I.M. Young

Tillage, habitat space and function of soil microbes

This review examines the effect of tillage on microbial habitat space, and the roles of microbes in influencing N-transformation processes within a heterogeneous soil environment. Literature relating tillage to microbial processes is assessed critically focusing on (a) degrees of physical disruption and N-processes, (b) interactions between organisms and the soil pore network, and (c) the role of soil structure in mediating oxygen movement to sites of microbial activity in soil. Spatial heterogeneity is shown to be a key characteristic of soil structure and N-transformation processes, impacting on predator:prey relations, microbial habitable pore space, and the modelling of the soil system with respect to denitrification. The latter area is discussed with respect to the notion of how a functional appraisal of soil structure may be approached theoretically, at the aggregate and soil profile scale.

Quantification of fungal morphology, gaseous transport and microbial dynamics in soil: an integrated framework utilising fractal geometry

The consequences of heterogeneous structure for nutrient acquisition by soil fungi, microbial dynamics and transport in soil are studied. Fractal geometry provides the unifying theme and forms the basis of a theoretical framework for studying dynamics in heterogenous media. The interpretation of foraging strategies of soil fungi are presented which suggest that the processes governing branching and hyphal mass distribution are independent. Classical diffusion is shown to be inappropriate for the study of diffusion in heterogeneous soil and a new theory is proposed which incorporates heterogeneity and pore tortuosity. The consequences of structure for microbial spatial and temporal dynamics are examined and it is found that an understanding of these and related processes such as nutrient cycling must include the role of soil structure. While stressing the need to appreciate the relevance of the theory to any particular application, it is shown that quantitative fractal geometry can yield insights into the mechanism whereby spatial organisation influences the interaction between structure and biotic processes in the soil.

Microbiological factors affecting the colonisation of soil aggregates by Fusarium oxysporum f. sp. raphani

Abstract To elucidate factors that affect the colonisation of soil aggregates by Fusarium oxysporum f. sp. raphani PEG-4 (hereafter PEG-4), microcosms were designed which comprised a row of three soil aggregates 4–6 mm dia. placed 2 mm apart on silicone rubber podia. The central aggregate was sterile and inoculated with PEG-4. The lateral aggregates were non-sterile or treated in various ways to affect their microbiological status. Following incubation, propagule populations of PEG-4 in the lateral aggregates were enumerated. The degree to which PEG-4 colonised non-sterile aggregates was less than 1% of the control (sterile aggregates). When non-sterile aggregates were dried to −200 kPa or antibiotics were added to them to inhibit bacteria, colonisation by PEG-4 did not differ greatly from the control, suggesting that the suppressing factors might be of fungal origin. That PEG-4 colonised chloroform-fumigated aggregates, where there was a relative abundance of bacteria, supported this hypothesis. In addition, sterile aggregates which had been colonised by a range of soil fungi generally inhibited PEG-4 to a greater extent than aggregates which had been colonised by single strains of bacteria. The fungal strains that showed the greatest resistance to the establishment of PEG-4 were closely related to it ( F. oxysporum f. sp. raphani, F. nivale , F. oxysporum f. sp. cucumerinum ). An antibiotic-producing bacteria, Burkholdier cepacia MRT11, was also effective at inhibiting PEG-4. Outcomes of interactions between PEG-4 and the other microbes on agar plates were poorly correlated with the outcomes in the aggregate system. We suggest that such aggregate-based systems may be useful in screening putative biocontrol agents since they are soil-based and effects can be readily assessed.

An empirical stochastic model for the geometry of two-dimensional crack growth in soil (with Discussion) ☆

A model is presented for the fragmentation of drying soil, based on the geometry of two-dimensional crack growth. The model restricts itself to the formation of two-dimensional cracks. The pertinent parameters used to generate cracking patterns include those used to characterise crack growth development as a random walk, fragmentation of peds above a certain size threshold and attraction of cracks within defined distances. A sensitivity analysis is carried out to examine the effect of varying these parameters on final cracking patterns. The variation of curvature and threshold to aggregate splitting are shown to have significant effects. Simulations based on this model are illustrated, and compared with cracks in real soil.

