Bomb 14C enrichment indicates decadal C pool in deep soil?

Studies of changes in soil organic carbon (SOC) stocks normally limit their focus to the upper 20-30 cm of soil, yet 0-20 cm SOC stocks are only similar to 40% of 0-1 m SOC. Accounting for only the upper 20-30 cm of SOC has been justifiable assuming that deeper SOC is unreactive since it displays C-14-derived mean residence times of hundreds or thousands of years. The dramatic increase in the C-14 content of the atmosphere resulting from thermonuclear testing circa 1963 allows the unreactivity of deep SOC to be tested by examining whether deep soils show evidence of 'bomb-C-14' incorporation. At depths of 40-100 cm, a well-studied New Zealand soil under stable pastoral management displays progressive enrichment of over 200 parts per thousand across samplings in 1959, 1974 and 2002, indicating substantial incorporation of bomb C-14. This pattern of deep C-14 enrichment-previously observed in 2 well-drained California grassland soils-leads to the hypothesis that roots and/or dissolved organic C transport contribute to a decadally-reactive SOC pool comprising similar to 10-40% of SOC below 50 cm. Deep reactive SOC may be important in the global C cycle because it can react to land-use or vegetation change and may respond to different processes than the reactive SOC in the upper 20-30 cm of soil.