition. These strong effects were not consistent with the home field advantage phenomenon, as exposure to a specific litter did not favor the decomposition of the same litter when it was applied as a contemporary substrate. Reduction of the litter-history effects on soil biota decreased the impact of litter history on N immobilization, suggesting that plant litter impacts on the soil community may be an important component of plant litter legacies on N decomposition. In contrast to N, litter legacies appeared to be much less important for C decomposition, suggesting that legacy effects might uncouple contemporary C and N 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.     

Stellera chamaejasme L. increases soil N availability, turnover rates and microbial biomass in an alpine meadow ecosystem on the eastern Tibetan Plateau of China

Plant species have been shown to have significant effects on soil nutrient pools and dynamics. Stellera chamaejasme L., a toxic perennial weed, has established and is now abundant in the alpine meadow on the eastern Tibetan Plateau of China since the 1960s. We quantified the effects of Stellera on carbon and nitrogen cycling in two topographic habitats, a flat valley and a south-facing slope, where Stellera was favored to spread within the study area. Aboveground litter biomass and tissue chemistry of aboveground litter and root were measured to explain the likely effects of Stellera on soil carbon and nutrient cycling. The sizes of various soil pools, e.g. nitrate, ammonium, inorganic phosphorus, microbial biomass, soil respiration and turnover rates including net mineralization, gross nitrification and denitrification were determined. The results showed that Stellera produced more aboveground litter than each of the co-occurring species. Aboveground litter of Stellera had higher tissue N and lower lignin:N than the other species. Stellera significantly increased surface soil (0-15 cm) organic matter, whereas no significant differences were found for organic C and total P in subsoil (15-30 cm) within and between patches of Stellera. Soil extractable nitrate concentrations in Stellera surface soil were 113% and 90% higher on the flat valley and on the south-facing slope, respectively. Both microbial biomass C and N were significantly higher in Stellera surface soil. Gross nitrification and microbial respiration were significantly higher in Stellera surface soil both on the flat valley and on the south-facing slope, whereas significant differences of denitrification were found only on the flat valley. The differences in the quantity and quality of aboveground litter are a likely mechanism responsible for the changes of soil properties.     

Changes in soil organic carbon,total nitrogen,and abundance of arbuscular mycorrhizal fungi along a large-scale aridity gradient

Changes in soil organic carbon, total nitrogen, pH, and the abundance of arbuscular mycorrhizal fungi are examined along a large-scale aridity gradient from southeast to northwest in China. Soil organic carbon and total nitrogen decreased but pH increased with increased aridity. Aboveground plant biomass, spore abundance, and colonization of roots by arbuscular mycorrhizal fungi also declined as the aridity increased. Soil organic carbon and total nitrogen were positively correlated with aboveground plant biomass, and arbuscular mycorrhizal fungal spore number and root colonization were positively correlated with soil organic carbon, total nitrogen, and aboveground plant biomass but were negatively correlated with soil pH. A structural equation model suggested that aridity affected soil organic carbon and total nitrogen by limiting aboveground plant biomass. Aridity exerted a large direct effect and smaller indirect effects (via changes in aboveground plant biomass) on the abundance of arbuscular mycorrhizal fungi. Soil pH also directly influenced arbuscular mycorrhizal fungal abundance. These results suggest that aboveground plant biomass could be a key factor driving the changes of soil organic carbon, total nitrogen, and arbuscular mycorrhizal fungal abundance along this aridity gradient in China.     

Collembola species composition and diversity effects on ecosystem functioning vary with plant functional group identity

The relationship between decomposer diversity and ecosystem functioning is little understood although soils accommodate a significant proportion of worldwide biodiversity. Collembola are among the most abundant and diverse decomposers and are known to modify plant growth. We examined the effects of Collembola species diversity (one, two and three species belonging to different life history groups) and composition on litter decomposition and the performance of plant communities (above- and belowground productivity) of different functional groups (grasses, forbs and legumes). Collembola densities did not increase with diversity indicating niche overlap. Generally, Collembola species composition was a better predictor for ecosystem functioning than Collembola species number with the impacts of Collembola diversity and composition on ecosystem functioning strongly depending on plant functional group identity. Non-linear effects of Collembola diversity on litter decomposition and plant productivity suggest pronounced and context dependent species interactions and feeding habits. Net surface litter decomposition was decreased by Collembola, whereas root litter decomposition was at maximum in the highest Collembola diversity treatment. Forbs benefitted most from the presence of three Collembola species. Similarly, Collembola diversity influenced root depth distribution in a plant functional group specific way: while grass root biomass decreased with increasing Collembola diversity in the upper and lower soil layer, legume root biomass increased particularly in the lower soil layer. Idiosyncratic and context dependent effects of Collembola diversity and composition even in rather simple assemblages of one to three species suggest that changes in Collembola diversity may have unpredictable consequences for ecosystem functioning. The finding that changes in Collembola performance did not directly translate to alterations in ecosystem functioning indicates that response traits do not necessarily conform to effect traits. Distinct plant functional group specific impacts of Collembola diversity on root depth distribution are likely to modify plant competition in complex plant communities and add a novel mechanism how decomposers may affect plant community assembly.     

