Factors influencing the survivorship of the burrowing nematode,Radopholus similis (Cobb.) Thorne in two types of soil from banana plantations in Martinique

The burrowing nematode, Radopholus similis (Cobb.) Thorne, causes the most damage to bananas. To minimize nematicide applications, cropping systems that use fallow, crop rotation and clean planting material have been developed in the French West Indies. In order to optimize the benefit of the intercropping period, we studied the survivorship of R. similis in different soil types and conditions. We monitored the survivorship of calibrated populations of R. similis in the laboratory on a Nitisol and on an Andosol, two soils derived from volcanic ashes and pumices. We studied water potentials ranging from 0 to ?700 kPa on undisturbed soil and on soil previously frozen to get rid of living nematodes. Mortality of adult R. similis decreased regularly, and was fairly well described by Teissier's model. In the previously frozen soils, R. similis survived longer in wet soils (half-life of 21–46 days at 0 to ?5 kPa) than in dry soils (half-life of less than 10 days between ?80 and ?250 kPa). In contrast, in undisturbed soils, R. similis survived longer in dry soils: half-lives ranged from 57 days at ?273 kPa to 17 days at water saturation in the Andosol, and 36 days at ?660 kPa to 14 days at water saturation in the Nitisol. These results are consistent with the absence of anhydrobiosis in R. similis, unlike Pratylenchus coffeae. P. coffeae survivorship curves over time do not follow a model derived from exponential decrease like Teissier's model. These results also show that the recommended one year host-free period required to sanitize soils cannot be shortened without risk, even if flooding the soil could improve it.     

Effect of subsurface clay on nematode communities in a sandy soil

A variety of soil properties can directly or indirectly affect nematode community structure. The effects of subsurface clay content (at 20–40 cm depth) on nematodes in the surface layer (0–20 cm depth) of a sandy soil were examined in field experiments in Florida, USA. Plots were established in a site with a relatively uniform sandy upper soil layer (88–91% sand and 5–7% clay at 0–20 cm depth) but with varying levels of clay in the subsurface layer (3–35% clay at 20–40 cm depth). Nematode numbers in the surface soil layer were affected by the amount of clay in the subsurface layer. Population densities of a number of different nematode genera were greater in the surface layer of plots with 35% subsurface clay than in plots with 3% subsurface clay. Indices of nematode community structure were largely unaffected, since effects of subsurface clay were observed across all nematode groups. Most nematodes (70–80% of total numbers) occurred at 0–20 cm depth, although Teratocephalus was more common at 20–40 than at 0–20 cm. Subsurface clay content indirectly affected soil moisture and other environmental factors in the upper soil layer in which most nematodes reside.     

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.     

Effects of the earthworm Pontoscolex corethrurus on banana plants infected or not with the plant-parasitic nematode Radopholus similis

Radopholus similis is a worldwide endoparasitic nematode that greatly hampers banana (Musa acuminata, Cavendish subgroup) productivity. Earthworms are known to closely interact with above-ground and under-ground soil biota and particularly with plants and microfaunal communities. This study was aimed at investigating, under greenhouse conditions, the effects of the earthworm Pontoscolex corethrurus on banana growth and nutrient uptake, and assessing the influences of this earthworm on the development of an inoculated population of R. similis. Six-week-old tissue culture banana plants were submitted to four treatments: with P. corethrurus, R. similis, P. corethrurus+R. similis, and a control with no earthworms or nematodes. At the end of the experiment, the P. corethrurus treatments showed significantly higher leaf surface areas, shoot dry root weights, and root fresh weights than those without earthworms. This root growth enhancement probably contributed to the evident but non-significant decrease in the density of nematodes in the roots, even though earthworms did not reduce the total number of nematodes per whole root system. Moreover, the presence of earthworms slightly alleviated the severity of root damage. N bioavailability in the soil, along with N, Ca, and Mg content of banana plants, were also significantly increased in the presence of earthworms. Our results demonstrated that banana plant growth and nutrition were positively influenced by earthworms. Cropping practices that boost the development of earthworm communities in soil should therefore be promoted to enhance sustainability and to naturally alleviate nematode impact.     

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.     

