The direct or MIT route for nitrogen immobilization: A 15N mirror image study with leucine and glycine

Current models of nitrogen mineralization implicitly assume that all the N in organic substrates is mineralized to NH4+ + before assimilation by growing microorganisms — the so-called MIT hypothesis. In theory, however, N mineralization during the decomposition of low molecular weight substrates such as amino acids can occur by a direct route in which the substrate is assimilated into the cell, deaminated, and only the surplus released into the soil NH4+ pool. The decomposition in soil of the amino acids leucine and glycine was studied, using the techniques of 15N isotope dilution and enrichment, to determine whether the direct route of N assimilation was occurring in soil. The results showed clearly that the conventional MIT route was not operative and were consistent with the direct route. The relative proportions of N from leucine and glycine mineralized into the NH4+ pool were in close agreement with predictions based on the direct hypothesis. Results from other published work on amino acid decomposition, reinterpreted using these techniques, were also consistent with the direct route.