Murray H. Miller

Functional Ecology of Vesicular Arbuscular Mycorrhizas as Influenced by Phosphate Fertilization and Tillage in an Agricultural Ecosystem

AbstractThe importance of vesicular arbuscular mycorrhizas (VAM) in an agricultural crop production system depends largely on our ability, through soil management, to increase the effectiveness of the indigenous mycorrhizal fungal population. To do so requires a good understanding of the functional ecology of the symbiosis.In this article, we discuss primarily our programs at Guelph, which have focused on two aspects of the symbiosis: the influence of phosphate (P) fertilization on colonization and the influence of soil disturbance by tillage on colonization and P absorption.Although it is generally accepted that the level of colonization of roots by VAM fungi decreases with increased P availability, we have found that the decrease is not as marked as thought. A reasonable degree of colonization was observed at available P levels well above those required for maximum yield. We have also found that the reduction in colonization occurs to a greater extent in the roots growing in a fertilized zone than in th...

Tillage translocation and tillage erosion in the complex upland landscapes of southwestern Ontario, Canada

Tillage translocation and tillage erosion were measured throughout the topographically complex landscapes of two fields in the upland region of southwestern Ontario. Translocation of soil by tillage was measured by labelling plots of soil with chloride and measuring the tracers forward displacement in response to single passes by four tillage implements (mouldboard plough, chisel plough, tandem disc and field cultivator). The change in translocation within the landscape was used to measure tillage erosion. All four implements were erosive. A relationship between tillage translocation and slope gradient was observed; however, the variability in translocation could not be explained by slope gradient alone. Slope curvature was responsible for some translocation through the planning action of tillage implements. Tillage depth and speed were subject to considerable discontinuous and inconsistent manipulation by the operator in response to changing topographic and soil conditions. Tillage speed decreased by as much as 60% during upslope tillage and increased by as much as 30% during downslope tillage, relative to that on level ground. Tillage depth decreased by as much as 20% and increased by as much as 30%, relative to that on level ground. This manipulation is typical for tillage in complex landscapes and was presumed largely responsible for the variability in the results. The manipulation of tillage depth and speed are affected by the tractor-implement match and the responsiveness of the tillage operator.

Development of fungi below ground in association with plants growing in disturbed and undisturbed soils

It is well established that young maize plants take-up more P when they are sown in soil that has been left undisturbed after the removal of the shoots of previously grown maize plants. In a growth chamber experiment we eliminated arbuscular mycorrhizal (AM) fungi by pasteurization of soil and thus prevented any such stimulation of P uptake for plants growing in undisturbed soil. Arbuscules were absent from roots growing in the pasteurized soil. Arbuscular colonization (the % root length colonized by arbuscules) of roots growing in non-pasteurized soil that had been disturbed by breaking-up and mixing by hand was 32%, compared to arbuscular colonization of 51% in the corresponding undisturbed treatment. Following soil disturbance, non-mycorrhizal fungi showed a response that was similar to that of AM fungi. Colonization of roots in the pasteurized soil by parasitic hyphae in the disturbed treatment was one-third of that in the corresponding undisturbed treatment. At the end of the experiment, the total lengths of hyphae stained with trypan-blue (TB) in the non-pasteurized bulk soil around the roots were 42.5 and 28.7 m g−1 o.d. soil for the undisturbed and disturbed treatments, respectively; corresponding values in the pasteurized soil were almost identical, at 40.1 and 25.0 m g−1. These results show that the extent of production of both AM and parasitic fungi inside roots, and the net production of TB-staining hyphae in the bulk soil around those roots, are all lower during the period of growth following soil disturbance, as compared to the undisturbed situation. Caution should be taken in the interpretation of the functional status of TB-stained hyphae collected from bulk soil in the root zone.

Early phosphorus nutrition, mycorrhizae development, dry matter partitioning and yield of maize

We conducted a field experiment to test the hypothesis that improved phosphorus nutrition occurs in maize plants with rapid arbuscular (AM) mycorrhizae development at early developmental stages and that this also is reflected in dry matter allocation and final yield. A split-split plot design was used with previous crop (Zea mays L.-maize and Brassica napus L.-canola), tillage practices (no-tillage or conventional tillage) and P fertilization (5 levels) as factors chosen to modify mycorrhizae development at early developmental stages of maize. Previous cropping with canola resulted in decreased shoot-P concentration and shoot growth of maize at early stages. No-tillage resulted in higher shoot-P concentration but lower shoot weight than conventional tillage. Greater shoot-P uptake was related to a rapid intraradical development of mycorrhizae (previous crop of maize) or rapid connection to a mycorrhizal mycelium network (no-tillage treatments). Maize yield and harvest index were lower after cropping with canola. The yield for conventional tillage was higher than that for no-tillage but the harvest index was lower. The hypothesis was supported at early stages of maize growth by the effect of previous crop but not by results of tillage, because an unknown factor reduced growth in the no-tillage system. The hypothesis was supported at maturity by increased biomass allocation to grain relative to total shoot weight in treatments with greater shoot-P concentration at early stages.

