|Andrade, G. - OREGON STATE UNIVERSITY|
Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 15, 1998
Publication Date: N/A
Interpretive Summary: The roots of most crop plants are colonized by soil fungi that improve plant growth through an increased uptake of mineral nutrients from the soil. They also improve soil stability through the transport organic nutrients from the plant to the soil, where soil organisms use these nutrients to cement soil particles into stable soil aggregates. This job of soil stabilization is done by the plant, by the fungi and by soil bacteria working together. Some plants prefer certain fungi, and some of these fungi prefer to associate with certain bacteria. When the job is done, and the soil aggregates are formed, the aggregates provide a favored environment for the bacteria where they are protected from predators. In this protected environment the bacteria can best perform their important functions working together, such as the production of nitrogen fertilizer from air, and the making of phosphate available to the plant from insoluble rock. We performed an experiment, that demonstrated the processes described above. These processes are important for the management of agricultural systems, because only an understanding of the relationships between soil minerals and soil organisms can guarantee that beneficial organisms will dominate under benign soil conditions.
Technical 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 formation of water-stable soil aggregates (WSA) and on selected groups of soil microorganisms are interrelated. Soil containers consisting of four compartments were utilized. Two compartments on each side of a solid barrier were separated by a 43 m screen that permitted the passage of hyphae, but not of roots. Sorghum roots 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) 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 formation was significantly correlated with root or hyphal length. Numbers of colony-forming units of the microflora were in general not correlated with root or hyphal length, but in most cases were significantly correlated with WSA. Bacteria isolated from the water- stable soil 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 formation and suggest that they affect microorganisms indirectly by providing a favorable and protective habitat through the creation of habitable pore space in the WSA.