Améliorer la biodisponibilité du phosphore : comment valoriser les compétences des plantes et les mécanismes biologiques du sol ?

Les ions orthophosphates (Pi) représentent les seules formes de phosphore (P) utilisable par les cultures. Dans les sols, ils sont généralement présents à de faibles concentrations dans la solution, en raison des nombreux processus géochimiques contraignant leur mobilité et disponibilité. Les plantes et les micro - organismes associés, au travers de relations rhizosphériques, symbiotiques et par la prédation des populations microbiennes, modifient considérablement la quantité de P que la plante est capable d'acquérir tout au long de sa croissance (biodisponibilité). Cette revue décrit les différents processus (modifications des racines, rôle du pH, des anions organiques, des enzymes, de la microfaune et de la macrofaune) qui peuvent modifier la biodisponibilité du P dans la rhizosphère. Des pistes pour mieux valoriser le potentiel intrinsèque des végétaux et de l'écologie des organismes du sol et optimiser l'acquisition de P des cultures à partir du sol sont proposées. (Résumé d'auteur

Quantification of Organic P and Low Molecular Weight Organic Acids in Ferralsol Soil Extracts by Ion Chromatography

International audienc

N-fixing tree species introduced in Eucalyptus forest modify soil organic P and low molecular weight organic acid pools

Eucalypts, fast-growing trees, are planted in nutrient-poor soils such as sandy savannas in Congo, and deplete highly soil macronutrients such as N and P. To increase N inputs, the N-fixing tree species acacia was intercropped in eucalypt plantations. Increasing N availability could also change organic P (Po) cycling via different litter decomposition rate and increased contents of low molecular weight organic acids (LMWOAs) released by microbes. To investigate the effects of acacia on P cycling, we used first high performance ion chromatography to quantify Po forms and LMWOAs in soil from different plantations: pure stand of acacia, ofeucalyptus and the mixture of both species. We separated and identified six Po forms (glucose-6-P, AMP, ATP, phytate, fructose bis-P, uridine-diP) and four LMWOAs (malate, malonate, oxalate, citrate). Glucose-6-P and AMP were the dominant forms of Po in all stands, but their concentrations decreased in the mixture compared to the pure stands. Phytate was present in all soil samples and was much higher in eucalypt than in acacia stands, with intermediate values in the mixture. Oxalate and malate were the dominant LMWOAs, especially in eucalypt pure stands. Po mineralization was estimated in Olsen extract using two recombinant fungal enzymes, a phytase (from Aspergillus niger) and an acid phosphatase (from an ectomycorrhizal species). Both enzymes were able to release inorganic P from Po present in the soil extracts. To go further into the effect of LMWOAs on P bioavailability, two concentrations of organic acids were added to soil before running enzyme measurements. In conclusion, our results show that intercropping of acacia modified both organic P forms and LMWOAs concentrations in pure and mixed stands and presumably P cycling

Introducing N2-fixing trees (Acacia mangium) in eucalypt plantations rapidly modifies the pools of organic P and low molecular weight organic acids in tropical soils

Many studies have shown that introducing N2-fixing trees (e.g. Acacia mangium) in eucalypt plantations can increase soil N availability as a result of biological N2 fixation and faster N cycling. Some studies have also shown improved eucalypt P nutrition. However, the effects of N2-fixing trees on P cycling in tropical soils remain poorly understood and site-dependent. Our study aimed to assess the effects of planting A. mangium trees in areas managed over several decades with eucalypt plantations on soil organic P (Po) forms and low molecular weight organic acids (LMWOAs). Soil samples were collected from two tropical sites, one in Brazil and one in the Congo. Five different treatments were sampled at each site: monospecific acacia, monospecific eucalypt, below acacias in mixed-species, below eucalypts in mixed-species as well as native vegetation. Po forms and LMWOAs were identified in sodium hydroxide soil extracts using ion chromatography and relationships between these data and available P were determined. At both sites, the concentrations of most Po forms and LMWOAs were different between native ecosystems and monospecific eucalypt and acacia plots. Also, patterns of Po and LMWOAs were clearly separated, with glucose-6-P found mainly under acacia and phytate and oxalate mainly under eucalypt. Despite the strongest changes occurred at site with a higher N2 fixation and root development, acacia introduction was able to change the profile of organic P and LMWOAs in <10 years. The variations between available Pi, Po and LMWOA forms showed that P cycling was dominated by different processes at each site, that are rather physicochemical (via Pi desorption after LMWOAs release) at Itatinga and biological (via organic P mineralization) at Kissoko. Specific patterns of Po and LMWOAs forms found in soil sampled under acacia or eucalypt would therefore explain the effect of acacia introduction in both sites

