Frequent fires intensify soil crusting: physicochemical feedback in the pedoderm of long-term burn experiments in South Africa

Abstract

Savannas and grasslands in South Africa are adapted to fire, yet long-term effects of fire on soil, water and nutrients remain largely unknown. To determine whether frequent burning increases the tendency of soils to crust and alters soil chemistry, topsoils from 19 sites were examined in Mpumalanga, KwaZulu-Natal and the Eastern Cape where annual burning and fire exclusion experiments had been conducted for at least 28 years. A comparison of soils from burnt and unburnt plots revealed that composite samples taken to a depth of 10 cm from burnt plots had lower laboratory infiltration, electrical conductivity (EC) of 1:5 extracts, water-soluble K, water-soluble NH 4, total C, total N and labile C. Burnt plots had higher pH in KCl, water dispersible clay and modulus of rupture. Rate of soil respiration from composite samples (0–2 cm) from burnt plots was lower than from unburnt plots (means of 21 vs. 36 ng C g −1 s −1). Rainfall simulation on the same samples demonstrated that burnt plots crusted more rapidly than unburnt plots (19 vs. 35 mm h −1), while results from sampling at 1-cm intervals revealed that the greatest differences between burnt and unburnt plots were in the 0–1 cm layer. The top few centimetres of soil are likely to have a disproportionate effect on ecosystem functioning by influencing the rate of infiltration and mineralisation of soil organic matter. This thin surface layer has been named the pedoderm in this paper. Soil from 0–1 cm in burnt plots had lower total C (means of 0.8% vs. 2.7% for burnt and unburnt plots, respectively), total N (0.07% vs. 0.23%), (NH 4)OAc-extractable Ca (7 vs. 17 mmol c kg −1), Mg (2 vs. 7 mmol c kg −1), K (0.8 vs. 1.5 mmol c kg −1) and a greater exchangeable Na percentage (17% vs. 8%). The results indicate that burning increases soil crusting. This was ascribed to a decrease in humus content (and associated disaggregating effect) and an increase in the dispersion of clay. Nutrients are lost from burnt plots over time, probably by the removal of ash in surface runoff. Calcium, Mg, and K were lost more readily than Na probably because plants take up these nutrients in greater concentration than Na. The net effect was an increase in the exchangeable sodium percentage (ESP). Crusting on burnt plots may be self-perpetuating, because increased runoff is likely to increase the loss of soluble salts.