Effects of 2450 MHz microwave radiation on nitrification,respiration and S-oxidation in soil

A 20 s exposure to 2450 MHz microwave radiation had a marked differential effect on the viable count of soil micro-organisms, had little influence on numbers of heterotrophic bacteria, but reduced fungal colonies on dilution plates to zero. The growth of fungi from soil particles was also reduced following treatment. Prolonging the exposure to microwave radiation progressively inhibited nitrification and S-oxidation, but stimulated ammonification. Brief exposures (20s) also stimulated S-oxidation and increased the numbers of thiobacilli in soil. Most of these effects are explained by reference to the marked increase in soil temperature resulting from microwave treatment.     

Biological tools for the assessment of contaminated land: applied soil ecotoxicology

Abstract. Chemical analysis alone is inadequate for comprehensively assessing the impact of soil pollution on biota. The term bioavailability can only be applied in a context specific to a target biological receptor or a proven chemical surrogate. Integration of biological and chemical data can often yield significant advances in hazard assessment and act as a suitable baseline for making site-specific risk assessments. Here, the value of biological techniques is discussed and their application described. The relative merit of test selection is considered and the new direction being developed in sublethal assessments. Currently, however, one of the major limitations is the seeming lack of flexibility of many assays in that they are either applicable to agricultural systems or industrial scenarios, but rarely to both. As a consequence, few assays have internationally adopted protocols. The introduction of new methods and the continued improvement and refinement of assays make this area of soil science dynamic and responsive.     

The effect of freeze/thaw on gaseous nitrogen loss from upland soils

Abstract. The effect of a short-term freeze/thaw cycle (15°C to -8°C to 15°C) on gaseous N-loss (denitrification and NH₃-volatilization) from intact blocks of an upland soil is described. Rates of both denitrification and NH₃-volatilization were increased by the freeze/thaw cycle, particularly when the blocks had previously been fertilized with urea. Increased gaseous N-loss due to freeze/thaw is reported for soils under heather and under improved grass pasture.     

Influence of Soil Temperature and Matric Potential on Sugar Beet Seedling Colonization and Suppression of Pythium Damping-Off by the Antagonistic Bacteria Pseudomonas fluorescens and Bacillus subtilis

ABSTRACT Pseudomonas fluorescens B5 and Bacillus subtilis MBI 600 colonized sugar beet seedlings at matric potentials of -7 x 10(3), -140 x 10(3), and -330 x 10(3) Pa and under five temperature regimes ranging from 7 to 35 degrees C, with diurnal fluctuations of 5 to 22 degrees C. No interaction between matric potential and temperature was observed. In situ bioluminescence indicated physiological activity of Pseudomonas fluorescens B5. Colonization of the root at >/=4 cm below the seed decreased at very low matric potential (-330 x 10(3) Pa). Total population size of Pseudomonas fluorescens B5 per seedling was significantly increased at -140 x 10(3) Pa. However, matric potential had no significant effect on the population density of Pseudomonas fluorescens per gram of root fresh weight and did not affect the distribution of the population down the root. Total population size per seedling and downward colonization by Pseudomonas fluorescens B5 were significantly reduced at high temperatures (25 to 35 degrees C). Maximum colonization down the root occurred at intermediate temperature (15 degrees C) at both matric potentials (-7 x 10(3) and -140 x 10(3) Pa). Addition of B. subtilis MBI 600 to the seed had no effect on rhizosphere populations of Pseudomonas fluorescens B5. Populations of B. subtilis MBI 600, which consisted largely of spores, were slightly reduced at lower matric potentials and were not affected by temperature. Survival and dry weight of plants in soils infested with Pythium spp. decreased with increasing soil temperature and matric potential, indicating an increase in disease pressure. However, there was no significant interaction between the two factors. At -330 x 10(3) Pa, soil dryness but not Pythium infection was the limiting factor for plant emergence. At temperatures of 7 to 25 degrees C and matric potentials of -7 x 10(3) to 120 x 10(3) Pa, treatment with Pseudomonas fluorescens B5 increased plant survival and dry weight. At 7 degrees C and -120 x 10(3) Pa, there was almost complete emergence of seeds treated with Pseudomonas fluorescens B5. Antagonistic activity of Pseudomonas fluorescens B5 decreased with increasing soil temperature and decreasing matric potential. At 25 to 35 degrees C and -7 x 10(3) Pa, no effect was observed. In regimes with different day and night temperatures, the maximum (day) temperature was decisive for disease development and antagonistic activity. B. subtilis MBI 600 displayed no significant antagonistic effect against Pythium ultimum and did not influence the performance of Pseudomonas fluorescens B5 in combined inocula.     

Environmental and spatial characterisation of bacterial community composition in soil to inform sampling strategies

Soil physicochemical properties and microbial communities are highly heterogeneous and vary widely over spatial scales, necessitating careful consideration of sampling strategies to provide representative and reproducible soil samples across field sites. To achieve this, the study aimed to establish appropriate sampling methodology and to determine links between the variability of parameters, utilising two sampling strategies. The first (design 1) involved extracting 25 cores from random locations throughout the field and pooling them into five sets of five cores. The second (design 2) involved a further 25 cores within five 1 m² sub-plots. Sub-samples from each sub-plot were pooled in order to determine between and within sub-plot variability. All samples were analysed independently and as pooled sub-samples. Results indicate that pooling spatially separated samples significantly reduced the variability in pH, compared to individual samples. Pooling samples from a small area resulted in lower within sub-plot variability than between sub-plots for pH and bacterial community composition assessed by terminal-restriction fragment length polymorphism analysis. Following multivariate statistical analysis, a large amount of variation in community composition was explained by soil pH, which is remarkable given the relatively small size of the sampling area and minor differences in pH. Moisture content was also important in determining bacterial communities in the random design (design 1). In the 1 m² sub-plot design (design 2), the spatial location of the plots explained a large degree of the variation in bacterial community composition between plots, which was due to spatial autocorrelation of pH and possible additional environmental parameters. This study emphasises the importance of sampling design for obtaining representative samples from soil.     

