Thermokarst processes increase the supply of stabilizing surfaces and elements (Fe, Mn, Al, and Ca) for mineral–organic carbon interactions

The stabilizing properties of mineral–organic carbon (OC) interactions have been studied in many soil environments (temperate soils, podzol lateritic soils, and paddy soils). Recently, interest in their role in permafrost regions is increasing as permafrost was identified as a hotspot of change. In thawing ice-rich permafrost regions, such as the Yedoma domain, 327–466 Gt of frozen OC is buried in deep sediments. Interactions between minerals and OC are important because OC is located very near the mineral matrix. Mineral surfaces and elements could mitigate recent and future greenhouse gas emissions through physical and/or physicochemical protection of OC. The dynamic changes in redox and pH conditions associated with thermokarst lake formation and drainage trigger metal-oxide dissolution and precipitation, likely influencing OC stabilization and microbial mineralization. However, the influence of thermokarst processes on mineral–OC interactions remains poorly constrained. In this study, we aim to characterize Fe, Mn, Al, and Ca minerals and their potential protective role for OC. Total and selective extractions were used to assess the crystalline and amorphous oxides or complexed metal pools as well as the organic acids found within these pools. We analyzed four sediment cores from an ice-rich permafrost area in Central Yakutia, which were drilled (i) in undisturbed Yedoma uplands, (ii) beneath a recent lake formed within Yedoma deposits, (iii) in a drained thermokarst lake basin, and (iv) beneath a mature thermokarst lake from the early Holocene period. We find a decrease in the amount of reactive Fe, Mn, Al, and Ca in the deposits on lake formation (promoting reduction reactions), and this was largely balanced by an increase in the amount of reactive metals in the deposits on lake drainage (promoting oxidation reactions). We demonstrate an increase in the metal to C molar ratio on thermokarst process, which may indicate an increase in metal–C bindings and could provide a higher protective role against microbial mineralization of organic matter. Finally, we find that an increase in mineral–OC interactions corresponded to a decrease in CO2 and CH4 gas emissions on thermokarst process

Experimental study of DOC, nutrients and metals release from forest floors developed under beech (Fagus sylvatica L.) on a Cambisol and a Podzol

Forest floor in acid forest soils is a major source of dissolved organic carbon (DOC), nutrients and metals. We studied the release of DOC, nutrients and metals from two forest floors under beech stands (a moder from a Cambisol and a mor from a Podzol). We analyzed the composition of the solid phase, and we monitored the chemical properties of forest floor leachates through incubation-leaching experiments at 4 and 20 degrees C. Our data show that the release of DOC, nutrients and metals strongly differed between both forest floors. Whatever the temperature, the release of DOC, DIN, divalent cations and heavy metals, as compared to initial stocks of corresponding elements, were the largest in moder leachates. Instead, the release of K+, Na+, Si, Al and Fe were the largest in mor leachates at 20 degrees C, and were generally similar for both forest floors at 4 degrees C. The ionic signature of the leachates was strongly affected by DOC release and nitrogen mineralization, which was mostly directed towards the production of ammonium in mor, and of nitrate in moder. In moder leachates at 20 degrees C, Ca2+ was the dominant cation, and nitrate the major anion. Instead, in mor leachates, NH4+ was the major cation, and organic anions dominated the anionic charge. Despite the dominance of organic anions in mor leachates, the concentrations of cations and Si were strongly correlated to the concentrations of inorganic anions in leachates from both forest floors. The concentration of Al, Fe, Cu and Pb were strongly correlated to the concentration of organic anions, whereas Zn concentration was poorly correlated with either organic or inorganic anions. (C) 2008 Elsevier B.V. All rights reserved

Properties of successive horizons in a thick forest floor (mor) reflect a sequence of soil acidification

