Rhizosphere Microflora as Affected by Soil Moisture Changes

F the report by Hellriegel and Wilfarth (3) on the symbiosis between root nodule bacteria and leguminous plants, the contributions to plant welfare by the microflora near, on, or within plant roots became a subject for intensive study. The demonstration of the role of symbioticbacteria in the fixation of atmospheric nitrogen led soil microbiologists to believe that equally dramatic relationships might exist between other bacteria and other plants. In the decade following Hellriegel and Wilfarth's publication, claims were made that various crops could be inoculated profitably with nonsymbiotic bacteria, but such claims failed to win general acceptance. However, it did become established that microbes were encouraged to develop on or around plant roots, and there resulted a definition by Hiltner (4) of this ecologic region, the rhizosphere, as ''that soil region inside which the soil flora is subjected to the specific influence of plant roots". There appeared almost simultaneously with Hiltner's publication a controversy as to whether there was a special rhizosphere flora, or whether there was merely a region of stimulation of the general soil population. Inadequacy of knowledge both of microbiological technique and of taxonomy of the soil flora contributed to a slow development of the biology of the rhizosphere during the first quarter of the current century. A series of papers by Starkey (7) in the period 1929-31 again focused attention on the microbiology of the rhizosphere. This renewal of attention to the rhizosphere, beyond the closely related but more specialized problems of (a) root diseases, (b) legume — bacteria symbioses, and (c) 'mycorrhizal formations, each of these characterized by the development of its owri extensive literature, was fostered by a diversity of interests. Russian microbiologists developed a program of seed bacterization with nonsymbiotic bacteria; they also became interested in the relation of the microflora of the rhizosphere to the formation of a desirable soil structure. Some American workers studied the saprophytic microflora of the rhizosphere and its relationship to the susceptibility or resistance of plant roots to soil-borne diseases. Canadian investigators became similarly interested, and extended their studies to consider the rhizosphere in relation to the nutritive requirements of soil bacteria. The diversity of interests led to a diversity of experimental approaches and techniques as well as to nonuniformity in terminology and in the expression and interpretation of observations. In illustration, it may be noted that densities of microbial populations in the rhizosphere have been expressed as numbers per gram of gross sample (roots and adhering soil), as numbers per gram of root surface scrapings, as numbers per gram of the soil fraction of the gross root-soil sample, as numbers in successive washings of the root sample, and as numbers per square centimeter of root surface. In the course of rhizosphere studies, Timonin (9) noted higher microbial populations for the rhizosphere in soil maintained at 30% moisture-holding capacity than at 60%. Similarly, Clark (i) noted increasing bacterial numbers for roots of wheat as moisture content at sampling decreased through several intermediate gradations from 24.5% to 12.0%. In the investigations cited, the effects of soil moisture were considered only very briefly; also, differing bases were used for the expression of microbiological data. Inasmuch as there are now accumulating in the literature reports on the microbiology of the rhizospheres of various plants under widely differing soil conditions, it appeared desirable to make some investigation of the influence of soil moisture on the microbial density of the rhizosphere.