The production of exudates by plant roots and microbes in the rhizosphere, together with intense wetting and drying cycles due to evapotranspiration, stimulate changes in soil structure. We have attempted to separate these two processes using an experimental model with bacterial exopolysaccharides (dextran and xanthan) and root mucilage analogues (polygalacturonic acid, PGA), and up to 10 cycles of wetting and drying. To characterize the soil structure, tensile strength, water sorptivity and ethanol sorptivity of the amended soils were measured, and thin sections were made. Xanthan and PGA induced greater tensile strength of the amended soil, suggesting that they increased the bond energy between particles. Porosity increased with each cycle of wetting and drying, and this increase was less pronounced for the PGA 2 g l ±1 than for the xanthan and dextran. This suggests that PGA stabilized the soil against the disruptive effect caused by the wetting and drying. The PGA was the only polysaccharide that in¯uenced water sorptivity and repellency, resulting in slower wetting of the treated soil. Wetting and drying led to an increase of the sorptivity and a decrease of the repellency for all treatments with the exception of the PGA-amended soils. The PGA may therefore stabilize the soil structure in the rhizosphere by increasing the strength of bonds between particles and decreasing the wetting rate.

International audienceNear infrared reflectance spectroscopy (NIRS) was used to predict six biological properties of soil and earthworm casts including extracellular soil enzymes, microbial carbon, potential nitrification and denitrification. Partial least squares regression (PLSR) models were developed with a selection of the most important near infrared wavelengths. They reached coefficients of determination ranging from 0.81 to 0.91 and ratios of performance-to-deviation above 2.3. Variable selection with the variable importance in the projection (VIP) method increased dramatically the prediction performance of all models with an important contribution from the 1750–2500 nm region. We discuss whether selected wavelengths can be attributed to macronutrient availability or to microbial biomass. Wavelength selection in NIR spectra is recommended for improving PLSR models in soil research

Plant Growth-Promoting Bacteria (PGPB) of the genus Azospirillum are known to enhance root growth and yield in many plant species including cereals. To probe the underlying mechanisms, correlations between modifications of yield and 6-leaf plantlet characteristics were estimated on maize in four fields with contrasting soil properties over two consecutive years using the commercial isolate A. lipoferum CRT1. In both years, plantlet metabolome, photosynthetic potential and organ morphology were found to display field-and inoculation-specific signatures. Metabolomic analyses revealed that A. lipoferum CRT1 mostly affected sugar metabolism with no suggested impact on N and P assimilation. Mineral nitrogen feeding increased yield but did not affect yield enhancement by the bacterial partner. However, greater improvements of leaf photosynthetic potential correlated with yield diminutions and larger plantlets in all of their proportions correlated with yield enhancements. Bacterial inoculation restored proper seed-to-adult plant ratio when it accidentally dropped below 80%. Only in these cases did it raise yield. All in all, securing mature plant density is hypothesized as being the primary driver of A. lipoferum CRT1-mediated yield enhancement in maize fields.Over the past decades, conventional agriculture has become highly dependent on mineral nitrogen, phosphorus and potassium fertilization to increase production and meet world demand 1 . However, these mineral nutrients originate from limited resources and mineral nitrogen lixiviation into fresh water systems is a serious hazard 2 . Despite improvements in plant selection, soil structure and nutrient formulations, current levels of mineral fertilization remain unsustainable [3][4][5] . Since their early description in 1978 by Kloepper and Schroth 6 , Plant Growth-Promoting Bacteria (PGPB) represent an attractive source of technologies to maintain yields while reducing mineral nutrition with additional benefits in plant protection against pests and abiotic stresses [7][8][9][10] . The majority of PGPB comes from the Firmicutes and Proteobacteria. They engage in associative symbioses on plant root surfaces, or as root endophytes, without the development of specialized structures such as nodules [11][12][13] . Among PGPB, members of the genus Azospirillum have attracted particular attention since they are innocuous and are able to increase yields of many crop species including cereals 14 . A meta-analysis of field trials conducted between 1981 and 2008 notably revealed average enhancements of grain and forage yields of 15 and 27% (n = 91) after wheat inoculation with Azospirillum in the absence of nitrogen fertilization . A review of field assays conducted worldwide 18 found that all reports of assays conducted in Brazil and Mexico described positive yield-enhancements by Azospirillum. However, up to as low as 60% of field assays conducted in Egypt, India and Uruguay concluded to yield-enhancements while only 5 such cases were seen out of 6 years of experimentations ...

Inoculation of wheat roots with Paenibacillus (formerly Bacillus) polymyxa CF43 increases the mass of root-adhering soil. We tested the role of levan, a fructosyl polymer produced by strain CF43, in the aggregation of soil adhering to wheat roots. The P. polymyxa gene homologous to the Bacillus subtilis sacB gene encoding levansucrase was cloned and sequenced. The corresponding gene product synthesises high molecular weight levan. A P. polymyxa mutant strain, SB03, whose sacB gene is disrupted, was constructed using heterogramic conjugation. Effects of wheat inoculation with the wild type and the mutant strain were compared using two different cultivated silt loam soils in four independent pot experiments. Roots of wheat plantlets inoculated with CF43 or SB03 were colonized after 7-14 days at the same level, and root and shoot masses were not significantly different from those of the non-inoculated control plants. The ratio of root-adhering soil dry mass to root tissue dry mass was significantly higher for plants inoculated with strain CF43 than for those inoculated with mutant strain SB03: +30% in Orgeval soil and +100% in Dieulouard soil. Thus the levan produced by P. polymyxa is implicated in the aggregation of root-adhering soil on wheat.

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The eVect of direct inoculation of seeds with the plant growth promoting rhizobacteria (PGPR) Azospirillum lipoferum CRT1 was assessed on maize (Zea mays) grown for 35 days after sowing (d.a.s.) in controlled conditions (greenhouse) in a luvisol soil from south-eastern France. WhinRhizo ® software was used to describe the following changes in the root system morphology for each plant: distribution and average root diameter, root surface and the number of tips. The stress at breakage and stiVness of the roots in tension were also determined. Evaluation of biochemical components of roots was achieved by direct Attenuated Total ReXectance (or reXection) (ATR)-Fourier transform infrared (FTIR) on root section. Inoculated roots exhibited signiWcantly larger numbers of tips and extending surface to rhizosphere when compared to controls. Measured mechanical parameters of inoculated roots showed a slight increase in rupture stress up to the largest diameter (1.2 mm) when compared to controls. StiVness (Young's modulus) values were nearly constant for inoculated plants with higher values than for non-inoculated plants at day 26 and day 35. Using Principal Components Analysis of ATR-FTIR proWles, the polysaccharide enrichment of inoculated roots compared to controls was found at day 35. Noticeable absorbance at wavenumber speciWc to aromatic ether (lignin) was observed in control plants. All these data had a pattern of immature root properties, when maize was inoculated with Azospirillum lipoferum CRT1. Observed modiWcations of root development are possibly conducive to unseen beneWcial eVects, like water retention, resistance to mechanical stress, or root litter quality. Studies on more mature plants are required to assess if the diVerences between inoculated and control plants would persist or become accentuated with time until harvest.

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