Effect of tea plantation age on the distribution of soil organic carbon and nutrient within micro-aggregates in the hilly region of western Sichuan, China

Pool of micro-aggregate-associated carbon (C) play a highly accurate and general diagnostic role for change in soil organic C pool in response to changes in management practices. However, effects of different chronosequence phases on the pools of organic C and nutrient (nitrogen, phosphorus, and potassium) in micro-aggregates of different sizes in tea (Camellia sinensis L.) plantations have not been studied in detail. This study was to investigate the organic C and nutrient pools in micro-aggregates of different sizes as affected by age of tea plantation. Surface (0–20 cm) soil samples were collected from four tea plantations with different ages (16, 23, 31, and 53 years) in Zhongfeng town of Mingshan county, which is in the hilly region of western Sichuan, China. Micro-aggregates were isolated from samples through a standard wet-sieving procedure and then separated by ultrasonic dispersion into five fractions (250–50, 50–10, 10–5, 5–1, and <1 μm). For all tea plantations, the distribution of micro-aggregate fractions showed that the dominant class was 250–50 μm fractions with a mean value of 52.98%, and these fractions were the predominant pools of organic C and nutrient in micro-aggregates. Mean weight diameter (MWD) in 53 years of tea plantation was the highest in all tea plantations, suggesting that micro-aggregates in 53 years of tea plantation had more stability than other tea plantations. More important fractions for organic C and total nitrogen (N) retention would be the 250–50 fractions, and higher levels of available phosphorus (P) and potassium (K) were observed in the <1 μm fractions. Successive planting of tea (from 16 years to 53 years) induced a significant increase in pools of organic C and nutrient in whole soils, except for the available K pool that showed an inverse trend. These changes were reflected in parallel and predominant changes in pools of these elements in micro-aggregates, especially in the 250–50 μm fractions, which validated the importance of micro-aggregates (in particular the 250–50 μm fractions) as soil organic C and nutrient protection and stabilization sites in such tea plantations.