FIRE SEVERITY, ASH DEPOSITION, AND CLIPPING EFFECTS ON SOIL NUTRIENTS IN CHAPARRAL

Fire may affect soil nutrient status by direct addition of nutrients and by indirectly altering the soil environment. The objective of this study was to examine how fire severity, ash deposition, and clipping affect posttreatment soil nutrient status. There were eight experimental treatments designed to examine increasing fire severity, ash (nutrient) addition independent of fire severity, clipping to study competition for nutrients per se, and untreated controls. Fire severity affected both the quality and quantity of ash. Increasing fire severity increased the concentrations of Mg and K and decreased the concentrations of NH4-N and NO3-N in the ash. As fire severity increased, there was an increasing recovery of ash relative to standing biomass (10-18 g kg-1). Fire increased the availability of all nutrients (NH4-N, NO3-N, PO4-P, Ca, Mg and K). This nutrient enhancement was largely restricted to the surface soil (0-5 cm); only soluble N appeared to increase in the subsurface soil (5-10 cm). The soil quantities of NH4-N increased and NO3-N decreased with increasing fire severity, suggesting either a direct addition of variable amounts of these N ions or an indirect effect on postfire nitrification rates. Clipping had no effect on enhancing soil nutrient availability. The apparent recovery of the basic cations in the water extracts of the ash samples was in the order: K >> Mg > Ca; this was different from the subsequent recovery in the soil: Ca > K > Mg. It was hypothesized that the relative recovery of basic cations in the water extracts of the ash samples was controlled by the solubility of Ca, Mg, and K salts.