Impact on soil fauna of sheep faeces containing a range of parasite control agents

To compare the impact of parasite control agents in sheep faeces, 1 kg quantities of fresh faeces were spread uniformly over 1 m² pasture plots in June 2001 (winter; a time of high earthworm activity). Faecal treatments applied to five replicate plots were C− (none), C+ (from untreated sheep), B (from sheep with an intra-ruminal bolus releasing a benzimidazole anthelmintic—‘albendazole’), ML (from sheep with a bolus releasing a macrocyclic lactone anthelmintic—‘ivermectin’), F (from sheep receiving a daily feed supplement containing chlamydospores of the nematophagous fungus, Duddingtonia flagrans). The disappearance of faeces was assessed visually over the 50 days following faecal application, then soil samples were taken to assess: (a) populations of earthworms and other soil macrofauna, (b) nematodes and other soil microfauna, and (c) the presence of D. flagrans in soil. Faecal disappearance was greatest in F and C+ plots and least in ML and B plots at 12 and 23 days (P < 0.05). Earthworm casting after 23 and 50 days was greater (P < 0.05) in plots with faeces (C+, ML, F, but not B) than in plots without faeces (C−). Greater earthworm activity in plots with faeces was reflected in greater numbers of earthworms, cocoons and greater biomass m⁻² than in C− plots. On the basis of faecal dry weight applied, F plots had most earthworms and ML plots the least. After 50 days total nematodes in 0–5 cm soil showed a treatment effect (P < 0.001), being more abundant in F, C+ and B than in C− and ML plots; enchytraeids, rotifers, tardigrades and copepods showed no treatment effects. A few nematode taxa (Acrobeles, Alaimus, Pungentus, Tylencholaimus) showed significant treatment effects. The greatest effect among nematodes was in nematode channel ratio (NCR) (P < 0.008), with a decrease in F plots; changes in NCR may reflect the impact of earthworm activity on soil processes rather than a direct effect of the fungal treatment on nematodes. D. flagrans did not become established in the soil. During the trial conditions were favourable for earthworms and their activity was high in all treatments receiving faeces, with F and ML plots being the extremes. There was an apparent shift towards fungal-mediated decomposition in F plots. At the end of the 50-day trial, in a period when earthworms were active, there was no evidence of differential effects of any of the anthelmintic treatments on environmental indicators.     

Soil moisture effects on infectivity and persistence of the entomopathogenic nematodes Steinernema scarabaei,S. glaseri,Heterorhabditis zealandica,and H. bacteriophora

We tested the effect of soil moisture on the performance of four entomopathogenic nematodes species that have recently shown promise for the control of white grubs, i.e., Heterorhabditis bacteriophora, H. zealandica, Steinernema scarabaei, and S. glaseri. Experiments for all four nematodes were conducted in sandy loam, for S. scarabaei also in loamy sand and silt loam. Infectivity was tested by exposing third-instar Japanese beetle, Popillia japonica, to nematodes in laboratory experiments and determining nematode establishment in the larvae and larval mortality. Nematode infectivity was the highest at moderate soil moistures (−10 to −100 kPa), and tended to be lower in wet (−1 kPa) and moderately dry (−1000 kPa) soil. In dry soil (−3000 kPa), only S. scarabaei showed some activity. S. scarabaei was active from −1 to −3000 kPa in all soil types but the range of highest activity was wider in loamy sand (−1 to −1000 kPa) than in loamy sand and silt loam (−10 to −100 kPa). Persistence was determined in laboratory experiments by baiting nematode-inoculated soil with larvae of the greater wax moth, Galleria mellonella. For both Heterorhabditis spp. persistence was short at −10 kPa, improved slightly at −100 kPa, significantly at −1000 kPa, and was the highest at −3000 kPa. Both Steinernema spp. persisted very well at −10 kPa. However, S. glaseri persistence was the shortest at −10 kPa but did not differ significantly at −100 to −3000 kPa, whereas S. scarabaei persistence was not affected by soil moisture. Our observations concur with previous observations on the effect of soil moisture on entomopathogenic nematodes but also show that moisture ranges for infectivity and persistence vary among species. Differences among species may be based on differences in size and behavioral and physiological adaptations.     

