Danielle Prévost

ECOLOGY OF PLANT GROWTH PROMOTING RHIZOBACTERIA

Chapter presents a discussion on the term PGPR which underlines the need to have a uniform definition to be used by all authors. The actual biodiversity of PGPR will be illustrated by examples of genera and species chosen from the literature and their mechanisms of action for the following different groups: diazotrophs, bacilli, pseudomonads, and rhizobia. As PGPR are introduced in an ecosystem where intense interactions are taking place, we describe how plants, mycorrhiza, and soil fauna can influence the microbial diversity in the rhizosphere. Finally, the beneficial interactions between PGPR and symbiotic microorganisms in the Rhizobium-legume symbiosis, and in mycorrhizal plants are discussed. Interactions of PGPR with protozoa and nematodes are also examined.

Dynamics of soluble organic C and C mineralization in cultivated soils with varying N fertilization

Laboratory studies have shown that N availability may affect C decomposition in soils. A field study was undertaken to determine to what extent water-soluble organic C (WSOC) and C mineralization could be affected by mineral N fertilizers. Soil cores were collected in the 1993 and 1994 growing seasons, from a sandy loam (loamy, mixed, frigid, Typic Haplorthod) and a sandy clay (fine, mixed, frigid, Typic Humaquept) under early-maturing maize (Zea mays L.). Soils were fertilized with NH4NO3 at reduced (10, 60, 120 kg N ha−1) or recommended (180 kg N ha−1) rates. For both soils, C mineralization rate increased significantly (P<0.05) with amount of N fertilizer only at two of the nine sampling dates. However, WSOC contents generally decreased (P<0.05) with increasing N fertilizer rate. Overall, a decrease in soil mineral N content was consistently associated with an increase in WSOC content. The relationship between soil WSOC and mineral N contents was logarithmic for both soils (r2=0.70 in the sandy loam and r2=0.89 in the sandy clay). For both soils, we estimated a critical level of N of about 60 mg kg−1, below which small decreases in mineral N were associated with large increases in WSOC contents.

Bio-encapsulation of microbial cells for targeted agricultural delivery

Biofertilizers, namely Rhizobium and biocontrol agents such as Pseudomonas and Trichoderma have been well established in the field of agricultural practices for many decades. Nevertheless, research is still going on in the field of inoculant production to find methods to improve advanced formulation and application in fields. Conventionally used solid and liquid formulations encompass several problems with respect to the low viability of microorganisms during storage and field application. There is also lack of knowledge regarding the best carrier in conventional formulations. Immobilization of microorganisms however improves their shelf-life and field efficacy. In this context, microencapsulation is an advanced technology which has the possibility to overcome the drawbacks of other formulations, results in extended shelf-life, and controlled microbial release from formulations enhancing their application efficacy. This review discusses different microencapsulation technologies including the production strategies and application thereof in agricultural practices.

Winter fluxes of greenhouse gases from snow‐covered agricultural soil:intra‐annual and interannual variations

Despite the length of winter in cold temperate climates, few studies refer to greenhouse gas emissions from soils during the nongrowing season. In this study, N2O and CO2 fluxes from agricultural and forest soils in southeastern Quebec (Canada) were measured during winter and spring from 1994 to 1997, and the influences of climate, soil, and snow properties on the gaseous emissions were examined. N2O fluxes were far greater from the agricultural soil (2-187 ng N2O m−2 s−1) than from the forest soil (< 3 ng N2O m−2 s−1), but CO2 fluxes were equivalent for both soil systems (2-102 μ CO2 m−2 s−1). The higher N2O concentrations in the lower soil horizons could be explained by positive temperature gradients with depth and concomitant negative gas solubility gradients. However, the higher N2O concentrations could also be explained by variations in the expression of N2O reductase with depth, which can modify the N2/N2O ratios in relation to the availability of O2. Calculated N2O-N fluxes showed that N losses by gaseous emissions from soils during winter and spring were comparable to, or exceeded, similar reported N losses during the growing season. The highest winter fluxes observed in 1997 were interpreted to be due to favorable meteorological conditions that prevailed for denitrification through high soil water content in summer and fall of 1996. Although interannual and nterseasonal variations of fluxes are important, this study shows that wintertime losses of N2O from agricultural soil can be up to 2 to 4 times greater than emissions measured during the growing season in similar agroecosystems.

