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

Abstract

The 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 those outside this zone. Thus, although a band application of fertilizer may markedly reduce colonization in the fertilized zone, the remainder of the root system would be well colonized, and have an increased ability to acquire phosphorus.

That soil disturbance by tillage reduces the effectiveness of the VAM symbiosis in maize was first observed in the early 1980s. Since then we have conducted numerous field and growth chamber experiments to determine the practical importance of this effect in crop production and to elucidate the mechanisms involved. Systems such as no-till and ridge-till have been shown to result in greater P absorption during early maize growth. They also result in earlier colonization in some, but not all, cases. We have not, as yet, observed a yield advantage as a result of the earlier P absorption. This may be because factors other than P nutrition are limiting yields with the reduced tillage systems. Our results do indicate quite clearly, however, that greater rates of P fertilizer are not required in reduced tillage systems, compared with systems that cause a greater degree of soil disturbance. We suggest that lesser rates of P fertilizer may be required, which would have both an economic and environmental impact.

Our studies to elucidate the mechanisms have led to the conclusion that the integrity of the extraradical mycelium from a previous crop is a critical factor in the greater early P absorption in undisturbed systems. It may also be important in rapid colonization of roots of newly developing seedlings. This latter effect, however, has been quite inconsistent in our studies. It is apparent that some unknown factor or factors, in addition to the integrity of the mycelium, is also critical.

The fact that a preexisting extraradical mycelium is important for early P nutrition under our field conditions led to the hypothesis that the mycelium from a previous crop retains its infectivity and is able to absorb and transport P to newly attached roots following an extensive period during which the soil is frozen. Experiments in which pouches containing soil with a root-free mycelium were exposed to freezing for differing lengths of time have provided evidence to support this hypothesis. Roots growing in undisturbed pouches removed from the field when the soil was frozen were rapidly colonized during a bioassay. Disturbance of the soil in these pouches invariably reduced P absorption by the bioassay plants and sometimes, but not always, reduced colonization.

A more thorough understanding of the mechanisms involved in the effect of disturbance on colonization would increase the potential for increasing the effectiveness of the VAM symbiosis in crop production through management practices.