The influence of the root-knot nematode Meloidogyne incognita, the nematicide aldicarb and the nematophagous fungus Pochonia chlamydosporia on heterotrophic bacteria in soil and the rhizosphere

Plant‐pathogenic nematodes are a major cause of crop damage worldwide, the current chemical nematicides cause environmental damage, but alternatives such as biological control are less effective, so further understanding of the relationship between nematodes, nematicides, biological control agents and soil and rhizosphere microorganisms is needed. Microbial populations from roots of cabbage and tomato plants infested with the root‐knot nematode Meloidogyne incognita were compared with those from plants where the nematode was controlled by the nematicide aldicarb, or a nematophagous fungus with biological control potential, Pochonia chlamydosporia. The total numbers of culturable bacteria and fungi in rhizosphere soil were similar in all three treatments for both plants, around 100‐fold more than in control soil in which there were no plants. However, there were clear differences in the catabolic diversity, assessed by Biolog EcoPlate? carbon substrate utilization assays, between microbial populations from unplanted soil and the rhizosphere. In cabbage, a poor host for M. incognita, the rhizosphere population from P. chlamydosporia‐treated plants was distinct from the population from untreated and aldicarb‐treated plants. In tomato, a host susceptible to the nematode, the catabolic diversity of populations from aldicarb‐ and P. chlamydosporia‐treated plants was similar and differed from the untreated, nematode‐infested plants. The genetic diversity of the fast‐growing heterotrophic bacteria in the tomato rhizosphere, indicated by PCR fingerprinting with ERIC primers, was very different in the infested roots, whereas the profiles of isolates from both aldicarb‐ and P. chlamydosporia‐treated roots were similar. Evidently, nematodes have a greater impact on the rhizosphere population of a susceptible host, tomato, than a poor one, cabbage, and nematode‐infested roots are colonized by a different subpopulation of soil microbes from that on plants where infection is controlled, illustrating differences in root morphology and physiology.     

Effects of a plant parasitic nematode (Heterodera trifolii) on clover roots and soil microbial communities

We studied the effects of the root endoparasitic nematode Heterodera trifolii on rhizodeposition and the root architecture of white clover (Trifolium repens). Rhizosphere solutions were collected from the root systems of plants growing with and without H. trifolii (200 juveniles per inoculated plant) in sand-based microlysimeters. The organic carbon (C) content of these solutions was analyzed, and they were applied to plant-free soils to investigate microbial responses. Although plant biomass was unaffected by nematodes, the architecture of the root systems was significantly altered, with a decrease in overall root length and an increase in the density of lateral branches from the primary root. The presence of nematodes reduced the concentration of organic compounds in the rhizosphere solutions but only on the final sampling date (75 days). Analysis of microbial signature phospholipid fatty acids revealed no change in the structure of the microbial communities in soils to which rhizosphere solutions were applied. However, these microorganisms did respond with changes in substrate utilization patterns (community-level physiological profiles). Microbes in soils that received rhizosphere solutions from the nematode-infected clover showed lower utilization of most substrates but higher utilization of oligosugars. These responses appear to be related to changes in roots and rhizodeposition associated with nematode infection of clover roots. The results of this study suggest that root herbivory can negatively impact carbon-limited soil microbial communities via changes in root architecture that moderate rhizodeposition.     

Do bacterial-feeding nematodes stimulate root proliferation through hormonal effects?

We prepared soil with greater populations of bacterial-feeding nematodes either by stimulating the native populations of the soil, adding an additional mixed community of nematodes, or by adding Caenorhabditis elegans, to investigate the effects of bacterial-feeding nematodes on root morphology, soil auxin (indolyl-3-acetic acid—IAA) concentrations and microbial community structure. In the presence of enhanced bacterial-feeding nematode populations, tomato plants had a more highly branched root system with longer and thinner roots. Root system development was greater with native nematodes than C. elegans. The changes of root morphology were accompanied by an increase of soil IAA content and an altered microbial community structure. Bacterial-feeding nematodes may have affected plant growth by stimulating hormone production through grazing-induced changes to the soil microbial 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.     

