[Effects of extraneous inorganic nitrogen forms on the dynamics of soil amino sugars]

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.