Biochar addition leads to more soil organic carbon sequestration under a maize-rice cropping system than continuous flooded rice

•SOC and grain yield was measured with biochar and straw addition in maize-rice vs rice-rice system.•Straw addition has no effect on SOC accumulation for both cropping systems over a 4-year periods.•Biochar significantly improved more SOC in rice-maize system than rice-rice system.•Higher SOC under rice-maize system with biochar addition were linked increased soil aggregation.•Both biochar and straw addition led to higher late rice yield.