Succession of soil nematodes in pine forests on coal-mining sands near Cottbus,Germany

The succession of soil nematodes from initial planting with Pinus sylvestris seedling to about 30-year-old pine plantations on coal mining sands in the Lusatian lignite-mining district near Cottbus (Germany) was studied and compared with the nematode fauna of a 40-year-old semi-natural pine forest on naturally formed sandy soil. The initial stage was primarily characterised by a very low abundance (20×10³ individuals/m²), which increased over a period of two years to values common in older pine plantations (500–600×10³ individuals/m²). In the semi-natural forest the mean abundance of nematodes was about 1300×10³ individuals/m². Populations of Tardigrada, Rotifera and Enchytraeidae also increased with stand age. Nematode biomass increased from 49 to 543 mg m⁻² in pine plantations and slightly decreased in the semi-natural forest to 301 mg m⁻² over the period of investigation. The early colonisation of the initial stage was by bacterivorous (Acrobeloides) and fungal feeding (Aphelenchoides) nematodes, but the communities diversified as succession progressed with bacterivorous nematodes of the genera Plectus, Wilsonema and Metateratocephalus, root-fungal feeding Filenchus, omnivorous Aporcelaimellus and Eudorylaimus, and predacious Prionchulus becoming abundant. The abundance of plant-parasitic nematodes was very low. The greatest number of nematode genera was found in the semi-natural forest.     

An outline of soil nematode succession on abandoned fields in South Bohemia

Secondary succession of nematodes was studied in 1–48-year-old abandoned fields on cambisols in South Bohemia, Czech Republic, and compared with cultivated field and sub-climax oak forests. Bacterivores were the predominant group in the cultivated field whereas in forests root-fungal feeders (mainly Filenchus) were almost as abundant as bacterivores. The total abundance of nematodes in the cultivated field averaged 868 × 10³ ind m⁻². During the first three years of succession the abundance practically did not change (775 × 10³ ind m⁻²), the fauna was still similar to that in cultivated field but the biomass increased mainly due to Aporcelaimellus. Then the abundance increased up to 3731 × 10³ ind m⁻² in 7–8-year-old abandoned fields, plant parasites (Helicotylenchus) dominated and the fungal-based decomposition channel was activated. Later the abundance stabilised at between 1086 and 1478 × 10³ ind m⁻² in 13–25-year-old successional meadow stages with high population densities of omnivores and predators. The total abundance of nematodes was low in the 12–13-year-old willow shrub stage (594 × 10³ ind m⁻²), increased in the 35–48-year-old birch shrub stage (1760 × 10³ ind m⁻²) and the nematode fauna developed towards a forest community. The diversity and maturity of nematode communities generally increased with the age of abandoned fields but the highest values were in meadow stages (81–113 species, 57–68 genera, MI 2.73–3.30). The development of meadow arrested succession towards forests or diverted succession towards a waterlogged ecosystem. The succession of nematodes was influenced by the method of field abandonment (bare soil vs. legume cover, mowing) that affected the formation of either a shrub or meadow stage, and by the soil water status. The composition of the nematode fauna indicated that the soil food web could recover faster from agricultural disturbance under successive meadows than under shrubs.     

Responses of soil nematode community structure to different long-term fertilizer strategies in the soybean phase of a soybean–wheat–corn rotation

The impact of long-term application of fertilizers in soybean fields on soil nematode community structure was studied. The long-term application model of fertilizers lasted 13 years in a soybean–wheat–corn rotation, and included three treatments: no fertilizer (NF), chemical fertilizer (urea and ammonium phosphate, CF), and pig manure combined with chemical fertilizer (MCF). The soil nematode community structures and ecological indices were determined from soil samples taken at five soybean growth stages from May to October in the soybean phase of the rotation. Fertilizer application had significant effects on abundance of plant parasites, bacterivores and fungivores (P < 0.05), but had no significant effects on total nematodes and omnivores-predators. Abundance of plant parasites was higher in NF than in MCF and CF, and abundance of bacterivores was highest in MCF. Fertilizer application significantly affected Plant-parasitic Nematode Maturity Index (PPI) and Nematode Channel Ratio (NCR) ecological indices (P < 0.05). Shannon–Weaver Index (H′) and Species Richness (SR) indices were higher in MCF than in either NF or CF. The abundances of total nematode and plant parasites showed increasing trend with soybean growth in all three treatments. This is probably due to soil environment being more suitable for soil nematode survival with more food available for plant parasites as the soybean grows. Soybean growth stage significantly affected the H′, Free Living Nematode Maturity Index (MI) and PPI. Bacterivores significantly correlated with soil nutrient status suggesting that they could be used as a potential indicator of soil fertility.     

