Quantity and quality of carbon from ash deposits associated with desert fire

Large-scale fire in the desert Southwest was historically rare but is
becoming more common owing to human activity. In June 2020, a large wildfire burned the
eastern half of the Sycamore Creek, AZ watershed, leaving the mainstem and western half
unburned. During storms, which occurred twice during the year of the fire and not again until a
year later, large quantities of ash were transported to the stream and deposited on stream banks
as well as along upland flowpaths. Because up to 70% of this fire-associated carbon transported
during floods was consumed over 21 d, we investigated the properties of the ash, including
amounts of carbon and nutrients leached from the ash, utilization (measured as loss of carbon
across 3- and 21-d incubations), and carbon quality of the DOC leached from the ash and of that
remaining after incubations.
We found that heavy but episodic subsidies of DOC to desert stream ecosystems occur following
fire. Spatial and temporal patterns of rainfall that produced runoff determined the amount of
these subsidies. Ash collected from upland and riparian depositional areas contained measurable
quantities of carbon that were consumed during laboratory incubations with an inoculum of
microbial communities from stream sediments. Leached ash also released inorganic and organic
nitrogen and other materials. These data suggest that catastrophic release of materials during fire,
when transported to desert streams, can support microbial metabolism by enhancing nutrient-
limited primary production or supplying a novel source of organic matter to heterotrophic microbes.