Effects of drought on isolates of <Emphasis Type="Italic">Bradyrhizobium elkanii</Emphasis> cultured from <Emphasis Type="Italic">Albizia adianthifolia</Emphasis> seedlings of different provenances

Albizia adianthifolia (Schumach) W. F. Wright, a N-fixing legume tree, has a wide distribution in Africa, in Ghana occurring in high rainfall forests and in seasonally droughted forests, and is associated in the Ghanaian forest zone with dry, infertile sites. We hypothesised that A. adianthifolia hosted different rhizobial strains in different forest types, and that these different strains would show different growth responses to moisture stress and different motility and mortality in droughted soil. Three isolates, extracted from seedlings of A. adianthifolia growing in three forest types differing in seasonal drought, were identified as Bradyrhizobium elkanii and exposed to varying levels of osmotic stress. Growth responses varied between the three strains, one of which displayed clear signs of drought tolerance. A novel approach using soil leaching columns was used to test the effects of soil pore water (in terms of neck diameter) on both the survival and movement of wet and dry forest rhizobial isolates through soil columns. The responses of the isolates were significantly different, the pore neck diameter, marginally insignificant and the drought treatment insignificant. Thus the dry forest isolate survived better in all treatments, and showed less response to the treatments, than the isolate from the wet forest. The results offer preliminary evidence that Bradyrhizobium elkanii strains from A. adianthifolia in Ghana have evolved in response to local differences in seasonal water availability. These differences could assist the selection of A. adianthifolia provenances for agroforestry or land rehabilitation.     

Influence of soil type and pH on the colonisation of sugar beet seedlings by antagonistic Pseudomonas and Bacillus strains,and on their control of Pythium damping-off

In five different soils originating from Scotland (Craibstone and Cruden Bay), Germany (Magdeburg and Uelzen) and Greece (Tymbaki), Pseudomonas fluorescens B5 reached higher population sizes (4.7–5.7logCFU/plant) on 12-day-old sugar beet seedlings than Bacillus subtilis MBI 600 (4.1–4.8logCFU/plant). Total population size per plant was not affected by soil type. In all five soils, the antagonists reached highest population densities in the hypocotyl and the upper 2cm root section (P. fluorescensB5: 5.2–6.8log₁₀CFU/g plant fresh weight, Bacillus subtilisMBI 600: 5.2–6.1log₁₀CFU/g plant fresh weight) and declined to 0–3log₁₀CFU below 4cm root depth. Colonisation by P. fluorescens B5 down the root was slightly increased in the soils from Craibstone, Magdeburg, and Uelzen compared to the sandy clay loam from Tymbaki. In lux-marked P. fluorescens B5, population density was positively correlated with light emission in all soils; the light emission indicated physiological activity of the strains. However, P. fluorescens B5 reduced Pythium damping-off (measurement after 14 days plant growth) only in three of the five soils (Craibstone, Cruden Bay and Magdeburg). Co-inoculation of B. subtilis MBI 600 increased downward colonisation of the root by P. fluorescens B5, but not the total population ofP. fluorescens B5 per plant. Bacillus subtilisMBI 600 did not reduce Pythium damping-off in any of the soils nor did it influence the efficiency of co-inoculated P. fluorescens B5; its population consisted mainly of physiologically inactive spores. In Craibstone soil, pH did not affect population density, distribution along the root or biocontrol activity against P. ultimum of P. fluorescens B5 or B. subtilis MBI 600.     

A partial chloroform-fumigation technique to characterise the spatial location of bacteria introduced into soil

Bioluminescence-marked cells of Pseudomonas fluorescens were inoculated into soil by introduction into pores of two different size classes (< 6 and 30–60 m neck diameter). Partial chloroform fumigation resulted in a differential killing of cells depending on the placement of the inoculum within the soil pore network and on the period of fumigation. Reduced survival was associated with increasing periods (30–120 min) of fumigation, and with inoculum placement into larger rather than smaller pores. Comparison of the effects of partial fumigation on cells introduced into four soils of contrasting pore-size distribution highlighted the need to calibrate the method on the basis of air/water-filled pore space and chloroform diffusion dynamics for different soil types. It is proposed that partial fumigation facilitates spatial characterisation of the distribution of bacterial cells introduced into soils.     

Meeting the challenge of scaling up processes in the plant–soil–microbe system

The scaling up of processes in the plant–soil–microbe system represents one of the greatest challenges facing environmental scientists and yet is essential for sustainable land management worldwide. The latter encompasses, for example, the mitigation of and adaptation to anthropogenic climate change, the bioremediation of industrially contaminated sites, catchment management of human pathogens such as Escherichia coli O157 and integrated crop management on the farm. Scaling up is also essential for the regional and global biogeochemical modelling that will inform policy-makers of the critical environmental factors driving climate change. Despite increasing understanding of the links between gene expression and process on a microscale, there is still much progress to be made when relating this to processes at the macroscale. In this paper, we explore the challenges this poses and examine key case studies of successful up-scaling.