[LTER-sbc_all] PHD Seminar - Erin Hanan - Thursday, December 10th, 8:30am

[Jenny Dugan]

 ***PHD Seminar***
 
 Erin Hanan
 

Thursday, December 10th, 8:30am 
 
Bren room 1424 
 Title: Biogeochemical responses to fire in coastal chaparral ecosystems

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Fire is a major restructuring force in chaparral and other Mediterranean-type ecosystems. Following fire, heavy winter rains can leach nitrogen into streams, particularly from slopes that have been denuded. The extent to which nitrogen is transported from burned slopes to streams depends on how rapidly soil microbes metabolize nitrogen into mobile forms such as nitrate and how rapidly recovering plants take up mineral nitrogen. 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 nitrogen is leached into streams and from watersheds. 
 
Fieldwork focused on how nitrogen-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 nitrogen relative to annual net nitrogen mineralized. Microbial biomass on the other hand decreased substantially in the first growing season, and remained low through the following year. I also conducted laboratory incubations to experimentally examine the influence of pH, charcoal, and ammonium supply on nitrogen cycling and microbial dynamics at varying stages of recovery from fire. 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 nitrogen that was nitrified, while char did not have a strong impact on nitrogen cycling. To project the effects of changing precipitation regimes, I used the ecohydrologic model RHESSys to simulate mineralization, nitrification, nitrogen leaching, and plant uptake under a range of climate scenarios surrounding a simulated fire event. Results suggest that nitrogen 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 nitrogen loss relative to average conditions because it reduced the recovery rate of post-fire vegetation. These results suggest that climate can regulate nitrogen balance by influencing how quickly plants “turn on” and begin to take up nutrients mobilized by fire.
 

 



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Jenny Dugan

Marine Science Institute
University of California
Santa Barbara, CA 93106-6150
Phone: 805-893-2675
email: j_dugan at lifesci.ucsb.edu
http://msi.ucsb.edu/people/research-scientists/jenny-dugan () 
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