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

Strydom, Tercia, Riddell, Eddie S., Rowe, Thomas, Govender, Navashni, Lorentz, Simon A., le Roux, Pieter A.L., Wigley-Coetsee, Corli
Geoderma 2019 v.345 pp. 114-122
basaltic soils, burnt soils, dewpoint, ecosystems, fire frequency, hydraulic conductivity, infiltration rate, infiltrometers, landscapes, nutrient availability, nutrients, prescribed burning, savanna soils, savannas, soil water, soil water potential, soil water retention, spatial variation, temporal variation, wildfires
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 (Kunsat) was measured using a Tension Disc Infiltrometer to infer infiltration rates, saturated hydraulic conductivity (Ksat) 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 Kunsat 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 Kunsat which were as low as <1 mm hr−1 compared to a mean Kunsat 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 Kunsat than fire frequency. Fires did not affect Ksat within the initial 2–5 cm of the soil surface. In general, the granitic soils had faster Kunsat and Ksat 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.