Life after a fiery death: Fire and plant biomass loading affect dissolved organic matter in experimental ponds

Abstract

Drier and hotter conditions linked with anthropogenic climate change can increase wildfire frequency and severity, influencing terrestrial and aquatic carbon cycles at broad spatial and temporal scales. The impacts of wildfire are complex and dependent on several factors that may increase terrestrial deposition and the influx of dissolved organic matter (DOM) from plants into nearby aquatic systems, resulting in the darkening of water color. We tested the effects of plant biomass quantity and its interaction with fire (burned vs. unburned plant biomass) on dissolved organic carbon (DOC) concentration and degradation (biological vs. photochemical) and DOM composition in 400 L freshwater ponds using a gradient experimental design. DOC concentration increased nonlinearly with plant biomass loading in both treatments, with overall higher concentrations (>56 mg/L) in the unburned treatment shortly after plant addition. We also observed nonlinear trends in fluorescence and UV‐visible absorbance spectroscopic indices as a function of fire treatment and plant biomass, such as greater humification and specific UV absorbance at 254 nm (a proxy for aromatic DOM) over time. DOM humification occurred gradually over time with less humification in the burned treatment compared to the unburned treatment. Both burned and unburned biomass released noncolored, low molecular weight carbon compounds that were rapidly consumed by microbes. DOC decomposition exhibited a unimodal relationship with plant biomass, with microbes contributing more to DOC loss than photodegradation at intermediate biomass levels (100–300 g). Our findings demonstrate that the quantity of plant biomass leads to nonlinear responses in the dynamics and composition of DOM in experimental ponds that are altered by fire, indicating how disturbances interactively affect DOM processing and its role in aquatic environments.

Fire transforms effects of terrestrial subsidies on aquatic ecosystem structure and function

“…A companion study (Spiegel et al, 2023) found that DOC decomposition in our experiment was strongly attributed to microbial activity than photodegradation, and that the highest rates of DOC decomposition occurred at intermediate concentrations. The saturating relationship between detrital loading and CO 2 concentrations also indicates that organic carbon was mineralized at slower rates at the highest concentrations where DOC accumulated, and DOC respiration was lower in burned treatments (Figure 6), possibly due to fire-effects on DOC composition such as increased aromaticity.…”

“…Indeed, over the short period of our experiment (~90 days) we observed substantial temporal variability in water quality and productivity, suggesting a critical transition between terrestrial loading/DOC concentrations and the stimulatory effect of limiting nutrients on aquatic productivity (Stetler et al, 2021). In a companion study, we show that detrital loading and burning produce non-linear effects on the chemical composition of DOM in these tanks, with greater humification and specific ultraviolet absorbance (i.e., SUVA 254 nm, a proxy for aromaticity); however, humification decreased more rapidly in burned treatments compared to unburned (Spiegel et al, 2023). Therefore, greater NPP and lower DOC in the burned treatment may be a result of fire effects on the organic chemistry of the parent materials and the processing of DOM in aquatic ecosystems.…”

“…In a companion study (Spiegel et al, 2023), decomposition (% mass loss) of burned sage was twofold greater than burned willow after ~90 days, but the decomposition of burned and unburned sage was largely equivalent across the detrital-loading gradient. Therefore, lower % sage-15 N in burned treatments overall and the marked decline at high detrital-loading, is likely not driven by difference in rates of 15 N release from burned/unburned sage, but instead changes in dissolved compounds, their lability, and composition (e.g., aromaticity) (Hampton et al, 2022;Rodríguez-Cardona et al, 2020).…”

“…However, at high detrital-loading, dissolved phosphorus accumulated and this effect was accentuated by burning (Figure S6), indicating a reduction of energy transfer across trophic levels . This effect may be due to a combination of (1) fire-induced changes in DOC chemistry (increased aromaticity) (Hampton et al, 2022;Spiegel et al, 2023), (2) plant polyphenols affecting enzyme activity (Hättenschwiler & Vitousek, 2000), (3) and/or changes to nutrient cycling between autotrophic and heterotrophic micro organisms (Cole et al, 1988).…”