HR: 10:20h
AN: B12B-01 INVITED    [Abstracts]
TI: Progress, Potential and Pitfalls in the Use of Bomb ¹⁴C to Constrain Soil Carbon Dynamics
AU: * Baisden, W T
EM: t.baisden@gns.cri.nz
AF: Rafter Radiocarbon Laboratory, National Isotope Centre, GNS Science, Lower Hutt, - 5040, New Zealand
AB: Forty four years have passed since atmospheric testing of thermonuclear weapons injected a major ¹⁴C spike into the atmosphere-biosphere-hydrosphere system. The use of bomb ¹⁴C, in combination with millennial decay of ¹⁴C, remains the most effective empirical tool for constraining rates of carbon (C) cycling in soils at timescales beyond experimental manipulations (>5 years). In the last 20 years, accelerator mass spectrometry has greatly increased the potential and throughput of soil ¹⁴C studies. At present, atmospheric Δ¹⁴C appears to be stabilizing at more constant values as a result of reinjection of bomb ¹⁴C from decadal storage in forests and soils. This means that current and future studies using bomb ¹⁴C have different sensitivities and uncertainties compared to those carried out during periods of rapid Δ¹⁴C decline such as the 1970s, 80s and 90s. Bomb ¹⁴C proves most effective when archived soil samples are available: simply using bulk Δ¹⁴C from samples collected at two or more times can surpass single time point Δ¹⁴C from soil fractions in providing robust C cycling rates. Of course, measurement of Δ¹⁴C in soil fractions from time series samples can significantly improve estimates of C cycling parameters. Samples collected between ca. 1965 and 1995 have now greatly surpassed pre-bomb samples in utility, although pre-bomb samples retain considerable usefulness for estimating the size of inert (millennial) C pools. Major pitfalls in the use of bomb ¹⁴C, particularly for single time point samples and fractions, are mainly associated with model assumptions. For example, calculated residence times can be highly sensitive to a minor component of old C (<10% of total C). Similarly, calculated residence times are also highly dependent upon rates of soil C accumulation or loss. A final key source of error is lag times between C fixation from atmospheric CO₂ and incorporation in the measured soil C pool, either due to long-lived plant tissue, or residence times in other soil pools/horizons. All work using Δ¹⁴C should consider sensitivity and uncertainty related to these issues. Major potential exists in the use of Δ¹⁴C to constrain soil C dynamics as a function of soil depth, in relation to major unexplained losses of soil C, and to probe the mechanisms and rates of soil organic matter stabilization. These areas of major potential all lay outside conventional use of Δ¹⁴C to calculate simple residence times.
UR: http://www.gns.cri.nz/who/staff/2234.htm
DE: 0402 Agricultural systems
DE: 0428 Carbon cycling (4806)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0466 Modeling
DE: 0486 Soils/pedology (1865)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting