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.     

Nematodes as indicators of soil recovery in tailings of a lead/zinc mine

Trophic groups and functional guilds of soil nematodes were measured under four mine tailing subsystems in the Baoshan lead/zinc mine, Hunan Province, southern China to test the indicator value of nematodes for heavy metal pollution. No obvious correlation was found between heavy metal concentration and the total number of nematodes. However, the densities of c–p3, c–p4 and c–p5 nematodes were negatively correlated with Pb and Zn concentrations, suggesting that the abundance of nematode groups of high c–p values is useful indicators of heavy metal contamination. The “weighted faunal analysis” provided a better assessment of soil health condition than Maturity Index (MI) in situations where there were extremely low numbers of soil nematodes. Results showed that the effect of heavy metal contamination on soil nematodes might be strongly influenced by plants. Although the abundance of plant-feeding nematodes did not reflect the heavy metal conditions in the soil, it might be used as an index for assessing the soil remediation potential of pioneering plants. Patrinia villosa seems superior to Viola baoshanensis as a pioneer plant species for soil remediation based on analysis of rhizosphere nematode community.     

Local variation in belowground multitrophic interactions

A growing number of studies point at the involvement of root herbivores in influencing plant performance, community composition and succession. However, little is known about the factors that control root herbivore abundance and the role of local variation in the effectiveness of these factors. Here, we performed a full factorial experiment with plants, root-feeding nematodes and rhizosphere microbial communities from two dune sites, to test the hypothesis that the outcome of belowground multitrophic interactions depends on local differences between the interacting organisms. The organisms included the marram grass Ammophila arenaria, the cyst nematode Heterodera arenaria, microbial plant pathogens and natural enemies of the nematodes from two coastal foredune systems, one in The Netherlands and one in Wales. The two plant populations differed at the molecular and phenotypic level, and the microbial communities from the two dune sites differed in the composition of the dominant soil fungi but not of the dominant bacteria. Plants were negatively affected by the rhizosphere microorganisms from one of the sites. Nevertheless, nematode performance was not affected by the origin of both the host plants and the microbial communities. The reproductive output of the cyst nematode depended on the presence of microorganisms, as well as on inter-population variability in the response of the nematode to these natural enemies. In the absence of microorganisms, the two nematode populations differed in the number and size of the produced cysts, although maternal effects cannot be excluded. Inter-population differences in the host plant were a secondary factor in the nematode-microorganisms interactions, and did not influence bottom-up control of the cyst nematodes. Our results did not reveal strong signals of coevolution in belowground multitrophic interactions of plants, cyst nematodes and soil microbial communities. We conclude that the interactions between the studied organisms do not necessarily depend on their local vs. non-local origin. Nevertheless, we were able to show that local variation in soil organism community composition can be an important factor in determining the outcome of interactions in belowground multitrophic systems.     

Effects of agricultural management on nematode–mite assemblages: Soil food web indices as predictors of mite community composition

Biological indicators based on abundances of soil organisms are powerful tools for inferring functional and diversity changes in soils affected by agricultural perturbations. Field plots, combining organic and conventional practices with no tillage, conservation tillage and standard tillage maintained different nematode assemblages and soil food webs. Soil food web indices based on nematode assemblages were reliable predictors of the trophic composition of functional characteristics of soil mite assemblages. Bacterial-feeding and predatory nematodes, together with predatory mites, were abundant in the organic-no till treatments and were associated with high values of the Enrichment and the Structure Index based on nematode assemblages. Conventional-Standard tillage treatments had high abundances of fungal- and plant-feeding nematodes and algivorous mites, associated with high values of the Basal and Channel Index. This study validates the hypothesis that nematode-based soil food web indices are useful indicators of other soil organisms such as mites, with similar functional roles and environmental sensitivities.     

Host range as an axis of niche partitioning in the plant-feeding nematode community of banana agroecosystems

The stability and positive functions (such as pest regulation) of intensively managed agroecosystems can be enhanced by increasing biodiversity (in particular, by introducing cover plants or associated crops). Therefore, understanding modes of interspecific interactions among the phytophagous species, such as coexistence via host-plant partitioning, may allow manipulation of the balance between pest and non-pest species.In the present study, we tested the hypothesis that a community of six plant-feeding nematode species in banana agroecosystems is structured by host plants. This was done using extensive data on the abundance of the different nematodes species on banana and associated weeds in Martinique. Because the purpose of this study was to focus on host-plant range, we eliminated the effects of temperature, rainfall, and soil type using a partial canonical correspondence analysis.Host plants in banana agroecosystems have a marked influence on the plant-feeding nematode community structure. Host plants allow niche partitioning between some but not all pairs of species. The most evident pattern is the niche partitioning among the three nematodes considered long established in Martinique.For pairs of species showing no host-plant niche partitioning, additional information regarding the colonisation and population dynamics over time both at the root and agroecosystem scale would allow better understanding of the equilibrium among species of the plant-feeding nematode community.For management purposes, the modification of abundance of some host plants, which could be used as cover plants in the field, could change the balance among nematode species.     

