Wildfire occurrence and intensity are increasing worldwide causing severe disturbances to forest watersheds used for potable water supply. The effects of wildfire on drinking water quality are not well understood, especially in terms of terrestrial dissolved organic matter (DOM) and DOM-associated formation of disinfection byproducts (DBP). As the forest floor layer is a major source of terrestrial DOM, we investigated characteristics and DBP formation of water extractable organic matter (WEOM) from the 0−5 cm depth of nonburned detritus (control) and burned detritus with black ash (moderate severity) and white ash (high severity) associated with the 2013 Rim Fire in California. Spectroscopic results suggested that the aromaticity of WEOM followed white ash > control > black ash and fluorescence region II (excitation 220−250 nm; emission 330−380 nm) of the emission-excitation-matrix was identified as a potential burn severity indicator. Compared to the control, WEOM from white and black ashes had lower reactivity in forming trihalomethanes (55%-of-control) and haloacetic acids (67%-of-control), but higher reactivity in forming the more carcinogenic haloacetonitrile after chlorination (244%-of-control) and N-nitrosodimethylamine after chloramination (229%-of-control). There was no change in reactivity for chloral hydrate formation, while WEOM from black ash showed a higher reactivity for haloketone formation (150%-of-control). Because wildfire consumed a large portion of organic matter from the detritus layer, there was lower water extractable organic carbon (27%-of-control) and organic nitrogen (19%-of-control) yields in ashes. Consequently, the wildfire caused an overall reduction in water extractable terrestrial DBP precursor yield from detritus materials.

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Carbon (C) standing stocks, C mass balance, and soil C burial in tidal freshwater forested wetlands (TFFW) and TFFW transitioning to low‐salinity marshes along the upper estuary are not typically included in “blue carbon” accounting, but may represent a significant C sink. Results from two salinity transects along the tidal Waccamaw and Savannah rivers of the U.S. Atlantic Coast show that total C standing stocks were 322–1,264 Mg C/ha among all sites, generally shifting to greater soil storage as salinity increased. Carbon mass balance inputs (litterfall, woody growth, herbaceous growth, root growth, and surface accumulation) minus C outputs (surface litter and root decomposition, gaseous C) over a period of up to 11 years were 340–900 g C · m−2 · year−1. Soil C burial was variable (7–337 g C · m−2 · year−1), and lateral C export was estimated as C mass balance minus soil C burial as 267–849 g C · m−2 · year−1. This represents a large amount of C export to support aquatic biogeochemical transformations. Despite reduced C persistence within emergent vegetation, decomposition of organic matter, and higher lateral C export, total C storage increased as forests converted to marsh with salinization. These tidal river wetlands exhibited high N mineralization in salinity‐stressed forested sites and considerable P mineralization in low‐salinity marshes. Large C standing stocks and rates of C sequestration suggest that TFFW and oligohaline marshes are considerably important globally to coastal C dynamics and in facilitating energy transformations in areas of the world in which they occur.

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Forest fires occur with increasing frequency and severity in the western United States, potentially altering the chemistry and quantity of dissolved organic matter (DOM) and disinfection byproduct (DBP) precursors exported from forested watersheds. However, little is known concerning effects of the fire triangle (heat, oxygen, and fuel) on DOM alteration. Using detritus from Pinus ponderosa and Abies concolor (dominant species in forests in the western United States), we prepared DOM from unburned and burned detritus under hypoxic (pyrolysis) and oxic conditions (thermal oxidation) at 250 and 400°C. DOM characteristics and chlorine reactivity were evaluated by absorption and fluorescence spectroscopy and chlorination-based DBP formation potential tests. Spectroscopic results suggest that burned-detritus extracts had lower molecular weight (reflected by increased E2:E3 and fluorescence index) and divergent aromaticity (reflected by SUVA 254 ) depending on oxygen availability. Temperature and oxygen availability interacted to alter the chlorine reactivity of fire-affected DOM. Increasing temperature from 50 to 400°C resulted in decreased reactivities for trihalomethane and chloral hydrate formation and divergent reactivities for haloacetonitrile formation (unchanged for pyrolysis and increased for oxidation) and haloketone formation (increased for pyrolysis and decreased for oxidation). We demonstrate that DBP precursors in fire-affected forest detritus are highly dependent on temperature and oxygen availability.

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