Dynamic changes of bacterial community under the influence of bacterial-feeding nematodes grazing in prometryne contaminated soil

Microcosm experiments were carried out to study the effects of bacterial-feeding nematodes and prometryne on soil bacterial communities in contaminated soil. Prometryne (5 or 10 mg kg⁻¹ dry soil, that is, P5 or P10) and bacterial-feeding nematodes (5 or 10 individuals g⁻¹ dry soil, that is, N5 or N10), singly and in combination (P5N5, P5N10, P10N5, P10N10), were added to a nematode-free soil. An uncontaminated nematode-free soil was studied for comparison (Control). Bacterial-feeding nematode grazing boosted soil enzyme activities in contaminated soils, thus speeding up prometryne degradation. In the initial stage of the experiment, prometryne enhanced the soil enzyme activities too, but served the opposite purpose later. Denaturing gradient gel electrophoresis (DGGE) analysis indicated that prometryne contamination and nematode grazing over the incubation period exerted an obvious impact on Species richness (S), Shannon–Wiener index (H′) and Evenness (E_H) of soil bacteria, which increased initially, then decreased and increased again later. The cluster analysis of DGGE profiles showed that the similarity of soil bacterial communities in all treatments with indigenous microbes, P5, P5N5, P5N10, P10, P10N5, and P10N10 and the Control was 75%, 44%, 78% and 49% at Day 0, Day 8, Day 18 and Day 30, respectively. Compared to the Control, DGGE profiles displayed a varying characteristic bands pattern in all treatments over the incubation period with certain bands present in the treatments while not in the Control and vice versa, suggesting that bacterial-feeding nematode grazing and prometryne contamination affected soil bacterial communities evidently. Consequently, when added to contaminated soil, bacterial-feeding nematodes can contribute to restoration of contaminated sites by degrading toxic compounds like prometryne through enhanced microbial activity.     

Ring nematodes (Mesocriconema xenoplax) alter root colonization and function of arbuscular mycorrhizal fungi in grape roots in a low P soil

A reduction of arbuscules in roots of grapevines (Vitis vinifera) observed when ring nematodes were added to field microplots led to the hypothesis that nematode feeding suppresses arbuscules by competing for root carbohydrates. Support for this hypothesis was tested by growing ‘Pinot noir’ grapevines in a factorial experiment with three levels of initial nematode densities (0, 0.1, 1.0 nematodes g^?1 soil), two levels of light (full sun, 50% sun), and two levels of AMF (nonAMF, +AMF). Effects on plant growth were primarily driven by a light and AMF treatment interaction, such that low light increased stem dry matter accumulation at the expense of roots in +AMF vines only. Nematodes had only a minor influence on plant growth (leaf mass was reduced at the highest nematode density), but nematodes did not affect overall plant dry matter accumulation. Since nonAMF vines were severely limited by P and their growth was so poor, the impact of nematode and light treatments was further analyzed in +AMF plants only. Nematode populations, AMF colonization, and root carbohydrates were differentially affected by initial nematode density or light levels. Root biomass, and reducing sugar and starch concentrations in fine roots were reduced by low light, but the final nematode populations and arbuscule frequencies in roots were unaffected by light. Nematodes reduced arbuscules and starch concentrations in fine roots, but did not affect total colonization by AMF (hyphae, vesicles or arbuscules). Nematodes reduced plant P and K uptake at the highest density, and low light reduced Mg uptake. These findings are consistent with the hypothesis that ring nematodes suppress arbuscules in roots via competition for root carbohydrates. However, the lack of a treatment interaction between light and nematodes in our study suggests that ring nematode–AMF interactions in grape roots are controlled by more than competition for photosynthate.     

