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Two 15N labelling methods for assessing net rhizodeposition of nitrogen (N) in pea crop (Pisum sativum L.) were compared in the greenhouse and in the field: the cotton-wick (CW) and the split-root (SR) methods. Rhizodeposition is defined as the organic material lost from roots during their growth through the soil. CW is a method in which 15N urea was supplied to the plant in pulses via a wick threaded through the stem. In SR, the root system was divided between a hydroponic labelling compartment (LC) containing the labelling nutrient solution (1 or 5 mM 15NO3–15NH4) and a compartment filled with soil in which the amount of 15N rhizodeposition was assessed. The percentage of N derived from rhizodeposition (%Ndfr), was used to calculate the amount of N rhizodeposition which was obtained from the ratio of atom % 15N excess of the soil : atom % 15N excess of the roots. Above ground parts in the field accumulated markedly more dry matter and N than in the greenhouse, regardless of the labelling method. 15N enrichments of above ground parts were higher than those of roots recovered from the soil. Results indicated that amount of 15N applied to plants were lower in SR than in CW. Additionally, LC roots of SR tended to retain large amounts of 15N. As a consequence, atom % 15N excess of roots was less than 1% in SR, whereas most values varied from 1% to 4% in CW. However, relationships between enrichments of the soil and of the roots were different in SR and CW. It was not possible to compare the Ndfr:root-N ratio between the two methods, but the ratio of Ndfr:plant-N was found to be 10% higher in SR than in CW. Finally, relative to total plant-N, the total contribution of below ground parts to the N pool of the soil reached 22–25% at maturity for the two methods. From our experiments, we could not conclude that one method is better than the other for estimating either net rhizodeposition of N or the contribution of a pea plant to the soil N pool. However, CW is easier to adapt and monitor under field conditions than SR.

Field experiments were carried out during three successive years to study through a dynamic approach the competition for soil N and its interaction with N2 fixation, leaf expansion and crop growth in pea–barley intercrops. The intensity of competition for soil N varied between experiments according to soil N supply and plant densities. This study demonstrates the key role of competition for soil N which occurs early in the crop cycle and greatly influences the subsequent growth and final performance of both species. Relative yield values for grain yield and N accumulation increased with the intensity of competition for soil N. Barley competed strongly for soil N in the intercrop. Its competitive ability increased steadily during the vegetative phase and remained constant after the beginning of pea flowering. The period of strong competition for soil N (500–800 degree-days after sowing) also corresponded to the period of rapid growth in leaf area for both species and therefore an increasing N demand. For each species, the leaf area per plant at the beginning of pea flowering was well correlated with crop nitrogen status. Barley may meet its N needs more easily in intercrops (IC) and has greater leaf area per plant than in sole crops (SC). Barley having a greater soil N supply results in an even higher crop N status and greater competitive ability relative to pea in intercrop. Competition by barley for soil N increased the proportion of pea N derived from fixation. The nitrogen nutrition index (NNI) values of pea were close to 1 whatever the soil N availability in contrast to barley. However N2 fixation started later than soil N uptake of pea and barley and was low when barley was very competitive for soil N. Due to the time necessary for the progressive development and activity of nodules, N2 fixation could not completely satisfy N demand at the beginning of the crop cycle. The amount of N2 fixed per plant in intercrops was not only a response to soil N availability but was largely determined by pea growth and was greatly affected when barley was too competitive.

Two methods were developed and used to study the root system dynamics of two species grown together or separately under field conditions. The first method, based on herbicide injection at different soil depths, was used to determine the rooting depth penetration rate of each species in pea–barley and pea–mustard mixtures. The roots absorbed the herbicide when they reached the treated zone leading to visible symptoms on the leaves which could be readily monitored. The second method used differences in 15N natural abundance and N concentration between legume and non-legume species to quantify the contribution of each species to root biomass of a pea–barley mixture. Each contribution was calculated using 15N abundance and N concentration of root mixtures and of subsamples of roots of individual species within mixtures. Both methods can indeed be used to distinguish roots of species in mixtures and thus to study belowground competition between associated species. The use of these methods demonstrated species differences in root system dynamics between species but also significant effects of interactions between species in mixtures. The rooting depth penetration rate was mainly species specific whereas root biomass was dependant on plant growth, allocation of dry matter between shoot and root components and growth factors such as N fertilization. Root biomass of each species may vary therefore with the level of competition between species.

