Factors Regulating Nitrogen Retention During the Early Stages of Recovery from Fire in Coastal Chaparral Ecosystems

Fire is a fundamental reorganizing force in chaparral and other Mediterranean-type ecosystems. Postfire nutrient redistribution and cycling are frequently invoked as drivers of ecosystem recovery. The extent to which N is transported from slopes to streams following fire is a function of the balance between the rate at which soil microbes retain and metabolize N into forms that readily dissolve or leach, and how rapidly recovering plants sequester this mobilized N. To better understand how fire impacts this balance, we sampled soil and plant N dynamics in 17 plots distributed across two burned, chaparral-dominated watersheds in Santa Barbara County, California. We measured a variety of ecosystem properties in both burned and unburned plots on a periodic basis for 2 years, including soil water content, pH, soil and plant carbon and nitrogen, extractable inorganic nitrogen, dissolved organic nitrogen, and microbial biomass. In burned plots, nitrification was significantly enhanced relative to rates measured in unburned plots. Ephemeral herbs established quickly following the first postfire rain events. Aboveground plant biomass assimilated N commensurate with soil net mineralization, implying tight N cycling during the early stages of recovery. Microbial biomass N, on the other hand, remained low throughout the study. These findings highlight the importance of herbaceous species in conserving ecosystem nutrients as shrubs gradually recover.