Successive horizons of thick forest floors may be considered as a litterfall decomposition sequence, but the evolution of physico-chemical properties along such a sequence is poorly known. Here, we have characterized the physico-chemical properties within a thick forest floor and evaluated their potential impacts on the release of Dissolved Organic Carbon (DOC). Seven sub-horizons of a mor forest floor were sampled in a 200 year-old beech stand on Podzol. We performed total analysis, measured the concentrations of exchangeable cations and determined the concentration of ions and DOC in water extracts. From 01 to Oh, sharp gradients were observed: (1) total concentrations of Ca. Mg and Mn decreased, while total concentrations of Si, Al, K and Na increased, (2) exchangeable Ca, Mg, K and Na concentrations decreased while exchangeable H and Al concentrations increased and (3) leachate pH value dropped from 6.3 to 4.2. These gradients can be compared to the evolution of physico-chemical properties of a soil undergoing acidification. However, mineral incorporation into forest floor likely prevented a decline in total K and Na concentrations. Such a vertical heterogeneity of physico-chemical properties within the forest floor can affect DOC release: low pH value and high Mn concentrations in Of could promote DOC production by lignin degradation, while high Al concentrations in Oh could reduce DOC solubility. (C) 2010 Elsevier B.V. All rights reserved

Influence of in-situ storage conditions on the composition of soil solutions from Norway spruce and Douglas fir forest floors

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Metal complexing properties of forest floor leachates might promote incipient podzolization in a Cambisol under deciduous forest

The characteristics of both the mineral substrate and the humus layer are of key importance in podzol formation. However, the properties of the forest floor leachates are seldom considered. We measured the complexation capacity by Cu2+ titration (CuCC), the optical density (E-4/E-6), as well as the concentrations of dissolved organic carbon (DOC) and of major and trace ions in the liquid extracts of forest floors of two Cambisols and two Luvisols on loess under deciduous forest. The vegetation consists of a mixture of beech, oak and maple. The humus type is a fibrimor in one Cambisol and a moder in the three other soils. Both in the solid phase and in the liquid extracts of the forest floors, the Si/(Al + Fe) and C/(Al + Fe) atomic ratios are much larger in the fibrimor than in the moders. In the liquid extracts, the complexation capacity (CuCC) and the density of metal binding sites (CuCC/DOC are larger in the fibrimor than in the moders. These large CuCC and CuCC/DOC in the fibrimor leachate are linked with relatively small stability constant of the humus-Cu complex and relatively large E-4/E-6 ratio, i.e. with reactive organic ligands with low molecular weight and weak aromatic character. These characteristics are associated with current incipient podzolization in the Cambisol with a fibrimor, and might be considered as an indicator of a substantial podzolizing potential of the forest floor. (C) 2002 Elsevier Science B.V. All rights reserved

Incipient podzolization and weathering caused by complexation in a forest Cambisol on loess as revealed by a soil solution study

Surface podzolization has so far been diagnosed from morphological observations, selective extraction and mineralogical investigations. We studied this process in two Cambisol profiles developed in loess, one with a fibrimor humus and the other with a dysmoder humus, by characterizing the chemical composition and the complexing properties of the soil solution. The solutions were sampled bimonthly for 3 years at four depths (4, 8, 13, 25 cm) using both zero-tension and low-tension capillary-wick lysimeters. The leachates from the Ah horizon of the soil with fibrimor contained less nitrate, sulphate and calcium than those from the one with a dysmoder because there was less bioturbation and mineralization in it. Both the complexation capacity and the density of ligand binding sites were larger in the soil solutions of the Ah (4 cm) and AB (8 cm) horizons of the soil with the fibrimor. In this soil, the complexing properties of the liquid phase induced a depletion of inorganic monomeric aluminium. In this environment, the hydroxy interlayered 2:1 clay minerals lose their Al-interlayers and transform into vermiculite and smectite, which in turn weather, producing large amounts of magnesium in the soil solution. This was found to be a major characteristic of weathering by complexation and incipient podzolization in the Cambisol with the fibrimor. In this process, nitric acid probably contributes to mineral dissolution