Plant species richness leaves a legacy of enhanced root litter-induced decomposition in soil

Increasing plant species richness generally enhances plant biomass production, which may enhance accumulation of carbon (C) in soil. However, the net change in soil C also depends on the effect of plant diversity on C loss through decomposition of organic matter. Plant diversity can affect organic matter decomposition via changes in litter species diversity and composition, and via alteration of abiotic and/or biotic attributes of the soil (soil legacy effect). Previous studies examined the two effects on decomposition rates separately, and do therefore not elucidate the relative importance of the two effects, and their potential interaction. Here we separated the effects of litter mixing and litter identity from the soil legacy effect by conducting a factorial laboratory experiment where two fresh single root litters and their mixture were mixed with soils previously cultivated with single plant species or mixtures of two or four species. We found no evidence for litter-mixing effects. In contrast, root litter-induced CO₂ production was greater in soils from high diversity plots than in soils from monocultures, regardless of the type of root litter added. Soil microbial PLFA biomass and composition at the onset of the experiment was unaffected by plant species richness, whereas soil potential nitrogen (N) mineralization rate increased with plant species richness. Our results indicate that the soil legacy effect may be explained by changes in soil N availability. There was no effect of plant species richness on decomposition of a recalcitrant substrate (compost). This suggests that the soil legacy effect predominantly acted on the decomposition of labile organic matter. We thus demonstrated that plant species richness enhances root litter-induced soil respiration via a soil legacy effect but not via a litter-mixing effect. This implies that the positive impacts of species richness on soil C sequestration may be weakened by accelerated organic matter decomposition.     

降水变化和氮沉降对荒漠草原土壤丛枝菌根真菌群落结构的影响

降水变化和氮沉降是影响植物、微生物和土壤环境变化的两个重要方面。尽管丛枝菌根（AM）真菌在陆地生态系统中起着至关重要的作用，但人们对降水变化和氮添加如何交互影响AM真菌群落仍知之甚少。本研究以短花针茅荒漠草原为研究对象，采用裂区设计，主区为自然降雨(CK)、增雨30%(W)和减雨30%(R)三个水分梯度，副区为0(N0)，30(N30)，50(N50)和100(N100) kg?hm-2?a-1 四个氮素梯度共12个处理，通过高通量测序分析了土壤中AM真菌群落的多样性和组成。结果发现，水分处理对土壤AM真菌的Alpha多样性有促进作用，氮素处理抑制了土壤AM真菌的Alpha多样性，水分增加和氮素添加的交互作用促进了AM真菌的Alpha多样性增加，并改变了土壤AM真菌群落组成。水分和氮素刺激了各功能型植物生物量的增加，氮添加使多年生杂草和半灌木、小半灌木生物量显著增加，多年生禾草生物量显著减少。此外，多型孢子菌科的相对丰度与一二年生植物和半灌木、小半灌木生物量呈显著正相关，一二年生植物和半灌木、小半灌木生物量在氮添加和增雨处理下增加。本研究证明了AM真菌群落在短期气候变化下的稳定性。此外，AM真菌在科水平上的丰度与各生活型植物地上生物量的相关性证明了地上和地下生态系统的连通性。     

Soil arthropods beneficially rather than detrimentally impact plant performance in experimental grassland systems of different diversity