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。     

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.     

Responses of root-feeding nematodes (Helicotylenchus spp.) to local and non-local populations of the host plant Ammophila arenaria

The root-feeding nematode community of wild plants may vary throughout their natural range. Little is known about how the variation of wild plants along their range affects their relationship with root-feeding nematodes. In the present study, we examined local and non-local combinations of host plants and root-feeding nematodes to test the hypothesis that nematode reproduction is favoured by local hosts. In two indoor experiments, we exposed populations of the wild dune grass Ammophila arenaria from northern and southern European coastal sand dunes to plant parasitic nematode species (Helicotylenchus spp.) from those same geographical origins. First, we used the southern nematode species to determine whether the effect of a local versus a non-local host may depend on nematode density. Then, in a cross-inoculation experiment we investigated how both nematode species performed with their local, as compared to the non-local hosts.

In both experiments, plant biomass and ontogenetic characteristics were not significantly different between the northern and southern populations. The applied nematode densities did not have a negative impact on plant performance. This allowed us to consider the response of the two different nematode species and their host plants without co-varying differences in plant responses. Reproduction of the nematode species differed according to host origin, but contrary to what we expected, nematode species did not perform better on their local hosts. Helicotylenchus n. sp., the southern species originating from Portugal, performed better on the non-local than on the local host. Male to female ratios were significantly different between the two nematode populations and were lowest in Helicotylenchus pseudorobustus from The Netherlands. Female and juveniles percentages were also quite distinct, with more females in the nematode species from The Netherlands and more juveniles in the nematode species from Portugal.

We concluded that ectoparasitic root-feeding nematodes Helicotylenchus spp. do not necessarily perform best on their local host population of the foredune grass A. arenaria. Our results imply that the natural distribution of Helicotylenchus spp. along the European coast is determined by other factors than host populations. These other factors that could be abiotic, e.g. water availability, or biotic, e.g. local natural enemies adapted to Helicotylenchus spp. Introducing plant genotypes from other parts of the natural range will, therefore, not necessarily lead to reduced abundance of semi-endo or ectoparasitic root-feeding nematodes.     

Temporal dynamics of soil nematode communities in a grassland plant diversity experiment

We report here on an 8-year study examining links between plant and nematode communities in a grassland plant diversity experiment, located in the north of Sweden on previous agricultural soil. The examined plots contained 1, 4 and 12 common grassland plant species from three functional groups; grasses, legumes and forbs. The same plant species composition was maintained in the plots through weeding and resowing during the experimental period. The hypotheses were (i) that the nematode community would shift towards a more diverse and mature fauna over time and (ii) that the effects of both plant species identity and plant species richness would increase over time. As hypothesized, the Bongers’ Maturity Index (a measure of nematode responses to disturbance) increased over time, but not nematode diversity. Instead, the nematode community development in the present grassland experiment seemed to be more characterized by shifts in dominance patterns than by colonization of new taxa. Clear temporal trends were found for plant-feeders and Adenophorea bacterial-feeders which increased in abundance over time in almost all plant treatments. Rhabditidae bacterial-feeders decreased in abundance over time, in particular in plots with legumes. Fungal-feeders, omnivores/predators and the two nematode indices PPI (Bongers’ Plant Parasitic Index) and NCR (Nematode Channel Ratio) had significant interactions between plant composition and time giving some support to our second hypothesis. Our results highlight the need for long-term experiments to examine plant species effects on soil fauna, especially on taxa belonging to higher trophic levels. The results also stress the importance of plant composition for belowground soil faunal communities.     

