Changes in physicochemical properties and quality index of an Alfisol after three years of rice husk biochar amendment in rainfed rice - Maize cropping sequence

Biochar's potential in improving crop performance, soil quality and functioning has been extensively studied with promising results reported. This study addressed research gaps and updated existing knowledge on long term effect of single biochar application on soil physicochemical properties. A three year field trial was set up on an acidic Alfisol testing the effect of rice husk biochar on select soil quality indicators viz.; soil pH, electrical conductivity, bulk density, water stable aggregates, water holding capacity, soil organic carbon, total nitrogen, CEC and C:N ratio sampled at two soil depth (0-10 cm and 10-20 cm). A soil quality index was estimated to provide a quantitative tool for assessing the long term effects of biochar amendment on Alfisol productivity. Biochar amendment at 6-12 t ha(-1) proved more efficient at improving soil quality index (SQI = 1.00) on a long term basis. Further, biochar amendment significantly enhanced all determined soil quality indicators after one year of biochar amendment at sampled soil depths. However, the effect of biochar on some soil quality indicators such as electrical conductivity, organic carbon, total nitrogen and C:N ratio diminished after three years, while soil pH, CEC, bulk density - (decreased), water stable aggregates and water holding capacity remained consistently enhanced. Biochar showed more positive long term effect on improving soil physical quality indicators than chemical indicators. Particularly, decreasing soil electrical conductivity, organic carbon, total nitrogen and C/N ratio after three years suggest increased uptake of soluble ions by plants, carbon mineralization and nitrogen flux with plausible soil organic carbon and nitrogen losses. To mitigate these accelerated carbon and nitrogen flux and increase the efficiency of aged biochar in sequestering carbon and nitrogen, re-application of biochar after three years is recommended. Overall, biochar amendment at the dosage of 6-12 t ha(-1) is recommended for long term soil quality improvement in the study area and regions of the world with similar soil type and climatic conditions.