How Much Soil Carbon is Derived from Woody Detritus? A Ten-Year Study of ¹³C Incorporation into Soil Organic Matter

The importance of woody detritus as a source of soil organic matter is not well constrained. We quantified the recovery of ¹³C derived from isotopic-enriched sugar maple wood in various C fractions of two temperate forest soils in central New York, USA. Decay rates of small woody debris were quite rapid (k = 0.362 to 0.477 per year) and after 10 years less than 1% of the original wood mass remained in incubation bags. After six years we recovered only 0.26% (± 0.025) of the added ¹³C in the upper 5 cm of underlying soil. After 10 years this recovery declined to 0.11% (± 0.020) indicating substantial lability of retained SOC; most of this decline occurred from year 6 to 8 in the 1–5 cm depth increment, suggesting that the residue was quite stable at 10 years. The largest fraction of ¹³C was recovered in microaggregates (45%), especially those occluded within macroaggregates (30%), with a smaller proportion associated with the silt + clay fraction (20%). These proportions did not change significantly from year 6 to 10. Faster decay and higher ¹³C recovery were coincident with abundant saproxylic invertebrates from Scarabaeidae at one of the sites. We conclude that small woody debris is a minor source of stable SOC in these temperate forests (that is, less than 1% of annual SOC accumulation).