Abstract: The objective for this study was to provide a scientific basis for incorporation of straw and biochar into purple using the principle of ‘equivalent carbon’. The effects straw and biochar on soil organic carbon (SOC) and active carbon fractions were studied at the Experimental Farm of Southwest University. Five treatments including control (no organic material added, CK), straw (CS), straw plus decomposition agent (CSD), biochar (BC), 50% straw and 50% biochar (CSBC) were used (in maize season and rape season). SOC increased significantly in all treatments including added carbon, ranging from 31.26% to 107.82% but SOC content in the BC and CSBC treatments were significantly higher than the CS and CSD treatments, indicating a greater ability to sequester carbon sequestration from biochar. Soil active organic carbon reached the highest content during maize filling stage and rape pod ripening period. Microbial biomass carbon (MBC), dissolved organic carbon (DOC) and readily oxidizable carbon (ROC) in different treatment were ranked CSD>CD>CSBC>BC>CK, during the maize season. The MBC, DOC and ROC content of straw and biochar related treatments ranged from 125.53-172.02 mg·kg^-1, 102.73-79.07 mg·kg^-1 and 2.00-6.01 g·kg^-1, respectively. Microbial entropy, the DOC∶SOC and ROC∶SOC ratio for CS and CSD treatments were significantly higher than that in BC and CSBC treatments. Compared with the control, CS and CSD treatments significantly improved the soil ROC∶SOC ratio. However, BC and CSBC treatments were 14.33%-18.38% lower than that of the CK. In summary, straw and straw with added decomposition agent was able to promote the transformation of active carbon, able to be directly used by microorganisms. These treatments also increased SOC and soil fertility. Compared with straw, biochar resulted in lower soil microbial activity. However, the higher stability of SOC in biochar treatments stimulated the accumulation of SOC.

Key words: soil active organic, straw, biochar, purple soil