A novel GC/MS technique to assess N and C incorporation into soil amino sugars

Amino sugars have been used as biomarker to indicate microorganism contribution to soil organic matter turnover and sequestration. However, there is no direct gas chromatograph mass spectrometry (GC/MS) approach to assess microbial synthesis of amino sugars in soil. We developed a novel method which combines laboratory incubation of substrate containing ¹⁵N or ¹³C and a GC/MS technique to trace ¹⁵N or ¹³C isotope changes in three amino sugars, glucosamine, galactosamine, and muramic acid. Sample preparation followed the procedure of Zhang and Amelung (1996) Zhang, X., Amelung, W., 1996. Gas chromatographic determination of muramic acid, glucosamine, galactosamine, and mannosamine in soils. Soil Biology and Biochemistry 28, 1201-1206.]. The GC/MS determination was conducted using a full scan mode with both electronic ionization (EI) and chemical ionization (CI) sources. The CI source was suitable for all of the three amino sugars, while the EI source was not applicable to muramic acid due to its low sensitivity in the determination as well as low concentration of muramic acid in soil. The enrichment of ¹⁵N or ¹³C in amino sugars during incubation was estimated by calculating the atom percentage excess (APE). ¹⁵N incorporation was evaluated according to fragment (F) abundance ratio of mass F+1 to F, whilst ¹³C incorporation was estimated according to the ratio of mass F+n to F (n is skeleton carbon number in the fragment). This novel method was assessed by using two soil samples (a Kandiudult and a Udoll) incubated with either ¹⁵N-amonium or U-¹³C-glucose. The results indicate that the GC/MS determination is reproducible, thus this technique is useful in detecting the microbial synthesis of amino sugars in soil, and especially it should be possible when looking at the position or how much labeled carbon and nitrogen atoms have been incorporated.