RESUMO
Biochar is a chemically recalcitrant carbon-rich solid material used in soil for its potential to improve soil quality and sequester carbon. While the rate of application has implications for soil carbon dioxide (CO2) emission and the overall benefits of biochar, its effects are yet to be fully understood. To evaluate the effect of application rates of rice husk biochar on CO2 emissions, 91-day field experiments were conducted on three soil types (Anthraquic Ustorthent, Grossarenic Kandiustalf, and Ustic Quartzipsamment) in the southern Guinea Savannah of Nigeria, using three biochar application rates of 5, 15 and 25 t h-1, and control. A two-way ANOVA showed that cumulative CO2 emissions were significantly (p < 0.01) different between soil types and treatments, and soil type/treatment interactions were also significant at p = 0.05. The highest cumulative CO2-C emission of 2.77g/m2 was recorded in the Grossarenic Kandiustalf, while the least value of 2.11g/m2 was recorded in the Ustic Quartzipsamment. CO2 emission increased with increasing biochar application rates, with the highest (3.06 CO2-C g/m2) value recorded at 25 t/ha compared to 2.78 g/m2 and 1.52 g/m2 values recorded for 5 t/ha and control treatments respectively. While CO2 emissions increased with biochar application rate however, the percentage of biochar-C mineralized was higher at lower biochar rates, and differences were significant at p = 0.01. While 0.63 % of biochar C was mineralized under 5 t/ha biochar treatment, 0.15 % was recorded for 25 t/ha treatment. Factors that had significant correlation with CO2-C emission in the soils were biochar addition rate, soil pH, N, P, Ca, Mg and K. At day 91, there were no significant differences in CO2 emissions between amended treatments and control, and only a small percentage (<1) of biochar C had been mineralized.