Understanding Long-Term Boreal Soil Carbon Accumulation: Insight from a Dynamic Layered Soil Model

Despite repeated fire disturbance, low-lying boreal forests have managed to accumulate large and globally significant stocks of C. Furthermore, boreal soils have historically occupied discontinuous and continuous permafrost zones and currently are experiencing a warming climate. Combined, these factors may result in loss of permafrost and increase in decomposition of the soil C. However, before attempting to assess the potential for future change in boreal soil C, the decadal to millennial mechanisms of soil C cycling and accumulation must be well understood. To that end, we developed a dynamic layered long-term soil C model to specifically address: 1) the influence of the soil thermal regime, 2) substrate recalcitrance through humification, and 3) the role of permafrost on the preservation and accumulation of boreal soil C. We included radiocarbon in our model so that we could compare soil C stocks as well as radiocarbon distribution to observations from a well studied Old Black Spruce (OBS) site in Manitoba, Canada. Using parameters from the literature, our results indicate that the accumulation of boreal soil C is dependent on the interaction between the soil thermal regime and humification. While these factors account for much of the soil C accumulation observed at OBS, the weighted average radiocarbon in the modeled deep soil was too new, suggesting a lack of preservation and accumulation of old soil C. However, upon inclusion of a permafrost layer for several centuries, not only the C accumulation and distribution between shallow and deep soil layers improved, but the weighted average radiocarbon markedly improved with respect to the OBS site. Our results suggest that: 1) in the absence of permafrost, deep soil C turnover time is on the order of 200-300 y rather than the previous estimates of up to and greater than 1000 y and 2) the legacy from recent shifts in boreal climate and permafrost must be considered when evaluating current observations.