Interactions between root-feeding nematodes depend on plant species identity

Root-feeding nematodes play an important role in structuring the composition of natural plant communities. Little is known about the role of intra- and interspecific interactions in determining the abundance of root-feeding nematodes in natural ecosystems. We examined interactions between two ectoparasitic root-feeding nematodes on two plant species: a good host plant for both nematode species and a good host for only one of the nematodes. We tested the hypothesis that root herbivore competitiveness depends on host suitability and related the experimental results to field data. In a greenhouse, we added different densities of the nematodes Tylenchorhynchus microphasmis and Tylenchorhynchus ventralis to Ammophila arenaria (the good host for both) and Carex arenaria (a good host for T. microphasmis only). Addition of T. ventralis did not significantly affect multiplication of T. microphasmis on both plant species. In contrast, on A. arenaria, T. ventralis experienced interspecific competition. However, on C. arenaria, T. microphasmis facilitated multiplication of T. ventralis. To explain this effect, we studied systemic plant-mediated effects in a split-root experiment. Nematode addition to one root compartment did not significantly influence nematode multiplication in the other root compartment, irrespective of nematode species identity. Therefore, the observed nematode interactions were not related to induced changes in the roots. In a two-choice experiment we tested whether host suitability was related to root attractiveness. Both nematode species were attracted to seedlings of A. arenaria, but not to C. arenaria. The low multiplication of T. ventralis on C. arenaria could be related to poor attraction to the roots. However, the poor attraction of T. microphasmis cannot be related to poor host suitability. Adding T. ventralis reduced shoot biomass of A. arenaria more than T. microphasmis did, whereas for C. arenaria the effect was the reverse. The interaction of the two nematodes on A. arenaria and C. arenaria shoot biomass was insignificant. However, the effect on root biomass of A. arenaria was interactive; adding T. ventralis to plants with high inoculation densities of T. microphasmis further decreased root biomass. Adding T. microphasmis further decreased root biomass of plants inoculated with low levels of T. ventralis. Depending on host plant identity, interactions between root-feeding nematodes may lead to competition or facilitation. Our results suggest that facilitation by T. microphasmis contributes to persistence of T. ventralis on C. arenaria. Thus, the population dynamics of root-feeding nematodes is influenced both by host plant identity and the presence of other root-feeding nematodes.     

Impact of soil fumigation practices on soil nematodes and microbial biomass

This study was designed to understand the impact of methyl bromide (MB) (CHaBr) and its alternatives on both free-living and root-knot nematodes in the soil. A randomized complete block experiment with six treatments and 4 replicates (each replicate in a separate greenhouse) was established in Qingzhou, Shandong Province, China. In addition to MB and untreated control (CK) treatments there were four alternative soil fumigation practices including MB virtually impermeable films (VIF), metam sodium (MS), MS VIF and soil solarization combined with selected biological control agents (SS BCA). Two tomato (Lycopersicum esculentum Mill.) cultivars, cv. Maofen-802 from the Xian Institute of Vegetable Science, China, and cv. AF179 Brillante from the Israeli Hazera Quality Seeds, were selected as test crops. The results indicated that Rhabditidae was the most dominant population with percentage abundance as high as 85% of the total number of identified free-living nematodes, followed by that of Cephalobidae. Methyl bromide and its alternatives except for the non-chemical SS BCA treatment controlled the target pest, root-knot nematodes. Also, the impact of the three chemical alternatives on free-living nematode number and functional group abundance was similar to the impact associated with a typical methyl bromide application. Chemical fumigation practices, especially that with MB, significantly reduced the number of nematodes in the soil and simultaneously significantly reduced the number of nematode genera thereby reducing nematode diversity. All the four soil chemical fumigation activities decreased soil microbial biomass and had an obvious initial impact on microorganism biomass. Furthermore, both plant-parasitic and fungivore nematodes were positively correlated with soil microbial biomass.     

Effect of the earthworm Lumbricus rubellus on the nematode community in a peat meadow soil

The effect of different densities of Lumbricus rubellus on the nematode community was studied in a field experiment. The stand, a cultivated meadow on peat-muck soil developed from moderately decomposed alder peat, was located on drained fens in the central basin of the Biebrza valley in the north-eastern part of Poland. Samples were taken from soil isolators into which 0, 2, 4 or 6 earthworms were previously introduced. The abundance and trophic structure of the nematode fauna in the different treatments were compared 30, 60, 90 and 120 d after the introduction of L. rubellus. The effect of L. rubellus on soil nematodes was most pronounced at the beginning of the experiment. Thirty days after introduction of L. rubellus, the total number of nematodes in all trials with earthworms was significantly lower than in the control, and nematode numbers decreased with increasing L. rubellus density. On later sampling dates, the results were less clear and 120 d after introduction of L. rubellus no significant effect on the total number of nematodes was observed in the treatments. The abundance of the bacterial-, fungal- and plant-feeding nematodes in the treatments with earthworms was lower than in the control 30 d after introduction of L. rubellus, but significant differences were found only in the case of bacterial feeders.     

