Soil bacterial community structure as affected by stand age in Chinese fir plantations: Insights at the aggregate scale

Exploring the variations of soil bacterial communities can contribute to the sustainability of Chinese fir plantations (CFPs). As the living environments of soil bacteria, soil aggregates significantly affect the diversity and composition regarding soil bacterial communities in CFPs. However, the distribution characteristics exhibited by soil bacteria at the aggregate scale are still not clear. In the present research, the diversity and composition regarding soil bacterial communities (using high-flux sequencing) in CFPs with diverse stand ages (3, 9, 17, and 26 years) were analyzed from the perspective of soil aggregates (>2, 1-2, 0.25-1, <0.25 mm) in Guangxi, China. According to the results, regardless of the aggregate size and the stand age, bacterial communities in soil were mainly composed of Proteobacteria, Chloroflexi, and Acidobacteria. During Chinese fir growth, the relative abundance (RA) of soil Proteobacteria first elevated and afterwards reduced, with the greatest level in the 17 years, while that of soil Chloroflexi exhibited an inverse trend. In addition, the RA of soil Acidobacteria increased over time. At the aggregate scale, the 17-year-old CFPs presented the greatest soil bacterial community diversity (based on the Chao 1 and Shannon indices). After 17 years, the destruction of coarse macroaggregates (>2 mm) could induce the reduction of soil bacterial community diversity, as coarse macroaggregates, taking up the majority of bulk soil, offered suitable environments for soil bacteria to survive and thrive. Moreover, soil organic C and total N concentrations and soil pH also significantly affected the diversity and composition regarding soil bacterial communities over time. Hence, the conservation of coarse macroaggregates after 17 years of Chinese fir growth is important to sustain the soil health as well as the bacterial community diversity in Guangxi, China.