Impact of tillage, stubble management and crop rotation on nematode populations in a long-term field experiment

The population abundance of free-living and plant-parasitic nematodes was investigated in a long-term rotation/tillage/stubble management experiment at Wagga Wagga Agricultural Institute, New South Wales (NSW), Australia. The treatments were a combination of two crop rotations: wheat (Triticum aestivum)–wheat and wheat–lupin (Lupinus angustifolius); two tillage systems: conventional cultivation (CC) and direct drill (DD); and two stubble management practices: stubble retention (SR) and stubble burnt (SB). Plots of one of the wheat–wheat treatments received urea at 100 kg N ha⁻¹ during the cropping season. Soil samples from 0–5 and 5–10 cm depths were collected in September (maximum tillering), October (flowering) and December (after harvest), 2001, to analyse nematode abundance. Soil collected in September was also analysed for concentrations of total and labile C, and pH levels.Three nematode trophic groups, namely bacteria-feeders (primarily Rhabditidae), omnivores (primarily Dorylaimidae excluding plant-parasites and predators) and plant-parasites (Pratylenchus spp. and Paratylenchus spp.) were recorded in each soil sample. Of them, bacteria-feeders (53–99%, population range 933–2750 kg⁻¹ soil) dominated in all soil samples. There was no difference in nematode abundance and community composition between the 0–5 cm and 5–10 cm layers of soil. The mean population of free-living and plant-parasitic nematodes varied significantly between the treatments in all sampling months. In most cases, total free-living nematode densities (Rhabditidae and Dorylaimidae) were significantly (P < 0.001) greater in wheat–lupin rotation than the wheat–wheat rotation irrespective of tillage and stubble management practices. In contrast, a greater population of plant-parasitic nematodes was recorded from plots with wheat–wheat than the wheat–lupin rotation. For treatments with wheat–wheat, total plant-parasitic nematode (Pratylenchus spp. and Paratylenchus spp.) densities were greater in plots without N-fertiliser (295–741 kg⁻¹ soil) than the plots with N-fertiliser (14–158 kg⁻¹ soil).Tillage practices had significant (P < 0.05) effects mostly on the population densities of plant-parasitic nematodes while stubble management had significant effects (P < 0.05) on free-living nematodes. However, interaction effects of tillage and stubble were significant (P < 0.01) for the population densities of free-living nematodes only. Population of Rhabditidae was significantly higher in conventional cultivated plots (7244 kg⁻¹ soil) than the direct drilled (3981 kg⁻¹ soil) plots under stubble retention. In contrast, plots with direct drill and stubble burnt had significantly higher populations of Dorylaimidae than the conventional cultivation with similar stubble management practice. No correlations between abundance of free-living nematodes, and concentration of total C and labile C in soil were observed in this study. These results showed that stubble retention contributed for enormous population density of free-living (beneficial) nematodes while conventional cultivation, irrespective of stubble management, contributed for suppressing plant-parasitic nematodes.     

Nematode communities in the artificially vegetated belt with or without irrigation in the Tengger Desert,China

Shifting sand dunes threatened the Baotou-Lanzhou railway being kept on operation smoothly seriously crossing Shapotou on the southeastern edge of the Tengger Desert (China). Artificial vegetation input was employed in Shapotou and a vegetated belt was established for stabilizing the shifting sand dunes. Nematode communities from bare soil between vegetation (ST, STI) and soil under the vegetation (ST-V, STI-V) in the vegetation belt, were investigated. All sites received natural rainfall, of which STI and STI-V received irrigation supplements. Total 43 genera were found in four types of soil samples, and Acrobeles, Acrobeloides, Chiloplacus, Panagrolaimus, Aphelenchoides and Ditylenchus were dominant genera in our study. Nematode abundance per 100 g fresh soil (47–552), the number of taxa identified (8–20), species richness (1.51–3.41), the proportion of plant feeders (0.3%–4.2%) and abundance of all cp groups responded to vegetation. The proportion of omnivores (0.5%–7%), the number of taxa identified, Shannon index (1.26–2.12), species richness, genus dominance (0.18–0.40) and abundance of cp3–5 responded to irrigation supplements. The application of PCA helped to reveal that almost all nematode taxa exhibited a positive loading on the horizontal axis, it implied that nematode abundance was generally higher on sites with planted input. Besides, it showed a vegetation gradient from bare soil between vegetation to soil under vegetation along the first axis, while an irrigation gradient along the second axis. Similarly, classification analysis based on all cased involved indicated that two-main group of nematodes was distinguished by their habitats under vegetation from bare soil. Those results suggested that vegetation input might play a more important role on nematode community composition than irrigation supplements for stabilization of shifting sand dunes. Therefore, nematode community composition could be as an indicator of stabilization of sand dune approaches of plant input with only natural rainfall versus with natural rainfall in combination with irrigation supplements in our system. However, the apparent anhydrobiotic state of the nematode fauna required special consideration in desert system, thus the utilization of the nematodes as an indicator of stabilization of sand dune regimes should be further explored on anhydrobiotic nematodes.     

