MYCORRHIZA AND SOIL ECOLOGY
Authors: HARLEY J. L. 
Published: January 2008

Summary1. Micro‐organisms are not evenly distributed throughout the soil, and different local soil variations and horizons are inhabited by populations differing in activity and number. The root regions of green plants constitute an important group of these microhabitats, and the population of the root regions of some plants have been examined experimentally. In the root region a population differing from the general soil microflora is found, and here increased rates of activity of certain soil processes, changing the availability of essential plant foods, have been described. Micro‐organisms of many types are subject to this rhizosphere effect, and roots differing in age and in genetic origin exert different effects on various types of micro‐organisms.2. The phenomena examined in mycorrhizal studies can be grouped naturally with general rhizosphere and root region phenomena. They differ essentially only in the relative dominance of one or few particular members of the root flora, but in typical cases of ectotrophic mycorrhiza the presence of the dominant fungus is associated with morphological changes in the root. Part of the confusion in the presentation of experimental results, and in the theoretical discussion of these results, is due to lack of appreciation of the general and widespread nature of rhizosphere effects and their dependence upon the nature and physiological state of the roots.3. Similar hypotheses, based on experimental results, have been put forward to explain both the general case of association of roots and micro‐organisms and the special cases of ectotrophic mycorrhiza. The excretion of substances causing stimulation of microorganisms, such as amino‐acids and vitamins, of food materials such as sugar, and of inhibiting substances, have all been suggested. Root excretions and extracts have been shown to affect the growth both of members of the flora of the root region and of mycorrhiza fungi.4. No explanation is at present available as to why there is intercellular penetration, morphological change and the formation of composite organs of root and fungus, in typical cases of ectotrophic mycorrhiza; nor is there again any explanation of the variability of such associations with soil conditions.5. The effect of the rhizosphere flora on the growth of the host plant has been examined. There may be a stimulation of growth by non‐mycorrhizal rhizosphere fungi and bacteria. Similar stimulation of growth by mycorrhizal fungi has frequently been claimed. A great majority of observers agree that under some conditions the growth of tree seedlings bearing mycorrhizal roots greatly surpasses that of those lacking these roots. Very few experiments on this point are, however, quite satisfactory, owing to the difficulty of providing adequate controls. These few, together with the large body of imperfect experiments, are sufficient to enforce the acceptance of the fact that mycorrhizal fungi frequently stimulate growth.6. The completeness of the living fungal sheath, and its intimate connexion with the root cortex in fully developed mycorrhizas, must be a fundamental consideration in the elaboration of hypotheses to explain the effects of infection on the host. Many of the existing theories do not take this sufficiently into account, because variations of the morphology of mycorrhizas between experimental treatments have frequently not been sufficiently described. The existing hypotheses are not all mutually exclusive, but cannot be further co‐ordinated or extended except by means of experiments more perfectly controlled than those yet described. Moreover, the activities or organisms associated with the true mycorrhiza‐former require examination. Recent reports of the growth of excised mycorrhiza roots cannot be fully accepted until the results have been confirmed.