Multi-decadal impacts of high-severity wildfire on ecosystem nitrogen cycling
Date
2022
Authors
Rhea, Allison Elizabeth, author
Covino, Tim, advisor
Rhoades, Charles, advisor
Kampf, Stephanie, committee member
Rathburn, Sara, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Wildfires modify the amount, form, and distribution of nitrogen (N) throughout an ecosystem. Though N stocks are lost during the combustion of vegetation and surface organic matter, there is often a subsequent increase in inorganic N delivery to streams that provide drinking water to the Western US. This can make streams and reservoirs more susceptible to eutrophication and algal blooms, threatening the delivery of clean drinking water. While many post-fire studies have documented short-term (<5 years) increases in soil and stream inorganic N, long-term monitoring after the Hayman fire has revealed that increases in stream N can persist for decades. This dissertation investigates the long-term controls of elevated post-fire N across spatial scales. Chapter 2 describes the stream biotic response to the Hayman and High Park fires that burned along the Colorado Front Range. I evaluated stream water chemistry, algal nutrient limitation, benthic biomass, and stream metabolism along stream reaches within three burned and three unburned watersheds. Although the two high-severity wildfires occurred five and 15 years prior to the study, the streams draining burned watersheds still had 23-times higher nitrate (NO3-) concentrations than unburned watersheds, a trend that is consistent across seasons and throughout the 15-year post-fire record. Autotrophic N-limitation was reduced in these nitrate-rich burned streams. Consequently, autotrophic biomass and primary productivity were 2.5 and 20-times greater, respectively, in burned relative to unburned streams which indicates post-fire increases in stream N demand. However, the continued export of N out of these burned streams suggests that terrestrial N supply exceeds in-stream N demand. This suggests that streams have a limited capacity to attenuate N exports from burned watersheds. It was unclear if terrestrial N delivery to streams was 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. I address this knowledge gap in chapter 3, by measuring 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 17 years prior in 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. 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. Finally, in chapter 4, I investigated the landscape and stream network drivers of persistent elevated stream NO3- in nine watersheds that were burned to varying degrees by the Hayman fire. I evaluated the ability of multiple linear regression and spatial stream network modeling approaches to predict observed concentrations of the biologically active solute NO3- compared to the conservative solute sodium (Na+). No landscape variables were strong predictors of stream Na+. Rather, stream Na+ variability was largely attributed to flow-connected spatial autocorrelation, indicating that downstream hydrologic transport was the primary driver of spatially distributed Na+ concentrations. In contrast, vegetation cover, measured as mean normalized differenced water index (NDMI) was the strongest predictor of spatially distributed stream NO3- concentrations. Furthermore, stream NO3- had weak flow-connected spatial autocorrelation and exhibited high spatial variability. This pattern is likely the result of spatially heterogeneous wildfire behavior that leaves intact forest patches interspersed with high burn severity patches that are dominated by shrubs and grasses. Post-fire vegetation also interacts with watershed structure to influence stream NO3- patterns. For example, severely burned convergent hillslopes in headwaters positions were associated with the highest stream NO3- concentrations due to the high proportional influence of hillslope water in these locations. My findings help characterize the potential magnitude, duration, and location of water quality concerns following fire. Slow forest recovery in large, high severity burn patches will likely sustain post-fire N export by limiting vegetation N uptake. As regeneration failures become more common with increasing fire severity and climate aridity, ecosystems will be more susceptible to sustained NO3- losses. If reforestation is desired, targeted plantings in riparian corridors, severely burned convergent hillslopes, and headwater positions will likely have the largest impact on stream NO3- concentrations.
Description
Rights Access
Subject
nitrate
watershed biogeochemistry
geospatial analysis
wildfire
water quality