This work studied the effects of P fertilization on nodulation of field-grown soybean by two Bradyrhizobium strains (SMGS1 and THA7), and checked if differences between strains were consistent with bacterial growth and growth pouch nodulation ability in response to P availability. In the field, nodule dry weight and nitrogen fixation activity of inoculated soybean were studied on typical acid soils of Thailand at the flowering (R1) stage and at the end of grain filling. Grain yield, growth and phosphorus content were recorded. The bradyrhizobial strains were cultivated in culture medium, and growth parameters recorded. Nodulation patterns were observed during growth pouch experiments: infective root cells were inoculated with strains cultivated at two P concentrations in their culture media, namely 1 μM and 1 mM. Ten days after inoculation, the position of each nodule was measured relative to the root tip (RT) mark, expressed relative to the smallest emerging root hairs-RT distance in the nodulation frequency profile, and the consistency of responses was tested. In the field, on P deficient soils, dry weight of nodules was higher with Bradyrhizobium japonicum strain SMGS1 than with strain THA7. P supply increased the number and dry weight of nodules for both strains, with a higher dry weight response for THA7 than for SMGS1. It also had a positive effect on tissue phosphorus status and grain yield at R8 stage. In growth media, significant differences were recorded between strains under P-limiting conditions: The growth rate was higher for strain SMGS1, as well as the maximal number of bacterial cells supported. With growth pouch, inoculating plants with bacteria grown in P-deficient medium resulted in a less intense nodulation of roots by THA7, and with nodules appearing earlier on roots than in the case of SMGS1. At 1 mM P, there was no significant difference between strains. Thus, strain THA7 is more affected by P deficiency than strain SMGS1. Although P was not supplied in the same way in the soil and in the growth pouch experiments, this consistency of behaviour between work scales indicates that phosphorus availability is a key component for a successful inoculation. Furthermore, the study of bacterial growth rates and nodulation profile represents an interesting step for bacterial screening for low P soils. [-11pt]

Development of the root system, appearance of nodules, and relationships between these two processes were studied on pea (Pisum sativum L., cv. Solara). Plants were grown in growth cabinets for 4 weeks on a nitrogen-free nutrient solution inoculated with Rhizobium leguminosarum. Plant stages, primary root length, distance from the primary root base to the most distal first-order lateral root, and distance from the root base to the most distal nodule, were recorded daily. Distribution of nodules along the primary root and distribution of laterals were recorded by sampling root systems at two plant stages. Primary root elongation rate was variable, and declined roughly in conjunction with the exhaustion of seed reserves. First-order laterals appeared acropetally on the primary root. A linear relationship was found between the length of the apical unbranched zone and root elongation rate, supporting the hypothesis of a constant time lag between the differentiation of first-order lateral's primordia and their emergence. Decline of the primary root elongation rate was preceded by a reduction in density and length of first-order laterals. Nodules appeared not strictly but roughly acropetally on the primary root. A linear relationship was found between the length of the apical zone without nodule and root elongation rate, supporting the hypothesis of a constant time lag between infection and appearance of a visible nodule. A relationship was found between the presence/absence of nodules on a root segment and the root elongation rate between infection and appearance of nodules on the considered root segment. Regulation of both processes by carbohydrate availability, as a causal mechanism, is proposed.

Publication Inra prise en compte dans l'analyse bibliométrique des publications scientifiques mondiales sur les Fruits, les Légumes et la Pomme de terre. Période 2000-2012. http://prodinra.inra.fr/record/256699; The design of more sustainable cropping systems requires increasing N-input from symbiotic N(2) fixation (SNF). However, SNF can be inhibited by nitrate exposure (e.g., soil N-mineralization). Although the effect of nitrate on SNF has been extensively investigated at the cell scale, few studies have highlighted the impact of nitrate exposure on nodule number and biomass, nodule activity of the SNF apparatus or its ability to recover. Pea plants were grown in greenhouse conditions in a N-free nutrient solution and exposed to nitrate (5 mM NO (3) (-) L(-1)) for one week during either early vegetative growth, flowering or seed filling. After nitrate removal, the plants were grown either under natural light or shade. Nitrate exposure reduced the rate of nodule establishment during vegetative growth, whereas it caused damage to existing nodules when applied during the reproductive stages. Nitrate decreased the specific activity of nodules regardless of the stage of the exposure. After nitrate removal, an extra wave of nodulation was observed on plants grown under natural light but only when nitrate exposure occurred before the seed filling stage. The recovery of SNF activity after nitrate removal depended on the amount of carbon available to nodules.

