Abstract: Soil aggregate is an important indicator of soil structure and fertility. Soil compaction and nutrient deficiency in tea gardens in subtropical red soil areas seriously affect the stability of soil carbon pools. Evaluating the effects of biochar-based fertilizer application on the organic carbon structure characteristics and physical stability of soil aggregates in tea gardens can help to reduce soil barriers and improve productivity in intensive tea gardens. In this study, a 40-year old Xishan tea garden in Guiping, Guangxi, located along the Tropic of Cancer, was selected as the research object. Three treatments [biochar-based fertilizer (BF), chemical fertilizer (F), and no fertilizer (CK)] were used for a field positioning test for two consecutive years to explore the soil aggregate structure, organic carbon distribution and physical stability of the tea garden. The results show that: (1) the application of biochar-based fertilizer increased soil pH value and cation exchange capacity (C_EC), and tended to decrease soil bulk density (B_D). The soil pH value under the treatment of biochar-based fertilizer increased by 0.18 and 0.31 respectively compared with the no fertilizer and chemical fertilizer treatments. The soil bulk density under the biochar-based fertilizer treatment decreased by 4.52% compared with the chemical fertilizer treatment. The cation exchange capacity under the biochar-based fertilizer treatment increased by 12.12% and 15.09% respectively compared with the no fertilizer and chemical fertilizer treatments. (2) The application of biochar-based fertilizer improved the structural stability of soil aggregates and promoted the formation of large aggregates with soil water stability. The values of water stability large aggregates (R_W0.25), mean mass diameter (D_MW) and geometric mean diameter (D_GM) of aggregates >0.25 mm under the biochar-based fertilizer treatment were increased by 15.34%, 23.94% and 34.48% compared with the chemical fertilizer treatment. (3) The application of biochar-based fertilizer increased the organic carbon content of soil macroaggregates, which was conducive to the storage of organic carbon in soil macroaggregates. The organic carbon content of the aggregate with particle size >2.00 mm under the biochar-based fertilizer treatment was significantly higher than that under the chemical fertilizer and no fertilizer treatments, increasing by 45.23% and 17.28%, respectively. The contribution rate of organic carbon in biochar-based fertilizer was 77.48% and 13.11% higher than that of chemical fertilizer and no fertilizer, respectively. The contribution rate of active organic carbon in aggregates under biochar-based fertilizer treatment was 50.40% higher than that under chemical fertilizer treatment. (4) The application of biochar-based fertilizer improved the stability of organic carbon in microaggregates. For aggregates with a particle size of >0.053-0.25 mm and ≤0.053 mm, the organic carbon oxidation stability coefficient (K_OS) under biochar-based fertilizer treatment increased by 82.42% and 77.78%, respectively, compared with that under no fertilizer treatment. For aggregates with particle size of >0.25-2.00 mm, the organic carbon oxidation stability coefficient under biochar-based fertilizer treatment decreased by 40.79% and 49.58%, respectively, compared to the chemical fertilizer and no fertilizer treatments. Biochar-based fertilizer treatment had a tendency to reduce the stability of organic carbon in large soil aggregates. The application of biochar-based fertilizer is conducive to the optimized management and carbon pool stability of tea gardens.

Key words: soil aggregate, soil organic carbon, carbon sequestration in tea garden, biochar-based fertilizer