Vertical distribution of nematodes in arable soil under grass (Festuca pratensis) and barley (Hordeum distichum)

Summary The connection between faunal composition and soil factors is discussed in this study on vertical distribution of soil nematodes under grass and barley. The investigation was undertaken on the field site of a Swedish integrated research project Ecology of Arable Land. The Role of Organisms in Nitrogen Cycling. Higher nematode number (7.6 × 10⁶ m^–2) and biomass (340 mg dry wt. m^–2) were found under a 4-year-old grass ley than under barley (5.0 × 10⁶ m^–2; biomass, 136 mg dry wt. m^–2). Plant feeders dominated under the grass ley (3.2 × 10⁶ m^–2 whereas under barley the bacterial feeders (2.4 × 10⁶ m^–2) were the most abundant feeding group. Number, biomass, mean individual size and various community parameters indicated a much better nutritive situation for the nematodes under grass than under barley. The vertical changes in the various parameters, including proportion of egg-carrying females, indicated an increasing food shortage for the nematode populations towards greater depths. In the top soil, predation could be an important factor in regulating nematode number.Dedicated to the late Prof. Dr. M.S. Ghilarov     

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     

Diurnal migration and responses to simulated rainfall in desert soil microarthropods and nematodes

Diurnal patterns of microarthropod abundance in surface leaf litter were related to its moisture content. Leaf litter moisture was nearly 7% by weight at 0800h but fell to less than 1% by mid-day. Oribatid and tydeid mites moved into litter in the early morning and back into the soil before mid-day. There were no significant differences in numbers of nematodes in litter or soil and 78–98% of the nematodes were anhydrobiotic (coiled) in soil and litter at all times sampled.Following simulated rainfall there were fewer microarthropods in litter at mid-day in the absence of marked decreases in soil and litter moisture content. During drying, there were gradual reductions in numbers and species diversity of litter microarthropods. Nematode numbers did not change as litter dried. Anhydrobiotic nematodes in the soil increased from 14% on day 1 to 85% on day 4. Between 24 and 36 h after simulated rainfall, the proportion of anhydrobiotic litter nematodes increased from 35 to 80%,.Within 1 h after simulated rainfall, there were marked increases in numbers and diversity of microarthropods in surface litter. No collembolans were extracted from dry litter controls but the wet litter was dominated by isotomid, sminthurid and onychiurid collembolans. There were increases in numbers and diversity of oribatid, tydeid and gamasid mites in the wet surface litter within l h after wetting compared to controls.     

Soil nematode assemblages as bioindicators of primary succession along a 120-year-old chronosequence on the Hailuogou Glacier forefield,SW China