Effects of dazomet on soil organisms and recolonisation of fumigated soil

Fumigation is a common practice to control soil pathogens, but little is known about the impacts of fumigation on other soil biota groups. The purpose of this study was to investigate the effects of fumigation on soil biota, including microorganisms, nematodes, and microarthropods. Bacteria were the most resistant group and some survived following treatment with 2000 mg kg⁻¹ dazomet. Some soil fungi survived 100 mg kg⁻¹ dazomet, although they were mainly Trichoderma. The fungi pathogenic to ginseng were all killed at 100 mg kg⁻¹, and showed both inter- and intra-species variation with respect to dazomet susceptibility. Among the nematodes, Aphelenchus was relatively resistant. The results suggested that susceptibility of soil organisms to dazomet differs between species, and that tolerant organisms may engage in recolonisation. In microcosm experiments, the microbial biomass and community were assessed using phospholipid fatty acid (PLFA) analysis while recolonisation of soil organisms was controlled by mesh size. The bacterial PLFA levels were changed little after fumigation, whereas the fungal PLFA levels gradually increased after fumigation. Principal analysis of the PLFA levels and the ratio of gram-negative to gram-positive bacteria showed that fumigation altered the microbial community. The number of nematodes did not recover even at 12 weeks after fumigation. The increased Collembolan numbers suggest that fumigated soil could be recolonised by specific organisms that have adapted to the conditions. In field experiments, we tested the ability of organic materials to enhance the recolonisation of fumigated soil by soil organisms. Bean powder and rice bran increased the microbial PLFA levels and nematode numbers at 6 weeks and 12 weeks after treatment, and the abundance of nematodes continued to increase 42 weeks after fumigation. The abundance of microarthropods was only slightly affected by the presence of the organic materials. We suggest that treating fumigated soils with organic materials is an effective technique to promote soil organism numbers. In addition, Trichoderma was observed to be relatively resistant to fumigation, and therefore, we propose that the fumigation effect can be improved by using a combination of resistant Trichoderma and dazomet.     

An emerging energetic soil contaminant,CL-20, can affect the soil invertebrate community in a sandy loam soil

We investigated the effects of nitramine explosive CL-20 (China Lake compound 20) on the indigenous soil invertebrate community in Sassafras sandy loam (SSL) soil using a 12-week soil microcosm assay. Freshly collected SSL soil was amended with CL-20 to prepare multiple treatment concentrations ranging from 0 (acetone control) to 10,300 mg kg⁻¹. The selected concentration range of CL-20 adequately assessed the concentration–response relationships for total microarthropods, and for individual microarthropod groups. The overall composition of microarthropod community in SSL soil was not affected by exposure to CL-20, based on the number of taxonomic groups present in the individual treatments after 12 weeks. However, community structure analysis revealed greater sensitivity to CL-20 by predatory mesostigmatid mites. Microarthropod and nematode communities showed contrasting sensitivities to CL-20 in SSL soil. Total numbers of nematodes were either unaffected or significantly (p < 0.05) increased in CL-20 treatments compared with control. Only predator group among nematodes was consistently adversely affected by exposure to CL-20. The abundance of predatory nematodes decreased in a concentration-dependent manner throughout the 12-week exposure. Microcosm assay with corresponding community structure analysis can provide the means for validating the ecotoxicity data from standardized laboratory tests, both complimenting and expanding upon the ecotoxicological significance of data from standardized single-species toxicity tests.     

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.     

Effects of cover crop quality and quantity on nematode-based soil food webs and nutrient cycling

Soil food webs cycle nutrients and regulate parasites and pathogens, services essential for both agricultural productivity and ecosystem health. Nematodes provide useful indicators of soil food web dynamics. This study was conducted to determine if nematode soil food web indicators and crop yield can be enhanced by combinations of cover crops in a conservation tillage system. The effects of three cover crop treatments (vetch/pea, oat/wheat and oat/wheat/pea/vetch) with low, medium and high C:N and a bare fallow control were investigated in Davis, CA. Nematode fauna, soil properties and plant productivity were measured. Soil food web indices, including the Enrichment Index (EI), Structure Index (SI), Basal Index (BI), and Channel Index (CI), based on the composition of nematode assemblages, were calculated to infer soil food web condition. Cover cropped tomato/corn rotations had twice the number of enrichment opportunist bacterial feeding nematodes, active participants in nitrogen mineralization, than fallowed tomato/corn rotations (opportunist bacterial feeders = 163 versus 98). In winter fallowed plots food webs were basal, common in disturbed, nutrient-poor conditions (BI = 37). Total number of enrichment opportunist nematodes, soil NH₄-N levels, and inferred nitrogen mineralization, were higher in cover crop treatments with low to mid C:N ratios. Omnivore and predator nematodes were scarce, averaging less than 6 nematodes 100 g^?1 in all treatments. In year one, plant productivity was highest after fallow. In contrast, in year two productivity was highest after cover crops with high nitrogen content and productivity significantly correlated with the structure of the soil fauna. Monitoring the abundance of enrichment opportunists may provide managers with a new tool to evaluate soil food web nitrogen mineralization and plant productivity.     

