Use of geostatistical models to evaluate landscape and stream network controls on post-fire stream nitrate concentrations

Forested watersheds provide many ecosystem services, such as the filtration of sediment, pollutants, and nutrients, which are increasingly threatened by wildfire. For example, stream nutrient concentrations often increase following wildfire and can remain elevated for decades, making downstream waters susceptible to eutrophication. We investigated the drivers of persistent elevated stream nutrients, specifically nitrate (NO3-), in nine watersheds that were burned 16 years prior by the Hayman fire, Colorado, USA. We evaluated the ability of multiple linear regression and spatial stream network modeling approaches to predict observed concentrations of the biologically active solute NO3- and the conservative solute sodium (Na+) which serves as a partial control. Specifically, we quantified the degree to which landscape and stream network characteristics predict stream solute concentrations. Stream Na+ exhibited strong spatial autocorrelation that was primarily controlled by topography and hydrology. In contrast, stream NO3- had higher spatial variability and was inversely correlated to vegetation cover, measured as mean normalized differenced moisture index (NDMI). Spatially heterogeneous wildfire behaviour left intact forest patches interspersed with high burn severity patches dominated by shrubs and grasses which contributes to the spatial variability in stream NO3- concentrations. 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. Our findings suggest that targeted reforestation in severely burned convergent hillslopes in headwater positions may enhance the recovery of stream NO3- concentrations to pre-fire levels.

Automated summary

Forested watersheds are important for providing ecosystem services, but they are increasingly threatened by wildfires. This study investigated the drivers of persistently elevated nitrate (NO3-) concentrations in burned watersheds and found that vegetation cover and landscape characteristics play a significant role. Additionally, targeted reforestation in severely burned areas may help restore stream nitrate concentrations.