Response of soil nematode communities to tree girdling in a subtropical evergreen broad-leaved forest of southwest China

The impact of canopy photosynthates on soil microbial biomass and nematode trophic groups was studied in a subtropical evergreen broad-leaved forest by performing a large-scale tree girdling experiment. Total fungal biomass was unaffected by tree girdling. Bacterial biomass differed significantly between the girdled and control plots in the mineral soil, but was not affected by girdling treatment in the humus layer. Girdling reduced total nematode density in the humus layer. The reduced fungivorous nematode density in girdled plots in the humus layer suggested a modified energy flow through the fungal based pathways. There were no differences in the abundance of bacterial-feeding, herbivorous and omnivorous-predatory nematodes between the girdled and control plots in both humus and mineral soil layers. This study provides direct evidence that the termination of belowground photosynthate-C allocation achieved by tree girdling affects soil nematodes, and that different trophic groups vary in their responses to the reduction of C efflux into the soil.     

Nematode communities of grazed and ungrazed semi-natural steppe grasslands in Eastern Austria

Soil nematode communities were investigated at eight semi-natural steppe grasslands in the National Park Seewinkel, eastern Austria. Four sites were moderately grazed by horses, cattle and donkeys, four were ungrazed. Nematodes were sampled on four occasions from mineral soil, and their total abundance, diversity of genera, trophic structure and functional guilds were determined. Altogether 58 nematode genera inhabited the grasslands, with Acrobeloides, Anaplectus, Heterocephalobus, Prismatolaimus, Aphelenchoides, Aphelenchus, Tylenchus and Pratylenchus dominating. Mean total abundance at sites was 185–590 individuals per 100 g soil. Diversity indices did not separate communities well, but cluster analysis showed distinct site effects on nematode generic structure. Within feeding groups the relative proportion of bacterial-feeding nematodes was the highest, followed by the fungal- and plant-feeding group. Omnivores and predators occurred in low abundance. The maturity indices and plant parasite indices were characteristic for temperate grasslands, but the abundance of early colonizers (c-p 1 nematodes) was low. A high density of fungal-feeding c-p 2 families (Aphelenchoidae, Aphelenchoididae) resulted in remarkably high channel index values, suggesting that decomposition pathways are driven by fungi. Nematode community indices of all sites pointed towards a structured, non-enriched soil food web. At most sites, grazing showed little or no effect on nematode community parameters, but total abundance was higher at ungrazed areas. Significant differences in the percentage of omnivorous nematodes, the sum of the maturity index, the number of genera and Simpson's index of diversity were found at one long-term grazed pasture, and this site was also separated by multi-dimensional scaling (MDS).     

Comparing sugarcane fields and forest fragments: the effect of disturbance on soil physical properties and nematode assemblages

Comparisons of agricultural and natural ecosystems reveal the magnitude of the effects of agricultural practices on the diversity and abundance of soil nematodes. Consequently, there is the need for testing ecological hypotheses, specifically with regard to nematode ecology, in natural and agricultural soils to seek strategies for biological control and environmental monitoring. We studied soil nematode assemblages and soil physical attributes of five soil layers (0–10, 10–20, 20–30, 30–40 and 40–50 cm) from sugarcane plantations and forest remnants in the sugarcane zone of Pernambuco State, Brazil. Structure and composition of the nematode assemblage and soil properties differed between forest and sugarcane soils, even in the same locality. The soil bulk density and the abundance of all nematodes and the diversity of plant‐parasitic nematodes were greater whereas soil porosity, soil respiration and abundance of predator nematodes were smaller in sugarcane than in forest areas. We suggest that sugarcane management practices result in changes in the soil properties and concomitantly alter the composition and structure of the nematode assemblages. Co‐inertia analysis indicated that others environmental factors also might be affecting the nematofauna.     

Strong impacts of belowground tree inputs on soil nematode trophic composition

Trees have a key role in determining the composition of soil biota via both above and belowground resource-based mechanisms, and by altering abiotic conditions. We conducted an outdoor mesocosm experiment to investigate the relative impact of above and belowground tree inputs on soil nematode trophic composition, and examine whether tree-driven impacts differed between contrasting species (birch and pine). For both species, we created a factorial design of litter addition and root presence treatments. The litter addition treatment was equivalent to natural levels of litterfall; tree saplings were planted in mesocosms for the root presence treatment and an unplanted control treatment was established that had no litter or root inputs. Litter addition had a limited impact on soil nematode community composition: it primarily decreased omnivore and predatory nematode abundance in birch but had few other effects on the nematode community. By contrast, root presence markedly altered nematode community composition through changes in a range of trophic groups. For both birch and pine, there were significant increases in total, fungivore and predatory nematode abundance in root presence treatments, and furthermore, total and fungivore abundances were positively related to root biomass. Root presence of these contrasting tree species also had a distinctive impact on some specific nematode trophic groups; pine roots promoted bacterivore abundance while birch roots promoted root-hair feeding nematode abundance. These findings suggest strong bottom-up effects of belowground tree inputs, and indicate that particular components of the nematode community may be affected differently by resource quantity and quality. Consequently, we suggest that, in the short-term, belowground rather than aboveground tree inputs have a strong impact on soil food web structure and complexity.     

