Changes of microbial residues over century-long soil development after tidal flat reclamation in eastern China

Growing evidence indicates that microbial residue carbon (MRC) accounted for a substantial proportion of soil organic carbon (SOC) pool. SOC tends to increase rapidly with the improvement of soil physicochemical and biological properties during the early stage of soil development. However, the changes of MRC and its contribution to SOC accumulation along with soil development are still unclear. Here, using amino sugars as biomarkers, we explored the changes of MRC with reclamation age (0, 11, 37, 102, and 200 years) in the soil profile (0-10, 10-20, 20-40, and 40-60 cm) after tidal flat reclamation in eastern China. Soil SOC and MRC slightly decreased over the first 11 years and then gradually increased with reclamation age. The more rapid increase in MRC than SOC led to the contribution of MRC to SOC increasing with reclamation age. The proportion of MRC in SOC increased from 6.74 to 13.25 % in the tidal flat sediment to 13.17-39.22 % in the 102 years old reclaimed soil. The MRC accrual mainly depended on the increase in fungal residues. Soil MRC and its contribution to SOC decreased with soil depth, and the vertical difference increased with reclamation age. The temporal accumulation of soil MRC was driven primarily by soil nitrogen and salinity, while the vertical distribution of MRC in the soil profile was controlled mainly by SOC and soil pH. Our work highlighted the increasing role of microbial C sequestration in SOC accumulation during early soil development after tidal flat reclamation. Further studies are needed to investigate whether the temporal and vertical changes of MRC revealed in this study also widely occur under other soil-forming conditions.