Arbuscular mycorrhizal fungi and nematodes are involved in negative feedback on a dual culture of alfalfa and Russian wildrye

The complex biological interactions taking place in soil–plant systems may sometimes alter the functioning of an ecosystem. We examined the relationship between arbuscular mycorrhizal (AM) root colonization, nematode populations, and plant competition in an 8-year-old field experiment comparing alfalfa monoculture to dual culture of alfalfa (Medicago sativa L.) and Russian wildrye (Psathyostachys juncea Fisch. Nevski) (RWR) grown under different soil P fertility levels, in a Brown Chernozemic soil in Saskatchewan. The experiment included three P rates: 0, 20 and 40 kg P₂O₅ ha⁻¹ (0P, 20P and 40P) applied annually and was sampled three times during the cropping season: 30 June, 1 September and 30 September. Higher AM symbiotic development compensated for reduced soil P fertility in alfalfa stands without RWR and forage dry matter yield was not affected by P rates. But in the presence of RWR, reduced soil P fertility at 0P and 20P led to forage yield reduction. Fertilization treatments modified the soil microbial community structure only in the presence of RWR, as revealed by discriminant analysis of the profiles of microbial phospholipids fatty acid in soil lipid extracts and functional nematode groups. Arbuscular mycorrhizal root colonization level was reduced with P fertilizer both in the presence and absence of RWR. In the presence of RWR, lower plant AM root colonization was concurrent with higher abundance of total, fungivorous and omnivorous nematodes. Our results are consistent with a model of negative feedback from the plant-associated soil microflora where the presence of RWR increased the population of fungivorous nematodes and grazing of AM hyphae. Negative impacts were larger in low P fertility soils promoting AM symbiotic development. The unexpected decrease in alfalfa–RWR dual culture yield under 0P and 20P fertilization levels was attributed to a carbon drain created by enhanced nematode feeding on AM fungi in the presence of RWR.     

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

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

Synergists and antagonists in the rhizosphere modulate microbial communities and growth of Quercus robur L.

The synergistic and antagonistic interactions among biotic components in the rhizosphere play a crucial role in plant defence against soil-borne pathogens. We investigated if the rhizosphere helper bacterium Streptomyces sp. AcH 505 (HB) indirectly protects the plant from the parasitic nematode Pratylenchus penetrans by modifying the rhizosphere microbial community structure and whether these interactions are dependent on the growth stage of oaks. Changes in the abundance of Streptomyces sp. AcH 505 and the phospholipid fatty acid (PLFA) composition of the rhizosphere soil as well as oak shoot and root biomass were assessed. Investigated were the bud resting stage A and the bud swelling stage B with maximal root elongation of oak microcuttings at two successive harvest times. The deleterious effect of P. penetrans on oak biomass was dependent on plant development, being limited to oak microcuttings growing at the stage B. In comparison to control and HB inoculated soils, shoot biomass decreased by about 33% and 41%, and root biomass by about 33 and 48%, respectively. The antagonistic effect of Streptomyces against the nematode was linked to shifts in the rhizosphere microbial community. The Streptomyces AcH505 strain promoted growth of oak microcuttings at bud swelling stage B during maximal root elongation and enhanced the abundance of saprophytic and ectomycorrhizal fungi in the rhizosphere by 158% with respect to controls. Our results highlight the importance of Streptomyces for counteracting the damage of nematodes and promoting plant growth in natural ecosystems such as forests.     

连作番茄根区病土对番茄生长及土壤线虫与微生物的影响

探索连作番茄根区病土对番茄根结线虫病的诱导效果及引起连作障碍的微生态机制,可为深入了解番茄连作障碍发生机理及探究番茄连作障碍防治方法提供科学依据。本研究利用盆栽试验,测定了番茄在健康土壤及接种病土土壤中生物学特性变化及根结线虫侵染状况,并分析鉴定了土壤中微生物及线虫的种类与数量。结果表明,接种连作番茄根结线虫病株根区病土会对番茄生长及根结线虫侵染产生影响:1)番茄苗期根系根结数达9个?株~(-1),健康土壤无根结;土壤线虫数量较健康土壤增加390.4%;收获期番茄根结线虫侵染率达62.7%,病情指数为80.0%。2)番茄生长受到抑制,叶片防御酶活性降低,收获期茎叶及根系鲜质量较健康土壤分别减少50.2%及33.1%,苗期番茄叶片PPO活性较健康土壤降低15.8%,POD活性较健康土壤增加24.0%,差异均达显著水平(P0.05)。3)番茄根系更易感染有害菌,根系内病原菌甘蓝假单胞菌数量较健康土壤增加463倍,根区土壤细菌、真菌及放线菌总数分别增加46.3%、94.5%及134.0%。4)食细菌线虫、食真菌线虫及植物寄生性线虫数量分别为健康根区土壤的3.3倍、1.6倍及7.3倍,其中的植物寄生线虫95.6%为根结线虫。综上所述,接入连作番茄根结线虫病株根区病土不仅导致番茄遭受根结线虫侵染,而且会导致土壤线虫总量及植物寄生线虫所占比例大幅增加,并使番茄根系内有害细菌数量显著增加,对番茄生长造成显著抑制作用,同时影响番茄的生理生化特性,受线虫侵染番茄防御性酶活性降低,使其更易被根结线虫及病原菌侵染,番茄根区土壤线虫、微生物及根系内优势细菌的种类与数量及其之间的作用发生改变。     

