Soil microorganisms are capable of producing secondary metabolites such as auxins upon the addition of tryptophan (L-TRP) that may significantly influence plant growth and development. This study was conducted to determine the stability and availability of indole-3-acetic acid (IAA) and proposed intermediates in the production of auxins from the addition of L-TRP to soil. L-TRP metabolism was not observed with the addition of L-TRP to a steam-sterilized soil when incubated for up to 7 days, indicating a biotic mechanism in the production of soil auxins. Incubation of 3'-C-14-L-TRP in non-sterile soil resulted in the conversion of the L-TRP label into indole-3-acetamide, indole-3-lactic acid, indole-3-acetic acid, indole-3-ethanol, and indole-3-aldehyde by the soil microbiota. The production of indole derivatives was dependent on the amount of L-TRP added to the soil. Adsorption-desorption isotherms showed a low affinity of auxin derivatives (5-hydroxy-indole-3-acetic acid, indole-3-acetamide, indole-3-lactic acid, indole-3-acetic acid, indole-3-ethanol, and indole-3-aldehyde) for the soil colloids. The persistence of L-TRP in five soils, measured in half-life (t1/2), ranged from 22.8 to 28.7 h. The t1/2 measured for the intermediates of auxin production indicated that several auxin derivatives were stable in soil and may have a greater effect on plant growth and yield when compared with auxins of lower soil stability.
