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Abstract:
Hydromorphic Podzol soils in the Amazon Basin generally support low-stature forests with some of the lowest
amounts of aboveground net primary production (NPP) in
the region. However, they can also exhibit large values of belowground
NPP that can contribute significantly to the total
annual inputs of organic matter into the soil. These hydromorphic
Podzol soils also exhibit a horizon rich in organic
matter at around 1–2m depth, presumably as a result of eluviation
of dissolved organic matter and sesquioxides of Fe
and Al. Therefore, it is likely that these ecosystems store
large quantities of carbon by (1) large amounts of C inputs
to soils dominated by their high levels of fine-root production,
(2) stabilization of organic matter in an illuviation horizon
due to significant vertical transfers of C. To assess these
ideas we studied soil carbon dynamics using radiocarbon in
two adjacent Amazon forests growing on contrasting soils: a
hydromorphic Podzol and a well-drained Alisol supporting
a high-stature terra firme forest. Our measurements showed
similar concentrations of C and radiocarbon in the litter layer
and the first 5 cm of the mineral soil for both sites. This result
is consistent with the idea that the hydromorphic Podzol soil
has similar soil C storage and cycling rates compared to the
well-drained Alisol that supports a more opulent vegetation.
However, we found important differences in carbon dynamics
and transfers along the vertical profile. At both soils, we
found similar radiocarbon concentrations in the subsoil, but
the carbon released after incubating soil samples presented
radiocarbon concentrations of recent origin in the Alisol, but
not in the Podzol. There were no indications of incorporation
of C fixed after 1950 in the illuvial horizon of the Podzol.
With the aid of a simulation model, we predicted that
only a minor fraction (1.7 %) of the labile carbon decomposed
in the topsoil is transferred to the subsoil of the Podzol,
while this proportional transfer is about 30% in the Alisol.
Furthermore, our estimates were 8 times lower than previous
estimations of vertical C transfers in Amazon Podzols,
and question the validity of these previous estimations for all
Podzols within the Amazon Basin. Our results also challenge
our previous ideas about the genesis of these particular soils and suggest that either they are not true Podzols or the podzolization processes had already stopped
