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Selective stabilization of aliphatic organic carbon by iron oxide.

Adhikari D, Yang Y - Sci Rep (2015)

Bottom Line: Impacts of organic matter composition and conformation on its sorption by hematite and release during the reduction reaction were analyzed.We found that hematite-bound aliphatic carbon was more resistant to reduction release, although hematite preferred to sorb more aromatic carbon.Resistance to reductive release represents a new mechanism that aliphatic soil organic matter was stabilized by association with iron oxide.

View Article: PubMed Central - PubMed

Affiliation: Department of Civil and Environmental Engineering, University of Nevada, Reno, 89557, USA.

ABSTRACT
Stabilization of organic matter in soil is important for natural ecosystem to sequestrate carbon and mitigate greenhouse gas emission. It is largely unknown what factors govern the preservation of organic carbon in soil, casting shadow on predicting the response of soil to climate change. Iron oxide was suggested as an important mineral preserving soil organic carbon. However, ferric minerals are subject to reduction, potentially releasing iron and decreasing the stability of iron-bound organic carbon. Information about the stability of iron-bound organic carbon in the redox reaction is limited. Herein, we investigated the sorptive interactions of organic matter with hematite and reductive release of hematite-bound organic matter. Impacts of organic matter composition and conformation on its sorption by hematite and release during the reduction reaction were analyzed. We found that hematite-bound aliphatic carbon was more resistant to reduction release, although hematite preferred to sorb more aromatic carbon. Resistance to reductive release represents a new mechanism that aliphatic soil organic matter was stabilized by association with iron oxide. Selective stabilization of aliphatic over aromatic carbon can greatly contribute to the widely observed accumulation of aliphatic carbon in soil, which cannot be explained by sorptive interactions between minerals and organic matter.

No MeSH data available.


δ13C for hematite-bound organic carbon and the residual fractions after reduction-release experiments.
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f3: δ13C for hematite-bound organic carbon and the residual fractions after reduction-release experiments.

Mentions: There is large difference in the stabilities of different functional groups of iron-bound SOM during the reduction reaction. Based on the δ13C analysis, the reduction-resistant fractions of iron-bound HAs were relatively enriched in 13C compared to the original HAs (Fig. 3). At pH 5, δ13C for residual iron-bound HA1–HA3 was −14.56,−19.31, and−23.78‰, while the original values were −26.26, −25.30, and −25.03‰ for HA1, HA2, and HA3, respectively. Previous studies showed a strong negative correlation between the value of δ13C and old organic matter with higher lignin content, which is a major contributor for aromatic moieties in soil/sediment organic matter3132. In this study, we developed an index for the amount of aromatic carbon in original hematite-bound organic matter:where ArS is the index for the amount of aromatic carbon in hematite-HA complexes, OC is the organic carbon content bound by iron oxide obtained by the sorption isotherm analysis, and Ar0 is the original aromatic carbon fraction in HAs derived from our previous NMR analysis19 (Supplementary Table S1). As we discussed, SUVA254R represents the relative decrease in aromatic fraction of aqueous HAs after sorption, and therefore (1 −SUVA254R) is used to represent the degree, by which the aromatic carbon was enriched in iron-bound HAs. We found a negative correlation between ArS and δ13C for iron-bound organic matter before reduction (Pearson Correlation Coefficient = −0.41, p < 0.05, Supplementary Fig. S4), which further demonstrated the applicability of δ13C as an index for aromatic fractions in residual organic matter. Therefore, the increase in δ13C of HAs after the reduction reaction indicates that residual organic matter had less aromatic carbon compared to the original iron-bound HAs. These results suggest that the aromatic carbon-rich organic matter was relatively easily released in the reduction reaction, although hematite selectively sorbed a larger amount of aromatic organic matter compared to other components of HAs. This finding implies contrast stability and sorption of aliphatic and aromatic organic matter on hematite. Resistance to reduction release represents an un-presented mechanism that aliphatic carbon was reserved and stabilized in soil environment. Aromatic fractions were selectively released by the reduction reaction, although they were sorbed more preferentially on hematite. The susceptibility of aromatic organic matter to reduction may be a result of the steric effect of aromatic rings, which lead them to mainly locate on the outer surface of the organic matter-mineral complex, while the more flexible aliphatic components can reside in the inner-layer immediately bound to iron oxide and be preserved by outer-layer organic matter. Such multi-layer scheme is supported by our SEM/TEM-EDS analysis.