不同细菌饲喂细菌线虫对土壤细菌数量、活性以及群落结构的影响

The effects of bacterial-feeding nematodes on bacterial number, activity, and community composition were studied through a microcosm experiment using sterilized soil inoculated with soil bacteria （soil suspension） and with bacteria and three species of bacterial-feeding nematodes （ Cephalobus persegnis, Protorhabditis filiformis, and Caenorhabditis elegans）. Catalyzed reporter deposition-fluorescence in situ hybridization, CO2 evolution, and denaturing gradient gel electrophoresis （DGGE） of PCR ampli- fied 16S rRNA gene fragments were used to investigate bacterial numbers, antivity, and community composition, respectively. Our results showed that bacterial numbers and activity significantly increased in the presence of bacterial-feeding nematodes, which indicated that bacterial-feeding nematodes had a significant positive effect on soil bacteria. The different nematode species had different effects on bacterial numbers and activity. C. persegnis and P. filiformis, isolated from native soil, increased the bacterial number and activity more than C. elegans. The DGGE analysis results showed that dominant bacterial species significantly differed among the treatments, which suggested that bacterial-feeding nematode species modified the bacterial community composition in soil. Further gene sequence analysis results showed that the dominant bacterial species in this study were gram-negative bacteria. Given the completely same conditions except nematode species, the varied selective feeding behavior of different nematode species was the most likely reason for the altered bacterial community composition. Overall, the alteration of bacterial numbers, activity and community composition resulting from the bacterial-feeding nematodes may ult!mately affect soil ecological functioning and processes.     

Niche partitioning based on soil type and climate at the landscape scale in a community of plant-feeding nematodes

Understanding how environmental factors structure communities is important in conservation biology and ecosystem management. The aim of this study was to test the hypothesis that a plant-feeding nematode community composed of six species is structured by soil type and climate at the landscape scale, and that niche partitioning via these factors is consistent with the coexistence of the species. Martinique has an impressive diversity of abiotic factors (climate and soil type) over a relatively small land area, which facilitates the study of how soil type and climate affect the nematode community.We conducted this study by building an extensive data set containing the abundance of each nematode species on banana (3708 samples and 5,673,705 nematodes) in a wide range of sites in Martinique. The data set also contained environmental data (soil, climate) and plantation age. We analyzed the response of each nematode species to climate and soil type with a generalized linear model in order to understand whether niche partitioning of factors could contribute to the coexistence of the nematode species.Temperature, rainfall, soil type, and plantation age significantly affected the abundance of the six nematode species. While some pairs of species shared the same environmental niches, other showed clear niche partitioning along climatic axes. The two dominant species, Radopholus similis and Helicotylenchus multicinctus, have similar convergent ecological niches regarding climate, soil type, plantation age, and host range. These two species, which often co-occur, probably have different resources at the root scale. Soil type and climate structure plant-feeding nematode species community at the island scale. Further studies need to evaluate coexistence at the root scale among dominant species.     

Birch invasion of heather moorland increases nematode diversity and trophic complexity

To determine whether successional changes in plant communities may influence belowground community structure, we quantified nematode abundance, trophic structure and diversity along two separate chronosequences from heather moorland to birch woodland in the Scottish uplands. Tree invasion markedly altered plant community composition, and hence litter inputs, both directly, and indirectly through changes in understorey species. In turn, these changes in detrital inputs were reflected in consistent changes in nematode community structure. Nematode abundance increased from moorland to birch woodland, with moorland soils being dominated by a few taxa, notably root-hair and fungal feeders, compared to the more diverse composition of the birch woodland soils. Trophic structure was altered through an increase in the abundance of bacterial feeding relative to fungal-feeding nematodes, and an increase in the abundance of predatory nematodes. The increase in predators during the succession from moorland to woodland was associated with an increase in soil pH, highlighting that not only changes in the plant community, but also changes in soil properties associated with tree invasion may influence soil nematodes. Nematode diversity increased from moorland to birch woodland, with nematode richness being positively related to both plant species richness and soil pH. These results suggest that trees may control soil community structure through the manipulation of resources and the soil physico-chemical environment, promoting greater nematode diversity and trophic complexity.     

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.