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 (CO 2) 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 CO 2 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 À1 , and control. A two-way ANOVA showed that cumulative CO 2 emissions were significantly (p < 0.01) different between soil types and treatments, and soil type/treatment interactions were also significant at p ¼ 0.05. The highest cumulative CO 2-C emission of 2.77g/m 2 was recorded in the Grossarenic Kandiustalf, while the least value of 2.11g/m 2 was recorded in the Ustic Quartzipsamment. CO 2 emission increased with increasing biochar application rates, with the highest (3.06 CO 2-C g/m 2) value recorded at 25 t/ha compared to 2.78 g/m 2 and 1.52 g/m 2 values recorded for 5 t/ha and control treatments respectively. While CO 2 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 ¼ 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 CO 2-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 CO 2 emissions between amended treatments and control, and only a small percentage (<1) of biochar C had been mineralized.

A reconnaissance survey conducted at the University of Ilorin Sugar Research Farm (USRF) revealed four dominant soils at Site 1 (USRF1) and one at Site 2 (USRF2). The soils were characterized and classified according to both the Soil Taxonomy (ST) and the World Reference Base for Soil Resources (WRB). Also, the suitability of the soils for sugarcane cultivation was evaluated using the limitation approach. While the USRF1 soils were reddish, the USRF2 soil was greyish due to poor drainage. The USRF1 soils were loamy sand with the AB-horizons of pedons II and III being gravelly. Pedon V had sandy loam surface, sandy clay loam subsurface and clay loam subsoil. The USRF1 soils were moderately acid while the USRF2 soil was slightly acid to slightly alkaline. Exchangeable calcium (Ca2+) content of the USRF2 soil which averaged 4.00 cmolc kg–1 was 2-3 times higher than that of the USRF1 soils. The USRF2 soil also contained higher Mg2+, K+ and Na+, 2-3 folds higher effective cation exchange capacity and > 10 folds higher soil organic carbon (with mean of 11.60 g kg–1) and total nitrogen (mean of 0.94 g kg–1). Under ST, pedons I and IV classified as Typic Haplustepts, II and III as Lithic Haplustepts and V as a Kanhaplic Haplustalf. Under WRB, pedons I and IV classified as Eutric Regosols (arenic), II and III as Endo-pisoplinthic Cambisols (arenic) and V as a Gleyic Lixisol (loamic). Pedon V was highly suitable (85.25%), I and IV moderately suitable (64.53%), II marginally suitable (47.40%) and III unsuitable (35.62%) for sugarcane cultivation.