Cécile Villenave

Rhizosphere fauna: the functional and structural diversity of intimate interactions of soil fauna with plant roots

For decades, the term “rhizosphere fauna” has been used as a synonym to denote agricultural pests among root herbivores, mainly nematodes and insect larvae. We want to break with this constrictive view, since the connection between plants and rhizosphere fauna is far more complex than simply that of resource and consumer. For example, plant roots have been shown to be neither defenceless victims of root feeders, nor passive recipients of nutrients, but instead play a much more active role in defending themselves and in attracting beneficial soil microorganisms and soil fauna. Most importantly, significant indirect feed-backs exist between consumers of rhizosphere microorganisms and plant roots. In fact, the majority of soil invertebrates have been shown to rely profoundly on the carbon inputs from roots, breaking with the dogma of soil food webs being mainly fueled by plant litter input from aboveground. In this review we will highlight areas of recent exciting progress and point out the black boxes that still need to be illuminated by rhizosphere zoologists and ecologists.

Changes in nematode communities following cultivation of soils after fallow periods of different length

Abstract The composition of the nematofauna was studied in four soils that differed in the length of fallow restoration period since previous cultivation. The longest fallow period was 21 years. Plots were sampled for 2 years after starting cultivation of the fallow soils. The treatments were ranked following a restoration–exploitation gradient depending on fallow duration and the number of years of millet cultivation after fallow clearing; components of the nematofauna were analysed for correlation with this ranking. The nematode community structures at the first date of sampling during cultivation clearly reflected the length of the fallow period. Nematode community structures in the fallow soils rapidly approached those in the continuously cultivated soil; they were hardly distinguishable during the second year of cultivation. One-third of the recorded nematode taxa exhibited pronounced responses to the cultivation. Mononchidae, Anatonchidae, Tylencholaimoidea, Acrobeles , Pseudacrobeles , Tylenchidae and Helicotylenchus preferred sites of more mature successional status, while Dorylaimoidea, Tylenchorhynchus and Rhabdolaimidae dominated the cultivated sites. The maturity index (MI) did not distinguish the management regimes. The plant parasite index (PPI) tended to decrease with higher restoration status linked to greater abundance of the Tylenchidae in these situations. The decrease of fungal to bacterial feeders reflected a decreasing importance of the fungal decomposition pathway after resuming cultivation.

Changes in nematode communities after manuring in millet fields in Senegal

Changes in the soil nematofauna community structure were followed in nine millet fields on seven farms in two villages in Senegal during one cropping cycle. Cultivation practices were done by field owners. One plot in each field was divided into two subplots; in one of these, manure (20 t ha-1) was added at sowing. Before the manure input, at mid-cycle and at millet harvest, the structure of the nematode fauna was studied. Soil physico-chemical characteristics, microbial carbon and plant production were measured at sowing and at millet harvest. In the sub-plots where manure was added, millet yield increased by 155%, the mineral nitrogen content of the soil increased by about 45%, while nitrogen flux increased by 150% and microbial biomass by 65%. The significant enrichment of soil by manure led to a 75% increase in total nematode population density at mid-cycle and to a 30% increase at harvest time. The density of opportunistic bacterial-feeding and fungal-feeding nematodes was significantly larger with than without manure. This result is similar to those of comparable studies in temperate areas; however the relative abundance of enrichment opportunists was extremely low with regard to that found under similar conditions in temperate ecosystems. Furthermore, the abundance of the c-p 2 bacterialfeeding nematodes, belonging mainly to the family Cephalobidae, was strongly correlated with soil microbial biomass. The other c-p feeding guilds showed no correlation with nitrogen flux, or soil microbial biomass.

