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Zusammenfassung:
A widespread decrease of the topsoil carbon content
was observed over England and Wales during the period
1978–2003 in the National Soil Inventory (NSI), amounting
to a carbon loss of 4.44 Tg yr−1 over 141 550 km2. Subsequent
modelling studies have shown that changes in temperature
and precipitation could only account for a small part
of the observed decrease, and therefore that changes in land
use and management and resulting changes in heterotrophic
respiration or net primary productivity were the main causes.
So far, all the models used to reproduce the NSI data have
not accounted for plant–soil interactions and have only been
soil carbon models with carbon inputs forced by data. Here,
we use three different versions of a process-based coupled
soil–vegetation model called ORCHIDEE (Organizing Carbon
and Hydrology in Dynamic Ecosystems), in order to separate
the effect of trends in soil carbon input from soil carbon
mineralization induced by climate trends over 1978–2003.
The first version of the model (ORCHIDEE-AR5), used for
IPCC-AR5 CMIP5 Earth System simulations, is based on
three soil carbon pools defined with first-order decomposition
kinetics, as in the CENTURY model. The second version
(ORCHIDEE-AR5-PRIM) built for this study includes
a relationship between litter carbon and decomposition rates,
to reproduce a priming effect on decomposition. The last
version (O-CN) takes into account N-related processes. Soil
carbon decomposition in O-CN is based on CENTURY, but
adds N limitations on litter decomposition.We performed regional
gridded simulations with these three versions of the
ORCHIDEE model over England and Wales. None of the
three model versions was able to reproduce the observed NSI
soil carbon trend. This suggests either that climate change is
not the main driver for observed soil carbon losses or that the
ORCHIDEE model even with priming or N effects on decomposition
lacks the basic mechanisms to explain soil carbon
change in response to climate, which would raise a caution
flag about the ability of this type of model to project soil
carbon changes in response to future warming. A third possible
explanation could be that the NSI measurements made
on the topsoil are not representative of the total soil carbon
losses integrated over the entire soil depth, and thus cannot
be compared with the model output.
