Soil sustainability in organic agricultural production

Traditionally, the assessment of soil sustainability and the potential impact of cultivation are based upon the application of chemical procedures. In the absence of a biological context, these measurements offer little in understanding longterm changes in soil husbandry. Detailed microcosm investigations were applied as a predictive tool for management change. The microcosms were designed with homogenised soils treated with organic amendments. Key soil functional relationships were quantified using stable isotope techniques, biochemical measurements and traditional approaches

Effects of increased nitrate availability on the control of plant pathogenic fungi by the soil bacterium Bacillus subtilis

Abstract In wet soils, low oxygen conditions often develop that favour disease development by many soil-borne plant pathogens. The introduction of a biocontrol agent, to suppress disease development, would require that the agent remains metabolically active under such conditions. Denitrifying bacteria can maintain this metabolic activity by switching to nitrate respiration. In the rhizosphere, plant roots not only supply carbon as an electron donor, but also cause a localised lowering of oxygen concentrations, conditions favourable for nitrate respiration. Two strains of Bacillus subtilis, showing strong inhibition of a number of pathogenic fungi on agar plates, and the capacity to grow under anoxic and anaerobic conditions when provided with nitrate, were used to study the possible involvement of nitrate respiration in fungal disease control. The effect of the addition of nitrate on the activity of these antagonistic strains was studied under anoxic conditions using the sealed plate method of Fiddaman and Rossal [Fiddaman, P.J., Rossal, S., 1995. Plant Pathol. 44, 695-703]. The assay tests the activity, measured as a reduction in fungal growth, of antifungal volatiles (AFV) produced by the bacteria. The in vitro experiments showed that antagonism by the B. subtilis strains towards Fusarium oxysporum varied under anoxic conditions, depending on the nitrate availability and agar used as a growth medium. AFV activity was increased by the presence of nitrate in the medium at concentrations of 10 mM or more. Nitrate respiration may therefore have an important role in the control of fungal root diseases by allowing denitrifying soil-borne bacteria to remain metabolically active in wet soils with low oxygen concentrations

Development of an acute and chronic ecotoxicity assay using lux -marked Rhizobium leguminosarum biovar trifolii

A soil isolate of Rhizobium leguminosarum bv. trifolii was marked with a lux CDABE gene cassette to enable the expression of bioluminescence. The suitability of the bacterium as a soil pollution biosensor was assessed using acute and chronic assays. Bacterial bioluminescence responded sensitively to the metals studied, The order of sensitivity was found to be Cd > Ni = Zn > Cu for the acute test and Cd > Ni = Zn = Cu for the chronic test. The sensitive response of the biosensor highlighted its potential for use as an indicator of soil pollution

Bacterial and Fungal Contributions to Carbon Sequestration in Agroecosystems

This paper reviews the current knowledge of microbial processes affecting C sequestration in agroecosystems. The microbial contri-bution to soil C storage is directly related to microbial community dynamics and the balance between formation and degradation of mi-crobial byproducts. Soil microbes also indirectly influence C cycling by improving soil aggregation, which physically protects soil organic matter (SOM). Consequently, the microbial contribution to C seques-tration is governed by the interactions between the amount of micro-bial biomass, microbial community structure, microbial byproducts, and soil properties such as texture, clay mineralogy, pore-size distribu-tion, and aggregate dynamics. The capacity of a soil to protect micro-bial biomass and microbially derived organic matter (MOM) is directly and/or indirectly (i.e., through physical protection by aggregates) related to the reactive properties of clays. However, the stabilizatio

Proline transport increases growth efficiency in salt-stressed Streptomyces griseus.

Streptomyces griseus synthesizes proline for osmoregulation under salt stress. Uptake of exogenous [14C]proline and internal synthesis of proline were quantified in cells growing at salt concentrations from 0 to 1 M NaCl. Externally supplied proline accounted for an increased proportion of the intracellular pool of free proline as salt concentration was increased, but neither the concentration nor the composition of the internal amino acid pool was substantially altered by supply of exogenous proline. Uptake of exogenous proline significantly increased the specific growth yield of S. griseus growing under salt stress; the increased yield was proportional to reductions in proline synthesis