Effect of different biochar application rates on soil organic carbon in the semi-arid Loess Plateau, China

Biochar plays an important role in improving soil quality and carbon (C) sequestration. However, the effects of different biochar application rates on soil organic C content and its fractions in the Loess Plateau of northwest China are still unclear. In this work, six biochar treatments were studied, with application rates of 0, 10, 20, 30, 40 and 50 t·ha−1 (CK, T1, T2, T3, T4 and T5, respectively). Soil total organic C (TOC), readily oxidizable C (ROC), water-soluble organic C (WSOC), microbial biomass C (MBC), light fraction organic C (LFOC), heavy fraction organic C (HFOC) and the Carbon pool management index (CPMI) in the 0–30 cm layer were analyzed after 3 years of wheat field trials. The content of soil TOC and its fractions decreased with soil depth but was significantly (p < .05) increased by biochar. TOC, ROC and HFOC showed significant (p < .05) positive correlations with the rate of biochar application, with the highest content of these components in T5. MBC, WSOC and LFOC first increased and then decreased with the increasing rate of biochar application. MBC and WSOC of T3 and LFOC of T4 were the highest of all treatments. When compared with CK, the CPMI of all biochar treatments was >100% in the order of T5 > T4 > T3 > T2 > T1. This study provides scientific support for large-scale biochar application on the Loess Plateau to significantly improve soil quality via increasing the soil organic C fractions and enhancing the inert soil C pool.