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 (Santín 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.

The land-lake interface is a unique zone where terrestrial and aquatic ecosystems meet, forming part of the Earth’s most geochemically and biologically active zones. The unique characteristics of this interface are yet to be properly understood due to the inherently high spatiotemporal variability of subsurface properties, which are difficult to capture with the traditional soil sampling methods. Geophysical methods offer non-invasive techniques to capture variabilities in soil properties at a high resolution across various spatiotemporal scales. We combined electromagnetic induction (EMI), electrical resistivity tomography (ERT), and ground penetrating radar (GPR) with data from soil cores and in-situ sensors to investigate hydrostratigraphic heterogeneities across land-lake interfaces along the western basin of Lake Erie. Our Apparent electrical conductivity (ECa) maps matched soil maps from a public database with the hydric soil units delineated as high conductivity zones (ECa > 40 mS/m) and also detected additional soil units that were missed in the traditional soil maps. This implies that electromagnetic induction (EMI) could be relied upon for non-invasive characterization of soils in sampling-restricted sites where only non-invasive measurements are feasible. Results from ERT and GPR are consistent with the surficial geology of the study area and revealed variation in the vertical silty-clay and till sequence down to 3.5 m depth. These results indicate that multiple geophysical methods can be used to extrapolate soil properties and map stratigraphic structures at land-lake interfaces, thereby providing the missing information required to improve the earth system model (ESM) of coastal interfaces.

Authors: Peter Regier1, Kyongho Son2, Xingyuan Chen2, Yilin Fang2, Peishi Jiang2, Micah Taylor2, Wilfred M Wollheim3, James Stegen2Affiliations 1Marine and Coastal Research Laboratory, Pacific Northwest National Laboratory, Sequim, WA, United States2Pacific Northwest National Laboratory, Richland, WA, United States3University of New Hampshire, Durham, NH, United StatesAbstract: Hyporheic zones regulate biogeochemical processes in streams and rivers, but high spatiotemporal heterogeneity makes it difficult to predict how these processes scale from individual reaches to river basins. Recent work applying allometric scaling (i.e., power-law relationships between size and function) to river networks provides a new paradigm to develop a scalable understanding of hyporheic biogeochemical processes. We used reach-scale hyporheic aerobic respiration estimates to explore allometric scaling patterns across two basins, and related these patterns to watershed characteristics. We found consistent scaling behaviors at lowest and highest exchange flux (HEF) quantiles, and consistent but HEF-dependent relationships to watershed elevation, precipitation, and land-cover. Our results also suggest variability of hyporheic respiration allometry for middle exchange flux quantiles, and in relation to land-cover. Our findings provide initial evidence that allometric scaling may be useful for predicting hyporheic biogeochemical dynamics across watersheds from reach to basin scales.Scientific Significance Statement: The hyporheic zone is a biogeochemical control point in streams and rivers, and processes like hyporheic respiration are important determinants of how watersheds move and process carbon and nutrients. However, the hyporheic zone is also characterized by high spatial heterogeneity, which makes it difficult to predict how hyporheic functions like respiration change across watersheds from reach to basin scales. This study applies allometric scaling theory, which suggests that function scales in a predictable way with size, to determine if hyporheic respiration scales with watershed area in two basins with contrasting watershed characteristics. We found some consistent patterns between basins that suggest allometric scaling of hyporheic respiration may be a tool for transferable knowledge of hyporheic function between basins, but also note some site-specific relationships may constrain the generalizability of this method to other regions and watersheds.

The complex interactions among soil, vegetation, and site hydrologic conditions driven by precipitation and tidal cycles control biogeochemical transformations and bi-directional exchange of carbon and nutrients across the terrestrial-aquatic interfaces (TAIs) in the coastal regions. This study uses a highly mechanistic model, ATS-PFLOTRAN, to explore how these interactions impact the material exchanges and carbon and nitrogen cycling along a TAI transect in the Chesapeake Bay region that spans zones of open water, coastal wetland and upland forest. Several simulation scenarios are designed to parse the effects of the individual controlling factors and the sensitivity of carbon cycling to reaction constants derived from laboratory experiments. Our simulations revealed a hot zone for carbon cycling under the coastal wetland and the transition zones between the wetland and the upland. Evapotranspiration is found to enhance the exchange fluxes between the surface and subsurface domains, resulting in higher dissolved oxygen concentration in the TAI. The transport of organic carbon decomposed from leaves provides additional source of organic carbon for the aerobic respiration and denitrification processes in the TAI, while the variability in reaction rates mediated by microbial activities plays a dominant role in controlling the heterogeneity and dynamics of the simulated redox conditions. This modeling-focused exploratory study enabled us to better understand the complex interactions of various system components at the TAIs that control the hydro-biogeochemical processes, which is an important step towards representing coastal ecosystems in larger-scale Earth system models.