Organic carbon, total nitrogen, and microbial community distributions within aggregates of calcareous soil treated with biochar

Soil aggregate exerts an important part in carbon turnover as well as soil nutrients. Biochar (BC) has considerable potential as a soil amendment in enhancing the levels of soil total organic carbon (TOC) and total nitrogen (TN), as well as the microbial activity. However, it remains largely unclear about whether BC plus chemical fertilisers have synergistic effect on the soil properties within aggregates. In the present study, the 2-year field experiment was performed for examining distributions of TOC, TN, and microbial community compositions in the aggregate fractions in calcareous soil fertilised with BC and/or chemical fertilisers. The treatments comprised the application of no fertiliser (CK), chemical fertilisers (NPK), biochar (BC), and biochar plus chemical fertilisers (BC + NPK). Our study suggested that the BC + NPK treatment evidently enhanced the stability of aggregates (MWD) compared to other treatments. Compared with CK and NPK treatments, all BC-amended treatments elevated the TOC level in bulk soil and within each aggregate fraction by 12.99–126.40%, of which, 2–0.25 mm fraction displayed the largest increase. Similar increases in the TN level by 23.95–52.66% were observed in bulk soil and within 2–0.25 mm fraction. Moreover, all NPK-amended treatments significantly elevated the total PLFAs in bulk soil and within each fraction compared with CK. The BC + NPK treatments dramatically elevated the relative fungal abundance and fungi/bacteria ratio in bulk soil and within > 2 mm fraction relative to CK and NPK treatments, similar increases were observed in the 2–0.25 mm fraction following the application of BC alone. In addition, the 2–0.25 mm fraction showed the greatest levels of TN, TOC, C/N ratio, along with total phospholipid fatty acids (PLFAs). Redundancy analysis (RDA) revealed that the soil TN content is the primary parameter that affects microbial community compositions within aggregates. The present study indicated that the application of BC in combination with NPK fertilisers can serve as an efficient approach to improve soil structure and micro-ecological environment, as well as to enhance TOC and N storage in the aggregate fractions of calcareous soil.