Movement of Isotopically Depleted FACE Log C into Mineral Soil after Four Years of Decomposition.

Dead wood can be a significant forest C pool, and understanding the extent and mechanisms regulating its incorporation into stable mineral soil C fractions is important for assessing the effects of climate change and forest management practices on ecosystem C sequestration.  Of particular importance is how climatic conditions and different microbial and invertebrate communities affect decomposition pathways.  However, C from wood decomposition cannot easily be differentiated from other soil C using standard analyses. We addressed this problem using the depleted wood d¹³C signature in loblolly pine, trembling aspen and paper birch logs grown at the Duke and Rhinelander Free-Air Carbon dioxide Enrichment (FACE) experiments.   Logs of the three species were placed in nine forests across the U.S. in 2011. After four years of FACE log decomposition in the field, we sampled the forest floor and mineral soil beneath the decomposing logs.  The samples were analyzed for C and ¹³C, as were samples from adjacent soil not beneath FACE logs.  Two end-member mixing models were used to estimate the proportion of soil C that originated from the decomposing FACE logs.  After four years, FACE wood C was detectable (more negative d¹³C) in mineral soil at seven of the nine sites.  The amount of FACE wood C in soil was positively correlated with the rate of mass loss of overlying logs.   Less wood C tended to end up as soil C for birch than for aspen or loblolly pine.  During this initial decomposition period, climate was a strong driver of decomposition rate and movement of wood C into mineral soil, with rates being fastest for warm, moist sites.   We hypothesize that rot pathway (brown vs white) will become more important as the experiment proceeds, playing a role in determining the proportion of FACE wood that ultimately ends up in stable soil C fractions.