Effects of biochar amendment on rapeseed and sweet potato yields and water stable aggregate in upland red soil

• Biochar significantly increased rapeseed and sweet potato yields.
• Biochar can improve soil aggregate stability and content of macroaggregates.
• Biochar increased soil total N in soil water stable aggregate in red soil, China.
• Biochar increased soil C:N ratio in soil water stable aggregate in red soil, China.
• Biochar increased soil organic carbon in soil water stable aggregate in red soil.

A field experiment was conducted to study the effect of biochar amendment (0, 2.5, 5, 10, 20, 30 and 40 t·ha− 1) on rapeseed and sweet potato yields, aggregate particle size distribution, aggregate stability, soil organic carbon, total N content and C:N ratio in soil water stable aggregate under a rapeseed–sweet potato rotation system in upland red soil in Jiangxi Province of southern, China. The results were as follows: compared with control treatment (CK), when the amount of biochar was 40 t·ha− 1, rapeseed and sweet potato yields were increased by 36.02% and 53.77% respectively; the soil water stable aggregate (> 0.25 mm) in the 0–15 cm soil layer had a remarkable increase than other treatments, especially the macroaggregate with particle size larger than > 2 mm. In the rapeseed harvest season, the mean weight diameter of soil water stable aggregate was enhanced by 28.02% compared to CK with the application rate of biochar being 40 t·ha− 1. A significant increment in soil organic carbon, total N and C:N ratio was observed in the > 2 mm, 2–0.5 mm, 0.5–0.25 mm and < 0.25 mm aggregate fractions with respect to the maximum treatment. These results suggest that biochar's incorporation into upland red soil will increase crop productivity and improve soil structure.