Abstract: Substrate availability affects microbial growth, whereas extraneous nitrogen forms can significantly affect microbial metabolic processes. As for soil amino sugars, the stable residues in microbial cell wall, their synthesis, decomposition and turnover are closely related to the availability of extraneous carbon and nitrogen. Using isotope tracing technique to study soil amino sugars can further understand the substrate utilization profiles by soil microorganisms. In this study, two incubation tests were conducted, with glucose plus ¹⁵Nlabelled NH₄⁺ or NO₃^-as the substrates, respectively. The ¹⁵N enrichment in each kind of soil amino sugars was identified by gas chromatography/mass spectrometry (GC/MS) to trace the dynamics of soil ¹⁵Nlabelled and native amino sugars. During the incubation, the content of soil ¹⁵Nlabelled amino sugars increased significantly, and the transformation rate from NH₄+ to amino sugars was significantly higher than that from NO₃^-, suggesting the preferred utilization of NH₄⁺ than NO₃^- by soil microorganisms. Significant changes in the amounts of soil unlabelled amino sugars were observed. The amount of unlabelled glucosamine increased with NH₄⁺ addition, but decreased gradually with NO₃^- addition. The content of unlabelled muramic acid decreased gradually, especially with NO₃^-addition. Either the increase or the decrease of galactosamine did not exceed 20% to the original value. These compoundspecific changes showed that the heterogeneous microbial residues played different roles on the turnover and stabilization of nitrogen in soil matrix. Fungal cell wall residues were easily accumulated in soil matrix, which benefited the stabilization of soil organic matter, while bacterial cell wall residues were easily degraded, playing an important role in the turnover of soil organic matter.

Key words: inorganic nitrogen, isotope, amino sugar, microbial process, soil.