Humic Substances in Soils:  Are They Really Chemically Distinct?

Kelleher, Brian P.; Simpson, André J.

doi:10.1021/es0608085

Cited by 328 publications

(242 citation statements)

References 31 publications

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“…Such supramolecular associations may form through relatively weak intermolecular interactions, such as hydrophobic interactions or H bonds (Sutton and Sposito, 2005). This concept is supported by recent investigations of Kelleher and Simpson (2006) who were able to assign nearly all of the NMR signals in traditional fractions of humic substances to intact or degrading biopolymers and therefore conclude that humic substances are not chemically distinct, but a complex mixture of microbial and plant biopolymers.…”

Section: Ekschmittmentioning

confidence: 76%

“…Physical protection of SOM through occlusion within aggregates or small pores and chemical protection through interaction with mineral surfaces or with other organic molecules are considered as important mechanisms to reduce the bioavailability and accessibility of organic matter (OM) for soil microorganisms and soil enzymes (Sollins et al, 1996;Kelleher and Simpson, 2006;von Lützow et al, 2006;Bachmann et al, 2008, this issue, pp. 14-26).…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

How relevant is recalcitrance for the stabilization of organic matter in soils?

Marschner

Brodowski

Dreves

et al. 2008

Z. Pflanzenernähr. Bodenk.

626

359

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Traditionally, the selective preservation of certain recalcitrant organic compounds and the formation of recalcitrant humic substances have been regarded as an important mechanism for soil organic matter (SOM) stabilization. Based on a critical overview of available methods and on results from a cooperative research program, this paper evaluates how relevant recalcitrance is for the long-term stabilization of SOM or its fractions. Methodologically, recalcitrance is difficult to assess, since the persistence of certain SOM fractions or specific compounds may also be caused by other stabilization mechanisms, such as physical protection or chemical interactions with mineral surfaces. If only free particulate SOM obtained from density fractionation is considered, it rarely reaches ages exceeding 50 y. Older light particles have often been identified as charred plant residues or as fossil C. The degradability of the readily bioavailable dissolved or water-extractable OM fraction is often negatively correlated with its content in aromatic compounds, which therefore has been associated with recalcitrance. But in subsoils, dissolved organic matter aromaticity and biodegradability both are very low, indicating that other factors or compounds limit its degradation. Among the investigated specific compounds, lignin, lipids, and their derivatives have mean turnover times faster or similar as that of bulk SOM. Only a small fraction of the lignin inputs seems to persist in soils and is mainly found in the fine textural size fraction (<20 lm), indicating physico-chemical stabilization. Compound-specific analysis of 13 C : 12 C ratios of SOM pyrolysis products in soils with C3-C4 crop changes revealed no compounds with mean residence times of > 40-50 y, unless fossil C was present in substantial amounts, as at a site exposed to lignite inputs in the past. Here, turnover of pyrolysis products seemed to be much longer, even for those attributed to carbohydrates or proteins. Apparently, fossil C from lignite coal is also utilized by soil organisms, which is further evidenced by low 14 C concentrations in microbial phospholipid fatty acids from this site. Also, black C from charred plant materials was susceptible to microbial degradation in a short-term (60 d) and a long-term (2 y) incubation

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Section: Ekschmittmentioning

confidence: 76%

Section: Introductionmentioning

confidence: 99%

How relevant is recalcitrance for the stabilization of organic matter in soils?

Marschner

Brodowski

Dreves

et al. 2008

Z. Pflanzenernähr. Bodenk.

626

359

View full text Add to dashboard Cite

show abstract

“…It has traditionally been thought that HS consist of novel categories of cross-linked macromolecular structures that form a distinct class of chemical compounds (Stevenson, 1994). In contrast to traditional thinking however, it was recently concluded that the vast majority of humic material in soils is a very complex mixture of microbial and plant biopolymers and their degradation products, and not a distinct chemical category as is traditionally thought (Kelleher and Simpson, 2006). Furthermore, the concept that extractable SOM is comprised mainly of humic materials has also been challenged and it has been shown that the presence of organic material sourced to microbes (as extant organisms or necromass) far exceeds presently accepted values, with large contributions of microbial peptides/ proteins found in the HS fraction (Kögel-Knabner, 2002;Kiem and Kögel-Knabner, 2003;Kindler et al, 2006;Simpson et al, 2007;Potthoff et al, 2008).…”

