Podzolisation affects the spatial allocation and chemical composition of soil organic matter fractions

Abstract

Podzols are soils that display a unique vertical distribution of soil organic matter (SOM). We hypothesise that podzolisation, as a pedogenetic process, influences or even controls content, allocation and quality of SOM. We determined soil organic carbon (SOC) and nitrogen (N) contents in six SOM fractions obtained from mineral horizons of five soils with increasing degree of podzolisation: sand and stable aggregates (S + A), particulate organic matter (POM) > 63 mm and <63 mm, silt and clay (s + c), resistant SOC and dissolved organic matter. We applied infrared spectroscopy to evaluate SOM decomposition state, relative abundance of functional groups and SOM-metal complexation. In topsoil horizons, relative SOC allocation shifted from the larger to the smaller size POM fraction with increasing podzolisation. Accompanied with size reduction, the POM < 63 mm fraction was progressively less decomposed, as derived from infrared spectroscopy and C : N ratios. In illuvial subsoils, the proportion of SOC in the S + A fraction increased with increasing podzolisation, implying SOM accumulation in aggregates and coatings on sand grains. Elevated abundance of carboxylate and aromatic C in the s + c fractions of subsoil horizons indicated their preferred sorption. Additionally, metal-carboxyl complexation increased during podzolisation.

Mobilisation of Al, Fe, and DOM from topsoil during simulated early Podzol development and subsequent DOM adsorption on model minerals

“…Additionally, soil material of all cylinders was depleted in Fe and Al extracted by dithionite (Fe d , Al d ), oxalate (Fe o , Al o ), and citrate (Fe c , Al c ; Table 1 ), which differed significantly between the fast and the slow run (except for Fe d ). Contents of C, Fe d , Fe o, Fe c , Al d , Al o , and Al c were in line with previous findings on more progressively podzolised topsoils from the same study area 28 indicating that the experiments were carried out under rather realistic conditions. All cylinders were relatively more strongly depleted in Al d (43–60%) than in Fe d (10–15%), particularly in the slow run, indicating rate-limited dissolution of Fe oxides, releasing Fe ions together with structurally incorporated and adsorbed Al ions.…”

“…Then, relatively more Al than Fe is mobilised and leached from the topsoil during the course of podzolisation, while the Fe released may be transferred to a different fraction and transported more slowly from the eluvial horizon. Consistent with our experimental results, the molar Fe d :Al d ratios of topsoil horizons of four Arenosols in the study area with increasing degree of podzolisation amounted to 1.85, 3.47, 3.69, and 3.22 28 , indicating almost the same pattern of selective metal depletion in the field and under laboratory conditions. The molar Al d :(Al d + Fe d ) ratios calculated for the initial material, that after the fast run and after the slow run, were 0.39, 0.28, and 0.24, may point to the initial presence of rather poorly crystalline Fe oxides such as ‘soil goethites’ or ferrihydrite with a large degree of Al substitution 44 , which were successively depleted in Al.…”

“…Instead, the topsoil lost more Al and Fe in absolute numbers from oxalate- and citrate-soluble sources. We suggest a joint organic source of Al and Fe solubilised by the two extractants, as previously reported particularly for Podzols 28 , 45 , 46 . Dissolution of Fe oxides by citrate as used for extraction is negligible 46 .…”

“…Larger differences in the fast run may point to the stronger tendency for preferential flow, as physical non-equilibrium conditions are enforced at higher flow velocity 26 , or to greater release of particles from soil by hydrodynamic shear or abrasion 27 or both, resulting in less reproducible concentration patterns in parallel replicates. However, we consider preferential flow less important in our study, as the extremely sandy horizon (98% sand) 28 did neither show aggregation, nor textural change, nor burrows by earthworms, which were not present in the acidic soil. Furthermore, preferential flow in soil demands a minimum clay content of 8% 29 and a ratio of the clay and organic C contents > 0.35 30 .…”

“…Furthermore, preferential flow in soil demands a minimum clay content of 8% 29 and a ratio of the clay and organic C contents > 0.35 30 . For the horizon under study, the latter ratio is 0.017, and the clay content is 0.2% 28 . Both numbers are at least one order of magnitude smaller than the threshold values.…”