Soil aggregate-associated bacterial metabolic activity and community structure in different aged tea plantations. 
Revealing the dynamics of soil aggregate-associated microbial (particularly bacterial) metabolic activity and community structure is of great importance to maintain the soil health and microbial community stability in tea plantation ecosystems.  In this study, the bacterial metabolic activity (as measured by Biolog Eco MicroPlates) and community structure (as measured by high-throughput sequencing) were analyzed in soil aggregates, which were collected at the 0-20 cm depth in four tea plantations with different ages (16, 23, 31, and 53 yrs.)  in the areas of Western Sichuan, China.  A dry-sieving procedure was adopted to separate soil aggregates into four fractions, including >2, 2-1, 1-0.25, and <0.25 mm.  In all the tea plantations, the highest levels of soil bacterial metabolic activity (as indicated by average well color development, AWCD) and community diversity (as indicated by Chao 1 and Shannon indices) appeared in the >2 mm fractions, which indicated that these aggregate fractions with complex bacterial communities not only provided biological buffering, but also prevented the dominance of individual microorganisms through predation or competition.  Soil aggregates with >2 mm were concentrated in the 23 yrs.  tea plantation, implying that this tea plantation possessed the relatively suitable soil environments to the growth and proliferation of soil bacteria, thus increasing their metabolic activity and community diversity.  After 23 yrs.  of tea planting, the reduction of the >2 mm fractions in the whole-soil accounted for the degradation of soil bacterial communities to some extent.  In the meanwhile, soil microbial quotient (the ratio of soil microbial biomass C to organic C) and pH were also important drivers of the variations in soil bacterial communities during tea planting.  This study underscored the requirement for sustainable soil managements which could maintain the soil health and bacterial community stability after 23 yrs.  of tea planting in the areas of Western Sichuan, China.