Congratulations Dr. Hanan

Last month, PhD student Erin Hanan successfully defended her dissertation “Biogeochemical responses to fire in coastal chaparral ecosystems”. Erin was a student in The Department of Ecology, Evolution, and Marine Biology (EEMB) and is a valuable member of the Tague Team Lab. Dr. Naomi Tague has worked with Erin as a member on her committee and is collaborating on lab related research. Erin’s thesis research focus:

“Fire is a major restructuring force in chaparral and other Mediterranean-type ecosystems. Following fire, heavy winter rains can leach nitrogen (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 mobilization processes, a point that has not been well studied in chaparral ecosystems. 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. Laboratory incubations revealed that nitrification increased most rapidly following ammonium addition. 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. Modeling simulations suggest that 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 results suggest that climate can regulate N balance by influencing how quickly plants “turn on” and begin to take up nutrients mobilized by fire.”

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