Simulated Burn Impact on Soil Properties and Carbon Cycling

Authors

Dana Johnson¹*, Benjamin Sulman², Kara Yedinak³, Thea Whitman¹ (twhitman@wisc.edu)

Institutions

¹University of Wisconsin–Madison, WI; ²Oak Ridge National Laboratory, Oak Ridge, TN; ³Forest Products Laboratory, USDA Forest Service, Madison, WI

Abstract

Boreal forests hold a massive terrestrial reservoir of carbon (C), which is held both above- and belowground. Wildfire in the boreal forest can drive changes in quantity and composition of C held in the soil via heat-induced tree mortality and production of coarse woody debris at the soil surface, incomplete combustion of organic matter leading to production of pyrogenic organic matter, and volatilization and loss of C to the atmosphere. To understand how wildfires impact soil C cycling and how this may vary with burn severity, the team aims to determine the impact of burning and burn duration on soil properties, including pH, total C, and total nitrogen (N), and on C cycling in the weeks to months post-burn. Researchers collected soil cores from twelve sites across Wood Buffalo National Park, Alberta, Canada and allowed cores to air-dry to simulate drought conditions. Cores from each site were subjected to burn simulations in a cone calorimeter for either 30 s or 120 s or were kept as controls and not burned (N=4 for each treatment). Soil carbon dioxide efflux was measured daily using an automated sampling system connected to a cavity ring-down spectroscopy instrument for 10 weeks post-burn. The team fit a two-pool decay model to respiration curves to estimate changes in the fractional size and decomposition rates of fast- and slow-cycling C pools. The simulated burns altered both soil C and N stocks, driving shifts in C:N ratios. These changes were accompanied by immediate increases in soil pH, with larger effects in soil cores subjected to the longer burn duration. During the 10 weeks post-burn, overall respiration rates (per gram dry soil) were lower in burned vs. unburned soil. These results suggest that the immediate impact of burning on soil C and the soil environment may drive a slowdown in C cycling in the weeks following fire, and this effect may be larger with increasing burn durations, which may be related to increasing burn severity in the field. Next steps include linking changes in C cycling to changes in bacterial and fungal community composition and microbial C use efficiency post-fire.