Responses of nematode‐community structure in turfgrass soil to microbial filtrates from municipal–solid waste compost

Actinomycetes, Bacillus subtilis, and Bacillus thuringiensis were isolated from municipal–solid waste (MSW) compost, and different microbial liquid filtrates (MLF) were prepared. Sterile culture media with no microbes were used as their controls. The effects of MLF on soil nematode communities were examined in pot‐grown Festuca arundinacea Schreb. Fifteen genera of nematodes in background soil were identified, of which Helicotylenchus and Rotylenchus were dominant. The inoculation of MLF strongly affected the abundance and community structure of soil nematodes. Compared with their controls, lower total nematode numbers following MLF incorporation were found. Actinomycetes inoculation changed community structure of soil nematodes, transforming the dominant genera from Helicotylenchus and Rotylenchus into Cephalobus, Chiloplacus, and Aphelenchus. Actinomycetes incorporation resulted in a significant decrease of plant‐parasitic nematodes relative to control pots. Only plant‐parasitic and omnivorous‐predatory nematodes were found in treatments following B. subtilis inoculation, and Helicotylenchus, Rotylenchus were dominant genera with relative abundance of 76.2% and 14.3%, respectively. Although the dominant genera were still Helicotylenchus and Rotylenchus, B. thuringiensis inoculation led to a marked decrease in populations of plant‐parasitic nematodes and an increase in populations of fungivorous and bacterivorous nematodes relative to control. Shannon's diversity index (H′), evenness index (J′), richness index (SR), and Wasilewska index (WI) in pots treated with actinomycetes and B. thuringiensis filtrates were significantly higher than those of their controls, whereas significant lower dominance index (λ) in actinomycetes and B. thuringiensis treatments was observed than their controls. Plant growth was improved in the treatments inoculated with three microbes. The findings highlight that actinomycetes can most effectively suppress plant‐parasitic nematodes, increase community diversity, evenness, and richness, thus improving soil environment for turf growth.     

Nematode communities in continuous tomato-cropping field soil infested by root-knot nematodes

Abstract

Soil nematode communities in greenhouses with different duration of continuous cropping were investigated from October 2007 to June 2008. Total nematode populations and trophic groups were observed. Fourteen families and 24 genera were identified; the genera Protorhabditis, Diploscapter, Meloidogyne and Helicotylenchus comprised 74.4% of the total population (from all tested samples). Plant-parasitic and bacterivorous nematodes were most abundant among the trophic groups. Populations of both trophic groups increased with increasing times of continuous cropping. The numbers of soil nematodes at different soil depths were significantly different (p <0.05). Shannon–Wiener index (H′) and Simpson index of diversity (D) were highest in 0-yr soil of all soils. Plant parasite index (PPI) and PPI/MI (maturity index) of soil nematodes increased with the increasing times of continuous cropping suggesting that continuous cropping resulted in gradual shift of plant-parasitic nematodes from k-strategists to r-strategists.     

Integration of cover crops and vermicompost tea for soil and plant health management in a short-term vegetable cropping system

Short-term vegetable crop production often involves frequent tillage and other farm activities that results in disturbed soil food web communities. A less disturbed soil community would have a more structured soil food web which contains soil fauna higher up in the food web hierarchy, thus higher integrity in soil nutrient cycling. The objective of this study is to examine if strip-till cover cropping and drenching soil with vermicompost tea could improve soil food web structure in a short-term agroecosystem. Two field trials were conducted in Waialua, HI, USA to evaluate the effect of strip-till planting of sunn hemp (SH, Crotalaria juncea) or crimson clover (Trifolium incarnatum) cover crops in a zucchini (Cucurbita pepo) cropping system. At zucchini planting, each cover crop plot was split to receive four soil treatments: fertilizer (F, chicken pellet), compost tea (CT), fertilizer plus compost tea (F + CT), and none. Compost tea was prepared from chicken manure based vermicompost aerated overnight in water at 1:10 (v:v). Planting of SH increased bacterivorous nematodes and suppressed plant-parasitic nematodes throughout both zucchini cropping cycles, but did not enhance the numbers of omnivorous or predatory nematodes. Crimson clover did not enhance beneficial nematodes nor suppress plant-parasitic nematodes. Adding CT to F suppressed the key plant-parasitic nematodes only at the initial stage of the zucchini growth, increased percentage of predatory or omnivorous nematodes only toward the end of zucchini crops, and increased the structure index at harvest in the first trial. Zucchini yield was increased by planting of SH but not by drenching of CT. Despite the benefits of CT in improving the soil food web structure, a correlation analysis revealed that zucchini yields were correlated to the reduction in the percentage of fungivorous nematodes at planting, an increase in the percentage of bacterivorous nematodes at harvest, and to reduction in the percentage of plant-parasitic nematodes at harvest.     

