Combined addition of crop residues and their biochar
increase soil organic C content and mineralization rate

【Objectives】This study investigated effects of single or combined addition of crop residues and straw biochar on the contents of soil organic carbon components and their mineralization, and revealed their effects in terms of soil carbon sequestration and fertility improvement, therefore provided theoretical support for reasonable utilization of farmland organic resources.【Methods】 An incubation experiment was carried out for 83 days with totally 11 treatments, control soil (CK), single addition (WS, WR, WB, MS, MR and MB) and combined addition (WS+WB, WR+WB, MS+MB and MR+MB) of wheat or maize residues (root, straw) and straw biochar. The experiment measured soil CO2 emission during cultivation, at the end of the experiment, soil total organic carbon (TOC), dissolved organic carbon (DOC), microbial biomass carbon (MBC), particulate organic carbon (POC) and coarse, fine particulate organic carbon (CPOC and FPOC) were also determined.【Results】 The contents of soil TOC, MBC, POC, CPOC and FPOC under the adding maize residue are higher than those of adding wheat residue, and the addition of shoots increases the contents of soil TOC, POC, CPOC and FPOC more obviously compared to the addition of roots. The organic carbon mineralization rates are rapid at the beginning of 4, 8, 21 d under treatments of single addition biochar, combination of crop residues and biochar and single addition crop residues respectively, then slow down and gradually stabilize. The single addition of crop residues has the maximum cumulative mineralization rate of organic carbon (up to from 30% to 46%). Compared with the control, the treatments of adding organic materials all significantly increase the content of soil TOC, and the single addition of biochar has the largest increase (by 34.4% and 36.5% under the single addition of wheat or maize straw biochar respectively), but the cumulative mineralization rate of organic carbon is only 3%. The soil FPOC content and sensitivity index of the single addition biochar treatments are higher than those of the others. The treatments with mixed wheat or maize residue and their biochar significantly increase the contents of soil MBC and CPOC by 102.2%, 109.3% and 199.2%, 80.2% respectively and the cumulative mineralization rate of organic carbon is between 12% to 19%. The treatments of combining addition crop residues and biochar also have the highest soil CPOC contents and sensitivity indices.【Conclusions】 The complementary role of soil nutrients is obvious in adding crop residues alone treatments, while the CO2 released rates of the adding crop residues alone treatments are higher than those of the other treatments, and the single addition of biochar has the lowest cumulative mineralization rate, therefore, the supply of soil nutrients is limited. Over all, the combined addition of crop residues and their biochar can better overcome their drawbacks, especially for maize straw and its biochar mixture, which can both increase soil carbon reservoir and ensure the supply of soil nutrients, have significant effect on soil fertility enhancement.