Soil carbon stabilization along climate and stand productivity gradients in black spruce forests of interior Alaska

The amount of soil organic carbon (SOC) in stable, slow-turnover pools is likely to change in response to climate warming because processes mediating soil C balance (net primary production and decomposition) vary with environmental conditions. This is important to consider in boreal forests, which constitute one of the world's largest stocks of SOC. We investigated changes in soil C stabilization along four replicate gradients of black spruce productivity and soil temperature in interior Alaska to develop empirical relationships between SOC and stand and physiographic features. Total SOC harbored in mineral soil horizons decreased by 4.4 g C·m^-2 for every degree-day increase in heat sum within the organic soil across all sites. Furthermore, the proportion of relatively labile light-fraction (density <1.6 g·cm ^-3) soil organic matter decreased significantly with increased stand productivity and soil temperature. Mean residence times of SOC (as determined by Δ¹⁴C) in dense-fraction (>1.6 g·cm^-3) mineral soil ranged from 282 to 672 years. The oldest SOC occurred in the coolest sites, which also harbored the most C and had the lowest rates of stand production. These results suggest that temperature sensitivities of organic matter within discrete soil pools, and not just total soil C stocks, need to be examined to project the effects of changing climate and primary production on soil C balance.