Cover crops alter the soil nematode food web in banana agroecosystems

Cover crops are increasingly being used in agriculture, primarily for weed or erosion management. The addition of cover crops increases the primary productivity of the system and diversifies basal resources for higher trophic levels. How increases in the quality and quantity of basal resources affect bottom-up and top-down control remains a key question in soil food web ecology. We evaluated the response of the nematode community to the introduction of cover crops between rows of a banana plantation. We measured changes in nematode food web structure and inferred the prevalence of bottom-up and top-down effects on the abundance of phytophagous nematodes (i.e., plant-feeding and root-hair-feeding species) 1.5 years after plots with cover crops (Poaceae or Fabaceae species) or bare soil were established. The addition of a cover crop greatly affected the structure and the abundance of the soil nematode community 1.5 years after planting. The abundance of all trophic groups except for plant-feeding nematodes tended to increase with the addition of cover crops. The Shannon–Weaver diversity index and the enrichment index increased with the addition of cover crops, indicating that opportunistic, bacterivorous and fungivorous nematodes benefited from the added resources. Plant-feeding nematodes were least abundant in plots with Poaceae cover crops, while bacterivorous, omnivorous, and root-hair-feeding nematodes were more abundant with Fabaceae cover crops than with bare soil, indicating that cover crop identity or quality greatly affected soil food web structure. Bottom-up effects on all trophic groups other than plant-feeding nematodes were evident with Poaceae cover crops, suggesting an top-down control of plant-feeding nematodes by omnivorous nematodes. Conversely, plant-feeding nematodes were evidently not suppressed in Fabaceae cover crops, perhaps because bottom-up effects on omnivorous nematodes were weaker (hence, top-down control by omnivorous nematodes was weaker), and because Fabaceae cover crops probably served as good hosts for some plant-feeding nematodes.     

土壤线虫群落对过量施用农用化学品的响应

为研究施用过量的农用化学品对土壤线虫群落组成及多样性的影响,采用定点试验的方法,在哈尔滨市呼兰区选择典型农田生态系统进行试验,对比研究土壤线虫群落对施用过量的氮肥、磷肥、钾肥、除草剂及杀虫剂的响应。在试验田中共鉴定出土壤线虫27科45属,其中Cephalobus和Aphelenchus为优势属。施用不同浓度的各类农用化学品对土壤线虫群落组成、多样性均产生一定影响。线虫总数及食细菌线虫、食真菌线虫、植物寄生线虫数量在不同处理间均存在显著差异(P<0.05);其中,植物寄生线虫的相对丰度随化肥施用量的升高呈增加趋势。从土壤线虫的生态指数来看,除PPI(植物寄生线虫成熟指数)外,其他生态指数[MI(成熟度指数)、F/B(食真菌线虫与食细菌线虫数量比值)、Evenness(均匀度指数)、SR(丰富度指数)、H’(多样性指数)]在施用不同农用化学品处理之间也存在显著差异,并且,MI随着施用钾肥、氮肥浓度的增加而降低。土壤线虫可以作为揭示施用农用化学品过程中土壤质量变化的生物学指标,其群落及多样性的变化表明土壤线虫群落对农用化学品的过量施用产生了响应,过量施用农用化学品会增加土壤生态系统的干扰,对土壤环境造成威胁。     

Effects of organic and other management practices on soil nematode communities in tea plantation: a case study in southern China

Four major tea management practices (organic, pollution‐free, conventional, and semi‐natural) are employed in Chinese tea plantations at present. These practices can induce changes in the physiochemical parameters, microorganism community and enzyme activity in tea plantation soil. However, understanding of their effects on soil nematodes is still scarce. This study aimed to investigate whether and how different management practices affect the biodiversity, function, and structure of soil nematode communities in tea plantation habitats. The soil nematode community structures and ecological indices were determined from the soil samples collected more than 6 y after their respective farming practices were first applied, and different management practices did not greatly affect soil nematode community evenness or species diversity, but organic practice increased nematode trophic diversity, common species diversity, and species richness. Pollution‐free practice considerably increased fungivorous nematodes, and both pollution‐free and conventional practice decreased bacterivorous nematodes markedly in the subsurface layers of soil. Predator and omnivorous nematodes were found to be more abundant in semi‐natural plantation. Organic practice was more sustainable and suitable for tea cultivation, with the greatest biodiversity, best nutrient conditions, higher and more stable C/N ratio and lower interference in the food web.     

