The crucial factors of soil fertility and rapeseed yield - A five year field trial with biochar addition in upland red soil, China

Biochar has been used as an amendment to improve soil fertility and increase crop yield. However, the effects of biochar on soil properties and rapeseed yield in upland red soil have not been thoroughly investigated, and the factors crucial for rapeseed yield are not yet clear. A five-year field trial was conducted to investigate the effects the of biochar (biochar application rates of 0, 2.5, 5, 10, 20, 30, and 40 t ha⁻¹, respectively) on soil physicochemical and microbial properties as well as rapeseed yield in upland red soil in Jiangxi Province, China. Results showed that biochar can significantly increase soil pH, available phosphorus, organic carbon, Ks, and water retention, however, the influences of biochar on these indexes declined over time. Soil total nitrogen increased significantly when the dose of biochar exceeded 5 t ha⁻¹, and the content of total nitrogen in the 40 t ha⁻¹ biochar treatment increased each year. While the application of biochar gradually increased the contents of NH₄⁺-N, NO₃⁻-N and enhanced the soil microorganism and enzymatic activities during the first three years, they had returned nearly to their starting values by the end of this study. Rapeseed yield and yield components were significantly improved relative to the control for all biochar amendments in the first year, but the rapeseed yield in all biochar treatments decreased steadily after 2012. According to the principal components analysis and path analysis, the most responsive parameters in the upland red soil were soil acidity and hydraulic properties, meanwhile, soil acidity and hydraulic properties had greater impacts on rapeseed yield than did other indexes. Taken together, these results suggest that biochar can significantly improve soil fertility and rapeseed yield, but the improvements are not permanent. Soil acidity and hydraulic properties were the crucial factors that determined soil fertility and rapeseed yield in upland red soil.