The effects of fire on the soil in a degraded semi-arid woodland .III. Nutrient pool sizes, biological activity and herbage response

Abstract

The effect of experimental fire regimes on soil nutrient pool values, surface condition status and herbage growth was studied in a shrub-invaded woodland on massive red earths near Coolabah, New South Wales. Artificial fuel was burnt in spring on plots to achieve intervals of 1, 2, 3, 4, 5 and 10 years between fires. No differences between fire regimes were detected below 1 cm for any of the measured soil variables. Soil respiration declined linearly with increasing number of fires and increasing total fuel load. There were no differences or trend for available or total soil nitrogen, organic carbon or soil wettability in the 0-1 cm horizon, but relatively refractory elements such as P and K, derived from the ash from the artificial fuel, built up in the soil proportionately with increased fire number and total fuel load. The surface condition status of the soil as assessed by a monitoring method was strongly inversely related to the number of fires and total fuel load. Differences in herbage growth between fire treatments were not significant; however, where plots had been burnt five times, herbage growth was greater with increased variability (35.8 g me2, s.e. 19.0) compared with unburnt controls (10.7 g m-2, s.e. 1.7). Plots burnt seven times annually had similar means and variance to controls. Low herbage yields in control plots were attributed to the high biomass of mature shrubs, whilst increasing variability with number of fires was caused by the combination of patchy decreased shrub biomass and small-scale erosion-deposition cells which differentially distribute seeds across the soil surface. Plots burnt annually seven times had deposition beyond the plot area.