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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.
