Biochar Amendment was Less Effective for Rice Yield Improvement but More Effective for Soil Quality Relative to Inorganic Fertilization in a Low Fertility Paddy Soil

Biochar as a soil amendment has recently gained popularity in China. However, the effect sizes of biochar application on rice yield and soil quality in a low-productivity paddy soil shows great uncertainties. A 4-year field experiment was conducted to evaluate how biochar and inorganic NPK fertilizers applied affect rice yield and soil quality variables. The experiment included four treatments: Control (without biochar and NPK fertilizers applied), NPK (inorganic fertilizers applied alone), Biochar (biochar addition alone), NPKB (both NPK fertilizers and biochar applied). Soil samples from both soil layers (0–15 cm and 15–30 cm) were collected and evaluated for sixteen soil physicochemical and biological parameters. Compared to NPK fertilizers, biochar was less effective for increasing 4-year average rice yield (5.9% vs. 47.6%). Yet, biochar was more effective than NPK fertilizers for increasing soil quality variables (e.g. soil organic carbon, total N, available P, and pH). Soil exchangeable K, NH₄⁺, and pH were important predictors of rice yield response. Partial least squares path modelling analysis showed that both biochar and chemical fertilization increased rice yield mainly via improving nutrients status of soil. The co-application of biochar and NPK fertilizers had the highest rice yield (9.81 t ha^–1) and soil quality index (SQI). A significantly positive relationship was observed between SQI and rice yield, indicating that soil quality played an essential role in boosting rice productivity.The NPKB was a benefit practice for improving both rice grain yield and soil quality in yellow clayey paddy soil.