Impacts of belowground insecticide application on plant performance and changes in plant community structure almost uniformly have been ascribed to reduced belowground herbivory, although recent studies reported distinct side effects on detritivore soil animals, particularly on Collembola. Consequently, it remains controversial if the resulting soil feedbacks on plants are due to alterations in arthropod herbivory or to changes in the activity of detritivores. We investigated the impacts of the application of a commonly used belowground insecticide (chlorpyrifos) on soil animals and soil feedbacks on model plant species representing two main plant functional groups of grassland communities, the grass Lolium perenne and the forb Centaurea jacea.Insecticide application decreased soil insect herbivore densities considerably. However, also Collembola densities and diversity decreased markedly due to insecticide application and this was most pronounced in Entomobryidae, Isotomidae, Hypogastruridae, and Sminthuridae. While densities of other detritivore taxa were not affected or even increased (Oribatida) in insecticide subplots, that of predators mostly decreased.Both model plant species built considerably more biomass in control subplots than in insecticide subplots irrespective of characteristics of the resident plant community. This suggests that soil feedbacks on plants were not due to belowground herbivory and highlights the significance of alternative mechanisms responsible for insecticide-mediated soil feedbacks on plants. The deterioration of model plant species’ performances in insecticide subplots most likely was due to decreased densities of Collembola resulting in the deceleration of nutrient cycling and plant nutrition. The results suggest that it is oversimplistic to only ascribe insecticide-mediated soil feedbacks on plants to belowground herbivores. The results further indicate that in the present study the impact of arthropod detritivores on plant productivity was more important than that of belowground herbivores. This emphasizes that plant-soil arthropod interactions in grassland might be based on both facilitative and antagonistic interrelationships.     

长期氮沉降和地上凋落物处理对半干旱区沙质草地表层土壤碳氮组分的影响

为揭示半干旱区沙质草地生态系统中表层土壤C、N组分对长期氮添加和地上凋落物处理的响应特征,以科尔沁沙地西南部国家野外科学观测研究站建立的长期(9年)氮添加和凋落物处理样地为平台,测定并分析该样地表层土壤环境因子、铵态氮、硝态氮、总有机碳、不同碳氮组分。结果表明:(1)持续9年的氮添加和地上凋落物处理对表层土壤环境因子和不同碳氮组分无交互作用;(2)氮添加处理显著降低土壤pH(p<0.01),增加土壤中硝态氮的含量(p<0.05),其增长幅度为37.57%,并显著增加溶解性有机氮(DON)和易变活性氮(LON)的含量(p<0.01,p<0.05);(3)地上凋落物去除显著降低土壤总有机碳(TOC)、易变缓性碳(IOC)、微生物生物量碳(MBC)和微生物生物量氮(MBN)含量(p<0.05);(4)经过9年氮添加和地上凋落物处理,半干旱区沙质草地表层土壤中不同碳氮组分与土壤环境因子间相关性并不密切。即长期氮添加和地上凋落物处理会改变表层土壤不同碳、氮组分的含量,但并未显著改变各碳、氮组分的比值。研究结果为揭示长期氮添加和地上凋落物处理对半干旱区沙质草地土壤C、N贮存和预测未来土壤生物地球化学元素动态研究提供参考资料。     

Responses of soil microarthropods to warming and increased precipitation in a semiarid temperate steppe

Soil microarthropods are an important component in soil food webs and their responses to climate change could have profound impacts on ecosystem functions. As part of a long-term manipulative experiment, with increased temperature and precipitation in a semiarid temperate steppe in the Mongolian Plateau which started in 2005, this study was conducted to examine effects of climate change on the abundance of soil microarthropods. Experimental warming had slightly negative but insignificant effects on the abundance of mites (−14.6%) and Collembola (−11.7%). Increased precipitation greatly enhanced the abundance of mites and Collembola by 117 and 45.3%, respectively. The response direction and magnitude of mites to warming and increased precipitation varied with suborder, leading to shifts in community structure. The positive relationships of mite abundance with plant cover, plant species richness, and soil microbial biomass nitrogen suggest that the responses of soil microarthropods to climate change are largely regulated by food resource availability. The findings of positive dependence of soil respiration upon mite abundance indicate that the potential contribution of soil fauna to soil CO₂ efflux should be considered when assessing carbon cycling of semiarid grassland ecosystems under climate change scenarios.     

Effect of Collembola on mineralization of litter and soil organic matter

Although soil Collembola are known to contribute to soil carbon (C) cycling, their contribution to the mineralization of C sources that differ in bioavailability, such as soil organic C (SOC) and leaf litter, is unknown. Stable C isotopes are often used to quantify the effects of both soil C and litter C on C mineralization. Here, ¹³C-labeled litter was used to investigate the effects of Collembola (Folsomia candida) on the mineralization of both SOC and litter C in laboratory microcosms. The three microcosm treatments were soil alone (S); soil treated with δ¹³C-labeled litter (SL); and soil treated with δ¹³C-labeled litter and Collembola (SLC). The presence of Collembola did not significantly affect soil microbial biomass or litter mass loss and only had a small effect on CO₂ release during the first week of the experiment, when most of the CO₂ was derived from litter rather than from SOC. Later, during the experiment (days 21 and 63), when litter-derived labile C had been depleted and when numbers of Collembola had greatly increased, Collembola substantially increased the emission of SOC-derived CO₂. These results suggest that the effect of Collembola on soil organic C mineralization is negatively related to C availability.     

