The effect of experimental fires on soil hydrology and nutrients in an African savanna

Savannas make up about 20% of the global land-surface and are dependent on fires to maintain a balanced ecosystem. Fires in other fire-driven landscapes, particularly wildfires, were found to have negative effects on various soil properties. However, there is a lack of studies confirming the effect of fires on soil hydrology in African savanna soils. A long-term fire experiment in a South African savanna provided an opportunity to investigate the effect of different prescribed fire frequencies on soil properties in situ across coarse-grained granitic and fine-textured basalt-derived soils. Soil properties were compared between soils exposed to annual fires, fires every 2-4 years and where fires have been excluded for approximately 60 years. Across all three fire treatments, unsaturated hydraulic conductivity (K-unsat) was measured using a Tension Disc Infiltrometer to infer infiltration rates, saturated hydraulic conductivity (K-sat) measured with a Guelph Permeameter, soil water potential calculated using a Decagon WP4-T Dewpoint Potentiometer to infer soil water retention and soil total C and N measured using a LECO CNS TruMac Series Analyser. Our study found that K-unsat is not affected by frequent annual fires which have infiltration rates similar to soils where fires have been excluded for nearly 6 decades. However, recently burnt granitic soils, i.e. three months prior, have significantly slower K-unsat which were as low as < 1 mm hr(-1) compared to a mean K-unsat of 30 mm hr(-1) on annually burnt soils, alluding to short term fire impacts on soil infiltration. Hence, we believe that time following a fire plays a greater role on K-unsat than fire frequency. Fires did not affect K-sat within the initial 2-5 cm of the soil surface. In general, the granitic soils had faster K-unsat and K-sat than the basaltic soils. Soil water potential, total C and N was significantly greater in the fire exclusion sites over both parent materials. Soil water and nutrient availability is critical in a post-fire environment to facilitate vegetation recovery in African savannas. These systems are resilient to fires which do not have long-term negative impacts on soil hydrology and nutrients, but instead increases the spatio-temporal variation in soil properties necessary in maintaining savanna heterogeneity.