Mechanisms of soil carbon stabilization in black spruce forests of interior Alaska: Soil temperature, soil water, and wildfire

The likely direction of change in soil organic carbon (SOC) in the boreal forest biome, which harbors roughly 22% of the global soil carbon pool, is of marked concern because climate warming is projected to be greatest in high latitudes and temperature is the cardinal determinant of soil C mineralization. Moreover, the majority of boreal forest SOC is harbored in surficial organic horizons which are the most susceptible to consumption in wildfire. This research focuses on mechanisms of soil C accumulation in recently burned (2004) and unburned (∼1850-1950) black spruce (Picea mariana [Mill.] BSP) forests along gradients in stand productivity and soil temperature. The primary research questions in these three chapters address: (1) how the interaction between stand production and temperature effect the stabilization of C throughout the soil profile, (2) the quantity and composition of water soluble organic carbon (WSOC) as it is leached from the soil across gradients in productivity and climate, and (3) physiographic controls on organic matter consumption in wildfire and the legacy of wildfire in stable C formation (pyrogenic C, or black carbon). Soil WSOC concentrations increased while SOC stocks decreased with increasing soil temperature and stand production along the gradients studied. Stocks of BC were minuscule in comparison to organic horizon SOC stocks, and therefore the C stabilizing effect of wildfire was small in comparison to SOC accumulation through arrested decomposition. We conclude that C stocks are likely to be more vulnerable to burning as soil C stocks decline relative to C sequestered in aboveground woody tissues in a warmer climate. These findings contribute to refining estimates of potential changes in boreal soil C stocks in the context of a changing climate.