Elimination of Transamination during Chloroform Fumigation to Determine Uptake of Labeled Amino Acids by Soil Microorganisms

Compound specific stable isotope analysis combined with chloroform fumigation extraction was used to investigate amino acid utilization by soil microorganisms. Double-labeled glycine and L-leucine (98% C-13(2), 99% N-15) were added together with ammonium to soil samples. The concentration of amino acids was determined by ion trap gas chromatography-mass spectrometry (GC-MS). In preliminary tests, the concentration of double-labeled leucine in fumigated samples was lower than in the unfumigated control, even when the samples were extracted or fumigated immediately after its addition. At the same time, the concentration of single-labeled leucine increased. Tests revealed that enzymes, released into soil solution during fumigation, replaced the labeled amino group of leucine with an unlabeled one taken from other organic molecules. The addition of sodium dodecyl sulfate to soil samples before fumigation stopped the interference by these enzymes. The proposed method allows determining the uptake of intact amino acids by soil microorganisms.