Mycorrhizae, crop growth, and crop phosphorus nutrition in maize-soybean rotations given various tillage treatments

Previously, tillage has been found to reduce early-season phosphorus (P) uptake from soil in continuous maize cropping systems. This reduced P uptake has often been associated with delayed colonization of roots by arbuscular mycorrhizal (AM) fungi. Our aim was to determine if similar responses occur in maize-soybean rotations, which are more typical of current farming in Ontario, Canada. Similar responses were expected because both are AM crops, and the mechanism by which tillage reduces P uptake is thought to be a negative impact on the development of effective mycorrhizae. Simultaneous field experiments with either maize-soybean-maize or soybean-maize-soybean rotations were conducted in 1992–4. Treatments imposed were no-till (NT), ridge-tillage (RT), and conventional tillage using a moldboard plow (MP). In 1993, early-season dry mass of maize was similar among treatments, but colonization of maize roots by AM fungi and P uptake of maize were stimulated by NT and RT, compared with MP. In 1994, early growth was more rapid overall than in 1993, but it was reduced in the NT and RT treatments compared with MP for reasons not related to P. For soybean, AM colonization in NT and RT systems was higher than with MP, but P uptake was unchanged. As was found for maize in 1994, early-season shoot dry mass of soybean was higher in the MP treatment than with NT, but both in 1993 and 1994. We conclude that colonization of both maize and soybean by AM fungi is susceptible to slower development in tilled systems, and that for maize, stimulation of P uptake under reduced tillage can occur in rotations with soybean just as easily as it does with continuous maize. Taken with other studies, the data here suggest that responses to tillage of colonization of roots by AM fungi and of P uptake could apply to many cropping systems. The slow early-season shoot growth seen in some years in response to reduced tillage is discussed.

Survival of the external mycelium of a VAM fungus in frozen soil over winter

The present investigation examines (1) whether the external VAM mycelium survives winter freezing to act as a source of inoculum in the spring, and (2) whether soil disturbance reduces the infectivity of the external VAM mycelium following freezing of the soil. Sealed pouches of fine nylon mesh were placed in pots containing soil inoculated with a Glomus species. The mesh was impervious to roots but not to hyphae. Following two 3-week growth cycles of maize in the pots, the pouches were transplanted to the field. Pouches were removed from the field once during the 4 months when the soil was frozen, and once after spring thaw. Measurements were made of VAM spore density, hyphal length and viability in the pouches. Bioassays for infectivity were conducted on all pouches. Some VAM hyphae survived freezing and remained infective following winter freezing, in the absence of plant roots. Soil disturbance did not reduce the infectivity of hyphae following exposure to freezing temperatures. We observed a change in the distribution of viable cytoplasm within hyphae over winter, which we hypothesize represents an adaptation allowing hyphae to survive freezing temperatures. We suggest that the effect of disturbance on hyphal infectivity may be related to this seasonal change in the distribution of hyphal viability.

An economic approach to evaluate the role of mycorrhizas in managed ecosystems

The benefits of management of mycorrhizas in agricultural and horticultural croppiing systems remains problematic except where the indigenous fungal population is low. Most experiments have focused on the introduction of exotic fungal isolates. Promotion of plant growth by mycorrhizas can be enhanced by increasing the effectiveness of the indigenous fungi as well as by introducing more effective species. Lack of reliable methods for identification of fungal species colonizing roots is a major limitation to characterizing the change in mycorrhizal populations. Assessment of the role of mycorrhizas in commerical food production systems must include an economic analysis. To do so requires an evaluation of the response to increasing the effectiveness of the mycorrhizal symbiosis relative to increasing yield with addition of phosphorus fertilizer. Thus field experiments should be designed to measure the response to phosphorus addition with the existing mycorrhizal population as well as with the more effectively managed population.In this paper we discuss changes that may be induced in mycorrhizal fungi by management to increase their effectiveness in promoting plant growth. We then suggest an economic analysis approach to assessing the potential benefits of this increase in effectiveness. We conclude with a discussion of research approaches needed to determine, in a more objective manner, the role of mycorrhizas in managed ecosystems.