Belowground responses by AM fungi and animal trophic groups to repeated defoliation in an experimental grassland community

We tested a hypothesis that the effects of defoliation on plants and soil organisms vary with the number of successive defoliations. We established a 23-week greenhouse experiment using replicated grassland microcosms that were composed of three plant species, Trifolium repens, Plantago lanceolata and Phleum pratense, growing together in grassland soil with a diverse soil community. The experiment consisted of two treatment factors-defoliation and harvest time-in a fully factorial design. The defoliation treatment had two levels, i.e. no trimming and trimming of plants every 2 weeks, and the harvest time five levels, i.e. harvests after 1-3, 5 and 7 trimmings. Shoot production (trimmed plus harvested shoot mass), harvested shoot and root mass and root N and C concentrations increased with time but were reduced by defoliation. Colonization rates of arbuscular mycorrhizal (AM) fungi decreased with time in T. repens roots but were enhanced by defoliation, whereas AM colonization rates in P. pratense roots were not affected by harvest time or defoliation. The abundance of bacterivorous and fungivorous nematodes decreased and that of herbivorous and predatory nematodes increased with time, while the abundance of omnivorous nematodes and detritivorous enchytraeids varied in time without a linear trend. Defoliation had no effect on fungivores and predators but increased the abundance of bacterivores. Defoliation also increased the abundance of herbivores, omnivores and detritivores after 2 trimmings and that of omnivores and detritivores after 5 trimmings, but had a negative effect on omnivores after 3 trimmings and on herbivores after 7 trimmings. Among nematode genera, some deviation from the trophic group responses existed: for instance, defoliation reduced the abundance of bacterivorous Acrobeloides spp. and did not affect the abundance of herbivorous Filenchus spp. and Paratylenchus spp. Our results show that the effects of defoliation on plants, AM fungi and some soil animal trophic groups may remain constant all the way through several defoliations, whereas other animal trophic groups may have different and even opposite responses to defoliation depending on the length of the defoliation period before monitoring. This shows how separate studies with defoliation periods of different length can produce contradictory results of the effects of defoliation on the abundance of soil animals.     

Responses of soil nematode community to monoculture or mixed culture of a grass and a legume forage species in China

Mixed cultivation of fast-growing grasses and nitrogen (N)-fixing legumes for forage production is widely considered effective for obtaining sustained high forage yields without depleting soil N levels. However, the effects of monoculture and mixed culture of these species on soil food webs are poorly understood. In this study, soil nematode communities were examined as indicators of the soil food web structure of monoculture and mixed culture of grass and legume at three N levels, i.e., 338 (low), 450 (moderate), and 675 (high) kg N ha^-1 year^-1, across 2 years in wet and dry seasons, using the grass Paspalum wetsfeteini and the legume Medicago sativa (alfalfa), both commonly cultivated worldwide. Repeated-measures analysis of covariance showed that compared with grass monoculture, legume monoculture and grass-legume mixture increased abundances of herbivorous, bacterivorous, and fungivorous nematodes in the soil food web under the low and moderate N fertilization levels. Principal response curve results showed that the abundance of Helicotylenchus, a plant parasite, was significantly higher under legume monoculture than other planting systems at the low N fertilization level. Structural equation model analysis indicated that the legume increased bacterivore abundance, while increasing N fertilization decreased omnivore abundance. The legume might increase the quantity and quality of food resources for soil biota, resulting in the bottom-up control of soil nematode communities. Our results indicate that targeted control of a soilborne pathogen, Helicotylenchus, is required in alfalfa-based planting systems. In addition, high inorganic N application, which is detrimental to legume-rhizobia symbiosis, nullified the otherwise positive effects of legumes on soil nematodes.     