Successional dynamics in terrestrial ecosystems is important for interactions between aboveground and belowground subsystems. In this study, nematode communities in a Hailuogou Glacier Chronosequence from seven stages were investigated to determine whether changes in soil phosphorus (P) and nematode assemblages parallel those observed in aboveground communities, and whether the primary succession in this chronosequence has entered a retrogressive phase after 120 years of succession. The initial 40-year succession, including stages 2, 3 and 4, can be viewed as a build-up phase. Especially at stage 3, vegetation succession from grassland to forest accelerated the accumulation of plant litter and bioavailable P, paralleled with a sharp increase in nematode abundance. The mature phases covering stages 5, 6 and 7 displayed most balanced nematode communities, in which abundance, taxon richness, maturity index and structure index were at highest. However, the last stage 7 appeared to show some retrogressive characteristics, as suggested by the reduced bioavailability of P and a significant decrease in nematode densities, along with the disappearance of some rare genera of nematodes from higher trophic guilds, resulting in decreases in the nematode channel ratio, plant parasite index and enrichment index. Thus, the Hailuogou Glacier Chronosequence may enter its retrogressive phase during the next decade or century. A bacterial-based nematode energy channel dominated the chronosequence during the development; by contrast, a fungivore-based channel was activated at the early and late stages, because fungivores are better adapted to nutrient-poor environments. Our results demonstrated that different nematode guilds have contrasting responses to chronosequence stages, possibly due to their different responses to bottom-up and top-down controls. Furthermore, soil nematode communities could be used as sensitive bioindicators of soil health in glacial-retreat areas.     

Spider predation on forest-floor Collembola and evidence for indirect effects on decomposition

Spiders are major predators in forest-floor leaf litter, yet little is known about their impact on prey populations and on forest-floor processes such as litter decomposition. This experiment investigated the effect of removing spiders on Collembola densities. We also examined the potential indirect effect of spider removal on rate of litter disappearance. Twenty-eight 1-m² plots were randomly assigned to one of four treatments: O – open, no manipulations; F – fenced, no litter sifted, no spiders removed; FS – fenced, litter sifted, no spiders removed; and FSR – fenced, litter sifted, spiders removed. In early August, we sifted the litter in the FS and FSR plots, removing all encountered spiders from the FSR treatment. A month later, we placed into each plot one 15 × 15-cm litterbag filled with a known amount (ca. 3 g) of dried straw. After six weeks, litterbags were collected and fauna were extracted in a Kempson-McFadyen apparatus. Decreasing spider predation increased densities of Collembola, and increased the rate at which straw disappeared from litterbags. These results indicate that spider predation may reduce Collembola densities enough to lower rates of litter disappearance on the forest floor.     

Biodiversity and trophic structure of soil nematode communities are altered following woody plant invasion of grassland

Woody plant encroachment is an important land cover change in dryland ecosystems throughout the world, and frequently alters above and belowground primary productivity, hydrology, and soil microbial biomass and activity. However, there is little known regarding the impact of this geographically widespread vegetation change on the biodiversity and trophic structure of soil fauna. Nematodes represent a major component of the soil microfauna whose community composition and trophic structure could be strongly influenced by the changes in ecosystem structure and function that accompany woody encroachment. Our purpose was to characterize nematode community composition and trophic structure along a grassland to woodland chronosequence in the Rio Grande Plains of southern Texas. Research was conducted at the La Copita Research Area where woody encroachment has been documented previously. Soil cores (0–10 cm) were collected in fall 2006 and spring 2007 from remnant grasslands and woody plant stands ranging in age from 15 to 86 years, and nematodes were extracted by sugar centrifugation. Neither nematode densities (3200–13,800 individuals kg⁻¹ soil) nor family richness (15–19 families 100 g⁻¹ soil) were altered by woody encroachment. However, family evenness decreased dramatically in woody stands >30 years old. This change in evenness corresponded to modifications in the trophic structure of nematode communities following grassland to woodland conversion. Although root biomass was 2–5× greater in wooded areas, root-parasitic nematodes decreased from 40% of all nematodes in grasslands to <10% in the older wooded areas, suggesting the quality (C:N or biochemical defenses) of woody plant root tissue could be limiting root-parasites. In contrast, bacterivores increased from 30% of nematodes in grasslands to 70–80% in older woody patches. This large increase in bacterivores may be a response to the 1.5–2.5× increase in soil microbial biomass (bacteria + fungi) following woody encroachment. Therefore, while energy flow through grassland nematode communities appears to be distributed nearly equally among herbivory, fungivory and bacterivory, the energy flow through nematode communities in wooded areas appears to be based primarily on bacterivory. We speculate that these shifts in nematode community composition and trophic structure could have important implications for ecosystem patterns and processes. First, the low abundance of root-parasitic nematodes (and presumably root herbivory) under woody plants may be one mechanism by which woody plants are able to establish and compete effectively with grasses during succession from grassland to woodland. Second, the large increase in bacterivores following woody encroachment likely accelerates microbial turnover and the mineralization of N, thereby providing a feedback that enables the persistence of N-rich woody plant communities.     

