Marsh Kerri L, Mulvaney Richard L, Sims Gerald K
University of Illinois, Department of Natural Resources and Environmental Sciences, Urbana, IL 61801, USA.
J AOAC Int. 2003 Nov-Dec;86(6):1106-11.
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 degrees 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 alpha-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.