Fungal endophyte infection increases carbon sequestration potential of southeastern USA tall fescue stands

Tall fescue (Schedonorus arundinaceous (Schreb.)) is often infected with a common toxic fungal endophyte (Neotyphodium coenophialum) capable of producing alkaloids that affect grazing animal health, insect herbivory, plant production, and litter decomposition. The strength of these endophyte-associated effects is thought to depend on the abiotic and biotic conditions of a specific site. Prior work from Georgia, USA, has demonstrated that fungal endophyte infection can increase soil carbon pools of tall fescue pastures; however, for endophyte infection to contribute substantially to regional carbon sequestration, this result would have to hold true across the broad range of environmental conditions that support tall fescue growth. In this study, we evaluated whether endophyte infection consistently alters various soil parameters, including carbon storage, of tall fescue stands located throughout the southeastern United States. Soil samples were collected from nine sites with established paired high- and low- endophyte-infected tall fescue stands. These samples were analyzed for basic soil parameters, soil organic carbon (SOC), soil total nitrogen (TN), particulate and non-particulate organic matter-C and -N (POM, n-POM), C and N mineralization rates, and microbial biomass and community composition. Averaged across all sites, endophyte-infected tall fescue stands had 6% greater SOC and 5% greater TN pools in surface soil than adjacent endophyte-free stands. The lack of a significant interaction between site and endophyte infection status indicated that this result was relatively consistent across sites, despite differences in stand age, climate, and other environmental conditions. While POM C and POM N tended to be higher in endophyte-infected than endophyte-free stands, this result was not significant. However, greater pools of n-POM C and N were observed in endophyte-infected vs. endophyte-free stands when averaged across all the sites, suggesting increased retention of recalcitrant substrates occurred in response to fungal endophyte infection. Total microbial biomass, measured via phospholipid fatty acid (PLFA) analysis, was greater in endophyte-infected than endophyte-free soils when averaged across sites, reflecting the trends observed with SOC and TN. Microbial community composition shifted somewhat in response to fungal endophyte infection: significantly higher fungal to bacterial ratios were observed in endophyte-free compared to endophyte-infected stands. However, ordinations of the PLFA data demonstrated only slight separation of endophyte-infected and endophyte-free microbial communities at some sites and no clear separation at others. Enhanced SOC, TN, recalcitrant n-POM C and N pools, and altered microbial biomass and communities suggest that this aboveground fungal endophyte symbiosis has widespread effects on soil biology and biochemistry, and that high prevalence of the aboveground endophyte increases C sequestration capacity of tall fescue stands throughout the southeastern USA.     

Effects of seasonal grazing and precipitation regime on the soil macroinvertebrates of a Mediterranean old-field

Soil macroinvertebrate communities (SMC) are well known to influence major ecosystem processes, but relatively few investigations have examined the mechanisms and factors involved in SMC regulation. We conducted a factorial experiment with combinations of seasonal grazing by sheep and irrigation (simulating different precipitation regimes) to assess their effects on the SMC of a semiarid Mediterranean old-field. We also analyzed effects on plant species richness, total aboveground biomass, and litter. The data were collected in autumn and spring, the two favorable seasons for SMC and primary production in the region, and season was included as an additional random factor. Main results were: 1) Ungrazed plots accumulated more aboveground plant biomass and litter during spring, providing extra food for soil biota. However, grazing during autumn or spring did not affect SMC characteristics. 2) Reduction of inter-annual precipitation variability in autumn and spring increased the abundance of two decomposer taxa: Oligochaeta and Diplopoda. Additionally, if summer drought was reduced, plant species richness, litter and the abundance of Isopoda were increased. 3) Oligochaeta and Diplopoda increase their abundance in spring, particularly, the most abundant taxon (Oligochaeta). We conclude that inter- and intra-annual variability in precipitation is a key environmental factor for the decomposer soil fauna in Mediterranean ecosystems, modifying the physical characteristics of the soils (humidity, hardness, etc.), as well as affecting the amount or characteristics of plant biomass or litter. The respiration system of the macroinvertebrates (cutaneous, tracheal or branquial) and the capacity to migrate vertically into the soil may determine the decomposers' responses to precipitation.     