Potential of Bioassays to Assess Consequences of Cultivation of <i>Acacia mangium</i> Trees on Nitrogen Bioavailability to <i>Eucalyptus</i> Trees: Two Case-Studies in Contrasting Tropical Soils

We hypothesized that the nitrogen-fixing tree Acacia mangium could improve the growth and nitrogen nutrition of non-fixing tree species such as Eucalyptus. We measured the N-mineralization and respiration rates of soils sampled from plots covered with Acacia, Eucalyptus or native vegetation at two tropical sites (Itatinga in Brazil and Kissoko in the Congo) in the laboratory. We used a bioassay to assess N bioavailability to eucalypt seedlings grown with and without chemical fertilization for at least 6 months. At each site, Eucalyptus seedling growth and N bioavailability followed the same trends as the N-mineralization rates in soil samples. However, despite lower soil N-mineralization rates under Acacia in the Congo than in Brazil, Eucalyptus seedling growth and N bioavailability were much greater in the Congo, indicating that bioassays in pots are more accurate than N-mineralization rates when predicting the growth of eucalypt seedlings. Hence, in the Congo, planting Acacia mangium could be an attractive option to maintain the growth and N bioavailability of the non-fixing species Eucalyptus while decreasing chemical fertilization. Plant bioassays could help determine if the introduction of N2-fixing trees will improve the growth and mineral nutrition of non-fixing tree species in tropical planted forests

Organic phosphorus immobilization in microbial biomass controls how N2-fixing trees affect phosphorus bioavailability in two tropical soils

International audienceEucalyptus is the tree most widely planted in tropical countries to satisfy growing demand for wood products, but high yields require high fertilizer inputs. Introducing N 2-fixing trees (NFT), such as Acacia mangium, has been proposed to improve soil fertility and aboveground tree biomass in Eucalyptus plantations. In addition to N inputs, NFT species may increase plant P nutrition through increased rates of organic P (Po) cycling. However, the positive effect of acacia on soil P availability and plant P nutrition was found to vary substantially between sites. The ability of acacia to improve P bioavailability might mainly depend on Po sequestration in microbial biomass, preventing Po mineralization by phosphatases and efficient Po recycling. This hypothesis was tested at two tropical sites, Itatinga (Brazil) and Kissoko (Congo) by measuring inorganic phosphate (Pi), Po and enzyme-labile Po in bicarbonate extracts from the topsoil collected from plots with Eucalyptus, acacias, or native vegetation. We used bicarbonate enzyme-labile Po after soil autoclaving as an indicator of microbial Po, and a Eucalyptus bioassay to measure the actual P bioavailability for Eucalyptus seedlings. At Itatinga, bicarbonate-Pi was very low, while Po was the main P form. Enzyme-labile Po was very weak in intact soils and high in autoclaved soils, indicating high immobilization in microbial biomass. At Kissoko, Po was highly enzyme-labile in both intact and autoclaved soils, especially from acacia plots, suggesting very low Po immobilization in microbial biomass. Growth and P accumulations in Eucalyptus seedlings were low in all soils at Itatinga and were the highest in Eucalyptus plants grown in acacia soils at Kissoko. Our results highlight the potential of acacia trees for improving P bioavailability for other tree species if labile Po enrichment in the soil provided by this N 2-fixing tree is not locked into the microbial biomass