Effects of pine roots on microorganisms,fauna, and nitrogen availability in two soil horizons of a coniferous forest spodosol

Summary We investigated the effects of pitch pine seedling roots on extractable N, microbial growth rate, biomass C and N, and nematodes and microarthropods in microcosms with either organic (41% C, 1.14% N) or mineral (0.05% C, 0.01% N) horizon soils of a spondosol. Root quantity was manipulated by varying plant density (0, 1, 2, or 4 seedlings) and rhizosphere soil was separated from non-rhizosphere soil by a 1.2 m mesh fabric. In the rhizosphere of organic soil horizons, moisture, microbial growth rate, biomass C and N, and extractable N declined as root density was increased, but there was little effect on nematodes or microarthropods. High levels of extractable N remained after 5 months, suggesting that N mineralization was stimulated during the incubation. In the rhizosphere of mineral soil horizons, microbial growth rate, and nematode and microarthropod abundances increased at higher root density, and in the absence of roots faunal abundance approached zero. Faunal activity was concentrated in the rhizosphere compared to non-rhizosphere soil. In organic soil horizons, roots may limit microbial activity by reducing soil moisture and/or N availability. However, in mineral soil horizons, where nutrient levels are very low, root inputs can stimulate microbial growth and faunal abundance by providing important substrates for microbial growth. Our results demonstrate a rhizosphere effect for soil fauna in the mineral soil, and thus extends the rhizosphere concept to components of the soil community other than microbes for forest ecosystems. Although our results need to be verified by field manipulations, we suggest that the effects of pine roots on nutrient cycling processes in coniferous forests can vary with soil nutrient content and, therefore, position in the soil profile.     

Soil bacterial community response to sulfadiazine in the soil–root zone

Sulfonamide antibiotics reach soil via manure and adversely affect microbial diversity. Clear effects of these bacteriostatic, growth‐inhibiting antibiotics occur in the presence of a parallel input of microbial activity stimulating manure. Natural hot spots with already increased soil microbial activity are located in the rhizosphere, comprising microorganism such as Pseudomonas with plant growth promoting and pathogenic strains. The hypothesis was therefore that the antibiotic activity of sulfonamides is promoted in the rhizosphere even in the absence of manure, followed by shifts of the natural plant‐specific microbial community structure. This was evaluated by a laboratory experiment with Salix fragilis L. and Zea mays L. After 40 d of incubation, sub‐areas such as non‐rhizosphere soil, rhizosphere soil and plant roots were sampled. Effects on microbial community structure were analyzed using 16S rRNA gene fragment patterns of total bacteria community and Pseudomonas. Selected exoenzymes of N‐, P‐, and C‐cycling were used to test effects on microbial functions. Compared to the factors soil sub‐area and sulfadiazine (SDZ) content, plant species had the largest influence on the bacterial community structure and soil exoenzyme activity pattern. This was also reflected by an up to 1.5‐fold higher acid phosphatase activity in samples from maize‐ compared to willow‐planted soil. We conclude that antibiotic effects on the bacterial community structures are influenced by the antibiotic concentration and root influence.     