Effects of the annual invasive plant Impatiens glandulifera on the Collembola and Acari communities in a deciduous forest

Invasive plants can disturb interactions between soil organisms and native plants and thereby alter ecosystem functions and/or reduce local biodiversity. Collembola and Acari are the most abundant microarthropods in the leaf litter and soil playing a key role in the decomposition of organic material and nutrient cycling. We designed a field experiment to examine the potential effects of the annual invasive plant Impatiens glandulifera on species diversity, abundance and community composition of Collembola and Acari in leaf litter and soil in a deciduous forest in Switzerland. Leaf litter and soil samples were obtained from plots invaded by I. glandulifera since 6 years, from plots in which the invasive plant had been removed for 4 years and from plots which were not yet colonized by the invasive plant. The 45 leaf litter and soil samples were equally distributed over three forest areas, which were differently affected by a wind throw 12 years prior to sampling representing a natural gradient of disturbance. Collembola species richness and abundance in the leaf litter and soil samples were not affected by the presence of the invasive plant. However, the species composition of Collembola was altered in plots with I. glandulifera. The abundance of leaf-litter dwelling Acari was increased in invaded plots compared to the two other plot types. Furthermore, the presence of the invasive plant shifted the composition of Acari individuals belonging to different groups. Our field experiment demonstrates that an annual invasive plant can affect microarthropods which are important for nutrient cycling in various ecosystems.     

Dynamics and stratification of bacteria and fungi in the organic layers of a scots pine forest soil

The abundance and micro-stratification of bacteria and fungi inhabiting the organic layers of a Scots pine forest (Pinus sylvestris L.) were investigated. An experiment using stratified litterbags, containing organic material of four degradation stages (fresh litter, litter, fragmented litter and humus) was performed over a period of 2.5 years. Dynamics and stratification of fluorescent stained bacteria and fungi, ratios between bacterial and fungal biomass, and relationships with moisture and temperature are described. Average bacterial counts in litter and fragmented litter were similar, i.e., approximately 5×10⁹ bacteriag^–1 (dry weight) organic matter, and significantly exceeded those in humus. The mean bacterial biomass ranged from 0.338 to 0.252mg carbon (C) g^–1 (dry weight) organic matter. Lengths of mycelia were significantly below the usually recorded amounts for comparable temperate coniferous forests. The highest average hyphal length, 53mg^–1 (dry weight) organic matter, was recorded in litter and decreased significantly with depth. The corresponding mean fungal biomass ranged from 0.050 to 0.009mg Cg^–1 (dry weight). The abundance of bacteria and fungi was influenced by water content, that of fungi also by temperature. A litterbag series with freshly fallen litter of standard quality, renewed bimonthly, revealed a clear seasonal pattern with microbial biomass peaks in winter. The mean hyphal length was 104mg^–1 (dry weight) and mean number of bacteria, 2.40×10⁹ bacteria g^–1 (dry weight). Comparable bacterial and fungal biomass C were found in the freshly fallen litter [0.154 and 0.132mgCg^–1 (dry weight) organic material, respectively]. The ratio of bacterial-to-fungal biomass C increased from 1.2 in fresh litter to 28.0 in humus. The results indicate the existence of an environmental stress factor affecting the abundance of fungi in the second phase of decomposition. High atmospheric nitrogen deposition is discussed as a prime factor to explain low fungal biomass and the relatively short lengths of fungal hyphae in some of the forest soil layers under study. Received: 26 June 1997     

Collembola switch diet in presence of plant roots thereby functioning as herbivores

Collembola are abundant and ubiquitous soil decomposers, being particularly active in the rhizosphere of plants where they are assumed to be attracted by high microbial activity and biomass. While feeding on root associated microorganisms or organic matter they may also ingest plant roots, e.g. particularly root hairs and fine roots. Employing stable isotope analysis we investigated Collembola (Protaphorura fimata Gisin) feeding preferences and types of ingested resources. We offered Collembola two resources with distinct isotope signatures: a C4 plant (Zea mays L.) planted in soil mixed with ¹⁵N labelled litter of Lolium perenne L. (C3 plant). We hypothesised that Collembola obtain their nutrients (C and N) from different resources, with their carbon being mainly derived from resources that are closely associated to the plant root, e.g. root exudates, causing enrichment in ¹³C in Collembola tissue, while the incorporated nitrogen originating from litter resources. In contrast to our hypothesis, stable isotope analysis suggests that in absence of plant roots Collembola derived both the incorporated C and N predominantly from litter whereas in presence of plant roots they switched diet and obtained both C and N almost exclusively from plant roots.The results indicate that Collembola in the rhizosphere of plants, being assumed to be mainly decomposers, in fact predominately live on plant resources, presumably fine roots or root hairs, i.e. are herbivorous rather than detritivorous or fungivorous. These findings have major implications on the view how plants respond to decomposers in the rhizosphere.