Effects of organic mulches on soil microfauna in the root zone of apple: implications for nutrient fluxes and functional diversity of the soil food web

A variety of organic materials (e.g. composts, paper recycling wastes, hay) can be used as in-row mulches in perennial horticultural cropping systems such as high density apple orchards. As organic materials with variable properties, such mulches can be expected to have variable effects on structure of the soil food web and mineralisation of N and P in the root zone. The overall objectives of this study were to: (1) assess the effects of a selection of organic mulches on the abundance of protozoa and nematode trophic groups; (2) use the model of Hunt et al. [Biol. Fertil. Soils 3 (1987) 393] to assess the implications of changes in microfaunal abundance for microbial turnover and N mineralisation; and (3) determine effects of the mulches on nematode indicators of increased microbial production/turnover and functional diversity of the soil food web. Organic mulch treatments commenced in 1994 and included shredded office paper, municipal biosolids, shredded paper applied over municipal biosolids, shredded paper applied over municipal compost, alfalfa hay, and black polyethylene fabric. The control was conventional tree-row weed management with glyphosate. Biosolids and municipal compost treatments were re-applied in 1997. Protozoan abundance was determined in 1998, 1999 and 2000. Nematode community structure was assessed in 1998, 1999, and twice in 2000. Nematode community parameters evaluated included: abundance of bacterivorous, fungivorous, omnivorous and predacious nematodes; abundance of the root-lesion nematode, Pratylenchus penetrans; absolute and relative abundances of enrichment opportunist nematodes (Rhabditidae+Diplogasteridae+Panagrolaimidae); Simpson’s diversity; evenness; and the indices of nematode community enrichment (EI) and structure (SI) described by Ferris et al. [Appl. Soil Ecol. 18 (2001) 13]. Measurements of the abundance of enrichment opportunists and the EI were evaluated as indicators of enhanced nutrient fluxes. Diversity and the SI were evaluated as indicators of changes in functional diversity of the soil food web.The abundance of protozoa and bacterivorous nematodes, and estimated fluxes of N and P through the microfauna, were greater under all combinations of biosolids or municipal compost and shredded paper than under the control and plastic mulch. The abundance of enrichment opportunist nematodes and the EI were also consistently greater under combinations of biosolids or municipal compost and shredded paper. The abundance of enrichment opportunists and EI were both also correlated with leaf P, providing additional evidence to support the use of these parameters as indicators of enhanced turnover of microbial biomass and nutrients. The SI was greatest under shredded paper and shredded paper applied over municipal compost, and least under municipal biosolids and alfalfa hay. Population densities of P. penetrans were reduced under shredded paper mulch relative to the control and biosolids alone.     

Effects of fungivorous and predatory arthropods on nematodes and tardigrades in microcosms with coniferous forest soil

Effects of fungivorous and predatory soil arthropods on free-living nematodes and tardigrades were studied in a factorial microcosm experiment. A stepwise increase in faunal complexity was obtained by adding soil arthropods to defaunated humus samples from an irrigated+fertilized and an untreated stand of Scots pine. The effects were assessed after 103 and 201 days at 15°C and a soil moisture content of 50% water-holding capacity. The study showed that a diverse community of fungivorous arthropods (collembola and oribatid mites), present in numbers similar to those in the field, reduced the abundance of nematodes. A complete community of fungivorous and predatory arthropods (e.g., gamasides, spiders, and cantharid larvae) further strengthened this repressive effect. Certain nematode genera were more affected than others. Tardigrades seemed to be efficient predators on nematodes, but their numbers were, in turn, strongly reduced by predatory arthropods. Because predatory arthropods fed on both nematodes and their tardigrade predators, the impact of arthropod predators on nematode regulation was greater than it appeared to be on the basis of nematode numbers. Humus type also interacted with the other factors. Nematode numbers were initially higher in the untreated humus than in the irrigated+fertilized humus. However, because tardigrade populations increased only in the untreated humus, nematode numbers decreased more in this humus than in the irrigated+fertilized humus. The study demonstrates that nematode abundance can be regulated by a number of types of interacting predators.     

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.     

