Soil degradation and nutrient availability in fire-prone Mediterranean shrublands of southeastern Spain

Chemical properties and nutrient availability were measured in soil profiles of fire-prone, semiarid, Mediterranean-type shrublands in Sierra de Almijara (SE Spain). The area is steep and has a xeric moisture regime. In such conditions, soils developed on kakiritized (sugarlike microstructure) dolomitic marble have distinctive features such as a very coarse texture, extremely high carbonate content, and low nutrient availability. These soils support site-specific shrubland communities with many dolomitophile plant endemisms. During the last decades, the area has been subjected to a high fire frequency and intensive erosion, causing deforestation and soil degradation. To assess the problem, we chose a four-stage chronosequence representative of these changes, which included examples ranging from a long-unburned, dense, mature garigue stand, to an open, frequently burned, regenerating, gorse scrubland. The linear increase in the fire frequency along the chronosequence was associated to a sharp depletion of soil-available nutrients, namely, exchangeable cations, extractable mineral N, and especially NaHCO3-extractable inorganic P. The NH4+/NO3- ratio decreased through the sequence. Organic matter contents were >10%, and total N concentrations were >4 mg N g(-1) in surface soils of the long-unburned garigue site, which contrasted with values of <4% and 1 mg N g(-1), respectively, in the most frequently burned, regenerating scrubland site. This decline of available nutrients is related to the slow recovery of the original garigue vegetation, or even substitution by other more degraded vegetation formations, that is observed in eroded sites, as well as with the lack of successful reforestation programs currently under way at the study site.