Soil Properties Dynamics Induced by Passage of Fire During Agricultural Burning

Aims: Effects of fire on soil properties was performed in experimental plots, whose fuel amount was altered in order to obtain different heating intensities with the aim of assessing changes in soil physical conditions at varying fire temperature and also the fire temperatures within which soil quality attributes are depleted.
Study Design: The experiment consisted of two treatments (burned and un-burned plots) arranged in a RCBD with three replicates. Data were statistically analyzed for variance (ANOVA). A post Hoc Duncan multiple comparisons test was performed to compare the severity of fire temperature on soil properties. Paired t-test was used to compare means of the unburnt and burnt plots. For all tests, a threshold of P = 0.05 was used to define statistical significance. All statistical analyses were performed using SigmaStat (3.5 Edition) and validated using SPSS 17.0. Pearson correlation coefficients were used to assess the degree of relationships among variables
Place and Duration of Study: The research was conducted in a continuous cropped arable experimental plots located at the University of Uyo Teaching and Research Farm (UUTRF), Use-Offot, Uyo, Nigeria for four growing seasons, between March, 2010 to October, 2011.
Methodology: The severity of burning in each site was measured qualitatively from the degree of litter consumption of the applied biomass. Immediately after burning, soil temperatures were read from the installed temperature sensors at the surface and subsurface of the respective plots. To ensure representative sampling, bulk soil samples, which were analyzed for soil physico-chemical properties, were composite of five random samples taken at 0–15 and 15-30 cm depths within replicated plots. Particle-size distribution was determined in the soil samples using hydrometer method. Bulk density was estimated by dividing the oven-dry mass of the soil by the volume of the soil. In addition, core samples were also used to determine saturated hydraulic conductivity (Ks) in the laboratory using a constant head permeameter. pH was determined with the use of glass electrode pH meter to read the suspension of 10g soil sample with 20 ml 0.01 N CaCl2. Available phosphorus was determined using bicarbonate extraction, with acid reductant. Meanwhile, the exchangeable cations (calcium, Ca; magnesium, Mg; potassium, K; and sodium, Na) in the soil were determined by first extracting the soil sediment with 1M NH4 OAc (ammonium acetate) solution. The amounts of exchangeable Na and K in the extract were determined by flame photometry while Ca and Mg were determined by atomic absorption spectrometry.
Results: Temperature differences significantly (P < 0.05) affected sand, total nitrogen, organic carbon and pH contents of the soils positively (r = 0.518* 0.478*, 0.582*, 0.595** respectively), whereas a reduction in the soil temperature increased the concentrations of clay, 1mm, 0.05mm and 0.25 mm stable soil aggregates in the soil (r = -0.619**, -0.578*, -0.780, -0.526* respectively) after burning. Exchange acidity increased to 5.12 cmolkg-1 at 400C from 0.80 cmolkg-1 at initial temperature of 250C at the surface soil.
Conclusion: Though aggregates formation was significantly higher (P = 0.05) after burning than the control soil locations, both organic matter and ECEC increased at increasing fire temperature. Potassium content remained surprisingly constant as the fire temperature increased. However, despite the merits of quick release of occluded nutrients, heating temperatures of slash-and-burn method of land clearing altered the soil quality attributes, this soil will easily be distressed with the least application of force.