Responses of grassland soil nematodes and protozoa to multi-year and single-year applications of dairy manure slurry and fertilizer

In order to improve understanding of how long-term use of manure affects nitrogen cycling processes, the effects of multiple years of manure applications on abundance of protozoa and nematode community structure were assessed. Plots of a grass sward in the Fraser Valley of British Columbia were either left untreated or were treated with dairy manure slurry or fertilizer, each at 50 or 100 kg NH₄-N ha⁻¹, two to four times per year for six consecutive years. Nematode community structure and protozoan abundance were determined at 19 sample dates during the fourth (1997), fifth (1998) and sixth (1999) years of application. Protozoa, bacterivorous nematodes and fungivorous nematodes were consistently more abundant in soil treated with manure at both rates than in fertilized and untreated soil, indicating that microbial turnover and flux of nutrients through the soil food web was enhanced in manured soil relative to fertilized or untreated soil. The Maturity Index (MI) and the MI2-5 were both reduced by fertilization and manure, relative to the control. The MI for the manure treatment was lower than for the fertilizer treatment as a result of greater relative abundance of enrichment opportunist nematodes in manure-treated soil. Accordingly, the MI2-5 did not differ between the manure and fertilizer treatments, suggesting that with the exception of enrichment opportunists fertilizer and manure have similar effects on structural complexity of the soil food web.Populations of micro-fauna were also assessed through 1998 and 1999 in subplots that had been treated with manure or fertilizer for four years and stopped receiving manure or fertilizer in 1998, and in subplots given manure in 1998 that had previously either been fertilized or left untreated. Protozoa and bacterivorous and fungivorous nematodes remained more abundant through 1998 and 1999 in previously manure-treated plots than in previously fertilized plots, indicating that the cumulative effects of manure application on enhancement of microbial production can be detected through at least two growing seasons after applications cease. Application of manure for one year to previously non-treated or fertilized soil raised the abundance of protozoa and bacterivorous and fungivorous nematodes to levels comparable to continuously manured soil.     

Legacy effects of elevated ozone on soil biota and plant growth

Many studies have examined how human-induced atmospheric changes will influence ecosystems. The long-term consequences of human induced climate changes on terrestrial ecosystems may be determined to a large extend by how the belowground compartment will respond to these changes. In a free-air ozone enrichment experiment running for 5 years, we reciprocally transplanted soil cores from ambient and elevated ozone rings to test whether exposure to elevated ozone results in persistent changes in the soil biota when the plant and soil are no longer exposed to elevated ozone, and how these legacy effects of elevated ozone influenced plant growth as compared to current effects of elevated ozone. After one growing season, the current ozone treatment enhanced plant growth, but in soil with a historical legacy of elevated ozone the plant biomass in that soil was reduced compared to the cores originated from ambient rings. Current exposure to ozone increased the phospholipid fatty acids of actinomycetes and protozoa, however, it decreased dissolved organic carbon, bacterivorous and fungivorous nematodes. Interestingly, numbers of bacterivorous and fungivorous nematodes were enhanced when soils with a legacy of elevated ozone were placed under elevated ozone conditions. We conclude that exposure to elevated [O₃] results in a legacy effect in soil. This legacy effect most likely influenced plant growth and soil characteristics via responses of bacteria and fungi, and nematodes that feed upon these microbes. These soil legacies induced by changes in soil biotic community after long-term exposure of elevated ozone can alter the responses of ecosystems to current climatic changes.     

Influence of organic Crotalaria juncea hay and ammonium nitrate fertilizers on soil nematode communities

Comparisons of organic and inorganic fertilizer effects on nematode communities depend on the specific organic fertilizer used. Field experiments were conducted during 2001 and 2002 in a squash (Cucurbita pepo) agroecosystem to determine if applying sunn hemp (Crotalaria juncea) hay as an organic fertilizer improved nematode communities involved in soil nutrient cycling compared to an equivalent N rate (100 kg N/ha) of ammonium nitrate. Fertilizer source had minimal effect on nematode communities in 2001 when treatments were applied after a winter cover crop of oats (Avena sativa), but differences (P ≤ 0.05) between the fertilizer sources occurred in 2002 when no winter cover cropping preceded squash. Fertilization with sunn hemp hay increased abundance of the bacterivore guilds Ba₁ and Ba₂, and increased fungivores at the end of the experiment. Compared to ammonium nitrate, fertilization with sunn hemp hay resulted in a community with lower maturity index, higher enrichment index, and lower channel index, consistent with a disturbed and nutrient-enriched soil food web undergoing bacterial decomposition. Sunn hemp hay occasionally stimulated omnivorous nematodes, but suppressed plant-parasitic nematodes relative to ammonium nitrate fertilizer. Increasing the sunn hemp hay rate to 200 kg N/ha increased the abundance of bacterivores, fungivores, and predatory nematodes, and total nematode abundance compared to hay at 100 kg N/ha. Fertilization with ammonium nitrate increased the percentage of herbivores, but reduced percentage and abundance of omnivores. In conclusion, sunn hemp fertilizer maintained greater numbers of nematodes involved in nutrient cycling as compared to ammonium nitrate.     

