A technique to recover tracer as Carboxyl-carbon and α-nitrogen from amino acids in soil hydrolysates

Isotope analysis of biochemical compounds provides an unequivocal means for detecting assimilation of tracer C and N into microbial biomass. A diffusion method recently developed to determine amino acid-N by ninhydrin oxidation of soil hydrolysates was modified to permit simultaneous collection of the CO₂ liberated during this oxidation. In the technique described herein, this is accomplished after removal of (NH₄⁺ + amino sugar)-N, by performing ninhydrin oxidation at 90°C for 7 h in a 1.9 L Mason jar sealed with a lid equipped to support a petri dish containing 5 mL 0.2N NaOH. Recoveries of carboxyl-C and α-N ranged from 98 to 101% in evaluations with standard solutions of amino acids, whereas these recoveries exceeded 90% for ¹⁴C and 94% for ¹⁵N when soil hydrolysates amended with labeled amino acids were analyzed.