Chaparral wildfire shifts the functional potential for soil pyrogenic organic matter and nitrogen cycling

Wildfires alter soil chemistry and microbial communities, opening niche space for pyrophilous or "fire-loving" microbes to initiate secondary succession. However, wildfires also increase soil nitrogen (N) and transform woody vegetation into difficult-to-degrade pyrogenic organic matter (PyOM), a potential carbon (C) source for pyrophilous microbes. Whether pyrophilous microbes can degrade these post-fire resources has significant implications for global C and N cycling, including C sequestration and greenhouse gas emissions. To determine temporal patterns in the functional potential for PyOM degradation and N cycling, we performed shotgun metagenomics on 30 burned and unburned soils collected at 17, 25, 34, 131, and 376 days after a high-severity wildfire in a Southern California fire-adapted chaparral shrubland. Not only did overall profiles of C and N cycling genes differ between burned and unburned plots, but burned plots showed dramatic increases from 17 to 376 days in genes responsible for degrading PyOM (167%) and cycling N (117%). In contrast, gene abundances for PyOM degradation and N cycling remained stable over time in unburned plots. Genes encoding the degradation pathways for two PyOM degradation intermediate pathways, catechol and protocatechuate, revealed that the easier-to-degrade ortho-cleavage pathways dominated all time points in the burned plots. Genes encoding N retention pathways (assimilatory and dissimilatory nitrate reduction) dominated N cycling pathways in burned plots for the entire year. Finally, dominant pyrophilous bacterial taxa in the genera Massilia and Noviherbaspirillium encoded different PyOM and N processing pathways, suggesting that traits enabling adaptation to wildfires, like post-fire resource acquisition, drive microbial secondary succession.