Does plant growth phase determine the response of plants and soil organisms to defoliation?

To test a hypothesis that the effects of defoliation on plant ecophysiology and soil organisms depend on the timing of defoliation within a growing season, we established a greenhouse experiment using replicated grassland microcosms. Each microcosms was composed of three plant species, Trifolium repens, Plantago lanceolata and Phleum pratense, growing in grassland soil with a diverse soil community. The experiment consisted of two treatment factors—defoliation and plant growth phase (PGP)—in a fully factorial design. Defoliation had two categories, i.e. no trimming or trimming a total of four times at 2 week intervals. The PGP treatment had four categories, i.e. 1, 3, 7 or 13 weeks growth following planting before the first defoliation (subsequently referred to as PGP1, PGP2, PGP3 and PGP4, respectively). In each PGP treatment category, microcosms were harvested 1 week after the final defoliation. Harvested shoot and root mass and total shoot production (including trimmed and harvested shoot mass) increased with time and were lower in defoliated than in non-defoliated systems. The fraction of root biomass of harvested plant biomass decreased with time but was increased by defoliation at PGP3 and PGP4. The proportion of T. repens in total shoot production increased and those of P. lanceolata and P. pratense decreased with time. Defoliation increased the proportions of P. lanceolata and P. pratense in total shoot production at PGP3 and PGP4. Root N and C concentrations increased and root C-to-N ratio decreased with time in non-defoliated systems. Defoliation increased root N concentration by 38 and 33% at PGP1 and PGP2, respectively, but decreased the concentration by 22% at PGP4. In contrast, defoliation reduced root C concentration on average by 1.5% at each PGP. As with the effects on root N concentration, defoliation decreased the root C-to-N ratio at PGP1 and PGP2 but increased the ratio at PGP4. Among soil animal trophic groups, the abundance of herbivorous nematodes was higher at PGP4 than at PGP1-3 and that of predacious nematodes higher at PGP2-4 than at PGP1, while the abundance of bacterivorous, fungivorous and omnivorous nematodes and that of detritivorous enchytraeids did not differ between the PGP categories. Among bacterivorous nematodes, however, Acrobeloides, Chiloplacus and Protorhabditis species decreased and that of Plectus spp. increased with time. Defoliation did not affect the abundance of soil animal trophic groups, but reduced the abundance of herbivorous Coslenchus spp. at each PGP and raised the abundance of herbivorous Rotylenchus spp. and bacterivorous Eucephalobus spp. at PGP4. Confirming our hypothesis, the results suggest that the effects of defoliation on the attributes of grassland plants, such as biomass allocation between roots and shoots and root quality, may depend on the timing of defoliation within a growing season. However, contradicting our hypothesis, the results suggest that significant changes in plant attributes after defoliation may not always lead to substantial changes in the abundance of belowground organisms.     

温室土壤食物网静态和动态结构特征

Soil food web structure is fundamental to ecosystem process and function; most studies on soil food web structure have focused on agro-ecosystems under different management practices and natural terrestrial ecosystems, but seldom on greenhouses. This study explored the static and temporal variability of soil food structure in two greenhouses of Shandong Province, North China over a two-year period. The static properties were measured directly by surveying functional group composition and a series of parameters portraying the species properties, link properties, chain properties and omnivory properties of the web, as well as indirectly through calculation of nematode indices, enrichment index （EI）, structure index （SI）, and channel index （CI）. The dynamic variability of greenhouse soil food structure was described by the dynamics of functional groups, Bray-Curtis （BC） similarity and cluster analysis. The results showed that the greenhouse soil food web contained 14 functional groups, with microbes having the highest mean biomass, followed by protozoa. Of the three functional groups of protozoa, flagellates were the dominant group on most sampling dates, amoebae only became the dominant group during the summer, while ciliates were the least prevalent group. All nematodes were assigned into one of the four functional groups, bacterivorous, fungivorous, herbivorous and omnivorous, and the fungivorous nematodes had the lowest mean biomass. Mites were assigned into three functional groups and the omnivorous noncryptostigmatic mites were the dominant group. All the functional groups showed significant seasonal changes. The soil food web connectance was 0.15, the maximum food chain length was 5, and the average food chain length was 3.6. The profiles of the EI and SI showed that the food web was resource- depleted with minimal structure. The results of CI indicated that the bacterial decomposition pathway was the dominant pathway in the food web of the greenhouse soils studied and the results of BC similarity showed that the soil food web had higher variability and instability over time. The cluster analysis showed that the functional groups located at high trophic levels with low biomass were in a cluster, whereas those at low trophic levels with high biomass were closer. Compared with the food web structure of agroecosystem and natural terrestrial ecosystem soils, the structure of greenhouse soil food web was simple and unstable, which was likely driven by high agricultural intensification, particularly over application of fertilizers.     

