FUNCTIONAL ECOLOGY OF VESICULAR-ARBUSCULAR MYCORRHIZAS AS INFLUENCED BY PHOSPHATE FERTILIZATION AND TILLAGE IN AN AGRICULTURAL ECOSYSTEM

The importance of vesicular arbuscular mycorrhizas (VAM) in an agricul tural crop production system depends largely on our ability, through s oil management, to increase the effectiveness of the indigenous mycorr hizal fungal population. To do so requires a good understanding of the functional ecology of the symbiosis. In this article, we discuss prim arily our programs at Guelph, which have focused on two aspects of the symbiosis: the influence of phosphate (P) fertilization on colonizati on and the influence of soil disturbance by tillage on colonization an d P absorption. Although it is generally accepted that the level of co lonization of roots by VAM fungi decreases with increased P availabili ty, we have found that the decrease is not as marked as thought. A rea sonable degree of colonization was observed at available P levels well above those required for maximum yield. We have also found that the r eduction in colonization occurs to a greater extent in the roots growi ng in a fertilized zone than in those outside this zone. Thus, althoug h a band application of fertilizer may markedly reduce colonization in the fertilized zone, the remainder of the root system would be well c olonized, and have an increased ability to acquire phosphorus. That so il disturbance by tillage reduces the effectiveness of the VAM symbios is in maize was first observed in the early 1980s. Since then we have conducted numerous field and growth chamber experiments to determine t he practical importance of this effect in crop production and to eluci date the mechanisms involved. Systems such as no-till and ridge-till h ave been shown to result in greater P absorption during early maize gr owth. They also result in earlier colonization in some, but not all, c ases. We have not, as yet, observed a yield advantage as a result of t he earlier P absorption. This may be because factors other than P nutr ition are limiting yields with the reduced tillage systems. Our result s do indicate quite clearly, however, that greater rates of P fertiliz er are not required in reduced tillage systems, compared with systems that cause a greater degree of soil disturbance. We suggest that lesse r rates of P fertilizer may be required, which would have both an econ omic and environmental impact. Our studies to elucidate the mechanisms have led to the conclusion that the integrity of the extraradical myc elium 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 effec t, 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 extraradic al mycelium is important for early P nutrition under our field conditi ons led to the hypothesis that the mycelium from a previous crop retai ns its infectivity and is able to absorb and transport P to newly atta ched roots following an extensive period during which the soil is froz en. Experiments in which pouches containing soil with a root-free myce lium were exposed to freezing for differing lengths of time have provi ded evidence to support this hypothesis. Roots growing in undisturbed pouches removed from the field when the soil was frozen were rapidly c olonized during a bioassay. Disturbance of the soil in these pouches i nvariably reduced P absorption by the bioassay plants and sometimes, b ut not always, reduced colonization. A more thorough understanding of the mechanisms involved in the effect of disturbance on colonization w ould increase the potential for increasing the effectiveness of the VA M symbiosis in crop production through management practices.