Ecological importance of soil bacterivores for ecosystem functions

BackgroundBacterivores, mostly represented by protists and nematodes, are a key component of soil biodiversity involved in soil fertility and plant productivity. In the current context of global change and soil biodiversity erosion, it becomes urgent to suitably recognize and quantify their ecological importance in ecosystem functioning.ScopeUsing meta-analysis tools, we aimed at providing a quantitative synthesis of the ecological importance of soil bacterivores on ecosystem functions. We also intended to produce an overview of the ecological factors that are expected to drive the magnitude of bacterivore effects on ecosystem functions.ConclusionsBacterivores in soil contributed significantly to numerous key ecosystem functions. We propose a new theoretical framework based on ecological stoichiometry stressing the role of C:N:P ratios in soil, microbial and plant biomass as important parameters driving bacterivore-effects on soil N and P availability for plants, immobilization of N and P in the bacterial biomass, and plant responses in nutrition and growth.

Successional trends in the characteristics of soil nematode communities in cropped and fallow lands in Senegal (Sonkorong)

Soil nematode communities in the 0‐15 cm soil layer are used as indicators for describing the processes of fallow succession in the semi-arid zone of West Africa (Senegal). Abundance of plant feeding nematodes, non-plant feeding nematodes, plant parasite index (PPI), species richness and Shannon evenness of plant parasitic nematodes were measured at five stages of succession: fields, early (1‐3 years), intermediate (8‐10 years), old (18‐20 years) fallows, and forest stage. These nematological indexes were analyzed simultaneously by multivariate analysis to show the indicative properties of nematode communities. Overall, changes in abundance of nematode groups, PPI and diversity, show continuous trends from early fallow to mature stages of the succession; these trends parallel theoretical trends in secondary succession. In addition, soil nematode parameters were meaningful and expressed interactions of various uncontrolled factors with successional processes, such as environmental conditions or cropping history. During the 3 years of the survey, the structure of nematode communities, described by index analysis, showed little temporal change and supported the use of nematodes as stable indicators. ©2000 Elsevier Science B.V. All rights reserved.

Phosphorus acquisition from phytate depends on efficient bacterial grazing, irrespective of the mycorrhizal status of Pinus pinaster

Background and aimsPhosphorus from phytate, although constituting the main proportion of organic soil P, is unavailable to plants. Despite the well-known effects of rhizosphere trophic relationships on N mineralization, no work has been done yet on P mineralization. We hypothesized that the interactions between phytate-mineralizing bacteria, mycorrhizal fungi and bacterial grazer nematodes are able to improve plant P use from phytate.MethodsWe tested this hypothesis by growing Pinus pinaster seedlings in agar containing phytate as P source. The plants, whether or not ectomycorrhizal with the basidiomycete Hebeloma cylindrosporum, were grown alone or with a phytase-producing bacteria Bacillus subtilis and two bacterial-feeder nematodes, Rhabditis sp. and Acrobeloides sp. The bacteria and the nematodes were isolated from ectomycorrhizal roots and soil from P. pinaster plantations.ResultsOnly the grazing of bacteria by nematodes enhanced plant P accumulation. Although plants increased the density of phytase-producing bacteria, these bacteria alone did not improve plant P nutrition. The seedlings, whether ectomycorrhizal or not, displayed a low capacity to use P from phytate.ConclusionsIn this experiment, the bacteria locked up the phosphorus, which was delivered to plant only by bacterial grazers like nematodes. Our results open an alternative route for better utilization of poorly available organic P by plants.

Impact of direct seeding mulch-based cropping systems on soil nematodes in a long-term experiment in Madagascar

The objective of this work was to assess the effects of conventional tillage and of different direct seeding mulch-based cropping systems (DMC) on soil nematofauna characteristics. The long-term field experiment was carried out in the highlands of Madagascar on an andic Dystrustept soil. Soil samples were taken once a year during three successive years (14 to 16 years after installation of the treatments) from a 0-5-cm soil layer of a conventional tillage system and of three kinds of DMC: direct seeding on mulch from rotation soybean-maize residues; direct seeding of maize-maize rotation on living mulch of silverleaf (Desmodium uncinatum); direct seeding of bean (Phaseolus vulgaris)-soybean rotation on living mulch of kikuyu grass (Pennisetum clandestinum). The samples were compared with samples from natural fallows. The soil nematofauna, characterized by the abundance of different trophic groups and indices (MI, maturity index; EI and SI, enrichment and structure indices), allowed the discrimination of the different cropping systems. The different DMC treatments had a more complex soil food web than the tillage treatment: SI and MI were significantly greater in DMC systems. Moreover, DMC with dead mulch had a lower density of free-living nematodes than DMC with living mulch, which suggested a lower microbial activity.

