Deep plant-derived carbon storage in Amazonian podzols

Abstract

Equatorial podzols are soils characterized by thick sandy horizons overlying more clayey horizons. Organic matter produced in the topsoil is transferred in depth through the sandy horizons and accumulate at the transition, at a depth varying from 1 to more than 3 m, forming deep horizons rich in organic matter (Bh horizons). Although they cover great surfaces in the equatorial zone, these soils are still poorly known. Studying podzols from Amazonia, we found out that the deep Bh horizons in poorly drained podzol areas have a thickness higher than 1 m and store unexpected amounts of carbon. The average for the studied area was 66.7 ± 5.8 kgC m<sup>−2</sup> for the deep Bh and 86.8 ± 7.1 kgC m<sup>−2</sup> for the whole profile. Extrapolating to the podzol areas of the whole Amazonian basin has been possible thanks to digital maps, giving an order of magnitude around 13.6 ± 1.1 PgC, at least 12.3 PgC higher than previous estimates. This assessment should be refined by additional investigations, not only in Amazonia but in all equatorial areas where podzols have been identified. Because of the lack of knowledge on the quality and behaviour of the podzol organic matter, the question of the feedback between the climate and the equatorial podzol carbon cycle is open

Modelling the genesis of equatorial podzols: age and implications for carbon fluxes

“…Sierra et al (2013) used a compartment model that was constrained by 14 C dating to estimate the carbon fluxes in a Colombian shallow podzol (Bh upper limit at 0.9 m). They showed that the C fluxes from topsoil horizons to the Bh horizon were smaller (2.1 gC m −2 year −1 ) than the fluxes estimated in Montes et al (2011). However, they did not account for the age and genesis time of the Bh horizon.…”

“…In wet tropical areas podzols can be very deep, with E horizons thicker than 10 m and Bh horizons thicker than 4 m (Chauvel et al, 1987;Dubroeucq and Volkoff, 1998;Montes et al, 2011). This means that they can store huge quantities of organic matter: Montes et al (2011) estimated the C stocks in Amazonian podzols to be around 13.6 Pg C.…”

“…As pointed out by Sierra et al (2013), in order to corroborate this conclusion, it is necessary to produce a model that accounts for C additions and losses over time. Montes et al (2011) roughly estimated the C flux to the Bh horizon to be 16.8 gC m −2 year −1 . Sierra et al (2013) used a compartment model that was constrained by 14 C dating to estimate the carbon fluxes in a Colombian shallow podzol (Bh upper limit at 0.9 m).…”

“…Such large amounts of carbon may play a central role in the global carbon balance (Raymond, 2005), which raises the question of the magnitude of the carbon fluxes during podzol genesis and in response to drier periods that might occur in the future due to climate change. A schematic of the main carbon fluxes in Amazonian podzols (Leenheer, 1980;Lucas et al, 2012;Montes et al, 2011) is presented in Fig. 1.…”

“…The high Rio Negro basin was chosen because it is a region that has the highest occurrence of podzol in the Amazon (Montes et al, 2011) (Fig. 2).…”