Decomposition of lysine and leucine in soil aggregates: Adsorption and compartmentalization

The mode of amino acid stabilization in soil was examined in the laboratory using synthetic aggregates prepared from a ⩽20 μm soil fraction. 14C-labelled lysine or leucine was added throughout the aggregate by mixing during preparation or surficially by means of spray application after aggregate preparation. Synthetic aggregates were inoculated with soil microbes. The 14CO 2 released by the microbes was measured and used as an index of decomposition. Greater quantities of lysine and leucine were decomposed in the spray-applied aggregates compared to the aggregates which had amino acid mixed throughout. A minimum depth of microbial activity into the aggregates was estimated. Binding to mineral surfaces suppressed the decomposition of lysine. The results imply that compounds with net positive charge originating from amino groups are stabilized in soils having permanent negative charge. Further, microbial decomposition of the amino acids depended upon their location relative to tne aggregate surface.