Fire is a major restructuring force in chaparral and other Mediterranean-type ecosystems. Following fire, heavy winter rains can leach N into streams, particularly from slopes that have been denuded. The extent to which N is transported from burned slopes to streams depends on how rapidly soil microbes metabolize N into mobile forms such as nitrate and how rapidly recovering plants take up mineral N. This dissertation research combines empirical analyses, remote sensing, and modeling to evaluate how ecosystems recover in the years following fire and how this affects the rates at which N is leached into streams and from watersheds.

Fieldwork focused on how N-retention mechanisms counterbalance post-fire N-mobilization processes, a point which has not been well studied in these shrubland ecosystems. I measured a variety of ecosystem properties in both burned and unburned sites on a periodic basis for two years. In burned sites, nitrification was significantly enhanced relative to rates measured in unburned sites, however ephemeral herbs established quickly, immobilizing large amounts of N relative to annual net N mineralized. Microbial biomass on the other hand decreased substantially in the first growing season, and remained low through the following year. I also measured mineralization and nitrification rates in chaparral ecosystems at varying stages of recovery from fire, and conducted laboratory incubations to experimentally examine the influence of pH, charcoal, and ammonium supply on N cycling and microbial dynamics. I found that nitrate concentrations increased over the course of incubation in soils from all age classes, especially with the addition of ammonium. When ammonium was sufficiently high, pH determined the relative proportion of inorganic N that was nitrified, while char did not have a strong impact on N cycling.

To project the effects of changing precipitation regimes, I used the ecohydrologic model RHESSys to simulate mineralization, nitrification, N leaching, and plant uptake under a range of climate scenarios surrounding a simulated fire event. Results suggest that chaparral systems are vulnerable to rapid nitrification and leaching immediately after fire, and N export is highest when fire is followed by drought. This occurs because dry conditions prolong the period during which nitrification is decoupled from plant uptake. Pre-fire drought also increased N loss relative to average conditions because it reduced the recovery rate of post-fire vegetation. These findings suggest that climate can regulate N balance by influencing how quickly plants "turn on" and begin to immobilize N.