Plant and Soil 202: 89–96, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 89 Soil aggregation status and rhizobacteria in the mycorrhizosphere G. Andrade 1 , K.L. Mihara 2 , R.G. Linderman 2 and G.J. Bethlenfalvay 2,3 1 Universidade Estadual de Londrine, Depto. de Microbiologia, Londrine, PR 86051-970, Brazil and 2 U.S. Depart- ment of Agriculture, Agricultural Research Service, Horticultural Crops Research Laboratory, 3420 NW Orchard Avenue, Corvallis, OR 97330, U.S.A. 3 Corresponding author ∗ Received 11 June 1997. Accepted in revised form 15 April 1998 Key words: actinomycete, anaerobic bacterium, arbuscular mycorrhiza, Glomus mosseae, hyphosphere, rhizobac- teria, rhizosphere, soil aggregation, soil hyphae, Sorghum Abstract Soil aggregation is a dynamic process in which plants and the soil microbiota play a major role. This experiment was conducted to determine whether the effects of mycorrhizae on the stability of water-stable soil aggregates (WSA) and on selected groups of soil microorganisms are interrelated. Soil containers consisting of four com- partments were utilized. Two compartments on each side of a solid barrier were separated by a 43 μm screen that permitted the passage of hyphae, but not of roots. The roots of Sorghum bicolor plants were split over the center barrier, and the roots on one side were inoculated with an arbuscular-mycorrhizal (AM) fungus. This design produced mycorrhizosphere soils (M) by AM roots or hyphosphere (H) soils by AM hyphae in the two compartments on the one side of the barrier, and rhizosphere soils (R) by nonAM roots or root- and hypha-free bulk soil (S) in the two compartments on the other side. At harvest (10 wk), there were significant differences in WSA between soils in the order: M>R>H>S, and WSA stability was significantly correlated with root or hyphal length. Numbers of colony-forming units of the microflora (total bacteria, actinomycetes, anaerobes, P solubilizers, and nonAM fungi) were in general not correlated with root or hyphal length, but in some cases were significantly correlated with WSA. Bacteria isolated from the water-stable soil-aggregate fraction tended to be more numerous than from the unstable fraction. The difference was significant in the M soil for total bacteria and P solubilizing bacteria. NonAM fungi were more numerous in the unstable fraction of the M soil. The data show that the root and fungal components of mycorrhizae enhance WSA stability individually and additively in concert, and suggest that they affect microorganism numbers indirectly by providing a favorable and protective habitat through the creation of habitable pore space in the WSA. Introduction Rhizobacteria can promote or inhibit plant growth ac- cording to environmental conditions, host genotype or mycorrhizal status (Germida and Walley, 1996; Nehl et al., 1996), and their effects may range from promoting the development of one plant organ to in- hibiting another at the same time (Andrade et al., 1995). Similarly, rhizobacteria can stimulate (Gryn- dler and Vosátka, 1996) or inhibit (Bethlenfalvay et al., 1997) mycorrhiza formation, while arbuscular- ∗ FAX No: 1 541 750 8764. E-mail: bethleng@ucs.orst.edu mycorrhizal (AM) fungi, in turn, may increase (Olsson et al., 1996) or decrease (Christensen and Jakobsen, 1993) soil bacterial populations. The activities of soil organisms depend to a large extent on the stability of the structural matrix in which they occur. Soil water content, drying and wetting cycles, and compaction of the soil fabric by root devel- opment are well-known factors that affect the stability of soil aggregates (Emerson and Greenland, 1990; Per- fect et al., 1990). Soil structure is one of the most important properties controlling plant growth (De Fre- itas et al., 1996), and although AM effects on the