1 INTRODUCTION:
Climate change is driving earlier seasonal onset of wildfire, increased
fire frequency, and larger fires in many regions globally
(Flannigan et al.,
2009; Westerling, 2016). Wildfires induce changes in ecohydrological
processes, including reduced infiltration from increased soil
hydrophobicity
(DeBano, 2000), and
reduced canopy cover that diminishes evapotranspiration and interception
of precipitation (Guo
et al., 2023; Wine et al., 2018). The resulting changes in streamflow
and terrestrial-aquatic connectivity from these shifts in
ecohydrological processes influence the composition and fluxes of
materials to stream networks, with the potential to degrade downstream
water quality (Ball et
al., 2021; Dahm et al., 2015; Hohner et al., 2019; Jones et al., 2022;
Paul et al., 2022; Rust et al., 2018; Santos et al., 2019). Thus, it is
important to improve our understanding of the spatio-temporal drivers of
water quality responses to wildfires
(Raoelison et al.,
2023).
Across spatial scales, wildfire has been documented to increase solute
concentrations by orders of magnitude in some receiving streams
(Hickenbottom et al.,
2023; Murphy et al., 2018), but lead to little response or decline in
others (Abbott et al.,
2021; Oliver et al., 2012). This may be due to differences in wildfire
and/or watershed characteristics. For example, previous literature has
identified a threshold of ~20% burn extent needed to
trigger a hydrologic response across different ecoregions
(Hallema et al.,
2018), yet identification of such responses for water quality
parameters is nascent
(Richardson et al.,
2024). While several previous studies have documented the effect of
wildfire on water quality parameters and biogeochemical processes across
broad spatial scales (e.g.,
Hampton et al., 2022;
Raoelison et al., 2023; Rust et al., 2018), few have sought to link
observed responses across time, climate, burn, and watershed
characteristics.
In particular, nitrate (NO3_) and
dissolved organic carbon (DOC) are key nutrients that underpin global
biogeochemical cycles and have the potential to degrade water quality
with increasing wildfire activity. For example, excess nitrate can lead
to downstream eutrophication
(Mast et al., 2016),
while DOC compositional shifts may influence water treatment processes
(Hohner et al.,
2019). Relationships with burn severity and extent have been observed
in some systems for nitrate
(Bladon et al., 2008;
Rhoades, Chow, et al., 2019), however, for DOC, little to no
relationships have been consistently observed across studies and systems
(Santos et al., 2019a;
Wei et al., 2021).
Observed differences in nitrate and DOC concentrations pre- and
post-fire were most pronounced in the first five years following
wildfire (Rust et al.,
2018). However, the persistence of fire effects on hydrologic and
biogeochemical processes are moderated by the rate of post-wildfire
vegetation recovery which can vary by ecosystem
(Guo et al., 2023;
Wine et al., 2018). Nitrate responses, for example, may lag due to the
shift in nitrogen speciation during combustion creating conditions that
increase nitrification post-fire
(Gustine et al., 2022;
Hanan, Schimel, et al., 2016). The magnitude and length of DOC
responses are likely a result of heterogeneous burn conditions that can
decrease and alter the chemistry of source pools
(SantÃn et al.,
2016).
Responses may be linked to changes in streamflow
(Richardson et al.,
2024), which is highly variable across climates post-fire
(Hallema et al.,
2017). This variability may co-vary with additional drivers, such as
drought (Murphy et
al., 2018; Newcomer et al., 2023) resulting in shifts in nitrate and
DOC export. For example, while the directionality of the relationship
between concentration and discharge may not be altered with wildfire,
the strength of that relationship has been shown to change for both
nitrate and DOC
(Murphy et al., 2018;
Richardson et al., 2024). While trends are emerging for streamflow
across time since fire, climate, and burn characteristics
(Hallema et al.,
2017), such trends have not yet emerged for nitrate and DOC.
Discerning biogeochemical responses post-fire are further complicated by
heterogeneous watershed characteristics
(Agbeshie et al.,
2022; Hallema et al., 2018). For example, catchment slope has a
dominant influence on biogeochemical linkages between terrestrial and
aquatic systems, primarily due to longer residence times of water and
constituents in lower-gradient catchments
(Lintern et al.,
2018). The biogeochemical signatures in steeper catchments typically
reflect that of surficial pathways, especially during periods of
enhanced hydrologic connectivity where a large proportion of material is
mobilized from the terrestrial landscape into receiving streams
(Laudon & Sponseller,
2018). Conversely, lower-gradient catchments are less responsive to
periods of enhanced hydrologic connectivity due to the greater
proportion of groundwater contributions
(Laudon & Sponseller,
2018). Lower-gradient catchments also promote longer residence times
that allow for transformations and provide a source of DOC available to
leach into receiving streams
(Tank et al., 2020).
Additionally, topography heavily influences terrestrial species
composition which influences carbon and nitrogen cycling, thus affecting
solutes available for export
(Weintraub et al.,
2017).
The objectives of this meta-analysis were to better constrain the
controls on stream water chemistry across broad spatial scales
post-fire. In this study, we synthesize biogeochemical responses of
nitrate and DOC to wildfires using meta-analytical techniques to
evaluate the effect sizes and the percent differences across reference
and fire-impacted sites spanning 3 biomes and 62 watersheds. We chose to
leverage reference-burn study designs to minimize the confounding
influence of interannual climate variability on our results
(Clausen & Spooner,
1993). We focused specifically on the importance of time-since-fire,
climate, and burn extent as factors of interest to assess post-fire
shifts in solute concentrations through space and time. Through time as
ecosystems recover, we hypothesize that there will be a decrease in the
effect size of wildfire impacts on nitrate and DOC, as concentrations
begin to reflect those in non-fire impacted systems. Furthermore, we
anticipate that there will be a systematic shift in nitrate and DOC
post-fire related to ranges in aridity and mean annual precipitation
with climate, which will be modulated by in-stream hydrologic responses
to local catchment characteristics. Lastly, we hypothesized that the
area of watershed burned will impact the relationships between watershed
characteristics and nitrate and DOC responses, influencing the magnitude
of wildfire effects on water quality.