How are nematode communities affected during a conversion from conventional to organic farming in southern French vineyards

The rate of conversion from conventional vineyards to organic farming practices is increasing. Organic farming improves some soil properties, although some organic practices have negative effects on soils. The objective of this work was to study the long-term effects of organic farming through the use of soil nematodes as bio-indicators of soil processes. Our experimentation was conducted in a commercial vineyard where plots belonged to two types of viticulture: conventional viticulture and organic viticulture (for 7, 11 and 17 years). The nematode community structure and nematode indices were determined. The main result was that organic practices increased soil nematode density. An increase in the available resources, as measured by a higher enrichment index (EI), led to an increase in the microbial feeder density and mainly opportunistic fungal-feeding nematodes. A greater density of plant-feeding nematodes was attributed to the presence of a grass cover. The functioning of the soil was shifted with the decomposition channel of the soil organic matter becoming more fungal than bacterial. Even though changes were observed in the nematode community structure following the conversion, the maturity index (MI), the plant-parasitic index (PPI) and the structure index (SI) remained constant. Consequently, the organic practices did not improve the soil food web length or complexity even though the biological activity, as measured by microbial biomass and total nematode density, increased.

Microcosm experiments on the development of different plant parasitic nematode fauna in two soils from the Soudanese-Sahelian zone of West Africa

Abstract To test the hypothesis that the structure of plant parasitic nematode communities is affected by soil characteristics, experiments were conducted in a greenhouse with two soils with different physical and chemical characteristics and land management histories (fallow and a cultivated field) from adjacent plots. The cultivated soil was more sandy and had lower organic matter and nutrient contents than the fallow soil. Four nematode assemblages of Scutellonema cavenessi, Helicotylenchus dihystera and Tylenchorhynchus gladiolatus were inoculated in the soils. The pot experiment was conducted on millet during 2 months. Multiplication rates of H. dihystera were not significantly different in the two soils. T. gladiolatus had a lower multiplication rate in the fine-textured soil. S. cavenessi seemed to reproduce better in the coarse-textured soil when inoculated in low density with H. dihystera. The presence of plant parasitic nematodes in the cultivated soil caused a significant decrease of millet biomass, whereas plants in the fallow soil were less sensitive to nematode damage and were only affected when the soil was inoculated with T. gladiolatus alone. This experiment did not explain the distribution of plant parasitic species observed in the field. However, parameters other than the presence of a favourable host plant and micro-climatic conditions were found to induce differences in the reproductive rates of several species of plant parasitic nematodes.

Influence of soil organic matter and ion concentration on some Senegalese plant-parasitic nematodes

Abstract To determine the effect of different physicochemical soil parameters on plant-parasitic nematodes, organic matter, KNO 3 and NaCl salts were added to sterilised sand or sandy soil. The five most frequently observed species in Senegal were inoculated on millet together or as individual species. The species were Helicotylenchus dihystera, Pratylenchus pseudopratensis, Tylenchorhynchus gladiolatus, Scutellonema cavenessi and Criconemella curvata. Shoot and root weights were measured at the end of the experiment. Organic matter had no effect on shoot growth but reduced root growth when present in a high proportion whereas KNO 3 improved both shoot and root growth. Individual populations of H. dihystera increased root growth. The reproductive index of species such as S. cavenessi and C. curvata was not affected by the substrate whereas that of H. dihystera, T. gladiolatus and P. pseudopratensis was affected. The addition of KNO 3 and NaCl had no influence on the reproductive index of T. gladiolatus , but reduced that of H. dihystera. These effects disappeared when nematodes were inoculated together. These experiments show that soil characteristics, as well as the soil solution influence nematode reproduction, but the response could be different if the species is present as a population or in a community. Results show that modifying the soil solution can influence nematode interactions, but this may not necessarily lead to reduction in their pathogenicity as plant susceptibility may increase in the changed environment.