Grain legumes, such as peas (Pisum sativum L.), are known to be weak competitors against weeds when grown as the sole crop. In this study, the weed-suppression effect of pea-barley (Hordeum vulgare L.) intercropping compared to the respective sole crops was examined in organic field experiments across Western Europe (i.e., Denmark, the United Kingdom, France, Germany and Italy). Spring pea (P) and barley (B) were sown either as the sole crop, at the recommended plant density (P100 and B100, respectively), or in replacement (P50B50) or additive (P100B50) intercropping designs for three seasons (2003–2005). The weed biomass was three times higher under the pea sole crops than under both the intercrops and barley sole crops at maturity. The inclusion of joint experiments in several countries and various growing conditions showed that intercrops maintain a highly asymmetric competition over weeds, regardless of the particular weed infestation (species and productivity), the crop biomass or the soil nitrogen availability. The intercropping weed suppression was highly resilient, whereas the weed suppression in pea sole crops was lower and more variable. The pea-barley intercrops exhibited high levels of weed suppression, even with a low percentage of barley in the total biomass. Despite a reduced leaf area in the case of a low soil N availability, the barley sole crops and intercrops displayed high weed suppression, probably because of their strong competitive capability to absorb soil N. Higher soil N availabilities entailed increased leaf areas and competitive ability for light, which contributed to the overall competitive ability against weeds for all of the treatments. The contribution of the weeds in the total dry matter and soil N acquisition was higher in the pea sole crop than in the other treatments, in spite of the higher leaf areas in the pea crops.

Correspondance: malezieux@cirad.fr; International audience; The evolution of natural ecosystems is controled by a high level of biodiversity, In sharp contrast, intensive agricultural systems involve monocultures associated with high input of chemical fertilisers and pesticides. Intensive agricultural systems have clearly negative impacts on soil and water quality and on biodiversity conservation. Alternatively, cropping systems based on carefully designed species mixtures reveal many potential advantages under various conditions, both in temperate and tropical agriculture. This article reviews those potential advantages by addressing the reasons for mixing plant species; the concepts and tools required for understanding and designing cropping systems with mixed species; and the ways of simulating multispecies cropping systems with models. Multispecies systems are diverse and may include annual and perennial crops on a gradient of complexity from 2 to n species. A literature survey shows potential advantages such as (1) higher overall productivity, (2) better control of pests and diseases, (3) enhanced ecological services and (4) greater economic profitability. Agronomic and ecological conceptual frameworks are examined for a clearer understanding of cropping systems, including the concepts of competition and facilitation, above- and belowground interactions and the types of biological interactions between species that enable better pest management in the system. After a review of existing models, future directions in modelling plant mixtures are proposed. We conclude on the need to enhance agricultural research on these multispecies systems, combining both agronomic and ecological concepts and tools.

International audience; The development of sustainable cropping systems is a key priority for agronomists and crop scientists. A first step involves understanding the relationship between cropping system performance and farmers’ practices. To complete this step, a methodological framework entitled Regional Agronomic Diagnosis (RAD) has been developed. During the last ten years, the scope of the regional agronomic diagnosis has been enlarged to include several factors describing crop quality and the environmental impact of cropping systems. Regional agronomic diagnosis has led to several major advances such as (1) the assessment of the effect of preceding crop and soil structure on malting barley quality in France and (2) the assessment of the effects of ploughing, nematicide use and fertilisers on soil properties in intensive banana plantations in the West Indies. Improvements have also been gained in methodology, particularly by the selection of indicators for assessing the effects of crop management, soil and weather conditions, and data analysis. Finally, regional agronomic diagnosis has been integrated into more general approaches of agricultural development. We review here this methodological progress.

The ureide assay method has been found to be suitable for soybean (Glycine max) to assess nitrogen ixation in a wide range of genotypes and Bradyrhizobium strains. The relationships between the relative abundance of ureide-N in xylem-sap and the proportion of plant N derived from nitrogen fixation were studied for early maturing soybeans grown under greenhouse and field conditions. Plants grown under greenhouse conditions were either supplemented with N-free nutrients or supplied with different levels of 15N-nitrate. In the field, soybeans were grown with or without inoculation, in the presence or absence of nitrogen fertilizer. Treatments were applied in order to obtain a wide range of nitrogen nutrition levels. The effect of the source and level of nitrogen nutrition on growth and nitrogen accumulation in plants, proportion of fixed nitrogen in the xylem-sap and in the shoots, and the proportions of N-compounds in the xylem-sap was determined. The greenhouse study demonstrated that the relative...

International audience; The competitiveness of two Bradyrhizobium japonicum strains was determined under controlled conditions. The relationship between the ratio of the nodules formed by the two bradyrhizobial strains and the ratio of the two strains in the inoculum was established and used as an internal standard to evaluate the equilibrium ratio between populations of the two strains in the soil. The fluorescent antibody (FA) technique was also used for counting the bradyrhizobial populations directly in the soil. Results with the FA technique were analysed and compared with the estimate obtained through nodule occupancy. The initial ratio of the two bradyrhizobial strains in populations introduced simultaneously into the soil changed during incubation time and the two strains reached the same equilibrium levels after 53 days. Never-theless, the strain introduced into the soil as an overpopulation remained clearly dominant for the nodule occupancy. We conclude that direct counts with the FA approach are selective and these counts could be mainly of the unattached bacteria.

