Iron-mediated organic matter decomposition in humid soils can counteract protection

Chen, Chunmei; Hall, Steven J.; Coward, Elizabeth K.; Thompson, Aaron

doi:10.1038/s41467-020-16071-5

Cited by 278 publications

(265 citation statements)

References 80 publications

(220 reference statements)

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“…This may be linked to the different oxidation levels of Fe. At higher precipitation levels, Fe oxides can be reduced, resulting in a release of associated SOC to the aqueous phase (Berhe et al 2012;Chen et al 2020;Thompson et al 2011). This mechanism is probably responsible for the low correlation between SOC and high Feox concentrations in our data (Figure 5e), pointing to the fact that Feox can act as pedogenic threshold, depending on its oxidation level in the soil system.…”

Section: Oxalate Al and Fementioning

confidence: 76%

Continental-scale controls on soil organic carbon across sub-Saharan Africa

Fromm¹,

Hoyt²,

Aynekulu³

et al. 2020

Preprint

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Abstract. Earlier studies have demonstrated that soil texture and geochemistry strongly affect soil organic carbon (SOC) content. However, those findings primarily rely on data from temperate regions with soil mineralogy, weathering status and climatic conditions that generally differ from tropical and sub-tropical regions. We investigated soil properties and climate variables influencing SOC concentrations across sub-Saharan Africa. A total of 1,601 samples were analyzed, collected from two depths (0–20 cm and 20–50 cm) at 45 sentinel sites from 17 countries as part of the Africa Soil Information Service (AfSIS) project. The dataset spans climatic conditions from arid to humid and includes soils with a wide range of pHH20 values, weathering status, soil texture, exchangeable cations, extractable metals and a variety of important land cover types. The most important SOC predictors were identified by linear mixed effects models, regression trees and random forest models. Our results indicate that SOC is primarily controlled by aridity index (PET/MAP), exchangeable calcium (Caex) and oxalate-extractable aluminum (Alox); this was found across both depth intervals. Oxalate-extractable iron (Feox) emerged as the most important predictor for both depth intervals in the regression tree and random forest analyses. However, its influence on SOC concentrations was strong only below Feox concentrations of 0.25 wt %. This suggests that Feox can act as a pedogenic threshold – even on a continental scale. Across model-ling approaches, clay and fine silt content (

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Section: Oxalate Al and Fementioning

confidence: 76%

Continental-scale controls on soil organic carbon across sub-Saharan Africa

Fromm¹,

Hoyt²,

Aynekulu³

et al. 2020

Preprint

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“…They found that on average 15% of organic C in the studied cores is directly bound to reactive Fe, corroborating the findings in estuarine and marine sediments. These results are counter-intuitive because ligand exchange was shown to be suppressed by SO 4 2− (Table 1) [40,41], which is abundant in seawater (0.03 M), and because Fe can undergo reductive dissolution as Fe(II) in reducing environments, leading to the disruption of the OM-Fe interactions [25,26]. Compared to marine sediments, mangrove soils are a much more redox dynamic environment, thus the interactions of OM and Fe at redox interfaces may be more important than in marine sediments.…”

Section: Mineral Soils: Interactions With Soil Minerals or Metalsmentioning

confidence: 99%

“…Moreover, the decomposition rate of mangrove litter and roots varies considerably within and between mangrove forests [11,20,21]. Soils with more saturated conditions can have higher OM decomposition rates compared to drier conditions, for instance by enhanced leaching or through the release of physico-chemically protected OM due to metal reduction [22][23][24][25][26]. As such, the postulated low decomposition rate of mangrove soil C due to the suboxic condition may not be universal to all mangroves.…”