Growth of a ciliate protozoan in model ballotini systems of different particle sizes

The effect of structure (i.e. particle size) on protozoan population development was studied using liquid culture, containing known amounts of the bacterium Erwinia cartovora subsp. carotovora or nutrient growth medium PPY, and the ciliate protozoan Tetrahymena pyriformis. Structure was introduced into each system in the form of different size ranges of ballotini (glass beads), or sand. Even with the smallest particle sizes used, all pore pathways were accessible to protozoa. Incorporation of structure into nutrient solution acted to lower significantly (P < 0.05) protozoan activity in the structured pore network, as compared to the nutrient solution without structure. There were no significant differences (at the 5% level) in the final protozoa population in either substrate system. As particle size decreased, the protozoan population also decreased. Structure was shown to introduce distances between protozoa and bacterial cells thus, in comparison with treatments without structure, protozoan populations were significantly reduced. Further, reducing particle size would increase the time taken to explore the available pore volume, and reduce the amount of food available in each pore. The outcome was that decreasing the particle size reduced the feeding rate and so reduced the rate of population increase. Other possible physical mechanisms which may limit protozoan movement, such as surface area, are examined.

A multiple scaled fractal tree

In a study of two species of oak tree, the distribution and arrangement of branch lengths are found to be governed by a simple algorithm. The algorithm which has its footing in a type of fractal self-similarity observed in other physiological structures ( West et al., 1986 . J. appl. Physiol. 60, 189–197; Goldberger & West, 1987 . Yale J. biol. Med. 60, 421–435), reproduces the observed power-law behaviour of mean branch length with order. Furthermore, the high degree of intra-order variability is accounted for as a natural consequence of the superposition of a multiplicity of scales in the structure. The conclusions of this study point to a genetic rather than environmental origin for the design.

Links between substrate additions, native microbes, and the structural complexity and stability of soils

Soil pore space influences and controls a vast array of soil processes, physical, chemical and biological. The geometry and dimensions of the pore space define the rates at which such processes occur. Using relatively simple structures, generated from the desiccation of soil slurries, we investigated the action of adding a range of substrates to the soil (e.g. glucose or ammonium nitrate), in relation to emerged cracking patterns and soil stability. Using probability and Monte Carlo techniques, we quantified the heterogeneity and connectivity of the cracking patterns. We hypothesise that the addition of substrates directly acts to alter microbial activity which then alters the cracking patterns of dried soil slurries. In addition, we show that the addition of substrates acted to decrease crack heterogeneity (1.30–1.47), and increase crack connectivity (1.15–1.27) and density (10–16%), (P<0.05). The addition of glucose decreased the number of aggregates created during desiccation and decreased the stability of cracks (P<0.05). The mechanisms controlling these effects are discussed.

Effects of soil matric potential and bulk density on the growth of Fusarium oxysporum f. sp. raphani

Abstract The effects of soil dry bulk density and matric potential on the growth of the pathogenic fungus Fusarium oxysporum f. sp. raphani PEG-4 (PEG-4) in the absence or presence of selected soil microorganisms were investigated. These ‘challenging’ organisms included F. oxysporum f. sp. cucumerinum , a soil fungus Trichoderma viride and a Gram-positive bacterium Pimelobacter sp. The degree of growth of PEG-4 in sterile soils previously colonised by, or simultaneously inoculated with, the soil microorganisms was determined. Soil extracts were also used to test for the effects of a mixed pupulation of soil microorganisms on the interactions. Soil bulk density, ranging from 1.1 to 1.5 Mg m −3 and matric potential (− 1000 to −1 kPa) significantly affected the PEG-4 propagule numbers: PEG-4 appeared to form fewer propagules in more compacted soil or soil at lower matric potential (i.e. drier soil). The potential for the prior colonisers to explore the soil and inhibit the growth of PEG-4 is discussed in relation to the soil structure and associated moisture conditions.

Water-suspensible solids and structural stability

The water suspensibility of a number of particle/microaggregate sizes ( 106 μm). After a period of saturation of approximately 1 min. significant amounts of water-suspensible material (WSM) < 106 μm in size were observed. After a 10-min period of saturation, the quantity of WSM < 60 μm doubled. whilst no significant change occurred in other sizes. No further WSM of < 60 μm was produced over a saturation period of 1000 min. The input of energy into the saturated soil by shaking produced significantly more WSM of all sizes. The WSM < 60 μm as a percent of the total soil in that size class was considerably greater than for other sizes. n nIt is shown that WSM < 60 μm as opposed to WSM < 2 μm is a more important size in the stability of structure for certain soils and deserves more attention. Results are discussed in connection with furrow irrigation and soil strength.

Hardsetting soils in the UK

Abstract The extent of hardsetting soils in the UK is unknown. A brief survey of relevant literature suggests that there are a significant number of soils in the UK that are hardsetting or potentially hardsetting. One soil in the UK (Big Ground) has been positively identified as hardsetting. A brief summary of the field and laboratory work carried out in this soil and a similar non-hardsetting soil (Plum Orchard) is presented. It is shown that Big Ground exhibits physical behaviour similar to the Australian hardsetting soils. The influence of UK hardsetting soils on plant growth and cultivations is discussed.