Experimental strategies to measure the microbial uptake and mineralization kinetics of dissolved organic carbon in soil

Soil organic matter turnover rates are typically estimated from mass loss of the material over time or from on rates of carbon dioxide production. In the study, we investigated a new way to characterize the concentration-dependent kinetics of amino acids used by measuring microbial uptake and mineralization of 14C-alanine. We measured the depletion from soil solution after additions 14C-alanine. The microbial uptake of 14C-alanine from soil solution was concentration-dependent and kinetic analysis indicated the operation of at least three distinct alanine transport systems of differing affinities. Most of the 14C-alanine depletion from the soil solution occurred rapidly within the first 10–30 min of the incubation after 10 µM to 1 mM substrate additions. At alanine concentrations less than 250 mM, the kinetic parameters for Km and Vmax of the higher-affinity transporter were 60.0 µM and 1.32 µmol g−1 DW soil h−1, respectively. The mineralization of alanine was determined and the half-time values for the rapid mineralization process were 45 min to 1.5 h after the addition at alanine concentrations below 1 mM. The time delay after its uptake into microbial biomass suggested that alanine uptake and subsequent respiration were uncoupled pattern. The microbial N uptake rate was calculated by microbial mineralization, and an estimated Km value of 1731.7±274.6 µM and Vmax value of 486.0±38.5 µmol kg−1 DW soil h−1. This study provides an alternative approach for measuring the rate of turnover of compounds that turnover very rapidly in soil.