Soil organic carbon mobility in equatorial podzols: soil column experiments

Abstract

Abstract. Transfer of organic carbon from topsoil horizons to deeper horizons and to the water table is still little documented, in particular in equatorial environments, despite the high primary productivity of the evergreen forest. Due to its complexing capacity, organic carbon also plays a key role in the transfer of metals in the soil profile and, therefore, in pedogenesis and for metal mobility. Here we focus on equatorial podzols, which are known to play an important role in carbon cycling. We carried out soil column experiments using soil material and percolating solution sampled in an Amazonian podzol area in order to better constrain the conditions of the transfer of organic carbon at depth. The dissolved organic matter (DOM) produced in the topsoil was not able to percolate through the clayey, kaolinitic material from the deep horizons and was retained in it. When it previously percolated through the Bh material, there was production of fulvic-like, protein-like compounds and small carboxylic acids able to percolate through the clayey material and increase the mobility of Al, Fe and Si. Podzolic processes in the Bh can, therefore, produce a DOM likely to be transferred to the deep water table, playing a role in the carbon balances at the profile scale and, owing to its complexing capacity, playing a role in deep horizon pedogenesis and weathering. The order of magnitude of carbon concentration in the solution percolating at depth was around 1.5–2.5 mg L−1. Our findings reveal a fundamental mechanism that favors the formation of very thick kaolinitic saprolites.

Selective sorption and desorption of DOM in podzol horizons — FTIR and Py-GC/MS of leachates from a column experiment

“…Comparing both DOM types, the Peat is more aliphatic as indicated by the positive loadings of aliphatic products (C:0, C:1, Al1-3) and the alkyl side-chains on the PAHs and benzenes. The grouping of the B leachates negative on PyDim1 with the highest negative scores for the Water samples agrees with the fact that the source of SOM from hydromorphic Bh-horizons is mainly from processed DOM instead of in-situ deposited root material (Bardy et al, 2008;Lopes-Mazzetto, 2018a), and with a more proteinaceous DOM desorbed from B columns (Merdy et al, 2021). A larger homogeneity of the Stream samples is indicated by the more compact clustering compared to other DOM types.…”

Evidence of humic acid-aluminium‑silicon complexes under controlled conditions

“…On the other hand, the presence of HA-Al-Si complexes suggested by our study is in agreement with the assumptions put forward to interpret the experiments of Merdy et al (2021). These authors showed, in laboratory column percolations of podzolic horizons, that certain types of organic matter can promote the transfer of Al and DOM through kaolinitic materials and presume the existence of DOM-Al-Si complexes.…”

Complementary effects of sorption and biochemical processing of dissolved organic matter for emerging structure formation controlled by soil texture

“…Although the depth of the direct influence by roots is limited to the extension of the root system, percolating DOM from upper soil horizons can transfer OC into much deeper soil regions. This depth migration is dependent of the water flow in the vadose zone and OC retention at mineral surfaces (Krettek & Rennert, 2021; McCarthy, 2005; Merdy et al., 2021), implying that soil texture can have a major influence on OC mobility.…”