MOBILIZATION OF NUTRIENTS BY FIRE IN A SEMIARID GORSE-SCRUBLAND ECOSYSTEM OF SOUTHERN SPAIN

The effect of fire on ash deposition, litter ignition, and short-term changes in soil nutrient availability and erosion losses was studied after prescribed burning of a semiarid, gorse-scrubland ecosystem of SE Spain. Ash deposition added important amounts of P and cations but negligible amounts of N to the soil. The water-soluble fraction was only 3% for P added in ash but represented 20-60% for cations (Ca, K, Na). On a weight per area basis, 63% of the litter mass and 43% of the litter total N pool were lost by ignition during the fire. However, no significant changes were found for litter P and cations. Litter C/N, C/P, and N/P ratios significantly decreased. A sharp increase in the concentrations of labile P fractions and exchangeable cations was found in surface soils immediately after the fire. However, this fertilization effect lasted only a few weeks at most. Soil total and mineral N showed no significant differences immediately after the fire. Following the first important rain, nitrate concentrations increased sharply from 5 to above 20 mu g NO3--N g(-1) throughout a 3-month sampling period; whereas NH4+ concentrations decreased. Total erosion losses during that period were significantly higher in the burned and the clipped plots (734 and 845 kg ha(-1), respectively) than in the control plot (46 kg ha(-1)). Erosion debris leaving the burned plot showed the highest nutrient concentrations shortly after the fire, particularly for total and water-soluble P. Such high initial concentrations tended to decrease progressively until they ranked with those found in the clipped plot for equivalent sampling periods. Total nutrient erosion losses from the clipped and burned plots were up to 1 order of magnitude higher than those from the control plot. Although total mass losses were higher after clipping, erosion removed 20% more N and 60% more P from the burned than from the clipped plot, which relates to the higher nutrient concentrations in debris originated at the burned plot. Data strongly suggest the nonconservative character of fire as a mineralizing agent in these steep, semiarid, oligotrophic ecosystems, and point out the risk of soil fertility depletion and desertification that high fire frequency imposes onto these areas.