Direct effects of prescribed fire on available nitrogen and phosphorus in an Arizona chaparral watershed

The effect of prescribed fire on soil nutrients in Arizona chaparral is limited even though prescribed fire is used extensively as a management tool. Concentrations of exchangeable NH4 +‐N and NO3 ‐‐N and extractable P were studied under two Arizona chaparral shrubs growing on a Lithic Ustorthent soil before and after a prescribed fire. The results show exchangeable NH4 +‐N increased from 5.19 to 48.74 mg kg‐1 and extractable P increased from 5.38 to 14.55 mg kg‐1 in the surface horizon. NH4 +‐N increased from 5.39 to 71.62 mg kg‐1 under Cercocarpus betuloides and from 5.23 to 36.10 mg kg‐1 under Quercus turbinella, while extractable P increased from 5.80 to 22.62 mg kg‐1 under C. betuloides and from 5.46 to 14.58 mg kg‐1 under Q. turbinella. The difference in soil nutrient status between species after prescribed fire is due to the amount of litter accumulated and the nutrient concentration difference in the litter and standing biomass of these two species. The majority of N and P is lost owing to volatilization of litter and biomass, yet small but significant amounts of N and P are translocated down and condensed in the surface soil. The condensation of N and P along with ash deposition increased soil fertility after the prescribed fire, which is essential to the long‐term productivity and sustainability of Arizona chaparral ecosystems.