Section: Introductionmentioning

confidence: 88%

CO2 uptake by a soil microcosm

Hart

Oppenheimer

Moran

et al. 2013

Soil Biology and Biochemistry

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CO 2 enrichment a b s t r a c t Sequestration of CO 2 via biological sinks is a matter of great scientific importance due to the potential lowering of atmospheric CO 2 . In this study, a custom built incubation chamber was used to cultivate a soil microbial community to instigate chemoautotrophy of a temperate soil. Real-time atmospheric CO 2 concentrations were monitored and estimations of total CO 2 uptake were made. After careful background flux corrections, 4.52 AE 0.05 g CO 2 kg À1 dry soil was sequestered from the chamber atmosphere over 40 h. Using isotopically labelled 13 CO 2 and GCMSeIRMS, labelled fatty acids were identified after only a short incubation, hence confirming CO 2 sequestration for soil. The results of this in vivo study provide the ground work for future studies intending to mimic the in situ environment by providing a reliable method for investigating CO 2 uptake by soil microorganisms.

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“…This view of carbon reactivity implies that recharge of anthropogenic waste fuels microbial reduction in As-rich aquifers, even if the amount of young carbon is small compared with other pools and difficult to detect. However, recent direct observations indicate the OC in the subsurface exists in smaller, simpler molecular structures than originally thought based on bulk extractions, and that often these reactive molecules persist in sedimentary systems much longer than previously believed (10,11). This different perspective suggests that the molecular properties do not in themselves control reaction rates but OC decomposition rates are affected by ecosystem properties such as nutrient limitation, energy scarcity, or carbon bioavailability due to physical limitation such as reactions with mineral surfaces (12)(13)(14).…”

mentioning

confidence: 88%

Advection of surface-derived organic carbon fuels microbial reduction in Bangladesh groundwater

Mailloux

Trembath‐Reichert

Cheung

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

105

103

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Chronic exposure to arsenic (As) by drinking shallow groundwater causes widespread disease in Bangladesh and neighboring countries. The release of As naturally present in sediment to groundwater has been linked to the reductive dissolution of iron oxides coupled to the microbial respiration of organic carbon (OC). The source of OC driving this microbial reduction-carbon deposited with the sediments or exogenous carbon transported by groundwater-is still debated despite its importance in regulating aquifer redox status and groundwater As levels. Here, we used the radiocarbon ( 14 C) signature of microbial DNA isolated from groundwater samples to determine the relative importance of surface and sediment-derived OC. Three DNA samples collected from the shallow, high-As aquifer and one sample from the underlying, low-As aquifer were consistently younger than the total sediment carbon, by as much as several thousand years. This difference and the dominance of heterotrophic microorganisms implies that younger, surface-derived OC is advected within the aquifer, albeit more slowly than groundwater, and represents a critical pool of OC for aquifer microbial communities. The vertical profile shows that downward transport of dissolved OC is occurring on anthropogenic timescales, but bomb 14 C-labeled dissolved OC has not yet accumulated in DNA and is not fueling reduction. These results indicate that advected OC controls aquifer redox status and confirm that As release is a natural process that predates human perturbations to groundwater flow. Anthropogenic perturbations, however, could affect groundwater redox conditions and As levels in the future.A quifer redox status is a major factor affecting groundwater composition and its suitability for human consumption. The most egregious example is the dire health impact of elevated levels of arsenic (As) in groundwater drawn with inexpensive handpumped tube wells by more than 100 million villagers across South, Southeast, and East Asia (1, 2). Aquifer redox status is largely controlled by microbial respiration of organic carbon (OC) coupled to the utilization of terminal electron acceptors. Reduction of iron (Fe) oxides containing As coupled with the oxidation of OC is the dominant process causing the accumulation of As in groundwater (1), but there is no consensus on the source of OC that fuels these transformations. Such information is critical to understanding subsurface carbon cycling, redox reactions, and the vulnerability of groundwater aquifers to perturbation.Human perturbations have long been suggested to exacerbate the problem of elevated As in groundwater in several ways. Given the connectivity between contaminated surface waters and aquifers, widespread pumping could redistribute As from the surface to depth (3, 4) or between aquifers (5). This pumping could potentially also deliver reactive OC from ponds and latrines to depth, causing microbial reduction and As release (3, 4). Fluorescent spectra of dissolved organic carbon (DOC), correlations between sedimentary ...

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Humic Substances in Soils: Are They Really Chemically Distinct?

Cited by 328 publications

References 31 publications

How relevant is recalcitrance for the stabilization of organic matter in soils?

How relevant is recalcitrance for the stabilization of organic matter in soils?

CO2 uptake by a soil microcosm

Advection of surface-derived organic carbon fuels microbial reduction in Bangladesh groundwater

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