Distribution of microbial biomass and activity within soil aggregates as affected by tea plantation age

Variations in microbial biomass and activity within aggregate fractions can provide valuable information regarding mechanisms for enhancing carbon (C) sequestration and nutrient status in agricultural soils. The effects of different chronosequence phases on these variations in tea (Camellia sinensis L.) plantations have not yet been well documented. In this study, we assessed the relationship between tea plantation age and microbial biomass and activity at the aggregate scale. Microbial biomass C and respiration rate were determined in different size fractions from soil depths of 0-20 and 20-40 cm under four tea plantations of varying ages (16, 23, 31, and 53 years) in the hilly region of Western Sichuan, China. Aggregates were separated by an improved dry-sieving procedure into four fractions: > 2 mm (large macro-aggregates), 2-1 mm (medium macro-aggregates), 1-0.25 mm (small macro-aggregates), and < 0.25 mm (micro-aggregates). All the tea plantations we observed were dominated by large macro-aggregates with values of 41.25%-61.12% at both soil depths. Their proportion and mean weight diameter (MWD) were higher in 23-year tea plantation than those in other plantations, indicating that the soil structure in the 23-year tea plantation was more stable than the others. Notably, we found that aggregate stability is closely correlated with microbial biomass and their relationship is dependent on aggregate size. Aggregates with different particle sizes exhibited different levels of microbial biomass C and respiration rate regardless of tea plantation age. The soil properties we examined were at their highest levels in large macro-aggregates, implying that microorganisms associated with these fractions are more biologically active there than elsewhere in the present ecosystem. Decreases in soil microbial biomass and activity after 23 years of tea planting occurred mainly due to reduction in large macro-aggregates in the whole soil over time. To this effect, it is crucial to establish appropriate management protocol tailored to the prevention of soil structure degradation after 23 years of tea planting in such plantations.