COMPETITIVE OUTCOME AMONG 4 PASTURE SPECIES IN STERILIZED AND UNSTERILIZED SOILS

Four pasture species (Dactylis glomerata, Holcus lanatus, Lolium perenne and Trifolium repens) were grown in monoculture and in all possible 2-, 3-, and 4-species combinations in pots. One set of pots was filled with sterilized soil in which most soil microorganisms and mycorrhiza had been eliminated, a second set was unsterilized and had an added Rhizobium inoculum. The experiment had four successive destructive harvests. For each plant species, regardless of the identity of its competitors, percentage survival was lowest in unsterilized soils, but the mean weight of survivors was unaffected, except for T. repens which had an increased biomass. In addition, at the first harvest the microorganisms and each of the plant species had a significant effect on the relative growth rates of each of the target plant species but this effect was not continued to the final harvest. It is argued that either (a) in the unsterilized soils microorganisms inhibit germination of some seeds or adversely affect young seedlings, and that they compete with growing plants for limited resources, (b) sterilization eliminates most of the bacteria present and this along with the added Rhizobium inoculum might contribute to the higher survival in sterilized soil, or (c) the nature of sterilization procedure alone increases the availability of essential resources to growing plants.