Nematode succession and microfauna-microorganism interactions during root residue decomposition

The quality of plant material affects the vigor of the decomposition process and composition of the decomposer biota. Root residues from hairy vetch (Vicia villosa Roth), rye (Secale cereale L.) and vetch+rye, packed in litterbags were placed in pots of soil at 15 C and the content of the bags was analyzed after 2, 4, 8 and 12 weeks. Bacterial biomass did not differ between residues with contrasting composition. Among bacterivores groups of nematodes that require high bacterial production dominated in fast decomposing resources whereas flagellates with smaller requirements prevail in slower decomposing resources. Biomass of bacterial feeding nematodes correlated positively with early phase (0-2 wk) decomposition that increased in the order: rye< vetch+rye<vetch. Bacterial biomass therefore seems to be under top-down (predation) control during early decomposition. In contrast, the fungal biomass differed between resources with highest values for rye. Moreover, this increase in fungal biomass occurred later during succession and was correlated with decomposition activity for rye in that period. Fungal biomass therefore seems to be under bottom-up (resource) control. The composition of the nematode assemblages (composed of 25 taxa) showed a clear relationship to initial plant resource quality as well as decomposition phase. Early successional microbivorous nematodes vary according to resource quality with demanding bacterivores+predators (Neodiplogasteridae) dominating in vetch and less demanding bacterivores (Rhabditidae) and fungivores (Aphelenchus) being equally common in vetch and rye. Later in the succession (2-4 wk) bacterivorous Cephalobidae and fungivorous Aphelenchoides prevailed similarly on the different root materials whereas bacterivorous protozoa and the amoebal fraction thereof dominated in rye. At week 12 no species dominated the nematode assemblages that were similar between the resources. The differences between nematode assemblages among plant resources at 2 week were similar to the results of a field study sampled after 6 weeks with the same soil and plant resources. This lends support to the relevance of the successional patterns observed in this incubation study.     

Interplay of omnivory, energy channels and C availability in a microbial-based soil food web

 To study the effects of omnivory on the structure and function of soil food webs and on the control of trophic-level biomasses in soil, two food webs were established in microcosms. The first one contained fungi, bacteria, a fungivorous nematode (Aphelenchoides saprophilus) and a bacterivorous nematode (Caenorhabditis elegans), and the second one fungi, bacteria, the fungivore and an omnivorous nematode (Mesodiplogaster sp.) feeding on both bacteria and the fungivore. Half of the replicates of each food web received additional glucose. The microcosms were sampled destructively at 5, 9, 13 and 19 weeks to estimate the biomass of microbes and nematodes and the soil NH₄ ⁺-N concentration. The evolution of CO₂ was measured to assess microbial respiration. Microbial respiration was increased and soil NH₄ ⁺-N concentration decreased by the addition of glucose, whereas neither was affected by the food-web structure. Supplementary energy increased the biomass of fungi and the fungivore, but decreased the biomass of bacteria, the bacterivore and the omnivore. The omnivore achieved greater biomass than the bacterivore and reduced the bacterial biomass less than the bacterivore. The biomass of the fungivore was smaller in the presence of the omnivore than in the presence of the bacterivore at three sampling occasions. Fungal biomass was not affected by food-web structure. The results show that the effects of the omnivore were restricted to its resources, whereas more remote organisms and soil processes were not substantially influenced. The results also indicate that the presence of an omnivore does not necessarily alter the control of populations as compared with a food web containing distinct trophic levels, and that the fungal and bacterial channels may respond differently to changes in energy supply. Received: 15 December 1997     

Rehabilitation of the nematode fauna in a phytostabilized,heavily zinc-contaminated,sandy soil

