Effect of humic amendments on inorganic N, dehydrogenase and alkaline phosphatase activities of a Mediterranean soil

Dehydrogenase activity, alkaline phosphatase activity and NH₄ ⁺, NO₂ ⁻ and NO₃ ⁻ concentrations were monitored in an aridisol treated with three commercially available humic amendments. The materials were of plant residue, lignite and peat origins. The humus plant residues, fulvic acids, with a high content of Kjeldahl-N, sustained high enzyme activities and highest levels of NH₄ ⁺, NO₂ ⁻ and NO₃ ⁻. Humus lignite (mainly humic acids) produced the highest dehydrogenase activity, whereas the alkaline phosphatase activity was not as high as that amendment with humus plant residues. The lower activity of alkaline phosphatase could not be attributed to the higher P content of humus lignite. Nitrification was also low, probably due to the low N content of this fertilizer. The amendment of humus peat origin (only humic acids) did not increase enzyme activity or inorganic N concentrations of soil. Our results show that although these materials are widely utilized and recommended as microbial and plant activators, they all behave very differently, and the effects on soil microbiological activity cannot be predicted solely on the basis of their humic and/or fulvic acid contents.