Topographic influences on wildfire consumption of soil organic carbon in interior Alaska: Implications for black carbon accumulation

We measured characteristics of soil organic carbon (SOC) and black carbon (BC) along opposed north‐ and south‐facing toposequences in recent (2004) and old (∼1860–1950) burn sites throughout interior Alaska. Surface fuel consumption did not vary between different topographic positions, with 3.3 (±0.5) kg C m−2 being consumed on toe slope sites, 4.0 (±0.2) kg C m−2 on north‐slope sites, and 3.6 (±0.3) kg C m−2 on south‐slope sites. On a relative scale, 43% of the organic matter depth was removed through biomass burning on toe slopes, 62% on north‐slopes, and 77% on south‐slope sites. Mineral soil BC stocks (measured using a chemical/thermal oxidation method isolating more graphitic BC) ranged from 112 ± 24 g C m−2 to 173 ± 43 g C m−2 on north‐ and south‐facing aspects, respectively. These data supported the expectation that deeper and wetter ground fuels occurring in north‐facing and toe‐slope forests burned less completely, which decreased the amount of BC incorporated into mineral soil where it could be protected from subsequent fires. Thus, warmer and drier forests harbored less total SOC, but following wildfire aggraded larger BC stocks. BC was therefore an important component of stable soil C in warmer and drier forests which had higher C turnover rates in other soil pools (determined by density fractionation, δ13C, and Δ14C). These data provide a basis for understanding how variable patterns of organic layer depths and soil moisture mediate the consumption of SOC in wildfire, as well as the long‐term accumulation of BC.