Prescribed fire and grass mulch impact on selected soil properties and amelioration potentials of amendments under an agricultural field in Ile-Ife, Nigeria

Prescribed fire (slash and burn) tends to impede the sustainable functionality of soil in agricultural systems. However, the use of amendment has a potential to reverse these negative effects. Prescribed fire is still used by farmers in Nigeria for land preparation before planting. This has continued to increase soil degradation in Nigeria with no agronomic activities that could ameliorate this effect in view. This study therefore examined the influence of prescribed fire and grass mulch on selected soil physical properties and organic carbon immediately after burning, and the potential of amendment to improve the soil properties negatively affected by fire. The prescribed fire treatments consisted of 200 °C and 500 °C fire intensities using Megathyrsus maximus (a large perennial bunch of grass that is dominant in the study area) as the fuel and also served as the grass mulch. Cured poultry manure at 7.5 t ha⁻¹ was combined with urea at 40 kg N ha⁻¹ and applied as a combined amendment while a single dose of urea was applied at 80 kg N ha⁻¹ as an inorganic amendment. The experiment was carried out across three cropping seasons on maize (Zea mays L.) in 2019 and 2020. Soil water repellency (SWR), bulk density (D_b), soil unsaturated hydraulic conductivity (K_unsat), water stable aggregates (WSA), soil strength (SS), and soil organic carbon (SOC) were determined immediately after burning and three months after amendments were applied. Data obtained were subjected to analysis of variance and means separated using the Duncan multiple range test at p≤0.05. Results showed that prescribed fires increased SWR by an average of 50.1 and 62.7 % for 200 °C and 500 °C intensities, respectively compared with the control. Three months later, the SWR was reduced by 25.9 % and 62.3 % for 200 °C and 500 °C, respectively when no amendment was added. Notably, the addition of sole urea and cured poultry manure + urea reduced the SWR by 50 % and 48.5 %, respectively in the 200 °C intensity, and 62.2 % and 62.7 % in the 500 °C intensity, respectively. Also, K_unsat was reduced by an average of 49.6 and 62.2 % by 200 °C and 500 °C intensities, respectively just after burning. However, it was not improved three months after the prescribed fires despite the applied amendments. The prescribed fire of 200 °C and grass mulch had no significant influence on OC just after the fire. Also, amendments did not significantly improve OC three months after prescribed fire. Therefore, the non-improvement of other soil properties, after the applied amendments, showed that further study is required to determine the required rate of the applied amendments that will significantly improve other soil properties negatively affected by prescribed fire in agricultural soils.