Influence of humic acids on laurel growth, associated rhizosphere microorganisms, and mycorrhizal fungi

Increasing concentrations of humic acids were tested in order to determine their effects on the microbial rhizosphere and the growth of laurel (Laurus nobilis L.). Plants that were treated with 300 mg kg-1 of humic acids had the heaviest weights of both fresh and dry shoots; however, doses of 3000 mg kg-1 were highly phytotoxic and inhibited the growth of laurel. Total aerobic bacteria and actinomycetes were stimulated by doses of 1500 and 3000 mg kg-1 of humic acids at the first harvest. Nevertheless, at the end of the experiment no significant differences were found among the different doses. The number of fungi living in the laurel rhizosphere was not affected by any concentration of humic acids. Vesicular-arbuscular mycorrhizal (VAM) colonization was only slightly affected by the addition of increasing concentrations of humic substances to the soil, while the hyphal growth of Glomus mosseae was reduced.