Investigating amino acid utilization by soil microorganisms using compound specific stable isotope analysis

• Glycine and l-leucine were tracked in soil solution and in the microbial biomass.
• The half-lives of the two amino acids in soil solution were less than 5.0 h.
• Keto acids were present in soil solution and in the microbial biomass.
• At least a fraction of the added amino acids were taken up as intact molecules.

Amino acids comprise a large proportion of soil organic nitrogen (N). Microorganisms can take up amino acids as intact molecules or as ammonium and α-keto acids after enzymatic deamination. The objective of this study was to trace double labeled (13C,15N) glycine and l-leucine from soil solution into microbial biomass using compound specific stable isotope analysis. The two amino acids were utilized rapidly by soil microorganisms, with the half-life of glycine and leucine in soil solution being 2.9 and 5.0 h, respectively. The highest concentration of added glycine and leucine in the microbial biomass was measured after 4 h and corresponded to 10 and 13% of the glycine and leucine added, respectively, showing that a part of the added amino acids was taken up as intact molecules. Based on the results from an accompanying isotope pool dilution experiment, at least 15% of the glycine and 50% of the leucine was taken up as intact molecules. Our results suggest that glycine mainly served as carbon (C) source, while leucine provided C as well as N for the soil microbial community. Labeled keto acids were detected in soil solution and in the microbial biomass; however, their concentration corresponded only to a small fraction of the C added with the amino acids. The fact that 30 and 65% of glycine and leucine N was mineralized within 12 h, while never more than 5% of the amino acid C was recovered in the form of keto acids highlights how quickly keto acids were utilized by soil microorganisms. The combination of compound specific stable isotope analysis and chloroform fumigation extraction proved to be a valuable tool to trace amino acids and keto acids in soil solution and the microbial biomass.