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| Abstract |
- Soils are globally significant sources and sinks of
atmospheric CO₂. Increasing the resolution of soil carbon
turnover estimates is important for predicting the response of
soil carbon cycling to environmental change. We show that
soil carbon turnover times can be more finely resolved using
a dual isotope label like the one provided by elevated CO₂
experiments that use fossil CO₂. We modeled each soil physical
fraction as two pools with different turnover times using
the atmospheric ¹⁴C bomb spike in combination with the label
in ¹⁴C and ¹³C provided by an elevated CO₂ experiment
in a California annual grassland.
In sandstone and serpentine soils, the light fraction carbon
was 21–54% fast cycling with 2–9 yr turnover, and 36–79%
slow cycling with turnover slower than 100 yr. This validates
model treatment of the light fraction as active and intermediate
cycling carbon. The dense, mineral-associated fraction
also had a very dynamic component, consisting of ~ 7%
fast-cycling carbon and ~93% very slow cycling carbon.
Similarly, half the microbial biomass carbon in the sandstone
soil was more than 5 yr old, and 40% of the carbon respired
by microbes had been fixed more than 5 yr ago.
Resolving each density fraction into two pools revealed
that only a small component of total soil carbon is responsible
for most CO₂ efflux from these soils. In the sandstone
soil, 11% of soil carbon contributes more than 90% of the
annual CO₂ efflux. The fact that soil physical fractions, designed
to isolate organic material of roughly homogeneous
physico-chemical state, contain material of dramatically different turnover times is consistent with recent observations
of rapid isotope incorporation into seemingly stable fractions
and with emerging evidence for hot spots or micro-site variation
of decomposition within the soil matrix. Predictions
of soil carbon storage using a turnover time estimated with
the assumption of a single pool per density fraction would
greatly overestimate the near-term response to changes in
productivity or decomposition rates. Therefore, these results
suggest a slower initial change in soil carbon storage due
to environmental change than has been assumed by simpler
(one-pool) mass balance calculations.
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| Citation |
- Torn, M. S., Kleber, M., Zavaleta, E. S., Zhu, B., Field, C. B., and Trumbore, S. E.: A dual isotope approach to isolate soil carbon pools of different turnover times, Biogeosciences, 10, 8067-8081, doi:10.5194/bg-10-8067-2013, 2013.
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| Funding Statement (additional comments about funding) |
- This work was supported by the Laboratory Directed Research and Development Program at Lawrence Berkeley National Laboratory, US Department of Energy under contract no. DE-AC03-76SF00098 and by the Office of Science, US Department of Energy under contract no. DE-AC02-05CH11231. The Jasper Ridge elevated CO₂ experiment was supported by grants from the US National Science Foundation to the Carnegie Institution for Science, Stanford University, and University of California, Berkeley.
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| Additional Information |
- description.provenance : Submitted by Erin Clark (erin.clark@oregonstate.edu) on 2014-03-06T21:52:48Z
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Previous issue date: 2013
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