Characteristics of rhizobia isolated from three legumes indigenous to the Canadian high arctic: Astragalus alpinus, Oxytropis maydelliana, and Oxytropis arctobia

SummaryForty-eight strains of rhizobia were isolated from the root nodules ofAstragalus alpinus (21),Oxytropis maydelliana (19) andOxytropis arctobia (8), three species of arctic legumes found in the Melville Peninsula, Northwest Territories, Canada. On the basis of 74 characteristics (cultural, physiological, biochemical and host nodulation range) the 48 arctic rhizobia could be divided into 11 distinct groups by numerical analysis techniques. All 48 arctic rhizobia were able to nodulate the three arctic legume species and also sainfoin (Onobrychis viciifolia), however, milkvetch (Astragalus cicer) was only nodulated by 33 strains. In general, the arctic rhizobia showed properties found in both Rhizobium and Bradyrhizobium. The adaptation of the arctic strains to low temperature is indicated by their ability to grow in liquid culture at 5°C.

Nitrous oxide production in soils cropped to corn with varying N fertilization

Mineral N fertilizers may contribute to N gas emissions to the atmosphere. Soil cores were collected in 1993 and 1994, in a sandy loam and a sandy clay cropped with an early-maturing corn (Zea mays L.) hybrid and fertilized with ammonium nitrate at rates of 10, 60, 120 or 180 kg N ha−1. Denitrification and N1O production rates, air-filled porosity (AFP), water-soluble mineral N (WSMN) and water-soluble organic C (WSOC) were measured. Denitrification and N2O production rates were generally small, but values >2 µg N2O-N kg−1 h−1 were measured (i) when WSMN contents exceeded 5 mg kg−1, and (ii) when AFP was <50 to 55% in the sandy loam, and <40 to 45% in the sandy clay. For most sampling dates, N2O production and denitrification rates increased with N fertilizer level. In 1993, AFP was relatively high and variable in soil cores, and regression analyses revealed that denitrification rates were closely related to AFP. In 1994, AFP was relatively low in soil cores, and regression analyses showed that denitrific...

Wastewater sludge as a substrate for growth and carrier for rhizobia: the effect of storage conditions on survival of Sinorhizobium meliloti

The inoculation of legumes with rhizobia is used to maximise nitrogen fixation and enhance the plant yield without using N fertilisers. For this reason many inoculant types were developed and optimised. In our study, the effects of the growth medium, the carrier, the temperature and the storage period were determined on the survival of Sinorhizobium meloliti. Secondary sludge from Communauté Urbaine de Quebec wastewater treatment plant and standard medium (YMB) were used for rhizobial growth. Dehydrated sludge from Jonquière wastewater treatment plant, peat and a mixture of peat and sludge were used as carrier materials. Results showed that the wastewater sludge offered better protection for rhizobia survival during freezing and thawing at -20 degrees C than the standard medium. In general, results also showed the suitability of using sludge as a carrier because it had the same or a higher potential than peat to support survival of S. meliloti. In the case of YMB-grown rhizobia, peat- and sludge-based carriers appeared to be similar in terms of survival rate during the storage at 4 and 25 degrees C. For secondary sludge-grown rhizobia, the survival was better in sludge than in peat based carrier. Generally, the cell count remained higher than 10(8) cells/g for up to 80 days at 4 and 25 degrees C in both carriers (sludge and peat). However, for the secondary sludge-grown cells stored in peat-based carrier at 4 degrees C, the viable cells decreased under 10(8) cells/g at the 81st day of storage but remained acceptable compared to the standard (10(7) cells/g of carrier).

