Large Macro-Aggregate Formation Improves Soil Bacterial Metabolic Activity and Diversity in a Chronosequence of Chinese Fir Plantations

Revealing the metabolic activity and diversity of soil bacteria is an effective way for evaluating soil fertility and health status. Despite this, it is still unclear how soil bacterial metabolism vary in a chronosequence of Chinese fir (Cunninghamia lanceolata) plantations. Based on the Biolog Eco micro-plate technology, this study focused on assessing the metabolic activity and diversity of soil bacteria in Chinese fir plantations with various stand ages (3, 9, 17, and 26 years) at the aggregate scales (>2, 2-1, 1-0.25, and <0.25 mm) in Guangxi, China. Regardless of stand age, large macro-aggregates (>2 mm) had the highest soil bacterial metabolic activity (based on the total average well color development) and diversity (based on the Shannon and McIntosh indices) in Chinese fir plantations. During late stage (from 17 to 26 years) of Chinese fir planting, large macro-aggregates being disintegrated into micro-aggregates (<0.25 mm) resulted in the decreases of soil bacterial metabolic activity and diversity. Besides, redundancy analysis and Pearson's correlation analysis indicated that the decreases of soil organic matter contents (based on the organic C and total N contents) and soil acidification (based on the pH) were also the main factors that inhibited the bacterial metabolism in soil during late stage. Therefore, during Chinese fir planting, increases in large macro-aggregates are conducive to the bacterial metabolism in soil, thus improving soil fertility and health status, especially during late stage, in Guangxi, China.