A mixture of grass and clover combines the positive effects of both plant species on selected soil biota

The introduction of N₂-fixing white clover (Trifolium repens) in grassland is a management measure that may contribute to sustainable grassland systems by making them less dependent on inorganic fertilizers. However, little is known about the impact of this measure on soil biota and ecosystem services. We investigated earthworms, nematodes, bacteria and fungi in an experiment in which white clover-only and a mixture of grass and white clover without fertilization were compared with grass-only with and without fertilization.In comparison with grass-only, white clover-only had a lower total root biomass and a lower C/N-ratio in the above- and below-ground plant biomass. These plant characteristics resulted in a lower bacterial biomass, a lower fungal biomass, a higher proportion of bacterivorous nematode dauerlarvae, a lesser proportion of herbivorous nematodes and a greater abundance of earthworms in clover-only.The quantity and quality (C/N-ratio) of the above- and below-ground plant biomass in the mixture of grass and white clover (20–30% clover in the DM) was comparable with grass fertilized with 150 kg N ha⁻¹ of inorganic fertilizer. Differences between these treatments might show specific clover effects in the grass–clover mixture on soil biota other than quantity and C/N-ratio of the litter. However, the only differences were a higher proportion of bacterivorous nematode dauerlarvae and a different nematode community composition in grass–clover.The soil structure in white clover-only showed a higher proportion of angular blocky elements, a lower penetration resistance, a higher number of earthworm burrows, a higher potential N-mineralization and respiration than the soil in grass-only. This suggests that clover stimulates the ecosystem services of water infiltration and supply of nutrients, but is less conducive to soil structure maintenance. The grass–clover mixture differed from grass-only in a higher respiration and from clover-only in a higher percentage of soil crumbs. We suggest that when clover is introduced in grassland to reduce the reliance on inorganic fertilizer, the mixture of grass and clover maintains the positive impact of grass roots on soil structure and increases the supply of nutrients via the soil food web. Thus, a grass–clover mixture combines the agronomic benefits of the two plant types.     

Vertical distribution of soil nematodes under different land use types in an aquic brown soil

A field investigation was conducted at the Shenyang Experimental Station of Ecology to study the vertical distribution of nematode communities down to a depth of 150 cm under four land use types (paddy field, maize field, fallow field and woodland) in an aquic brown soil of Northeast China. The results showed that the numbers of total nematodes and trophic groups exhibited a gradual decrease trend with depth under different land use types. The numbers of total nematodes, bacterivores, fungivores, and plant parasites were positively correlated with the contents of TOC, total N, and alkali N in the four land use types. The majority of nematodes were present in the 0–30 cm soil layers. No significant effects were found on the number of total nematodes at all depths among the maize field, fallow field and woodland. Bacterivores were found to be the most abundant group in the paddy field, while plant parasites were observed to be the most abundant group in the maize field, fallow field and woodland. The number of fungivores at the depths of 0–5 and 5–10 cm was higher in the maize field than in the other land use types. Omnivores-predators were found in relatively low numbers under each land use type, with a higher presence in the fallow field and woodland at the depths of 0–5, 5–10, 10–20 cm compared with the paddy and maize fields. 54 genera were observed throughout the four land use types in our study. The woodland treatment supporting greater basal resource inputs tended to result in a higher diversity of nematodes. The number of genera reached a maximum at the depth of 5–10 cm under each land use type. The faunal profiles showed that soil food webs in the fallow field and woodland were structured, and those in the paddy and maize fields were stressed. The faunal analysis provided a useful tool for diagnostic interpretation of the condition of upper soil layers.     

