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要旨:
The vertical distribution of soil organic matter (SOM) in the profile may constitute an important factor for soil carbon cycling. However, the formation of the SOM profile
is currently poorly understood due to equifinality, caused
by the entanglement of several processes: input from roots,
mixing due to bioturbation, and organic matter leaching. In
this study we quantified the contribution of these three processes
using Bayesian parameter estimation for the mechanistic
SOM profile model SOMPROF. Based on organic carbon
measurements, 13 parameters related to decomposition
and transport of organic matter were estimated for two temperate
forest soils: an Arenosol with a mor humus form (Loobos,
the Netherlands), and a Cambisol with mull-type humus
(Hainich, Germany). Furthermore, the use of the radioisotope
210Pbex as tracer for vertical SOM transport was studied. For
Loobos, the calibration results demonstrate the importance
of organic matter transport with the liquid phase for shaping
the vertical SOM profile, while the effects of bioturbation
are generally negligible. These results are in good agreement
with expectations given in situ conditions. For Hainich, the
calibration offered three distinct explanations for the observations
(three modes in the posterior distribution). With the
addition of 210Pbex data and prior knowledge, as well as additional
information about in situ conditions, we were able
to identify the most likely explanation, which indicated that
root litter input is a dominant process for the SOM profile.
For both sites the organic matter appears to comprise mainly
adsorbed but potentially leachable material, pointing to the
importance of organo-mineral interactions. Furthermore, organic
matter in the mineral soil appears to be mainly derived
from root litter, supporting previous studies that highlighted
the importance of root input for soil carbon sequestration.
The 210Pbex measurements added only slight additional constraint
on the estimated parameters. However, with sufficient
replicate measurements and possibly in combination with other tracers, this isotope may still hold value as tracer for SOM transport.
