The oxidation potential of dithionite (Na2S2O4) increases from 0.37 V to 0.73 V with increase in pH from 6 to 9, because hydroxyl is consumed during oxidation of dithionite. At the same time the amount of iron oxide dissolved in 15 minutes falls off (from 100 percent to less than 1 percent extracted) with increase in pH from 6 to 12 owing to solubility product relationships of iron oxides. An optimum pH for maximum reaction kinetics occurs at approximately pH 7.3. A buffer is needed to hold the pH at the optimum level because 4 moles of OH are used up in reaction with each mole of Na2S2O4 oxidized. Tests show that NaHCO3 effectively serves as a buffer in this application. Crystalline hematite dissolved in amounts of several hundred milligrams in 2 min. Crystalline goethite dissolved more slowly, but dissolved during the two or three 15 min treatments normally given for iron oxide removal from soils and clays. A series of methods for the extraction of iron oxides from soils and clays was tested with soils high in free iron oxides and with nontronite and other iron-bearing clays. It was found that the bicarbonate-buffered Na2S2O4-citrate system was the most effective in removal of free iron oxides from latosolic soils, and the least destructive of iron silicate clays as indicated by least loss in cation exchange capacity after the iron oxide removal treatment. With soils the decrease was very little but with the very susceptible Woody district nontronite, the decrease was about 17 percent as contrasted to 35–80 percent with other methods.

Iron Oxide Removal from Soils and Clays by a Dithionite-Citrate System Buffered with Sodium Bicarbonate

The oxidation potential of dithionite (Na 2 S 2 O 4 ) increases from 0.37 V to 0.73 V with increase in pH from 6 to 9, because hydroxyl is consumed during oxidation of dithionite. At the same time the amount of iron oxide dissolved in 15 minutes falls off (from 100 percent to less than 1 percent extracted) with increase in pH from 6 to 12 owing to solubility product relationships of iron oxides. An optimum pH for maximum reaction kinetics occurs at approximately pH 7.3. A buffer is needed to hold the pH at the optimum level because 4 moles of OH are used up in reaction with each mole of Na 2 S 2 O 4 oxidized. Tests show that NaHCO 3 effectively serves as a buffer in this application. Crystalline hematite dissolved in amounts of several hundred milligrams in 2 min. Crystalline geothite dissolved more slowly, but dissolved during the two or three 15 min treatments normally given for iron oxide removal from soils and clays. A series of methods for the extraction of iron oxides from soils and clays was tested with soils high in free iron oxides and with nontronite and other iron-bearing clays. It was found that the bicarbonate-buffered Na 2 S 2 O 4 -citrate system was the most effective in removal of free iron oxides from latosolic soils, and the least destructive of iron silicate clays as indicated by least loss in cation exchange capacity after the iron oxide removal treatment. With soils the decrease was very little but with the very susceptible Woody district nontronite, the decrease was aboout 17 percent as contrasted to 35-80 percent with other methods.

Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate

The world's soils store more carbon than is present in biomass and in the atmosphere. New experimental evidence suggests that the delivery of fresh plant-derived carbon to the subsoil stimulates microbial activity and results in mineralization of thousand-year-old carbon. This supports the recent proposal that the conservation of organic carbon at depth results from a lack of energy for decomposers. This large pool of deep carbon is unlikely to respond to future changes in temperature if no fresh carbon is supplied, limiting the predicted positive feedback between global warming and soil organic carbon decomposition. The results imply that management practices that increase the distribution of fresh carbon along the soil profile (such as deep ploughing and the use of drought-resistant crops with extensive root systems) will stimulate loss of this ancient buried carbon. It is shown that the supply of fresh plant-derived carbon to deep soil layers stimulated the microbial mineralization of carbon that is thousands of years old, and is suggested that a lack of supply of fresh-carbon may prevent the decomposition of the organic carbon pool in deep soil layers in response to future changes in temperature. The world’s soils store more carbon than is present in biomass and in the atmosphere1. Little is known, however, about the factors controlling the stability of soil organic carbon stocks2,3,4 and the response of the soil carbon pool to climate change remains uncertain5,6. We investigated the stability of carbon in deep soil layers in one soil profile by combining physical and chemical characterization of organic carbon, soil incubations and radiocarbon dating. Here we show that the supply of fresh plant-derived carbon to the subsoil (0.6–0.8 m depth) stimulated the microbial mineralization of 2,567 ± 226-year-old carbon. Our results support the previously suggested idea7 that in the absence of fresh organic carbon, an essential source of energy for soil microbes, the stability of organic carbon in deep soil layers is maintained. We propose that a lack of supply of fresh carbon may prevent the decomposition of the organic carbon pool in deep soil layers in response to future changes in temperature. Any change in land use and agricultural practice that increases the distribution of fresh carbon along the soil profile1,8,9 could however stimulate the loss of ancient buried carbon.

Stability of organic carbon in deep soil layers controlled by fresh carbon supply

http://directory.umm.ac.id/Data%20Elmu/jurnal/O/Organic%20Geochemistry/Vol31.Issue7-8.2000/9911.pdf

The role of DOM sorption to mineral surfaces in the preservation of organic matter in soils.

Oxides, Hydroxides, and Aluminosilicates

Iron oxide coatings or crystals must be removed from soils in which they are found in many mineralogical techniques for identification of colloidal layer silicates as well as the identification of silt and sand grains with the polarizing microscope. A procedure is presented which employs sodium dithionite (Na2S2O4, hyposulfite, or "hydrosulfite") as the reductor, and 0.3 molar citrate with or without Fe-3 specific Versene as the chelating reagent. It is a neutral system the pH of which is kept at 7.3. The reaction is fast, as much as 20% of iron oxides (hematite, geothite, or limonite but not magnetite or ilmenite) being removed from a soil in 15 minutes, and does not precipitate either elemental sulfur or iron sulfides. Like other procedures, it causes some decrease of exchange capacity of layer silicates which contain iron, and at the same time may increase the exchange capacity of kaolinic soils. View complete article To view this complete article, insert Disc 2 then click button8

Iron Oxide Removal from Soils and Clays1

Introduction Anthropic disturbances are driving unprecedented changes in forest ecosystem functions and biogeochemical processes, hindering the forests’ benefits to society. Litter decomposition is one of the most critical processes that regulate forests’ carbon and nutrient cycling. However, how forest degradation affects litter decomposition and elemental dynamics requires further examination. The main objective of this study was to evaluate the effect of forest degradation on the production and decomposition of litter and C,N, and P dynamics in a temperate forest in south-central Chile. Methods Litter traps and litter bags were installed in three Long Term Research Forest Plots (LTER) representing different conservation states: mature, secondary, and degraded Nothofagus forests. Results and Discussion The total litter input varied between 3.5 to 1.1 Mg ha–1 year–1 in the mature and degraded forests, respectively. We found the highest lignin and nutrient levels in the degraded forest and the lowest in the mature forest. In the mature forest, 44% of the initial litter was decomposed, while in the degraded forest it only reached 7%. Decomposing litter showed the lowest C:N and C:P ratios in the mature forest most of the year. The balance between inputs and outputs yielded a more substantial litter accumulation in the mature forests. Conclusion Our results strongly suggest that anthropogenic degradation altered litter quality and nutrient dynamics while decreasing litter production and decomposition.

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N. H. Aguilera

Papers

Iron Oxide Removal from Soils and Clays1

Forest degradation modifies litter production, quality, and decomposition dynamics in Southern temperate forests