Beaver ponds as biogeochemical hotspots in burned watersheds
American beaver at Big Stone National Wildlife Refuge.
As wildfire frequency and severity in the western U.S. increases, so does their ability to negatively impact watersheds. However, it remains unknown how the presence of beaver ponds in burned watersheds may impact retention and transformation of chemical constituents originating in burned uplands.
Beaver dams store sediment, particulate carbon and nutrients, and create anoxic environments. Heterotrophic microbes that inhabit these locations must rely on anaerobic respiration pathways during metabolism. This study evaluated sediment and water moving through a series of beaver ponds in a watershed burned during the 2018 Ryan Fire. Beaver ponds downstream of burned uplands exhibited an increased in concentration dissolved organic carbon and dissolved total nitrogen.
However, it appears the microbes in these ponds are using necromass derived from heat sensitive soil microbes and invertebrates lysed by the wildfire rather than pyrogenic organic matter as a primary source of carbon for respiration. Impoundment within beaver ponds enhances postfire sediment, carbon, and nitrogen storage compared to free-flowing streams. These burned beaver ponds had higher dissolved organic carbon, dissolved total nitrogen, and nitrate compared to an unburned stream reach, or free-flowing streams. Within these beaver ponds, microbial assemblages appear to forgo metabolic pathways associated with processing highly aromatic compounds in pyrogenically derived dissolved organic matter. Instead, the microbiome appears to source carbon through assemblages that produce enzymes for processing microbes and invertebrates lysed during soil heating. Other assemblages identified in the metagenome include anaerobic respiratory metabolic pathways associated with metal and sulfate reduction, which have consequences for carbon, nitrogen, and metals that are stored and transported.