Recovery of soil nematode populations from cropping stress by natural secondary succession to meadow land

The spectrum and densities of soil nematode species were studied in an extensively managed sub-thermophilous meadow and in conventionally managed and abandoned fallow fields left to natural succession. In the meadow, 115 species and 71 genera of soil nematodes were found and the total mean nematode abundance was 1019×10³ individuals/m². The dominant feeding groups were root-fungal feeders (31%, mainly Filenchus), bacterivores (26%; variety of genera, mainly Panagrolaimus, Rhabditis, Acrobeloides, Bursilla, Plectus, Anaplectus, and Eucephalobus), and fungivores (16%, Aphelenchoides). The cultivated field had 66 species and 41 genera of nematodes and a mean nematode abundance of 546×10³ individuals/m². The eudominant trophic group was bacterivores (53%; mainly Panagrolaimus and Acrobeloides) accompanied by fungivores (28%, Aphelenchoides). During 2 years of natural succession in an abandoned field, the total mean nematode abundance rose to 938×10³ individuals/m², and the number of species and genera increased to 73 and 46, respectively. Omnivores (25%, Aporcelaimellus and Eudorylaimus), fungivores (24%, Aphelenchus) and bacterivores (21%, Eucephalobus and Panagrolaimus) became the leading trophic groups, and the values of the Shannon Index of diversity and the Maturity Index increased. The development of soil nematode populations in the early successional stages of abandoned cambisol fields in Central Europe is discussed.     

Carbon and nitrogen budgets of nematodes in arable soil

Summary The amounts of C and N that pass through the nematode biomass in four cropping systems, barley without and with N fertilization, grass ley and lucerne, has been estimated. The nematodes were sampled at the field site of a Swedish integrated research project Ecology of Arable Land: The Role of Organisms in Nitrogen Cycling. The nematode biomass was lower (200 mg dry weight m^–2) in the annual (barley) than in the perennial (grass and lucerne, 350 mg dry weight m^–2) crops. For respiration, the nematodes used 4–71 O₂m^–2 year^–1 corresponding to C liberation of 1.3%–2.0% of the carbon input to the soil. A higher relative contribution by bacterial-feeding nematodes to the C and N fluxes and a higher turnover rate of the nematode biomass is an indication of more rapid nutrient circulation in the annual than in the perennial cropping systems.     

Biogeophysical factors influencing soil respiration and mineral nitrogen content in an old field soil

Microbivorous grazers are thought to enhance nutrient mineralization. The predicted effect of microbivory on nutrient cycling depends on the pore habitat model used. We evaluated CO₂ evolution and mineral N content of an old field soil to test two alternative habitat hypotheses. The exclusion hypothesis predicts that nematodes are separated from their microbial food resources in water-filled pores when soils dry, resulting in slower rates of biogeochemical transformations. The enclosure hypothesis predicts that nematode densities increase relative to their forage in smaller, isolated water volumes when soils dry, accelerating rates of biogeochemical transformations. We investigated the effect of soil moisture on the relationship between microbial biomass, microbivorous and predaceous nematodes, soil respiration and mineral N concentrations in an old field five times during the course of a year.We could evaluate the validity of the two habitat hypotheses for the entire field only in August 1997 because that was the only sampling date when maximum water-filled pore diameters were smaller than microbivorous nematode body diameters in all sampled field locations. The mean microbivorous and predaceous nematode abundances for the field in August were greater than 6300 kg⁻¹ and 80,000 kg⁻¹, respectively. Accordingly, the exclusion hypothesis was rejected. Predaceous nematode abundance was markedly higher in August than at any other sampling date. The high abundance of predators present suggests that detrital resources were not limiting productivity and that predators and microbivores were in enclosures, allowing predators to efficiently access their prey. Spatial maps, in agreement with linear correlation analyses, suggest that under our driest sampling conditions, soil respiration and mineral N content were controlled by microbivory and predation.     

A comparison of microbial-feeding nematodes and protozoa in the rhizosphere of different plants

Summary The biomass of microbial-feeding nematodes and protozoa was measured in the rhizospheres of peas, barley, grass and turnips grown for 10 weeks in pots containing a clay-loam soil; in the rhizospheres of peas and barley grown for 3 weeks in a sandy soil; and in the rhizosphere of barley grown for 11 weeks in an unfertilised and a fertilised clay-loam soil. The nematode biomass was consistently larger in the rhizosphere of all plants in both soils than in the bulk soil, but the protozoa biomass showed a rhizosphere effect only under pea and fertilised barley. The biomass of nematodes in the rhizosphere (1.2–22.3 g dry weight g^-1 dry soil) was greater than the biomass of protozoa (0.1–3.2 g g^-1), and greater under pea>barley>grass>turnip. It is suggested that nematodes are more able to exploit low bacterial densities than protozoa and that they initially migrate into the rhizosphere from the bulk soil. In samples of potato rhizosphere from field-grown plants, the nematode biomass was also greater than the active and total protozoan biomass. It is argued that in the rhizosphere the biomass of microbially feeding nematodes exceeds that of protozoa and that nematodes are more important in terms of nutrient cycling.     

