The effect of time since last fire occurrence on selected soil hydrological properties in a South African savanna

Fires are a natural occurrence in African savannas yet little is known about how fires affect soil hydrological properties over time. This study aimed to determine the short-term (1–9 months post fire) and long-term (> seven decades fire exclusion) impact of experimental fires on soil hydrology in a fire-prone African savanna. This study also investigated how soil hydrological properties may vary under different vegetation physiognomies.

Saturated hydraulic conductivity (K) was determined using a modified falling head method through macropores (K_Macro) while unsaturated K was measured using a tension disc infiltrometer (Ψ = 30 mm) to reflect the meso- and micro-pore conductivity (K_Meso+Micro). Soil hydrophobicity was measured using the water droplet penetration time (WDPT) test.

For the finer soil pores, fires significantly reduce K_Meso+Micro in the short-term one month post-burn but returns to Pre-burn K_Meso+Micro rates nine months later. After decades of fire suppression, K_Macro is significantly faster compared to burned soils while K_Meso+Micro was significantly slower through smaller pores compared to soils burned annually. K_Meso+Micro is slowest under shrubs in fire suppression plots compared to other vegetation physiognomies. Fires did not induce hydrophobicity in the short-term nor the long-term.

Whilst fire reduces K_Meso+Micro in the short-term, soils recovered nine months later, indicating that prescribed burning does not lead to lasting detrimental impacts on soil hydrological properties. Interestingly, decades of active fire suppression had either a positive or negative effect on the soil’s ability to allow water to infiltrate through the soil surface depending on the pore size and possible susceptibility to bioclogging and biocrusts.