Low vertical transfer rates of carbon inferred from radiocarbon analysis in an Amazon Podzol

[摘要] Hydromorphic Podzol soils in the Amazon Basin generally support low-statureforests with some of the lowest amounts of aboveground net primary production(NPP) in the region. However, they can also exhibit large values ofbelowground NPP that can contribute significantly to the total annual inputsof organic matter into the soil. These hydromorphic Podzol soils also exhibita horizon rich in organic matter at around 1–2 m depth, presumably as aresult of eluviation of dissolved organic matter and sesquioxides of Fe andAl. Therefore, it is likely that these ecosystems store large quantities ofcarbon by (1) large amounts of C inputs to soils dominated by their highlevels of fine-root production, (2) stabilization of organic matter in anilluviation horizon due to significant vertical transfers of C. To assessthese ideas we studied soil carbon dynamics using radiocarbon in two adjacentAmazon forests growing on contrasting soils: a hydromorphic Podzol and awell-drained Alisol supporting a high-stature terra firme forest.Our measurements showed similar concentrations of C and radiocarbon in thelitter 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 hassimilar soil C storage and cycling rates compared to the well-drained Alisolthat supports a more opulent vegetation. However, we found importantdifferences in carbon dynamics and transfers along the vertical profile. Atboth soils, we found similar radiocarbon concentrations in the subsoil, butthe carbon released after incubating soil samples presented radiocarbonconcentrations of recent origin in the Alisol, but not in the Podzol. Therewere no indications of incorporation of C fixed after 1950 in the illuvialhorizon of the Podzol. With the aid of a simulation model, we predicted thatonly a minor fraction (1.7%) of the labile carbon decomposed in thetopsoil is transferred to the subsoil of the Podzol, while this proportionaltransfer is about 30% in the Alisol. Furthermore, our estimates were 8times lower than previous estimations of vertical C transfers in AmazonPodzols, and question the validity of these previous estimations for allPodzols within the Amazon Basin. Our results also challenge our previousideas about the genesis of these particular soils and suggest that eitherthey are not true Podzols or the podzolization processes had already stopped.

[学科分类] 地球化学与岩石