Nitrogen Isotope Fractionation in Burned and Unburned Chaparral Soils

Plant and soil processes involving N‐transformations have been monitored by techniques measuring the 15 N/ 14 N ratio in a sample relative to the atmosphere (δ 15 N). Unusually low δ 15 N values have been reported in the tissues of chaparral shrubs. The primary objective of this study was to investigate soil N‐cycling processes which may yield the low plant tissue δ 15 N levels. Since the chaparral is subject to periodic brush fires, which in turn result in high levels of inorganic N‐forms in the soil during the first year following the burn, possible effects of burning on soil δ 15 N were also investigated. Incubations were conducted on soils from an area which had been subjected to a brush fire, and an adjacent unburned area; both soils are fine, thermic, schistose, very steep Ultic Haploxeralfs. Concentrations and δ 15 N of soil NH + 4 and NO ‐ 3 were periodically measured. A C‐ and N‐rich ash resulted in rapid mineralization of N in the burned soil; a substrate more resistant to biological degradation resulted in an initial loss, then subsequent slow accumulation of inorganic‐N in the unburned soil. Nitrate was the dominant mineral species in each soil after a few weeks. As nitrification progressed, δ 15 N of NH + 4 increased and δ 15 N of NO ‐ 3 decreased. Since the mineral pool in each soil became dominated by NO ‐ 3 , δ 15 N of the mineral pool became strongly negative. A mathematical model of isotope dynamics fits empirical data well.