Growth and nitrogen fixation ofAzolla pinnata andAzolla caroliniana as affected by urea fertilizer and their influence on rice yield

Soils from avocado (Persea americana Mill.) orchards in Israel (IS) and California (CA), both sites with a Mediterranean climate, were sampled and analyzed for the species and quantities of vesicular-arbuscular mycorrhizal fungus (VAMF) spores in them, and for soil physical and chemical characteristics.Numbers of spores were similar in soil from IS and CA but the dominant VAMF species were very different. In IS the most common fungi were Sclerocystis sinuosa and Glomus macrocarpum. In CA, Gl. constrictum was present in every orchard examined and Gl. fasciculatum was nearly as widespread. Acaulospora spp. and other Glomus spp. also were found, including A. elegans which has never before been reported from CA.The differences in VAMF populations and species constituents found on two continents but in areas with similar climates and soil types may be due to host or edaphic factors. Different avocado rootstocks are used in the two countries and lower pH and higher soil fertility levels were present in CA soils.The total VAMF spore populations in each orchard was about 275 per 100 mL soil. The population level was not correlated with any of the soil physical or chemical characteristics examined nor with avocado cultivar or age. In IS no fungus spores were found in three orchards; available P, Ca, Mg and Cu levels were high in these soils.