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 a fter prescribed burning of a semiarid, gorse-scrubland ecosystem of SE Spain. Ash deposition added important amounts of P and cations but ne gligible 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 lit ter total N pool were lost by ignition during the fire. However, no si gnificant changes were found for litter P and cations. Litter C/N, C/P , and N/P ratios significantly decreased. A sharp increase in the conc entrations 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 show ed no significant differences immediately after the fire. Following th e 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 tha t 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 nu trient concentrations shortly after the fire, particularly for total a nd water-soluble P. Such high initial concentrations tended to decreas e progressively until they ranked with those found in the clipped plot for equivalent sampling periods. Total nutrient erosion losses from t he clipped and burned plots were up to 1 order of magnitude higher tha n those from the control plot. Although total mass losses were higher after clipping, erosion removed 20% more N and 60% more P from the bur ned than from the clipped plot, which relates to the higher nutrient c oncentrations in debris originated at the burned plot. Data strongly s uggest the nonconservative character of fire as a mineralizing agent i n these steep, semiarid, oligotrophic ecosystems, and point out the ri sk of soil fertility depletion and desertification that high fire freq uency imposes onto these areas.