SOIL NITROGEN AND THE ROLE OF FIRE AS A MINERALIZING AGENT IN A SOUTH AFRICAN COASTAL FYNBOS ECOSYSTEM.

(1) The Clovelly soil studied under a mature coastal fynbos stand in the South-Western Cape, South Africa, showed low concentrations of exchangeable NH4+-N and NO3--N. (0.5–3.5 μg N g-1 for NH4+-N and 0.2–1.2 μg N g-1 NO3--N in the surface 10 cm) probably associated with the low total N content of this sandy soil. (2) The soil showed distinct gradients of decreasing quantities of organic matter, total N, exchangeable NH4+-N and NO3--N with depth. There was a linear relationship between organic matter content and total N concentration in the surface 100 cm. (3) Lateral differences in total N concentrations were not apparent between the soils of bare patches and soils of the rhizosphere regions of selected plants. (4) Lateral differences in NO3--N and exchangeable NH4+-N concentrations were significant (P < 0.01) and probably resulted from the N utilization patterns and soil microclimates differing beneath the species studied. (5) Significant seasonal changes in total N, NO3--N and NH4+-N were apparent in the soils. (6) Direct effects of fire were evidenced by the increased total N (input of 66 kg N ha-1) and exchangeable NH4+-N concentrations found at the soil surface immediately after a fire. The flush of available NH4+-N was of short duration. (7) The indirect effects of fire on soil N observed in the year following the fire were increased total N concentrations down the profile and increased concentrations of NO3--N which lasted nine months. (8) Fire is apparently an effective mineralizing agent in soils of the coastal fynbos and has substantial effects on the concentration, form and distribution of the low nitrogen reserves. [ABSTRACT FROM AUTHOR]