UCLA

Wetlands provide many critical ecosystem functions including cycling of elements such as nitrogen, sulfur, and carbon while providing other functions such as water treatment and flood management in urban areas. Climate change and related climate extremes such as wildfires have the potential to alter the ecosystem balance provided by wetlands. For instance, many wetlands are located downstream of wildfire-affected areas and receive stormwater runoff carrying wildfire residues. The deposition of the wildfire residues may affect some of the functions of the wetlands. Yet, no studies to date examine the effect of wildfire residues on the capacity of wetlands to reduce nitrate and sulfate. To investigate the effect of wildfire residues on microbially mediated denitrification and sulfate reduction, batch experiments were set up in the lab using wetland sediments, water, and wildfire ashes. Results reveal that the presence of wildfire residues accelerated both denitrification and sulfate reduction initially. While the denitrification rate remained consistent following repeated exposure to nitrate-rich water, sulfate reduction rates became slower following each exposure to sulfate-rich water. An increase in nitrate and sulfate reduction in the presence of wildfire residues was attributed to the potential change in water chemistry and microbial community. The presence of wildfire residues increased salinity, dissolved organic carbon, and the concentrations of nitrate and sulfate leached from wildfire residues— all of which could have affected the microbial reduction rate of nitrate and sulfate. Analysis of SUVA showed a possible increase in aromatic DOC in pore water. Analysis of functional genes also confirmed the higher abundance of denitrifying genes in wildfire batches following one week in a field experiment, but by three weeks, denitrifying genes were insignificant in both the field and the lab. Overall, the results suggest that wetlands could provide an effective barrier to removing excess sulfate and nitrate released from wildfire residues or other sources following the deposition of wildfire residues.