Microbial biomass and N transformations in two soils cropped with annual and perennial species

A field study was undertaken to determine the effects of different plant species on soil microbial biomass and N transformations in a well drained silty clay loam (Typic Dystrochrept) and a poorly drained clay loam (Typic Humaquept). The crop treatments were faba bean (Vicia faba L.), alfalfa (Medicago sativa L.), timothy (Phleum pratense L.), bromegrass (Bromus inermis L.), reed canarygrass (Phalaris arundinacea L.), and wheat (Triticum aestivum L.). Measurements of microbial biomass C, denitrification capacity, and nitrification capacity were performed periodically in the top 2–10 cm of soil. On most sampling dates, all three parameters were higher under perennial than under annual species. The nitrification capacity was positively affected by the level of N applied to each species (r=0.65** for the silty clay loam and 0.84*** for the clay loam) and not directly by the plant. The differences found in microbial biomass C were significantly correlated with the water-soluble organic C present under each plant species (r=0.74*** for the silty clay loam and 0.90*** for the clay loam), suggesting differences in C deposition in the soil among plant species. In the silty clay loam, the denitrification capacity was positively related to the amount of organic C found under each plant species, while in the clay loam, it was dependent on the amount of N applied to each species. There was less denitrification activity per unit biomass under legume species than under graminease, suggesting that, depending on their composition, root-derived materials may be used differently by soil microbes.

Symbiotic effectiveness of indigenous arctic rhizobia on a temperate forage legume: Sainfoin (Onobrychis viciifolia)

Sainfoin (Onobrychis viciifolia), a temperate perennial forage legume, can be nodulated by rhizobia isolated from 3 arctic legume species:Astragalus alpinus, oxytropis maydelliana andOxytropis arctobia. Arctic rhizobia, which are adapted to growth at low temperatures, may be useful in improving symbiotic nitrogen fixation during cold phases of the growing season, if they are effective on a temperate legume. In this study, we report on the symbiotic effectiveness of arctic rhizobia on sainfoin, as appraised by the total shoot dry matter yield obtained from 2 harvests. Under N-free conditions, 5 arctic strains at the first harvest and 8 at the second harvest were as effective as temperate standard strains. In the presence of 30 mgl−1 NO3-N, 7 arctic strains gave significantly higher yields than temperate strains at the second harvest. These results indicate that effective arctic rhizobia have a potential for use as inoculants on sainfoin.

Classification of bacteria nodulating Lathyrus japonicus and Lathyrus pratensis in northern Quebec as strains of Rhizobium leguminosarum biovar viciae.

The diversity of two populations of rhizobia isolated from Lathyrus japonicus (30 strains) and Lathyrus pratensis (49 strains) growing in northern regions of Quebec, Canada, was determined on the basis of phenotypic characteristics, multilocus enzyme electrophoresis, DNA-DNA homology, and 16S ribosomal DNA sequencing. According to numerical analysis of phenotypic characteristics, strains were divided into four groups. Strains isolated from L. pratensis fell in groups I to III; the latter included reference strains of Rhizobium leguminosarum. All strains isolated from L. japonicus were included in group IV. All strains had nodulation characteristics similar to those of R. leguminosarum bv. viciae. Strains isolated from L. japonicus originating from an arctic region were usually able to grow at 5 degrees C and were more likely to be tolerant to copper (CuCl2.H2O, 100 micrograms/ml) and lead [Pb(CH3COO)2, 500 micrograms/ml] than strains isolated from L. pratensis from a boreal zone. However, both populations of Lathyrus strains were adapted to the cold in comparison to reference strains from temperate regions. Each population had similar genetic diversity (H = 0.45), determined by multilocus enzyme electrophoresis of the loci encoding eight enzymes, but the diversity obtained by analyzing all strains including the reference strains (H = 0.58) was higher. Representative strains of both populations showed high levels of DNA homology among themselves and with R. leguminosarum. Partial sequences of the 16S ribosomal RNA genes were similar to those reported for R. leguminosarum bv. viciae. We conclude that the strains isolated from L. japonicus and L. pratensis belong to R. leguminosarum bv. viciae but are distinguishable by growth at 5 degrees C, which is a characteristic related to their geographic origin.