The effect of soil aggregate size on early shoot and root growth of maize (Zea mays L.)

Differences in plant growth arising from differences in aggregate size in the seedbed are normally atributed to limitations in nutrient or water supply during the early growth period. This study was initiated to determine if these were the only mechanisms by which aggregate size influences plant response.Four different aggregate size fractions (less than 1.6 mm, 1.6 to 3.2 mm, 3.2 to 6.4 mm and 6.4 to 12.8 mm diameter) were sieved from a silt loam soil. Nutrients were added to the soil and maize was grown in the aggregates for eighteen days after seedling emergence. Soil matric potential was maintained between — 3 and −20 kPa.Shoot dry weight declined by 18% as aggregate size increased from less than 1.6 mm to 1.6–3.2 mm. There was little further decline as aggregate size increased to 6.4–12.8 mm. Final leaf area showed a similar decline. The availability of nutrients or water were not limiting.Total root length in the coarsest aggregate system was less than 60% of that in the finest system. Main axes of seminal and nodal roots were longer in the coarser aggregate systems, the length of primary laterals was not affected, and length of secondary laterals was lower in the coarser systems. A greater proportion of the roots penetrated the larger aggregates than the smaller aggregates; however, the larger aggregates offered greater resistance to penetration by a rigid micropenetrometer (150 μ diameter probe). Diameter of the main axes roots were greatest in the largest two aggregate fractions. it is speculated that a combination of increased endogenous ethylene in roots in the finest aggregate system due to entrapment by water and increased mechanical resistance in the coarsest aggregate system accounts for the observed effects on root norphology.

Response of vesicular-arbuscular mycorrhizas of maize to various rates of P addition to different rooting zones

Colonization of plant roots by vesicular-arbuscular mycorrhizal fungi is known to be reduced as the phosphorus nutrition of the plant is increased. It is generally accepted that the concentration of P in the plant rather than the soil regulates VAM colonization. Whether it is the shoot P concentration, the mean P concentration in the root system or the P concentration in the specific root being colonized is not known, but is of agronomic significance because fertilizer P is frequently applied in concentrated zones which would be expected to result in higher P concentration in roots growing in the fertilized zone than in the remainder of the root system. Growth chamber and field experiments were conducted to determine the effect on colonization of supplying varying amounts of P to different portions of the rooting zone. In growth chamber studies using a split-pot technique, the proportion of maize (Zea mays L.) root length containing arbuscules in a high-P zone was lower than that of roots of the same plant growing in a low- or medium-P zone. Root P concentration was higher in the high-P zone. In a field experiment conducted over a two-year period, VAM colonization of roots of young maize plants growing in fertilized soil was affected differently than that of roots growing outside the fertilized zone. A small addition of fertilizer P increased colonization of roots in the fertilized soil, but further additions resulted in an abrupt decline followed by a slower further decline, although colonization was not eliminated even by rates of 1600 μg P g-1 soil. Colonization of roots growing outside the fertilized zone declined gradually with increasing P addition but the overall decline was less than for roots in the fertilized zone. The data support the hypothesis that it is P concentration in the portion of the root system being colonized rather than the general P status of the plant which regulates VAM colonization. The agronomic implication of this is that, although a fertilizer band may reduce VAM colonization of roots in the band volume, roots growing outside this volume may be well colonized so the mycorrhizal symbiosis may be an important contributor to P nutrition.

Responses of mycorrhizae and shoot phosphorus of maize to the frequency and timing of soil disturbance

In several growth chamber studies, both P absorption and mycorrhizal colonization of plants grown in soil left undisturbed after removal of the shoots of the previous crop were higher relative to plants in disturbed soil. However, in one of these studies the soil was disturbed only once instead of after each of three growth cycles, and this resulted in identical colonization in the undisturbed and disturbed treatments. The present study was conducted to systematically investigate the effect of varying the frequency and timing of soil disturbance on mycorrhizal colonization. Maize (Zea mays L.) was grown for four 3-week cycles in pots which initially contained disturbed soil. Five soil disturbance treatments were used to assess the impact of the frequency with which soil is disturbed and the impact of the timing of the disturbance. The frequency of soil disturbance had major effects on mycorrhizal colonization, while the timing of soil disturbance was more related to the reduction in shoot P absorption resulting from disturbance. These results suggest that the extraradical mycelium plays a key role in the mechanism by which soil disturbance reduces shoot P absorption.