SUMMARYPreviously described methods to quantify the proportion of root length colonized by vesicular-arbuscular (VA) mycorrhizal fungi are reviewed. It is argued that these methods give observer-dependent measures of colonization which cannot be used to compare, quantitatively, roots examined by different researchers. A modified method is described here to estimate VA mycorrhizal colonization on an objective scale of measurement, involving inspection of intersections between the microscope eyepiece crosshair and roots at magnification x 200; it is referred to as the magnified intersections method. Whether the vertical eyepiece crosshair crosses one or more arbuscules is noted at each intersection. The estimate of colonization is the proportion of root length containing arbuscules, called the arbuscular colonization (AC). The magnified intersections method also determines the proportion of root length containing vesicles, the vesicular colonization (VC), and the proportion of root length containing hyphae, the hyphal colonization (HC). However, VC and HC should be interpreted with caution because vesicles and hyphae, unlike arbuscules, can be produced in roots by non-mycorrhizal fungi.

SUMMARYThis study examined the mechanism by which vesicular-arbuscular (V.'\) mycorrhizal colonization of young maize {Zea mays L.) plants is inhibited by soil disturbance. Attention focussed upon the role of the external mycelial network.Plastic cylinders were divided into a central and two outer compartments by a fine nylon mesb screen w-hich was impervious to roots but not to mycorrhizal hyphae. Soil was packed into each compartment and maize plants grown in the outer compartments over a series of 3-week cycles, leaving root-free central sections. Prior to a final growth cycle, the root-free soil in the central compartment was either disturbed and repacked or left undisturbed. In a third treatment tbe hyphal connections between the outer and central sections were severed, while leaving the sou in all sections undisturbed. Plants were then grown in tbe central compartments.Mycorrhizai colonization and shoot P and Zn concentrations of plants grown in the root-free zones were substantially reduced by disturbance of tbe soil. The reduction in colonization was similar to that found in control pots in which tbe root system and mycelial network were disturbed. This suggested that destruction of the hypbal network is an important component in the disturbance phenomenon. Removal of hyphal connections with roots in the outer compartments had no effect on colonization or nutrient absorption. Isolation of the mycelial network from the root system from which it had originated did not then seem to affect its infectivity.I he mycelial network appears to be an important component of the inoculum potential of an undisturbed soil and It IS likely tbat its destruction is directly responsible for much of the effect of soil disturbance upon mycorrhizai colonization. Evidence is presented indicating that the undisrupted mycelial network may also increase the nutrient absorption capacity independent of the degree of colonization.

SUMMARYThis study was designed to test the hypothesis that disruption of the macrostrueture of a previously zero-tilled soil is capable of reducing vesicular-arhuscular (VA) mycorrhizal infection to a degree such that phosphorus (P) absorption is reduced. Undisturbed soil cores (within plastic cylinders) were excavated from long-term, zerotilled plots. Soil removed from these plots, which was first subjected to structurally disruptive forces and then packed into identical cylinders, formed the disturbed comparisons. Maize (Zea mays L.), wheat (Triticum aestivum L.), spinach (Spinacea oleracea 1>.) or rape (Brassica iiapiis L.) were grown in the cores and P and zinc (Zn) absorption patterns monitored.Soil disturbance significantly reduced P and Zn absorption by maize grown in soil originating from thtee sites differing in local geography and/or texture. Disturbance also reduced mycorrhizal infection in the three soils.A significant effect of disturbance upon the VA mycorrhizal infection of both maize and wheat roots (both mvcorrhizal) and also upon the P absorption by these species was observed. None was found with respect to spinach and rape comparisons (non-mycorrhizal).Injection of benomyl, a potent inhibitor of mycorrhizal fungi, into the soil surface significantly reduced the influence of soil disturbance upon P absorption. No significant difierences were found in VA mycorrhizal infection within fungicide-treated cores.We accepted our hypothesis based upon the above evidence.

SUMMARYPrior studies have established that phosphorus absorption by maize from undisturbed soil is greater than from soil which has been disturbed since the previous period of growth. Those experiments were conducted under controlled conditions and with only two disturbance treatments, namely disturbance of the soil by hand sufficient that it will pass a 5 mm sieve, and no disturbance at all. We present here two experiments in which additional treatments of intermediate degrees of disturbance are included.In a growth chamber experiment, cutting the soil into 1, 2 or 4 cm cubes reduced the phosphorus absorption and shoot dry matter, but the reduction was not as great as that after hand disturbance of the soil. In a field experiment, the phosphorus absorption and shoot dry matter found in a conventional tillage and no-till treatments were less than in hand-planted no-till plots, but greater than with severe soil disturbance. In both experiments increases in shoot dry mass and shoot phosphorus concentration in the less disturbed treatments were not accompanied by an increase in mycorrhizal colonization. This is inconsistent with earlier studies in which colonization was greater in undisturbed soil.Since it has now been demonstrated that increased colonization is not a necessary component in the process whereby lack of disturbance results in improved plant performance, the role of mycorrhizas in this effect should be reconsidered. If mycorrhizas are important in this respect then this can only be by means of changes in the external mycelium. However, without experimental confirmation this interpretation is speculative.

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