Selective stabilization of aliphatic organic carbon by iron oxide.

Adhikari D, Yang Y - Sci Rep (2015)

δ13C for hematite-bound organic carbon and the residual fractions after reduction-release experiments.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4462107&req=5

f3: δ13C for hematite-bound organic carbon and the residual fractions after reduction-release experiments.
Mentions: There is large difference in the stabilities of different functional groups of iron-bound SOM during the reduction reaction. Based on the δ13C analysis, the reduction-resistant fractions of iron-bound HAs were relatively enriched in 13C compared to the original HAs (Fig. 3). At pH 5, δ13C for residual iron-bound HA1–HA3 was −14.56,−19.31, and−23.78‰, while the original values were −26.26, −25.30, and −25.03‰ for HA1, HA2, and HA3, respectively. Previous studies showed a strong negative correlation between the value of δ13C and old organic matter with higher lignin content, which is a major contributor for aromatic moieties in soil/sediment organic matter3132. In this study, we developed an index for the amount of aromatic carbon in original hematite-bound organic matter:where ArS is the index for the amount of aromatic carbon in hematite-HA complexes, OC is the organic carbon content bound by iron oxide obtained by the sorption isotherm analysis, and Ar0 is the original aromatic carbon fraction in HAs derived from our previous NMR analysis19 (Supplementary Table S1). As we discussed, SUVA254R represents the relative decrease in aromatic fraction of aqueous HAs after sorption, and therefore (1 −SUVA254R) is used to represent the degree, by which the aromatic carbon was enriched in iron-bound HAs. We found a negative correlation between ArS and δ13C for iron-bound organic matter before reduction (Pearson Correlation Coefficient = −0.41, p < 0.05, Supplementary Fig. S4), which further demonstrated the applicability of δ13C as an index for aromatic fractions in residual organic matter. Therefore, the increase in δ13C of HAs after the reduction reaction indicates that residual organic matter had less aromatic carbon compared to the original iron-bound HAs. These results suggest that the aromatic carbon-rich organic matter was relatively easily released in the reduction reaction, although hematite selectively sorbed a larger amount of aromatic organic matter compared to other components of HAs. This finding implies contrast stability and sorption of aliphatic and aromatic organic matter on hematite. Resistance to reduction release represents an un-presented mechanism that aliphatic carbon was reserved and stabilized in soil environment. Aromatic fractions were selectively released by the reduction reaction, although they were sorbed more preferentially on hematite. The susceptibility of aromatic organic matter to reduction may be a result of the steric effect of aromatic rings, which lead them to mainly locate on the outer surface of the organic matter-mineral complex, while the more flexible aliphatic components can reside in the inner-layer immediately bound to iron oxide and be preserved by outer-layer organic matter. Such multi-layer scheme is supported by our SEM/TEM-EDS analysis.

Bottom Line: Impacts of organic matter composition and conformation on its sorption by hematite and release during the reduction reaction were analyzed.We found that hematite-bound aliphatic carbon was more resistant to reduction release, although hematite preferred to sorb more aromatic carbon.Resistance to reductive release represents a new mechanism that aliphatic soil organic matter was stabilized by association with iron oxide.

View Article: PubMed Central - PubMed

Affiliation: Department of Civil and Environmental Engineering, University of Nevada, Reno, 89557, USA.

ABSTRACT
Stabilization of organic matter in soil is important for natural ecosystem to sequestrate carbon and mitigate greenhouse gas emission. It is largely unknown what factors govern the preservation of organic carbon in soil, casting shadow on predicting the response of soil to climate change. Iron oxide was suggested as an important mineral preserving soil organic carbon. However, ferric minerals are subject to reduction, potentially releasing iron and decreasing the stability of iron-bound organic carbon. Information about the stability of iron-bound organic carbon in the redox reaction is limited. Herein, we investigated the sorptive interactions of organic matter with hematite and reductive release of hematite-bound organic matter. Impacts of organic matter composition and conformation on its sorption by hematite and release during the reduction reaction were analyzed. We found that hematite-bound aliphatic carbon was more resistant to reduction release, although hematite preferred to sorb more aromatic carbon. Resistance to reductive release represents a new mechanism that aliphatic soil organic matter was stabilized by association with iron oxide. Selective stabilization of aliphatic over aromatic carbon can greatly contribute to the widely observed accumulation of aliphatic carbon in soil, which cannot be explained by sorptive interactions between minerals and organic matter.

No MeSH data available.