The contribution and flow of microbial residual carbon in soil aggregates following forest restoration on the Loess Plateau, China

Understanding the dynamics of microbe-mediated soil organic carbon (SOC) is important for both soil carbon buildup and turnover. However, little is known about how forest restoration affects microbially-driven SOC dynamics, particularly at the aggregate level. This study assessed the distribution of bacterial and fungal residues among aggregate fractions (>0.25 mm, 0.25–0.053 mm, and < 0.053 mm), as well as how their contribution to SOC following forest restoration on the Loess Plateau, China. The results revealed that long-term forest restoration increased the SOC content, bacterial and fungal residual carbon contents in all aggregates size fractions. The highest contents of bacterial residues carbon (BRC) and fungal residue carbon (FRC) were found in the 0.25–0.053 mm aggregate fraction, followed by > 0.25 mm fraction. Forest restoration significantly enhanced the contribution of BRC and FRC to SOC in aggregates. Specifically, the contribution of BRC to SOC decreased with aggregate sizes, whereas the contribution of FRC to SOC increased with aggregate sizes. The fungal: bacterial ratio gradually decreased in aggregates along with forest restoration. The general direction of MRC flow was from silt and clay fractions, or micro-aggregates (MIA) to MIA or large macro-aggregates. These findings suggest that the increase in SOC after forest restoration was linked to changes in microbial communities, and the particle size of aggregates affect the contribution of MRC to SOC.