Temporal responses of soil microorganisms to substrate addition as indicated by amino sugar differentiation

Amino sugars are one of the important microbial residue biomarkers which are associated with soil organic matter cycling. However, little is known about their transformation kinetics in response to available substrates because living biomass only contributes a negligible portion to the total mass of amino sugars. By using 15N tracing technique, the newly synthesized (labeled) amino sugars can be differentiated from the native portions in soil matrix, making it possible to evaluate, in quantitative manner, the transformation pattern of amino sugars and to interpret the past and ongoing changes of microbial communities during the assimilation of extraneous 15N. In this study, laboratory incubations of soil samples were conducted by using 15NH4+ as nitrogen source with or without glucose addition. Both the 15N enrichment (expressed as atom percentage excess, APE) and the contents of amino sugars were determined by an isotope-based gas chromatography–mass spectrometry. The significant 15N incorporation into amino sugars was only observed in glucose plus 15NH4+ amendment with the APE arranged as: muramic acid (MurN) > glucosamine (GlcN) > galactosamine (GalN). The dynamics of 15N enrichment in bacterial-derived MurN and fungal-derived GlcN were fitted to the hyperbolic equations and indicative for the temporal responses of different soil microorganisms. The APE plateau of MurN and fungal-derived GlcN represented the maximal extent of bacterial and fungal populations, respectively, becoming active in response to the available substrates. The different dynamics of the 15N enrichment between MurN and GlcN indicated that bacteria reacted faster than fungi to assimilate the labile substrates initially, but fungus growth was dominant afterward, leading to integrated microbial community structure over time. Furthermore, the dynamics of labeled and unlabeled portions of amino sugars were compound-specific and substrate-dependent, suggesting their different stability in soil. GlcN tended to accumulate in soil while MurN was more likely degraded as a carbon source when nitrogen supply was excessive.

► The transformation velocity of amino sugars was evaluated for the first time by isotope technique. ► The 15N enrichment in amino sugars can clue on the renewal of bacterial and fungal cells in soils. ► Amino sugars can be decomposed as C sources, but being compound-specific and substrate-dependent.