Effect of long-term application of biochar and straw on soil organic carbon in purple soil aggregates of sloping uplands

The effects of biochar and straw application on the distribution of soil organic carbon (SOC) in purple soil aggregations of sloping uplands were explored from a long-term perspective. Five long-term fertilization treatments were included:no fertilizer (control, CK), mineral fertilizer application (NPK), crop straw residue application (RSD), a combined application of biochar and mineral fertilizer (BCNPK), and a combined application of crop straw residue and mineral fertilizer (RSDNPK). Soil samples were separated into > 2 mm large macroaggregates, 0.25-2 mm small macroaggregates, 0.053-0.25 mm microaggregates, and < 0.053 mm slit-clay microaggregates by sieving. The mass fraction of soil aggregations, organic carbon content, mean weight diameter, and geometric mean diameter of aggregations were measured. The results showed that the application of biochar and straw increased SOC content and stabilized the structure of aggregations compared with both CK and NPK treatments. In comparison with CK, SOC contents significantly increased by 43.1% under NPK treatment, 82.9% under RSD treatment, 79.1% under RSDNPK treatment, and 90.5% under BCNPK treatment. SOC stocks of topsoil increased significantly by 34.0% under NPK treatment, 68.2% under RSD treatment, 65.2% under RSDNPK treatment, and 74.3% under BCNPK treatment compared with CK treatment. In addition, the content of 0.25-2 mm aggregates of RSD treatment reached 45.5%, which was 57.7% higher than that of CK; more soil aggregations were concentrated on 0.25-2 mm aggregates. As compared with CK, the SOC content of 0.053-0.25 mm aggregates was reduced by straw and biochar treatments. The trends for NPK and RSD treatments were similar to those for CK, but RSDNPK and BCNPK treatment showed an increasing trend in organic carbon content in aggregates with decreased particle size. RSDNPK and BCNPK treatments significantly increased the SOC content and enhanced the stability of soil structure, but BCNPK treatment was superior to RSDNPK treatment in increasing SOC content, and RSDNPK treatment was more effective in stabilizing soil structure. BCNPK treatment and RSDNPK treatment are therefore effective management measures that can maintain and improve soil fertility of purple-soil-cultivated land.