Do trophic relationships in soil enhance organic P cycling and plant P nutrition? Phytate mineralization as a case study

Phosphorus (P) strongly limits plant productivity as plants can only absorb free inorganic orthophosphate (Pi) at very low concentrations in soil solution. However, soil contains high levels of poorly available P, especially phytate, considered as the most abundant plant-unavailable organic P source. Here, we investigated a new strategy based on rhizosphere trophic relationships to mobilize P from phytate. We hypothesized that the interactions between plant (Pinus pinaster), phytase-producing bacteria (Bacillus subtilis), mycorrhizal fungi (Hebeloma cylindrosporum), representing the more widespread strategy to improve plant P acquisition, and bacterial grazer nematodes (Rhabditis sp.) may improve plant P acquisition from phytate and thus P cycling from soil organic P. We grew seedlings jn microcosms containing soil with Pi or phytate, with or without the above-mentioned organisms for 2.5 months. With Pi, no significant differences were observed among inoculation or mycorrhizal treatments. In contrast, with phytate, nematode grazing was required for non-mycorrhizal plants to acquire P into their shoots. In mycorrhizal plants, bacteria alone improved net P accumulation and nematode grazing enhanced this positive effect. Soil microbial P contents and in situ probing of bacterial phytase gene expression are currently under study to understand better the mechanisms underlying the strong positive effects of nematode grazing on plant P nutrition. Our first results indicate that the use of trophic relationships should be considered as a sustainable strategy for plant P nutrition to enhance organic P cycling and P availability in the rhizosphere

Les relations trophiques microfaune bactéries rhizosphériques mycorhizes: quel rôle dans le recyclage des nutriments (N et P)?

National audienc

Does soil functional diversity enhance organic P cycling and plant P nutrition? phytate mineralization as a case study

absen

Agricultural Practices Modulate the Beneficial Activity of Bacterial-Feeding Nematodes for Plant Growth and Nutrition: Evidence from an Original Intact Soil Core Technique

International audienceFree-living nematodes have beneficial effects on plant growth and nutrition. Exploring how agricultural practices modulate these beneficial effects is still challenging. A study was conducted in Ferralsols from Madagascar from one unmanaged grassland and 16 upland rainfed rice fields, representative of different agricultural practices: rotation, agroforestry and monoculture. Intact soil cores in plastic cylinders were sampled in the field to assess the effects of agricultural practices on changes in plant growth and nutrition induced by the presence of bacterial-feeding nematodes. The soil cores were fumigated to kill the nematodes and moistened with a filtered fresh soil suspension containing only microbial cells. A rice seed was introduced in the core, which was then incubated under natural climatic conditions for 40 days with or without inoculation of the bacterial-feeding nematode Acrobeloides sp. The inoculation of the nematodes induced lower, similar or higher plant biomass and nutrient content in comparison to the control according to the agricultural practices. Positive effects of Acrobeloides sp. on plant functions were frequent in soil cores sampled from fields with high plant diversity, especially from agroforestry systems. The intact soil core technique appears to be a robust means of mimicking field conditions and constitutes a promising tool to assess effects on soil processes of the ecological intensification of agricultural practices

Effects of a bacterivorous nematode on rice 32 P uptake and root architecture in a high P-sorbing ferrallitic soil

International audienceSoil bacterivorous nematodes are key plant mutualists that increase nutrient availability for plants either by enhancing the mineralization of organic compounds (the “mineralization pathway”) or by increasing plant lateral root branching following shifts in internal plant metabolism, and subsequently leading to a higher volume of soil prospected by the roots (the “hormonal pathway”). The effects of these organisms on the nutrition of plants growing in strongly nutrient-deficient ferrallitic soils, especially in soils with limited available inorganic phosphorus (P), are poorly known, as are the pathways involved. In our study, using Oryza sativa (Poaceae) and Acrobeloides sp. (Cephalobidae), we tested the “mineralization” and “hormonal” hypotheses in an acidic P-depleted Ferralsol from the Madagascar highlands. We assessed the effect of nematode inoculation on (i) inorganic P flow from soil to plant using the 32P labelling technique and (ii) plant root architecture using a rhizobox device. We showed that the ability of Acrobeloides sp. to enhance P uptake in plants is strongly limited in Ferralsols. However, when the soil pH was corrected with dolomite, Acrobeloides sp. increased plant P uptake probably through the “mineralization” pathway (higher microbial turnover). Indeed, the L-value increased by 49% in the presence of nematodes and dolomite, suggesting the production of unlabelled plant-available P, probably through a higher net P mineralization when the nematodes were inoculated. Using the rhizobox technique, we also observed increased root length in the presence of nematodes but the specific root length, the tip number and the root branching density did not increase in the presence of nematodes, suggesting that nematodes did not increase plant P uptake and growth in this soil as proposed by the “hormonal” hypothesis. From an ecological intensification perspective, to promote agro-ecological development in tropical regions, our results suggest that amending ferrallitic soils with P-rich organic matter and correcting soil pH with an appropriate amount of dolomite may constitute suitable agronomic actionable triggers to drive the mutualistic activity of bacterivorous nematodes