LB 5-47 - Deadwood Carbon Movement to Soil Pools Varies by Species and Site During Long-Term Decomposition

Deadwood represents a major carbon (C) pool in forested ecosystems, with the long-term fate of the C known to depend on decomposition rate, wood species and site conditions. Still, variations in the amounts of different species’ deadwood C moving to soil pools in contrasting forest climates are poorly understood. We investigated the long-term proportions of C moving into soil pools by analyzing soil by depths of 0–10 cm, 10–20 cm, 20–30 cm and 30–50 cm beneath decomposing trembling aspen (aspen) and loblolly pine (pine) logs. The decomposing logs had a distinct δ13C that enabled tracking their C into soil and were deployed in 2011 at three bioclimatically diverse locations in the Coweeta (warm, moist), Fraser (cold, dry) and Marcell (cold, moist) Experimental Forests in the United States. We distinguished soil labile C from protected C by density-fractionation of bulk soils into free light (FLF), occluded light (OCC) and heavy (HF) fractions. The proportion of deadwood C in each fraction was estimated using a two-endpoint mixing model based on the δ13C of soil C fractions relative to the original woody material and that of control soil from the same depth.The proportion of soil C derived from logs was comparable between Coweeta (2.26 ± 0.22%) and Marcell (2.02 ± 0.29%), both of which were greater than Fraser (1.34 ± 0.22%). Aspen C (2.44 ± 1.32%) transferred to soil was almost twofold that of pine (1.32 ± 0.15%), but the difference between the two species was more apparent in cold, moist Marcell site. At all sites, soil C derived from logs was higher in topmost and deepest soil depths and mostly occurred in FLF (upper soil) and HF (deeper soil) fractions. The proportion of log’s C retained in soil varied by site, where accrual was greatest in the warm, moist Coweeta site, followed by the cold, dry Fraser site. However, log C was predominately retained in surface soil depths where more than half of the logs’ C occurred in HF, indicating long-term protection if forest disturbances are minimized. Log species had significant effect on log’s C stored in soil, as C retained in soil from aspen was over 3.5 times greater than for pine. These results suggested that deadwood C movement to soil pools varied by species and site, and indicating a need for management decisions to consider coarse-woody debris’ role in soil C sequestration and protection.