Assessing Soil Supply and Plant Demand to Interpret Long-Term Post-Fire Stream N Export
Abstract
Severe wildfire stimulates terrestrial nitrogen (N) losses during the combustion of vegetation and surface organic matter, which is often followed by increased inorganic N production and delivery to streams. Elevated post-fire stream nitrate (NO3-) has been shown to persist for years to decades and can threaten downstream ecosystems with eutrophication and algal blooms. However, it is unclear if terrestrial N delivery to streams is driven by long-term elevated soil inorganic N supply (i.e., pools and net transformation rates) or depressed post-fire vegetation recovery and plant nutrient demand. We measure inorganic N in surface mineral soils (0-15 cm), soil leachate (30 cm), and shallow groundwater (40-100 cm) in unburned watersheds dominated by ponderosa pine (Pinus ponderosa) and shrub-dominated watersheds that burned during the 2002 Hayman fire. Wildfire caused large C and N losses from soil O horizon during combustion (~1,500 and 50 g /m2 of C and N, respectively). However, total C and N stocks, soil-extractable inorganic N, plant-available inorganic N, and net N transformation rates (i.e., nitrification, and N mineralization) differed little between burned and unburned mineral soils. This indicates that there were no long-term post-fire increases in soil N supply. In contrast to the near surface patterns, NO3- concentrations were four- and ten-times higher, respectively in shallow groundwater and streams of burned watersheds with low post-fire vegetation cover. Tree regeneration has been slower than expected following the Hayman and other fires in the western US and these biogeochemical patterns suggest that low plant N demand may prolong the impacts of wildfires on stream nutrients where more extreme fire behavior and climatic conditions inhibit vegetation recovery.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFMGC53B..07R