Soil acidity, available phosphorus content, and optimal biochar and nitrogen fertilizer application rates: A five-year field trial in upland red soil, China

Biochar has been used as an amendment to reduce soil acidity and increase soil nutrients content and crop yields. While many laboratory studies have focused on the short-term effects of single application of biochar on soil properties, little is known about the long-term effects of biochar combined with nitrogen fertilizer on soil pH, available P content, and crop yields. Additionally, few studies have investigated the optimal biochar and nitrogen fertilizer application proportions in worldwide. A five-year field trial was conducted to study the effects of biochar (0, 5, 20 and 40 t ha(-1)) combined with nitrogen fertilizer (0, 60, 90, 120 kg ha(-1)) on soil pH, available P, rapeseed, and sweet potato yield. Multi-dimensional nonlinear regression models were built to predict the highest crop yields and the optimal economical biochar and nitrogen fertilizer application rate. The results were as follows: high dose of biochar (20 and 40 t ha(-1)) combined with nitrogen fertilizer can significantly increase soil pH, available P content, rapeseed and sweet potato yields in the first year; however, the effectiveness on these metrics weakened over time, biochar needed to be applied approximately every three years in the rapeseed-sweet potato rotation system in order to maintain the positive effects on crop yields. The multi-dimensional regression models showed that the highest yields of rapeseed and sweet potato are 3.34 t ha(-1) and 39.7 t ha(-1) when the application rates of biochar were 80.8 t ha(-1) and 59.4 t ha(-1) respectively. However, high dosages of biochar can increase crop yields without increasing net income. The highest net income was $29,206 ha(-1) 5y(-1) ($5841 ha(-1) y(-1)) when the application rates of biochar and nitrogen fertilizer are 11.4 t has and 102 kg ha(-1), respectively. The integrated use of biochar and nitrogen fertilizer was a promising strategy to obtain higher and sustained productivity of the rapeseed -sweet potato cropping system in upland red soil. From the point of view of crop yields and economic benefits, 11.4 t ha(-1) biochar was the optimal biochar application rates for the local farmers.