Cattle grazing increases microbial biomass and alters soil nematode communities in subtropical pastures

This study focused on examining the impacts of cattle grazing on belowground communities and soil processes in humid grasslands. Multiple components in the soil communities were examined in heavily grazed and ungrazed areas of unimproved and improved bahiagrass (Paspalum notatum Flugge) pastures in south-central Florida. By using small (1-m×1-m) sampling plots, we were able to detect critical differences in nematode communities, microbial biomass, and mineralized C and N, resulting from the patchy grazing pattern of cattle. Soil samples were collected on three occasions between June 2002 and June 2003. Microbial C and N were greater (P?0.01) in grazed than in ungrazed plots on all sampling dates. Effects of grazing varied among nematode genera. Most genera of colonizer bacterivores were decreased (P?0.10) by grazing, but more persistent bacterivores such as Euteratocephalus and Prismatolaimus were increased, as were omnivores and predators. Higher numbers of persisters indicated that grazing resulted in a more structured nematode community. Some herbivores, particularly Criconematidae, were decreased by grazing. Abundance of omnivores, predators, and especially fungivores were strongly associated with C mineralization potential. Strong correlation of microbial C and N with nematode canonical variables composed of five trophic groups illustrates important links between nematode community structure and soil microbial resources. Including the analysis of nematode trophic groups with soil microbial responses reveals detection of grazing impact deeper into the hierarchy of the decomposition process in soil, and illustrates the complexity of responses to grazing in the soil foodweb. Although highly sensitive to grazing impacts, small-scale sampling could not be used to generalize the overall impact of cattle grazing in large-scale pastures, which might be determined by the intensity and grazing patterns of various stocking densities at the whole pasture level.     

中国小麦田土壤线虫对生物炭添加的响应

While studies have focused on the use of biochar as soil amendment, little attention has been paid to its effect on soil fauna. The biochar was produced from slow pyrolysis of wheat straw in the present study. Four treatments, no addition (CK) and three rates of biochar addition at 2 400 (B1), 12 000 (B5) and 48 000 kg ha-1 (B20), were investigated to assess the effect of biochar addition to soil on nematode abundance and diversity in a microcosm trial in China. The B5 and B20 application significantly increased the total organic carbon and the C/N ratio. No significant difference in total nematode abundance was found among the treatments. The biochar addition to the soil significantly increased the abundance of fungivores, and decreased that of plant parasites. The diversity of soil nematodes was significantly increased by B1 compared to CK. Nematode trophic groups were more effectively indicative to biochar addition than total abundance.     

Response by soil nematode populations and the soil microbial biomass to a rain episode in the hot,dry Negev Desert

Since the amount, intensity, and frequency of rainfall in desert regions vary strongly over space and time, the response by soil biota to this variability is of great importance. We conducted a study in the Negev desert in order to examine the immediate response by the soil nematode populations and the microbial biomass to varying amounts of water applied in a single pulse. Soil samples from the 0–10-cm depth were collected from areas undergoing four different wetting treatments, comprising 5, 10, 15, and 20 mm of water, and from a non-irrigated control soil. There was a correlation between diurnal variations in nematode populations and the diurnal fluctuations in soil moisture. The greatest abundance of nematodes was found in the soil treated with 20 mm water (970 individuals 100 g^-1 dry soil) which was 2, 4, 5, and 14 times larger than that found in the soil treated with 15, 10, 5, and 0 mm of water, respectively. Bacterialfeeding and fungal-feeding nematodes accounted for approximately 95% of the total nematode population found in all treatments. The microbial biomass examined in the current study exhibited an immediate response to the wetting which was greater in soil treated with, 10, 15, and 20 mm of water compared with 0 and 5 mm. However, after 4 days (96 hours) the microbial biomass stabilized again at the basic level of the 0-mm control. However, our results indicated that the major trigger for changes in the nematode populations, and in the microbial biomass, was diurnal fluctuations in soil moisture, since peaks in nematode populations and in the microbial biomass were observed at various times of the day.     

