Are Newly Biosynthesized Microbial Residues More Stable Than Extant in Soil Carbon Storage?.

Microbial residues such as amino sugars are stabilized in soils and could be useful in investigating the fate and turnover of soil organic carbon. We conducted a 441 day microcosm study using three different soils (agricultural soil, grassland soil and forest soil) amended with (NH₄)₂SO₄ and U-¹³C-glucose as substrates. We used a GC-EI-MS technique to trace isotope carbon incorporation into the soil amino sugar pool during microbial transformation. The base peak of dominant amino sugar glucosamine derivative (m/z = 187, C₈H₁₁O₅) was selected for calculating the atom percentage excess (APE) to indicate isotopic enrichment. Specifically, ¹³C incorporation was assessed by the relative abundance change of F+n to F (n is the number of the carbon from added glucose-C to glucosamine-C; n is equal to 4 in this case). We found the APE value sharply increased within the initial 3-day or 21-day incubation in three soils, and then slowly changed or stabilized extending to 441 days, implying that the already ¹³C enriched glucosamine is unlikely to be further diluted. This may indicate that newly produced amino sugars are preferentially preserved in comparison with extant in-situ amino sugars. The stability difference between new and old residues may complicate the use of isotope tracer methods to study total carbon dynamics.