Section: Introductionmentioning

confidence: 99%

Organic Carbon Stabilization Mechanisms in Mangrove Soils: A Review

Kida

Fujitake

2020

Forests

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Despite the recognized organic carbon (OC) sequestration potential of mangrove forests, the ongoing climate change and anthropogenic disturbances pose a great threat to these ecosystems. However, we currently lack the ability to mechanically understand and predict the consequences of such impacts, primarily because mechanisms underlying OC stabilization in these ecosystems remain elusive. Research into OC stabilization has focused on terrestrial soils and marine sediments for decades, overlooking the vegetated coastal ecosystems including mangroves. In terrestrial soils and marine sediments, it is widely accepted that OC stabilization is the integrated consequence of OM’s inherent recalcitrance, physical protection, and interactions with minerals and metals. However, related discussion is rarely done in mangrove soils, and recalcitrance of roots and high net ecosystem production (high primary production and low heterotrophic respiration) have been considered as a primary OC sequestration mechanism in mangrove peat and mineral soils, respectively. This review presents the available information on the mechanisms underlying OC stabilization in mangrove soils and highlights research questions that warrant further investigation. Primary OC stabilization mechanisms differ between mangrove peat and mineral soils. In mangrove mineral soils, physico-chemical stabilization processes are important, yet grossly understudied OC stabilization mechanisms. In mangrove peat, recalcitrance of mangrove roots and the inhibition of phenoloxidase under the anoxic condition may be the primary OC stabilization mechanisms. Salinity-induced OC immobilization likely plays a role in both type of soils. Finally, this review argues that belowground production and allochthonous inputs in mangrove forests are likely underestimated. More studies are needed to constrain C budgets to explain the enigma that mangrove OC keeps accumulating despite much higher decomposition (especially by large lateral exports) than previously considered.

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“…The alternate tested method of using wet samples has largely been avoided, with only a few studies (e.g. van Bodegom et al, 2003;Chen et al, 2020) reporting the use of a wet slurry sample in soils and none for sediments. Chen et al (2020) justify their use of slurries as being to "minimise the physical-protection mechanisms of aggregation", acknowledging that drying methods are likely to result in superficial particle protection.…”

Section: Freeze Drying Of Samples As a Limiting Factor On Fe Reductionmentioning

confidence: 99%

“…Finally, sample preparation methods were examined since the commonly applied method of freeze drying is thought to cause particle aggregation, which may artificially shield Fe phases from reduction. The alternative approach of using a sediment slurry has been applied to soils (Chen et al, 2020) but has not been investigated in the context of marine sediments.…”

Section: Introductionmentioning

confidence: 99%

Technical Note: Uncovering the influence of methodological variations on the extractability of iron bound organic carbon

Fisher¹,

Faust²,

Moore³

et al. 2020

Preprint

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Abstract. Association of organic carbon (OC) with reactive iron (FeR) represents an important mechanism by which OC is protected against remineralisation in soils and marine sediments. Recent studies indicate that the molecular structure of organic compounds and/or the identity of associated FeR phases exerts a control on the ability of an OC-FeR complex to be extracted by the citrate-bicarbonate-dithionite (CBD) method. While many variations of this method exist in the literature, these are often uncalibrated to each other, rendering comparisons of OC-FeR values extracted by different method iterations impossible. Here, we created a synthetic ferrihdyrite sample coprecipitated with simple organic structures and subjected these to modifications of the most common CBD method. Method parameters (reagent concentration, time of the extraction and sample preparation methods) were altered and FeR recovery measured to determine which (if any) modifications resulted in the greatest release of FeR from the sediment sample. We provide an assessment of the reducing capacity of Na dithionite in the CBD method and find that the concentration of dithionite deployed can limit OC-FeR extractability for sediments with a high FeR content. Additionally, we show that extending the length of any CBD extraction offers no benefit in removing FeR. Finally, we demonstrate that for synthetic OC-FeR samples, the almost universal technique of freeze drying samples can significantly reduce OC-FeR extractability and we offer insight into how this may translate to environmental samples using Arctic Ocean sediments. These results provide a valuable perspective on how the efficiency of this extraction could be improved to provide a more accurate assessment of sediment OC-FeR content. Accurate determinations of OC-FeR in sediments and soils represents an important step in improving our understanding of, and ability to model, the global carbon cycle.

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Iron-mediated organic matter decomposition in humid soils can counteract protection

Cited by 278 publications

References 80 publications

Continental-scale controls on soil organic carbon across sub-Saharan Africa

Continental-scale controls on soil organic carbon across sub-Saharan Africa

Organic Carbon Stabilization Mechanisms in Mangrove Soils: A Review

Technical Note: Uncovering the influence of methodological variations on the extractability of iron bound organic carbon

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