The existence of a balance survival rate of different B. japonicum strains in soils has been well established. Balance levels (M) were shown to be independent of inoculum size but differences between strains in a soil could not be discussed partly because of the instability of the recovery's efficiency in the area of the plateau. In this paper, a dynamic model, the Gompertz equation, is fitted to the data, to estimate the rate (A) at which equilibrium is reached. For the three strains in the three soils previously studied, this rate is also independent of inoculum size and the soil-strain interaction is highly significant. Some data from the literature are also well fitted by the model. We propose to characterize the saprophytic potential of a strain with the pair of parameters A-M. An optimal experimental procedure is proposed to obtain the most precise estimates for these parameters. Their biological significance is discussed, but the fact that they are not dependent upon inoculation level is of basic importance for agronomic practice.

Afin d’améliorer la valeur quantitative de la technique de dénombrement des bactéries telluriques (Rhizobium japonicum) par immunofluorescence, on compare différents modes d’extraction et de récupération. L’utilisation de membranes en polycarbonates (Nucléopores) supprime les 40 p. 100 de pertes occasionnées par les membranes cellulosiques (Millipores) classiquement employées. Parmi les différents floculants testés, Ca(OH)2/MgCO3(2/5) permet d’obtenir les meilleurs résultats. Cependant pour les deux sols tropicaux étudiés, une floculation par centrifugation est nécessaire. Lorsque l’étape de la floculation n’est pas limitante, la dispersion du sol dans un extractif composé de gélatine diluée dans une solution de (NH4)2 HPO4 0,1 M améliore significativement le taux de récupération des bactéries., We have tested different procedures for releasing bacteria (Rhizobium japonicum) from soil particles and collecting them for fluorescent antibody quantification. The use of polycarbonate filters (Nucleopore) avoided the 40 % loss of bacteria usually found with classical cellulose filters (Millipore). A flocculant mixture of Ca(OH)2/MgCO3(2/5) gave the best results. However flocculation by centrifugation was needed for the two tropical soils tested. When the flocculation step was not the limiting factor, partially hydrolysed gelatin diluted in ammonium phosphate (0.1 M) could be used to suspend the soil and increase the recovery.

The ability of Bradyrhizobium strains to survive saprophytically in soil was studied by means of fluorescent antibodies (FA). It was found that the recovery rate may be considered a constant value although the limit of detection by the FA technique is approximately 103 bacteria g−1 soil. By studying the survival kinetics of B. japonicum strains introduced into soils, we observed that whichever soil-strain combination was tested in a given soil during the incubation all the different populations of a strain reached the same survival balance level, generally about 103–104Bradyrhizobium g−1 soil. When we reintroduced strains into a soil containing rhizobia of the same specificity, the new inhabitants reached the same equilibrium level as that of the initial population. The balance threshold level does not appear to be a very sensitive way to classify, strains on their saprophytic ability. We suggest that survival kinetics should be characterized by the rate at which the population reaches equilibrium.

La technique des anticorps marqués (immunofluorescence) a été utilisée pour l’étude de la persistance et de la compétition dans le sol de Rhizobium japonicum. La même technique a permis d’apprécier la multiplication de Rhizobium dans le sol rhizosphérique et dans le rhizoplan. Les populations de R. japonicum, souches G2 et G3 se maintiennent à un haut niveau (104 bactéries/g de sol) pendant tout le temps de l’expérimentation. Lorsque 2 souches sont introduites simultanément, on n’observe pas de compétition saprophytique entre celles-ci dans le sol. L’étude de R. japonicum dans la rhizosphère montre que seule la plante hôte de soja, var. « Chippewa nodulante », possède dans le sol proche de ses racines une population de Rhizobium plus importante que dans le sol normal. Dans une expérimentation portant sur le rhizoplan, 4 cultivars de soja sont testés. Les résultats montrent que R. Japonicum est 100 à 1 000 fois plus abondant dans le rhizoplan que dans le sol ne portant pas de végétation. Nous suggérons qu’au cours de la multiplication de Rhizobium dans le rhizoplan, l’équilibre entre 2 souches peut être modifié et une souche stimulée préférentiellement à l’autre., The fluorescent-antibody technique was used to study persistence and competition of Rhizobium japonicum strains in soils at different temperatures and to examine the effect of plants on this bacterium in thee rhizosphere and rhizoplane. The soil population of R. japonicum strains G2 and G3 remained the same (104 bacteria/g of soil) throughout the experiment, without any significant competition between the two strains. In the rhizosphere experiment, only the host legume (Glycine max, cv. « Chippewa » ) supported highh populations of strain G3. In another experiment with four cultivars of soybeans, the populations of R.japonicum 102,103 more numerous in the rhizoplane soil than in normal soil. We suggest that the multiplication of Rhizobium in the rhizoplane of the host plant is an important phenomenon, able to modify the equilibrium between two specific strains and thus to favour better nodulation by one strain rather than another.