4.2 Time-since-fire influences nitrate and DOC effect sizes
Nitrate increased in wildfire-impacted watersheds, and remained elevated
for up to 10-years post-burn, consistent with previous research
(Rhoades et al., 2019;
Rust et al., 2018), but with an observable lag effect (Fig. 2). Nitrate
effect sizes weren’t significantly higher compared to the unburned
watershed until one year following a wildfire, indicating a delay in
nutrient response (Fig. 2). This may be due, in part, to a lag in
increased soil nitrate pools that have been observed in the first year
post-fire (Wan et al.,
2001), which have been tied to shifts in key controllers of soil
nitrification post-burn, such as pH and ammonium availability
(Gustine et al.,
2022), limited vegetative uptake of nutrients
(Adkins et al., 2019;
Gustine et al., 2022; Smith et al., 2011), and/or climate related
drivers decoupling nitrate mobilization from its post-fire production in
soils (Hanan,
D’Antonio, et al., 2016). We observed that variation in nitrate
response was higher immediately following wildfire, and became more
constrained three years after wildfire, which may indicate that spatial
and temporal variability in soil nitrate pools are linked to hydrologic
export across the studied systems, as nitrate mobilization is often
tightly coupled to high hydrologic connectivity and flushing events
post-wildfire (Bladon
et al., 2008; Gustine et al., 2022).
While prior studies have generally reported increased nitrate export to
streams following a wildfire
(Coombs & Melack,
2013; Mast & Clow, 2008; Rhea et al., 2021), consensus regarding
post-fire DOC dynamics remains elusive, suggesting that mechanisms
driving DOC dynamics may vary more dramatically through time. We found
that DOC decreased significantly in burned watersheds compared to
unburned counterparts, and remained lower for at least five years
following wildfire (Fig. 2). Lower DOC following wildfire observed in
our meta-analysis may be related to alterations to DOC source pools
post-wildfire. For example, organic carbon (OC) source pools may become
depleted following a wildfire due to combustion of topsoil, thus
limiting the amount of DOC available for export
(Wagner et al., 2015;
Wang et al., 2012). Moreover, wildfire can transform biomass to
recalcitrant forms of pyrogenic carbon, which can be resistant to
microbial degradation and less soluble in water, resulting in lower
amounts of DOC released in burned watersheds compared to unburned
watersheds (Barton et
al., 2024).
The persistence of fire effects on in-stream concentrations indirectly
reflect terrestrial ecosystem recovery, thus we hypothesized DOC would
return to background conditions over time
(Clay et al., 2012;
Rhoades et al., 2019; Rodríguez-Cardona et al., 2020). Our
meta-analysis confirmed this, showing a non-significant difference in
DOC effect sizes greater than five years post-fire. Due to wildfire
effects on hydrologic and biotic processes, we expected greater
variability immediately following a wildfire and then becoming more
constrained through time. However, we did not observe a significant
difference between the variability of DOC response through time. This
may be due, in part, to hydrologic controls on DOC post-fire. Run-off
increased significantly in regions with a Mediterranean climate where
the shrub vegetation recovered slowly and transportation decreased for
extended periods of time
(Kinoshita & Hogue,
2011), while in more mesic climates, streamflow is linked to
interannual variability in precipitation
(Holden et al.,
2012). Variability in streamflow across climates is highest in the
first two years post-fire, and gradually decreases through time
(Saxe et al., 2018).