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Refractory Organic Carbon in Boreal Soils
Smittenberg, R.H., Eglinton, T.I., Schouten, S., Damste, J.S.S. 2006. Ongoing buildup of refractory organic carbon in boreal soils during the Holocene. Science 314: 1283-1286.

The authors note that "refractory organic matter makes up approximately half of the SOC [soil organic carbon] pool because of its resistance to degradation, and it is this pool that is ultimately responsible for long-term terrestrial carbon storage." However, they suggest that our current ideas about the long-term buildup of SOC are questionable, because our understanding of the phenomenon is largely derived from studies of present-day soils, due to a paucity of long-term temporal records of SOC dynamics. Hence, they attempt to rectify this situation in their present study.

What was done
Working with high-temporal-resolution sediments of Saanich Inlet, Canada, which were obtained from seven well-dated cores that contained layers ranging in age from recent to 5500 years before present, as well as layers just below the well-dated section and a late Pleistocene layer, Smittenberg et al. analyzed the distribution, stable carbon isotopic composition, and radiocarbon composition of long-chain n-alkanes that were derived predominantly from C3 vascular plant material produced upon boreal-forest-covered watersheds that drain into rivers that empty into the Inlet, which n-alkanes thus serve as proxies for recalcitrant terrigenous organic matter produced upon the watersheds.

What was learned
The researchers' work revealed that the average n-alkane ages have been increasing in a near-linear fashion toward the present, strongly suggesting that any loss due to erosion or mineralization is still largely outpaced by the accumulation." This finding is interpreted by them to imply that "the accumulation of refractory organic carbon in soils that developed after the deglaciation of the American Pacific Northwest is ongoing and may still be far from equilibrium with mineralization and erosion rates," which further suggests, in their words, that "the turnover time of this carbon pool is 10,000 to 100,000 years or more and not 1000 to 10,000 years as is often used in soil carbon models."

What it means
Smittenberg et al. say their findings "challenge the notion that the current production of refractory organic matter is balanced by decomposition and erosion after a few thousand years, as inferred via chronosequences or soil respiration measurements." They also say their findings place the terrestrial biosphere "in a more prominent position as a slow but progressively important atmospheric carbon sink on geologic time scales and may even influence current predictions about carbon cycling and soil carbon storage in response to elevated atmospheric CO2 levels," hinting that the potential for refractory organic carbon sequestration in a world with a CO2-accreting atmosphere may well be greater than what has previously been believed, but with the effect having its main impact on time scales in excess of a thousand years.

Reviewed 14 February 2007