PS 47-209 - Transfer of decaying wood C into mineral soil C pools: A long-term continental study

Abstract: Dead wood can be a significant forest C pool, and understanding the extent and mechanisms regulating its incorporation into mineral soil C fractions is needed to improve our ability to model effects of climate change and forest management practices on ecosystem C sequestration. However, C from wood decomposition cannot easily be differentiated from other soil C using standard analyses. We addressed this problem by using the depleted ¹³C signature in the wood of loblolly pine, trembling aspen and paper birch logs grown at 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, six and nine plus years of decomposition in the field, we sampled the surface mineral soil (< 10 cm) beneath the decomposing logs. In year six, soil adjacent to logs was also sampled, and in year nine+ soils to 50 cm were included. Samples were analyzed for C and ¹³C, as were samples from control soil distant from decomposing logs. Two end-member mixing models were used to estimate the proportion of soil C that originated from the decomposing logs. The amount of wood C found in surface soil beneath the logs increased from the fourth to sixth year of decomposition, doubling to quadrupling at most sites, but tended to decline thereafter. Averaged across species and sites, 3.4% of the original log C was in the top 10 cm of soil either beneath (1.4%) or adjacent to (2.0%) the logs at year six. The amounts found in the top 10 cm of soil were 2.2, 3.4 and 4.5%, respectively, for pine, aspen and birch. Across species and sites, about 52.5% of original C content remained in the logs, leaving approximately 44.1% of wood C that had moved to the atmosphere or was deeper in the soil than the sampling depth. Through six years, climate was a strong driver of decomposition rate and movement of wood C into mineral soil, with rates being fastest for warm, moist sites. In the early years of the experiment, much of the wood-derived C in soil was in the free light fraction (FLF), which was presumed to be susceptible to further decay. Over time, the amount of wood C in FLF declined and much of the wood C that resided in mineral soil was found in the mineral-associated heavy fraction (HF), which is more strongly protected against further decomposition.