Effects of tea plantation age on soil aggregate-associated C- and N-cycling enzyme activities in the hilly areas of Western Sichuan, China

Research on the changes of soil carbon (C)- and nitrogen (N)-cycling enzyme activities within aggregate fractions is essential for improving our understanding of soil organic matter (SOM) dynamics in agricultural ecosystems. However, the short- and long-term implications of the conversion of abandoned croplands to tea (Camellia sinensis L.) plantations on enzyme activities associated with aggregate fractions remains poorly understood. In the present study, we hypothesized that (i) the concentrations of soil organic C and total N would increase as the tea plantations continued to age, and (ii) the potential of SOM decomposition would differ with the stage of tea cultivation, as there would be significant changes in the activities of soil C- and N-cycling enzymes over time. The activities of beta-glucosidase, invertase, urease, and protease were analyzed in different size fractions of soil aggregates that were collected from 0 to 20 cm depth in four tea plantations with the same cultivar (Sichuan tea) of various ages (16-, 23-, 31-, and 53-years) in the hilly areas of Western Sichuan, China. The aggregates were separated using a dry-sieving procedure into four fractions, namely, large ( > 2 mm), medium (2-1 mm), small (1-0.25 mm) macro-aggregates, and micro-aggregates ( < 0.25 mm). In the early stage (in the first 23-years), tea cultivation contributed to a significant increase in organic C and total N stocks in the whole-soil, and these changes were primarily reflected in the increases of such elements stocks in the large macro-aggregates. For example, the increase in organic C stock associated with large macro-aggregates in the early stage accounted for 99.64% of the increase in organic C stock in the whole-soil. In the later stage (after 23-years), however, the increases in organic C and total N stocks in the whole-soil were primarily reflected in the increases of such elements stocks in the small macro-aggregates. Meanwhile, soil C- and N-cycling enzyme activities associated with the large macro-aggregates were significantly increased in the first 23-years of tea cultivation. These results indicated that young tea plantations in the early stage had limitations with respect to the potential for SOM sequestration; however, as the tea plantations matured under the present system of management, the SOM sequestration has improved.