Site condition, fertility gradients and soil biological activity in a New Zealand frost-flat heathland

Gradients in stressed areas potentially provide a powerful tool to interpret relations between soil biodiversity and site quality. We measured soil chemistry, soil microbiology and nematodes along three transects representing a fertility gradient and at a disturbed site near a road in a Dracophyllum subulatum-dominated shrubland in which frosts are a major factor in preventing succession to forest; we used D. subulatum size as a site-quality index. Significant correlations between both shrub height and shrub growth rate and volumetric measures of total soil phosphorus and anaerobically mineralisable nitrogen indicate that nitrogen and phosphorus regulate plant growth. Microbial biomass and total nematode abundance significantly increased with greater plant growth, presumably in response to greater litter input. Conversely, neither heterotrophic microbial diversity nor nematode diversity was correlated with shrub performance along the transects. Litter was from a single species and thus likely similar in quality so changes in microbial or nematode diversity might not be expected. In this oligotrophic environment, nutrient levels were not only the important regulators of plant growth but also appeared to have an indirect influence on the size of the microbial and nematode populations.     

Effects of chitin amendment of soil on microorganisms,nematodes, and growth of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.)

Effects of soil amendment with crabshell chitin on the growth of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.), and on populations of soil bacteria, fungi, and plant-parasitic and free-living nematodes were investigated in a pot trial. Five soil samples were collected from Te Puke (Paengaroa Shallow Sand, a Typic Hapludand) and five from Hamilton (Bruntwood silt loam, an Aquic Hapludand), New Zealand. Subsamples of each soil were either amended with chitin or unamended and planted with white clover and ryegrass. The ryegrass shoot weight in amended soil was greater (P<0.01), most probably due to N mineralised from chitin. A significantly lower (P<0.01) root: shoot ratio of ryegrass in the amended soil also suggested improved N availability, and therefore less root mass was needed to support a given shoot mass. A reduction in nodulation was observed in 12-day-old white clover seedlings (P<0.05) and also in 6-week-old seedlings (P<0.01). The shoot weight of white clover was significantly lower (P<0.05) in amended soil, possibly due to phytotoxic effects of chitin. Chitin increased (P<0.01) the populations of bacteria and fungi by 13-fold and 2.5-fold, respectively. The cyst nematode of white clover, Heterodera trifolii, was significantly reduced in chitin-amended soil, possibly due to increased levels of chitinase produced by rhizosphere microorganisms. Two other plant-parasitic nematodes, Pratylenchus spp. and Tylenchus spp., were also reduced in ryegrass roots and in soil as a result of the chitin amendment. However, the total number of free-living nematodes increased 5.4-fold in amended soil.     

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.     

Exploring patterns of Camellia seed cake application in relation to plant growth,soil nematodes and microbial biomass

ABSTRACT

The suppression of plant-parasitic nematodes is crucial for maintaining the worldwide development of the banana industry. In this study, different application patterns of Camellia seed cake previously reported to suppress root-knot nematode were conducted to manage pests and promote banana seedling growth. The results demonstrated seven days delay before transplanting was necessary after Camellia seed cake application. The dose 5 g/kg soil resulted in best plant growth promotion performance, which increased banana seedling height, stem diameter, shoot, and root fresh weight by upto 29%, 27%, 47%, and 21%, respectively. Plastic film mulching was beneficial when high amount (2%) of Camellia seed cake was added. The application of Camellia seed cake increased nutrient potassium amounts; the abundance of soil free-living nematodes, especially bacterivores; and the abundance of soil microbes and the soil catalase activity, while reduced plant-parasitic nematodes amounts. Further correlation analysis between the soil nematodes and microbial abundance showed that plant-parasite numbers had significant negative correlations with the bacterial biomass and a portion of the fungal biomass; bacterivores had significant positive correlations with the bacterial biomass; and omnivores had significant correlations with the bacterial biomass and fungal biomass. A fundamental challenge of root-knot nematode control is to sustain ecological services without losing biodiversity. This study provided an environmentally friendly strategy based on Camellia seed cake to regulate the soil health and quality.     

Nematode community structure as a sensitive indicator of microbial perturbations induced by a genetically modified Pseudomonas fluorescens strain