Background Aim and Scope  The Maatheide in Lommel, Belgium, is an extremely metal contaminated, sandy area where vegetation has disappeared over ca. 130 hectares due to the activities of a former pyrometallurgical zinc smelter. To reduce the environmental impact of this area a rehabilitation strategy had to be developed. Therefore, in the centre of this area, an experimental phytostabilization (grass) field of three hectares had been installed in 1990. After a grass cover had been established, the development of the nematode fauna in the phyto-stabilized soil was studied. Nematodes act at various levels in soil ecosystems: herbivorous species extract their food from plant roots, bacterivorous and fungivorous species feed on microbes, predatory species consume other nematodes, and omnivorous species have mixed diets. In a mature soil ecosystem that normally exercises its manifold functions, a diverse nematode fauna occurs, reflecting the intactness of the ecosystem. As such, this fauna is indicative of crop growth, vegetative diversity, organic matter decomposition, microbial activity and diversity, and the maturity of the soil ecosystem. Methods  A metal immobilizing soil amendment (beringite) and municipal waste compost (to improve the nutrient status and water-retaining capacity) were incorporated in the soil and metal tolerant ecotypes of grasses were sown. Soil samples for nematode analyses were taken four times. Results  As a result of the treatment, pH of the soil increased and the water extractable amount of Zn was strongly reduced. Grass growth revitalized the impoverished soil ecosystem, organisms as well as metabolic processes. The nematode fauna of the experimental field in the Maatheide has been studied since 1997 and the recovery of the various feeding guilds and taxa was checked again in 2002. Nematode densities and feeding guilds have normalized, with omnivores and predators as the last guilds to reappear. Up to 27 species with a current diversity were observed in the grass-covered experimental plot, but a number of ubiquitous species, present at a comparable site at some distance, remained absent. Conclusion  It can thus be concluded that rehabilitation of an impoverished soil ecosystem is possible in heavily contaminated soils by means of phytostabilization, but there are some limitations on rehabilitation, since a number of common nematode species remained absent. Further research should indicate if this absence is indicative of a loss of essential processes in the soil. Recommendation and Perspective. Phytostabilization of heavily zinc-contaminated, sandy soil also remediates impoverished soil ecosystems. In particular, the recovery of nematode feeding categories is indicative for the normalization of soil life. The absence/presence of a number of ubiquitous taxa should be checked again after some time to verify if recovery is completed.     

Diversity and distribution of nematode communities in grasslands from Romania in relation to vegetation and soil characteristics

The nematode communities of 36 grassland ecosystems in Romania, belonging to different plant associations and soil types, were studied. The abundance of nematodes, the species and trophic types present, as well as their distribution in relation to plant community and soil characteristics are analyzed and discussed.The abundance of nematodes from the 36 grasslands studied ranged between 0.41 × 10⁶ and 8.57 × 10⁶ individuals/m², and a total of 121 genera and 145 species of nematodes were found. The highest diversity was found in grasslands developed on brown earth soil (65–67 genera and 74–76 species), with least diversity in those evolving on podzol and lithosol (33–36 genera with 25–28 identified species). Most of the dominant taxa were found in specific soil layers; some obligate plant parasitic genera (e.g., Paratylenchus, Rotylenchus, Criconema) showed preference for deeper soil layers. The nematode diversity index (H′), with values ranging between 2.38 and 3.47, did not differ significantly between the different types of grasslands. Plant feeding, bacterial feeding, hyphal feeding and omnivorous nematodes were the main groups in mountainous grasslands developed on different soil types. Plant feeding and bacterial feeding nematodes dominated the trophic structure and more plant feeders (62–69%) were found in communities of subalpine and alpine grasslands developed on podzol and alpine meadow soil, than in those developed on rendzina and lithosol (27–33%). The ratio of hyphal feeding to bacterial feeding nematodes (Hf/Bf) is constantly in favour of the bacterial feeding group, the values being an indicator of good soil fertility for most studied grasslands. The nematode communities of grasslands are grouped into six main clusters according to their genera affinity and distinguished by different grassland and soil types. Communities from subalpine grasslands developed on rendzina, acid brown and lithosol have the greatest similarities. An ordination of nematode communities in relation to important environmental variables is presented. Environmental variables relevant in explaining the patterns of nematode composition in grasslands, using canonical correspondence analysis (CCA), are: humus, pH, total nitrogen, exchangeable bases and soil type. No single factor could be selected.