Microbial community structure in rhizosphere soil rather than that in bulk soil characterizes aggregate-associated organic carbon under long-term forest conversion in subtropical region

Forest conversion can strongly affect soil organic carbon (SOC) via modifying soil microbial community (SMC) structure. This process is particularly intense in rhizosphere since rhizosphere is the microbial hotspot in soils. However, the relationship between SMC structure in rhizosphere soil and aggregate-SOC regarding long-term land-use change remains unclear. Here, a 36-year conversion from natural forest to three typical plantations, i.e., Cunninghamia lanceolata (CL), Metasequoia glyptostroboides (MG) and Cryptomeria fortunei (CF), was investigated in subtropical region, with a natural forest (NF) as the control. SMC structures in bulk and rhizosphere soils (0–10 cm depth) were detected, and their potential linkages with bulk- and aggregate-SOC contents were analyzed. SMC structure in rhizosphere soil was distinctly differentiated from that in bulk soil and evidently separated between NF and the plantations, which was dominated by gram-negative bacteria (GN) and fungi. NF (1.61 g kg−1 soil) had significantly lower (P < 0.05) aggregate-SOC of <53 μm (F53–SOC) content than all the plantations while MG had the highest mass percentage of F53 (6%) among the forests. Bulk-SOC content showed nonsignificant differences among the forests. Furthermore, fungi and the ratios of fungi to bacteria and gram-positive bacteria to GN in rhizosphere soil had positive correlations with F53–SOC content. These results suggest that SMC structure in rhizosphere soil can effectively characterize the dynamics of aggregate-SOC content than that in bulk soil over more than three decades after forest conversion. The linkage between SMC structure in rhizosphere soil and SOC stability highlights the importance of rhizosphere microbial communities in determining soil C sequestration regarding long-term land-use change in subtropical region.