The effects of seed inoculation with the Pseudomonas fluorescens strains F113lacZY [a genetically marked biocontrol agent producing the anti-fungal agent 2,4-diacetylphloroglucinol (DAPG)] and F113G22 [a genetically modified (GM) derivative strain of F113lacZY incapable of producing DAPG] on associated nematode communities were investigated over 17 days of plant growth. Plant growth measurements and colony forming unit counts (CFU) derived from rhizosphere soil indicated only small and transient perturbations as a result of introductions of the GM bacteria. Total nematode numbers were increased significantly in the rhizosphere of inoculated plants compared with the non-inoculated control treatments. These increases were mainly due to increases in bacterial feeding nematodes. This indicates that inoculation with the GM P. fluorescens strains induced high bacterial growth rates in the rhizosphere of plants inoculated with these strains. No indication of greater root colonisation by fluorescent Pseudomonas spp. could be found using CFU counts on Pseudomonas-selective media. Numbers of fungal feeding nematodes decreased initially, probably as a result of lack of intact hyphae in the soil. However, inoculation with the two different GM P. fluorescens strains resulted in a rapid recovery of fungal feeding nematode populations, whereas in the non-inoculated control populations of fungal feeding nematodes remained small. This result is surprising as one of the strains (F113lacZY) produces the anti-fungal agent DAPG and it would be expected that this agent would result in a decrease in fungal activity.     

Influence of bacterial-feeding nematodes (Cephalobidae) on soil microbial communities during maize growth

The effect of several bacterial-feeding nematodes of the Cephalobidae family (Zeldia punctata, Acrobeloides nanus and Cephalobus pseudoparvus) on the microbial community of a Sahelian soil (Senegal) was investigated in microcosm. The consequences of the activity of these nematodes on the growth and nitrogen nutrition of young maize plants (aerial biomass, root biomass and nitrogen content) were also estimated. Laboratory-cultured nematodes were inoculated into soil containing maize seedlings where the natural nematofauna had been previously eliminated by alternately freezing and defrosting (five cycles). The microbial compartment of the soil community was characterised through total microbial biomass (using fumigation-extraction), density of bacteria (using colony forming units counts), microbial activity (using alkaline phosphatase) and genetic structure of soil microbial community (using denaturing gradient gel electrophoresis) at sowing and at 12, 26 and 47 days after planting. Final nematode densities in the different treatments (between 4 and 20 Ind g⁻¹ dry soil) demonstrated a high level of reproduction. The different types of nematodes tested induced similar trends in changes in the microbial pool of the soil and in maize seedling growth. Compared to control soils, the presence of nematodes led to an increase (+12%) in plant biomass and reduced concentrations of soil ammonium but had no effect on concentrations of nitrate by the end of the experiment. Sixty-three percent of the inorganic nitrogen initially present in the soil was incorporated into the maize plants with nematodes whereas only 47% was incorporated without nematodes. Nematode activity led to a significant decrease in microbial biomass (−28%) and density of cultivable bacteria (−55%), however, nematodes stimulated bacterial activity (+18%). The effects of Z. punctata were weakest compared to A. nanus and C. pseudoparvus. The presence of nematodes modified the genetic structure of the microbial community essentially by changing the relative abundance of dominant bacterial populations. Among nematode species tested, A. nanus modified the structure of the microbial communities the most compared with control soils without nematodes. Overall, results from this study provide evidence for the ability of microbial feeding nematodes to alter microbial activity, microbial community structure, nitrogen mineralisation and growth of maize seedlings in a Sahelian soil from Senegal, West Africa.     

Pressure plate studies to determine how moisture affects access of bacterial-feeding nematodes to food in soil

Nematode activity in the soil depends on the presence of free water. We conducted pressure plate experiments to understand better how soil matric potential and structural degradation affect the population growth of three bacterial‐feeding nematodes (Cephalobus, Pristionchus, Rhabditis). We took undisturbed cores from six soils (sand, silt loam and silty clay loam with four management regimes), and removed all fauna from them. Ten or 30 nematodes were added, and pressures corresponding to soil matric potentials of ?10, ?33, ?50, ?100 or ?1500 kPa were applied for 35 days. The nematodes were then counted. Significant reproduction of all bacterial‐feeding nematodes occurred when the diameters of water‐filled pores were approximately 1 μm. This confirms observations using repacked soils and field manipulations. Only for Pristionchus did declining populations match the reduction in total soil porosity related to intensification of land use on the silty clay loam. We had not expected Cephalobus to have the fastest increase in population of the three nematodes in intact soil cores, and our evidence questions the relative importance given to the three nematode families in soil processes. The differing rates of population increase of the three nematodes in the various soils reflect both habitable pore space and trophic interactions. This suggests that the very diversity of nematode assemblages is crucial in the resilience of biological soil processes. That water‐filled pores as small as 1 μm provide suitable spaces for sizeable populations of bacterial‐feeding nematodes accords with the observed migration of infective juveniles of trichostrongylid nematodes and mermithids in water films on herbage. Our results imply that assessment of the role of nematodes in soil processes may be a key to the understanding of biological interactions in water films, and the selection pressures on nematode morphology.     

