Baisden, W. Troy (Landcare Research, New Zealand Ltd, Private Bag 11052, Massey Univ Campus, Palmerston North, 5300, New Zealand; Phone: +64 6-356 7145; Fax: +64 6-355-9230; Email: baisdent@landcareresearch.co.nz)

Significant Bomb C-14 Responses in Soils Below 30 cm Depth: A Previously Unrecognized Decadal C Pool?

W.T. Baisden*

Globally, soil organic matter contains approximately 1500 Pg C to 1 m soil depth and 2300 Pg C to 3 m depth—more than biomass and atmospheric CO₂ combined. Efforts to account for the effects of land-use or vegetation change on soil organic carbon (SOC) stocks normally limit their focus to the upper 20–30 cm of the soil profile, yet 0–20 cm SOC stocks are only 42% of 0-1 m SOC. Accounting for only the upper 20–30 cm of SOC has been justifiable based on the assumption that deeper SOC is unreactive since it displays ¹⁴C-derived mean residence times of hundreds or thousands of years. Recent findings indicate that, at depths of 40–100 cm, a well-studied New Zealand silt-loam soil displays progressive Delta ¹⁴C enrichment of over 200 per mil across samplings in 1959, 1974 and 2002, indicating incorporation of bomb ¹⁴C during the last 40 years. This pattern of deep ¹⁴C enrichment – previously observed in two well-drained California grassland soils – suggests that roots and/or dissolved organic C (DOC) transport contribute to a Decadally-Reactive Deep Soil C (DRDSC) pool. Soil sampled in 1970 and 2002 from a chronosequence of New Zealand sand dunes deposited from ~100 to 15,000 years ago confirms this pattern of deep soil response to the bomb¹⁴C spike. However, in these better drained profiles in more humid climates, the bomb- ¹⁴C pulse has passed at depths of 55–80 cm, evidenced by a Delta ¹⁴C decline of ~70 per mil. This SOC pool can react to land-use or vegetation change.