Tea Plantation Age Shapes Soil Enzyme Activities Through Changes in Aggregate Size Distribution

Elucidating the response mechanisms of C-, N-, and P-cycling enzyme activities within soil aggregates to tea plantation age can provide a theoretical foundation for improving soil fertility, safeguarding soil health, and promoting the sustainable use of soil resources in tea plantations. In the present study, soil samples were collected from the 0–20 cm layer of tea plantations with different ages (3, 9, 16, and 24 years). Then, soil samples were separated into >2, 2–1, 1–0.25, and <0.25 mm aggregate size fractions using an optimal moisture sieving method, and the activities of β-glucosidase, invertase, urease, protease, and acid phosphatase were measured in each fraction. Across all tea plantation ages, the aggregate composition was dominated by the >2 mm fraction, whose content was significantly (p < 0.05) higher than that of other size fractions, averaging 54.47%. With increasing plantation age, the content of >2 mm aggregates first increased and then decreased, reaching a relatively high level at 16 years. The activities of β-glucosidase, invertase, urease, and protease in the tea plantation soils were predominantly distributed in the >2 mm aggregates, with average activities of 261.34, 585.31, 52.24, and 84.34 mg kg−1 h−1, respectively; in contrast, acid phosphatase activity was less affected by aggregate size. As plantation age increased, the activities of β-glucosidase, invertase, urease, and protease initially increased and then decreased, reaching relatively high levels at 16 years (322.98, 696.66, 67.00, and 100.98 mg kg−1 h−1, respectively), whereas acid phosphatase activity progressively increased with age. During the aggregate fractionation process, all enzyme activities were lost to varying degrees, with average recovery rates of 80.45% (β-glucosidase), 83.13% (invertase), 80.78% (urease), 82.16% (protease), and 81.66% (acid phosphatase). As the primary carriers of soil enzymes, the formation and stabilization of >2 mm aggregates are of great importance for promoting soil organic C and nutrient cycling. In tea plantation management, therefore, attention should be directed to the breakdown and disruption of >2 mm aggregates after 16 years of cultivation to maintain soil quality and sustain soil organic C and nutrient use efficiency.