Retention of Nitrogen Following Wildfire in a Chaparral Ecosystem

Abstract

Wildfires alter nitrogen (N) cycling in Mediterranean-type ecosystems, resetting plant and soil microbial growth, combusting plant biomass to ash, and enhancing N availability in the upper soil layer. This ash and soil N pool (that is, wildfire N) is susceptible to loss from watersheds via runoff and leaching during post-fire rains. Plant and soil microbial recovery may mitigate these losses by sequestering N compounds in new biomass, thereby promoting landscape N retention in N-limited chaparral ecosystems. We investigated the relative balance between wildfire N loss, and plant and soil microbial N uptake and stream N export for an upland chaparral watershed in southern California that burned (61%) in a high-intensity wildfire in 2009 by using a combination of stream, vegetation, soil microbial, and remote sensing analyses. Soil N in the burn scar was 440% higher than unburned soil N in the beginning of the first post-fire wet season and returned within 66 days to pre-fire levels. Stream N export was 1480% higher than pre-fire export during the first post-fire rain and returned within 106 days over the course of the following three rainstorms to pre-fire levels. A watershed-scale N mass balance revealed that 52% of wildfire N could be accounted for in plant and soil microbial growth, whereas 1% could be accounted for in stream export of dissolved nitrogen.