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.     

Detection and quantification of the nematophagous fungus Hirsutella minnesotensis in soil with real-time PCR

A real-time PCR assay was developed to quantify in soil the fungus Hirsutella minnesotensis, an important parasite of secondary-stage juvenile (J2) of the soybean cyst nematode. A primer pair 5′-GGGAGGCCCGGTGGA-3′ and 5′-TGATCCGAGGTCAACTTCTGAA-3′ and a TaqMan probe 5′-CGTCCGCCGTAAAACGCCCAAC-3′ were designed based on the sequence of the ITS region of the rRNA gene. The primers were highly species-specific. The PCR reaction system was very sensitive and able to detect as few as 4 conidia g^?1 soil. Regression analysis showed similar slopes and efficiency on DNA from pure culture (y = ?3.587x + 41.017, R² = 0.9971, E = 0.9055) and from Log conidia g^?1 soil (y = ?3.855x + 37.669, R² = 0.9139, E = 0.8172), indicating that the real-time PCR protocol can reliably quantify H. minnesotensis in the soil. The real-time PCR assay was applied to 20 soil samples from soybean fields, and compared with a parasitism assay. The real-time PCR assay detected H. minnesotensis in six of the soils, whereas the parasitism assay detected H. minnesotensis in the same six soils and three additional soils. The real-time PCR assay was weakly correlated (R² = 0.49) with the percentage of parasitized J2 in the six soils, indicating that different types of soil may interfere the efficiency of the real-time PCR assay, possibly due to the effect of soil types on efficacy of DNA extraction. The parasitism assay appeared to be more sensitive than real-time PCR in detecting presence of H. minnesotensis, but real-time PCR was much faster and less costly and provided a direct assessment of fungal biomass. Using the two assays in combination can obtain more complete information about the fungus in soil than either assay alone. Hirsutella parasitism was widespread and detected in 13 of the 20 field soils, indicating that these fungi may contribute to suppressiveness of soybean cyst nematode in nature and likely have high biological control potential for the nematode.     

Screening Musa germplasm for resistance to burrowing nematode populations from Uganda

Nematode pathogenicity experiments on Musa host plants were carried out in pot trials. Both the final nematode population densities and percentages root necrosis on different host plants were higher for the Radopholus similis population from Mbarara than for the populations from Namulonge, Ikulwe and Mukono (Uganda). The R. similis population from Mbarara managed to break the resistance of the diploid banana Pisang Jari Buaya, a known worldwide source of resistance to R. similis. The diploid banana hybrid TMB2× 9128-3 and the triploid dessert banana Yangambi km5 showed resistance against the four R. similis populations. These results indicate that differences in pathogenicity among different R. similis populations exist and should be taken into consideration for plantain and banana germplasm enhancement.     

Responses of the soil nematode community to management of hybrid napiergrass: The trade-off between positive and negative effects

An orthogonal experiment (form L₁₆(4⁵)) was used to investigate how the soil nematode community (density, diversity, and faunal structure) and soil health were affected by hybrid napiergrass management. The experiment included four levels of the each of the following main factors: nitrogen fertilization, cutting frequency, cutting intensity, and irrigation. The soil nematode community was affected more by nitrogen fertilization and irrigation than by cutting frequency and cutting intensity. Hybrid napiergrass develops a large root system and the carbon stored in the roots might have buffered any adverse effects of cutting on soil nematodes in the present study. The responses to fertilization indicated that fertilization had both positive and negative effects on the soil community and that the net effect depended on the level of fertilization. Additional water applied in irrigation was detrimental to soil nematode communities in that it might reduce the oxygen content of soil and also increases the potential for the leaching of nutrients from soil. Additionally, we suggest that moderate N fertilization (460 kg ha⁻¹ yr⁻¹), moderate irrigation (one time yr⁻¹ during the dry season), and cutting (three times per year at 20 cm height) will maintain soil health and provide substantial hybrid napiergrass yields.