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

Trophic relationships: effect of bacteria, fungi and nematodes interactions on P nutrition from phytate in the rhizosphere of Pinus pinaster seedlings

International audienceAims and Background: Phytate accounts for large P fractions accumulating in soil. However, plants are poorly able to access this source of P because of their low phytate mineralizing capacity. In contrast to plants, some rhizosphere bacteria are able to mineralize phytate. This activity could benefit first to bacterial grow th and plant P availability could be enhanced only through bacterial grazers liberating nutrients locked up in bacterial biomass. We hypothesize that phytase- producing bacteria could mobilize plant unavailable phytate P into new nutrient pools which become available to plants w hen bacteria are grazed by nematodes. Methods : We used a simplified system with phytate and combinations made of Pinus pinaster seedlings, an ectomycorrhizal fungus (Hebeloma cylindrosporum), populations of bacteria ( Bacillus subtilis) displaying phytase activity and one of their nematode grazer (Rhabditis sp). Results: The presence of bacteria, alone or in combination with the ectomycorrhizal fungus, was not sufficient to improve plant P status. In contrast, the nematode graz ing activity led to a dramatic increase of plant P accumulation up to 200% compared to control ones. Conclusion: Microbial grazing by nematodes represents therefore an important mechanism to improve P availability to plants, by accelerating phytate mineralization and/or releasing locked microbial P

Do multitrophic interactions matter for tree nutrition? A laboratory study involving bacteria, bacterial-feeding nematodes and ectomycorrhizal fungi

Soil microorganisms act as a sink and a source of available N and P by mediating key processes in the biogeochemical N and P cycling. The microbial loop, based upon the grazing of bacteria is thought to play a major role in the mineralization of nutrients such as nitrogen (N) and phosphorus (P) in terrestrial ecosystems. However, little is known about the impact of grazing by nematodes on mineral nutrition of ectomycorrhizal woody plants. We addressed this question in young seedlings of Pinus pinaster, whether or not associated with the ectomycorrhizal basidiomycete Hebeloma cylindrosporum. Plants, whether inoculated or not with Bacillus subtilis and bacterial-feeding nematodes, isolated from the same P. pinaster plantation in ectomycorrhizae and soil respectively, were grown in sterile agar gel. N was supplied equally as nitrate and bacterial N, and P as mineral P, bacterial P or phytate. Phytate is a well-known poorly available P source to plants but is used by B. subtilis able to release phytase in the medium. Nematode grazing upon 15N labeled bacteria increased plant growth and induced a low accumulation of 15N in shoots compared to medium N. This effect disappeared with bacterial P as the sole source of P. In contrast, nematode grazing considerably enhanced plant P access from phytate, and ectomycorrhizae increased plant P accumulation. These results underline the importance of multitrophic interactions to increase mineral nutrition of a woody species from nutrients locked up in bacteria and the prominent role of P availability for microbial loop efficiency

Grazing by nematodes on rhizosphere bacteria enhances nitrate and phosphorus availability to Pinus pinaster seedlings

The microbial loop is thought to play a major role in the mineralization of nutrients such as nitrogen (N) and phosphorus (P) in terrestrial ecosystems. This microbial loop is based on the grazing of bacteria by predators such as bacterial-feeding nematodes. However, little is known about the impact of grazing by nematodes on the mineral nutrition of woody plants. This study was undertaken to quantify the effect of nematode grazing on bacteria in the rhizosphere on the root architecture, growth and mineral nutrition (N and P) of a woody species (Pinus pinaster). Young P. pinaster seedlings were cultivated for 35 days in a simplified sterile experimental system with bacteria (Bacillus subtilis) and bacterivorous nematodes (Rhabditis sp.) isolated from soil samples collected from a 15-year old stand of maritime pine. To check the hypothesis that bacteria could be a source of nutrients, especially N, two N sources were supplied in the medium: (i) bacterial N labeled with (15)N and (ii) nitrate. Phosphorus was supplied as insoluble inorganic tri-calcium phosphate (TCP). The results showed that the (15)N flow from the bacteria to the plant shoots was only significant when nematodes were present, with an average accumulation of 14 +/- 5 mu g plant(-1) of (15)N. Plants cultivated with nematodes also accumulated significantly more total N in their shoots than sterile ones or inoculated with bacteria, resulting in a net average increase in N of 700 mu g plant(-1). The same result was observed for the total P accumulation in the shoots, as plants with nematodes accumulated an average of 300 mu g plant(-1) more P than sterile ones or inoculated with bacteria. However, the presence of bacteria, whether alone or with nematodes, did not modify the root architecture. These results demonstrated that the presence of bacterial-feeding nematodes significantly enhanced N and P availability to P. pinaster seedlings, probably by improving plant use of nitrate and insoluble P supplied in the medium

Studying soil quality is essential to evaluate the sustainability of a vineyard

International audienc

La caractérisation du fonctionnement biologique du sol en viticulture biologique peut être réalisée par l’analyse de la nématofaune du sol

Projet AIDY du programme INRA - AgriBio 3 (2010-2012) « Analyse Intégrée de laDYnamique de conversion à la viticulture bio »La nématofaune du sol est un bioindicateur qui permet de caractériser l’état biologique du sol. Les abondances des guildes fonctionnelles de nématodes et les indices nématofauniques mettent en lumière les changements de fonctionnement biologique lors de la conversion de vignobles en AB

Soil nematofauna as indicator of the effect of agricultural practices on soil biological functioning in apple orchards.

National audienc

Shifts in size, genetic structure and activity of the soil denitrifier community by nematode grazing

International audienceBacterial-feeding nematodes represent an important driver of the soil microbial activity and diversity. This study aimed at characterizing the impact of nematode grazing on a model functional bacterial guild involved in N-cycling, the denitrifiers. Bacterial-feeding nematodes (Cephalobus pseudoparvus) were inoculated into soil microcosms whose indigenous nematofauna had previously been removed. The size, genetic structure and activity of the soil denitrifier community were characterized 15 and 45 days after nematodes inoculation using quantitative PCR of the nirK, nirS and nosZ denitrification genes, fingerprinting of the nirK and nirS genes and denitrification enzyme activity measurements, respectively. A significant impact of C. pseudoparvus was observed on genetic structure of the nirK community, mainly due to shifts in the relative abundance of the dominant populations, but not on the nirS community. The grazing pressure also tended to decrease the density of all denitrification genes as well as that of 16S rRNA genes. Despite being non-significant, the extent of this decline in gene copy numbers ranged between 60 and 80% of the control microcosm genes densities. Finally, compared to non-inoculated microcosms, denitrification activity significantly decreased by 8% in response to the nematodes inoculation. The herewith data showed that predation by a single species of bacterial-feeding nematode can affect the soil denitrifier communit

Indicateurs de la qualité des sols viticoles

National audienc

Soil organic matter, aggregate stability and respiration: integrated measures for the evaluation of the soil sustainability in the vineyards

International audienc