Allelopathic effects of tree species on some soil microbial populations and herbaceous plants

The allelopathic potential of four tree species on soil microbial populatio ns and some herbaceous plants (two understory species and one general biote st species) was investigated. Effects of three nonindigenous tree species, Eucalyptus globulus Labill, Pinus radiata D.Don and Acacia melanoxylon R.Br ., on microorganisms participating in the cycle of nitrogen were evaluated, comparing them with those produced by the autochthonous Quercus robur L. I nfluence of the trees on Lactuca sativa L., Dactylis glomerata L, and Trifo lium repens L. was also checked in bioassays. Cell numbers of Nitrosomonas sp. were negatively affected by Acacia and Eucalyptus stands, mainly during spring, when flowers are especially abundant on the ground. Proteolytic mi croorganisms were also negatively affected by Eucalyptus and Pinus stands, whilst Quercus stand did not show any toxicity. Soil bioassays showed clear inhibitory effects on germination and growth of understory plants, particu larly soils from Eucalyptus and Acacia stands. The greatest effects had the soil from Acacia stand, which was phytotoxic during the whole period of ge rmination and growth of understory plants. Allelopathic phenomena could be, at least partially, responsible of the low species diversity in the unders tory of the nonindigenous tree stands.