栽培对黑土线虫营养群体的空间分布的影响

Geostatistics combined with GIS was applied to assess the spatial distribution of nematode trophic groups following two contrasting soil uses in the black soil region of Northeast China.Two plots,one with fallow for 12 years and the other cultivated,were marked on regular square grids with 2-m spacing.Soil samples were collected from each sampling point,nematodes were extracted from these samples and classified into four trophic groups:bacterivores,fungivores,plant parasites,and omnivores/predators.The numbers of total nematodes and trophic groups analyzed had normal distributions on both fallow and cultivated plots.The absolute abundances of total nematodes and trophic groups were observed to be much nore homogeneous on cultivated plot than on fallow one.Geostatistical analysis showed that the densities of total nematodes and trophic groups on both fallow and cultivated plots exhibited spatial dekpendence at the sampled scale and their experimental semivariograms were adjusted to a spherical or exponential model,except those of bacterivores and fungivores oncultivated plot.The spatial distribution of nematode trophic groups was found to be different for the two land uses,indicating that cultivation changed the native condition for soil nematode activities.     

Effect of compost and chemical fertilizer on soil nematode community in a Chinese maize field

Nematode density and biodiversity in maize field soil treated with compost, chemical fertilizer and with no amendments were investigated in a multi-year field experiment at the Qu-Zhou experimental station, China Agricultural University. The soils were collected from the upper (0–20 cm) soil layer during the maize growing stages in 2004. The results demonstrated that significant differences for the total nematode density, bacterivores, fungivores, plant parasites and omnivores-predators density were found between treatments and between dates. The total nematode density and bacterivores density were greater in compost-treated soil than in chemical fertilizer-treated soil, and were greater in chemical fertilizer-treated soil than in control soil during all sampling periods. The total nematodes density ranged from 106 to 657 individuals per 100 g dry soil in the present study. Total 40 nematode genera were found in all treatments and sampling periods, and 12 genera were bacterivores, 4 genera were fungivores, 16 genera were plant parasites and 8 genera were omnivores-predators. Cephalobus, Rhabditis, Tylenchorhynchus, Pratylenchus, Helicotylenchus and Rotylenchus were dominant genera in present study. The plant parasites and bacterivores were dominant trophic groups. The ratio of bacterivores plus fungivores to plant parasites was higher in compost-treated soil compared to chemical fertilizer-treated soil except October. Maturity index and combined maturity index were lower in compost-treated soil compared to chemical fertilizer-treated and control soil except July. The plant parasite index was higher in compost-treated soil compared to chemical fertilizer-treated soil except July. The multi-year application of compost and chemical fertilizer had effected on soil nematode population density and community structure.     

Nematode assemblages indicate soil restoration on colliery spoils afforested by planting different tree species and by natural succession

Soil nematodes were studied in 28 afforested sites on clay colliery spoil from opencast mining in the Czech Republic. The afforestations had four replicate sites and were represented by 20–33-year-old plantations of oak, alder, linden, spruce, larch, pine, and sites left to natural succession. The total abundance of nematodes in deciduous plantations plus natural successions was 1156 × 10³ ind/m² and in coniferous plantations 1236 × 10³ ind/m². The abundance of plant parasites and omnivores was significantly greater in deciduous than in coniferous plantations. The abundance of root–fungal feeders dominated by Filenchus was greater in spruce and pine plantations than in deciduous plantations. Bacterivorous Plectus predominated in larch plantations. Nematode assemblages in deciduous plantations had significantly greater numbers of species and genera, H’gen, MI, ΣMI, EI and SI values, and greater proportions of omnivores plus predators to other trophic groups than the assemblages in coniferous plantations. The nematode fauna indicated most advanced soil restoration on spoils under oak and alder plantations, intermediate under linden and spruce, and least under pine and larch. Nevertheless, some nematodes typical for developed forests were either found in low population densities (e.g. Teratocephalus) or were missing (e.g. Malenchus).