Season and Interval of Prescribed Burn Effects on Black Carbon in Eastern Oregon Soil.

In ponderosa pine (Pinus ponderosa) forests of the western United States, prescribed fires are used to reduce fuel loads and restore historical fire regimes. The season in which prescribed burns are performed and the interval between burns can have complex consequences for the ecosystem, including soil carbon cycling through the production of pyrogenic carbon (PyC). PyC, a broad term that refers to a spectrum of thermally-altered organic matter, plays a crucial role in soil carbon cycling, displaying turnover times that are orders of magnitude longer than unburned organic matter. This work investigated how the season of and interval between prescribed burns affects the formation and retention of pyrogenic carbon (PyC) in a ponderosa pine forest of eastern Oregon. In 1997 a season and interval of burn study was implemented in Malheur National Forest to examine the ecological effects of burning at 5 and 15-year intervals in either the spring or fall. In October 2015, both O-horizon and mineral soil (0-15 cm) samples were collected and analyzed for PyC concentration, content, and structure using the benzene polycarboxylic acid (BPCA) method. O-horizon depth, carbon and nitrogen concentration and content, pH, and bulk density were also measured. Compared to unburned controls, we estimate that fall burns increased the PyC concentration of the mineral soil by 8.4 g BPCA/kg C (95% CI: 4.2,12.6 g BPCA/kg C). No change in PyC concentration of the O-horizon was detected for the plots burned in either the spring or fall compared to the unburned controls; however, the chemical structure of the PyC in the O-horizon of the unburned control plots was significantly more condensed than that of the plots burned in either the spring or the fall.