Soil properties and nutrient relations in burned and unburned Mediterranean-climate shrublands of Baja California, Mexico

Abstract

We compared soil chemical properties and total N and P in ecosystem compartments (soil, litter, root, stem, and leaf) of adjacent burned and unburned shrublands in northwestern Baja California during the first annual cycle after burning. We sampled one stand of coastal sage scrub growing on soil derived from basalt, and two stands of mixed chaparral growing on soils developed from granitic rocks. In the coastal sage scrub site, total soil N (1350–2140 mg/kg) and P (360–540 mg/kg) were similar to concentrations reported for Mediterranean-climate shrublands growing in fertile soils of Alta California (USA), Chile and the Mediterranean Basin. But at the chaparral sites, total soil N (750–1180 mg/kg) and P (90–150 mg/kg) were as low as those of the relatively infertile Australian heath. There was no significant reduction in organic C, nor increase in pH or salinity, in the burned areas. Only inorganic N was slightly, but significantly, higher in the soil of all three burned areas. No significant differences were found in total N or P between burned and unburned ecosystem compartments across the three sites when the entire annual cycle was considered. But N was significantly higher in leaves and stems of burned areas, although only briefly during the middle of the first growing season after fire. Nutrient enrichment following wildland fire was thus less pronounced and more transitory than is typically reported for burned shrublands in Alta California. Fire suppression has not been effective in Baja California, and fuel loads and fire intensities tend to be lower than in Alta California. Lower fuel loads and fire intensities, and consequently lower ash deposition, may partly explain the lack of marked differences in soil properties found between burned and unburned areas.