Determination of the impact of continuous defoliation of Lolium perenne and Trifolium repens on bacterial and fungal community structure in rhizosphere soil

To determine the effects of defoliation on microbial community structure, rhizosphere soil samples were taken pre-, and post-defoliation from the root tip and mature root regions of Trifolium repens L. and Lolium perenne L. Microbial DNA isolated from samples was used to generate polymerase chain reaction–denaturing gradient gel electrophoresis molecular profiles of bacterial and fungal communities. Bacterial plate counts were also obtained. Neither plant species nor defoliation affected the bacterial and fungal community structures in both the root tip and mature root regions, but there were significant differences in the bacterial and fungal community profiles between the two root regions for each plant. Prior to defoliation, there was no difference between plants for bacterial plate counts of soils from the root tip regions; however, counts were greater in the mature root region of L. perenne than T. repens. Bacterial plate counts for T. repens were higher in the root tip than the mature root region. After defoliation, there was no effect of plant type, position along the root or defoliation status on bacterial plate counts, although there were significant increases in bacterial plate counts with time. The results indicate that a general effect existed during maturation in the root regions of each plant, which had a greater impact on microbial community structure than either plant type or the effect of defoliation. In addition there were no generic consequences with regard to microbial populations in the rhizosphere as a response to plant defoliation.     

Spatially tracking carbon through the root–rhizosphere–soil system using laser ablation‐IRMS

The intimate relationships between plant roots, rhizosphere, and soil are fostered by the release of organic compounds from the plant into soil through various forms of rhizodeposition and the simultaneous harvesting of nutrients from the soil to the plant. Here we present a method to spatially track and map the migration of plant‐derived carbon (C) through roots into the rhizosphere and surrounding soil using laser ablation‐isotope ratio mass spectrometry (LA‐IRMS). We used switchgrass microcosms containing soil from field plots at the Kellogg Biological Station (Hickory Corners, Michigan, USA) which have been cropped with switchgrass since 2008. We used a ¹³CO₂ tracer to isotopically label switchgrass plants for two diel cycles and tracked subsequent movement of labeled C using the spatially specific (< 100 µm resolution) δ¹³C analysis enabled by LA‐IRMS. This approach permitted assessment of variable C flow through different roots and enabled mapping of spatial variability of C allocation to the rhizosphere. Highly ¹³C‐enriched C (consistent with production during the ¹³CO₂ application period) extended ≈ 0.5–1 mm from the root into the soil, suggesting that the majority of recent plant‐derived C was within this distance of the root after 48 h. Tracking the physical extent of root exudation into the rhizosphere can help evaluate the localization of plant‐microbe interactions in highly variable subsurface environments, and the use of the isotopic label can differentiate freshly fixed C (presumably from root exudates) from other types of subsurface C (e.g., plant necromass and microbial turnover). The LA‐IRMS technique may also serve as a valuable screening technique to identify areas of high activity for additional microbial or geochemical assays.     

Continuous defoliation of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) and associated changes in the composition and activity of the microbial population of an upland grassland soil

A microcosm study was conducted to investigate the effect of continuons plant defoliation on the composition and activity of microbial populations in the rhizosphere of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). Continuons defoliation of ryegrass and clover resulted in sigmficant (P <0.01) increases in soil microbial biomass, although whilst increases were measured from day 2 in soil sown with clover significant increases were only seen from day 21 in soil sown with ryegrass. These increases were paralleled, from day 10 onwards, by increases in the numbers of culturable bacteria. Numbers ofPsendomonas spp. also increased in the later stages of the study. No influence on culturable fungal populations was detected. Whilst shifts in the composition of the microbial populations were measured in response to defoliation there was little effect on microbial activity. No changes in either dehydrogenase activity or microbial respiration in the rhizosphere of ryegrass or clover were measured in response to defoliation, but both dehydrogenase activity and microbial respiration were greater in ryegrass than clover when values over the whole study were combined. Continuous defoliation resulted in significant (P <0.001) reductions in the root dry weight of ryegrass and clover, of the order 19% and 16%, respectively.