Geogenic organic carbon in terrestrial sediments and its contribution to total soil carbon

Kalks, Fabian; Noren, Gabriel; Mueller, Carsten W.; Helfrich, Mirjam; Rethemeyer, Janet; Don, Axel

doi:10.5194/soil-2020-34

Cited by 3 publications

(3 citation statements)

References 49 publications

(69 reference statements)

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“…The analysis was restricted to topsoils (0-30 cm) since one of the critical assumptions of this MRT OC approach is that SOC is autochthonous, that is, it has been entirely built up in situ by the estimated carbon inputs. This is not always correct for topsoils, but is even more problematic for subsoils, which often contain allochthonous, translocated, relictic or geogenic OC (Kalks et al, 2020;Schneider et al, 2020). As mentioned above, SOC in soils with a C:N ratio >13, which have been classified as 'black sands' (Poeplau et al, 2020;Vos et al, 2018), were excluded since they constitute an extreme example of relictic SOC.…”

Section: Calculation Of the Mean Residence Time Of Organic Carbon Entering The Soilmentioning

confidence: 99%

Roots are key to increasing the mean residence time of organic carbon entering temperate agricultural soils

Poeplau

Don

Schneider

2021

Global Change Biology

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The ratio of soil organic carbon stock (SOC) to annual carbon input gives an estimate of the mean residence time of organic carbon that enters the soil (MRTOC). It indicates how efficiently biomass can be transformed into SOC, which is of particular relevance for mitigating climate change by means of SOC storage. There have been few comprehensive studies of MRTOC and their drivers, and these have mainly been restricted to the global scale, on which climatic drivers dominate. This study used the unique combination of regional‐scale cropland and grassland topsoil (0–30 cm) SOC stock data and average site‐specific OC input data derived from the German Agricultural Soil Inventory to elucidate the main drivers of MRTOC. Explanatory variables related to OC input composition and other soil‐forming factors were used to explain the variability in MRTOC by means of a machine‐learning approach. On average, OC entering German agricultural topsoils had an MRT of 21.5 ± 11.6 years, with grasslands (29.0 ± 11.2 years, n = 465) having significantly higher MRTOC than croplands (19.4 ± 10.7, n = 1635). This was explained by the higher proportion of root‐derived OC inputs in grassland soils, which was the most important variable for explaining MRTOC variability at a regional scale. Soil properties such as clay content, soil group, C:N ratio and groundwater level were also important, indicating that MRTOC is driven by a combination of site properties and OC input composition. However, the great importance of root‐derived OC inputs indicated that MRTOC can be actively managed, with maximization of root biomass input to the soil being a straightforward means to extend the time that assimilated C remains in the soil and consequently also increase SOC stocks.

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Section: Calculation Of the Mean Residence Time Of Organic Carbon Entering The Soilmentioning

confidence: 99%

Roots are key to increasing the mean residence time of organic carbon entering temperate agricultural soils

Poeplau

Don

Schneider

2021

Global Change Biology

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“…Following Cerri et al (1985) and Kalks et al (2020) we assume that the biogenic carbon in the different soil depths of all sites were relatively similar and overall several orders of magnitude younger than the FOC. The baseline values for this assumption are the depth explicit mean values of Δ 14 C of the mafic and felsic sites as they are free of FOC.…”

Section: Calculation Of Foc Contribution To Total Socbulkmentioning

confidence: 99%

The role of geochemistry in organic carbon stabilization in tropical rainforest soils

Reichenbach¹,

Fiener²,

Garland³

et al. 2021

Preprint

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Abstract. Stabilization of organic carbon in soils (SOC) depends on several soil properties, including the soil weathering stage and the mineralogy of parent material. As such, tropical SOC stabilization mechanisms likely differ from those in temperate soils due to contrasting soil development. To better understand these mechanisms, we investigated SOC dynamics at three soil depths under pristine tropical african mountain forest along a geochemical gradient from mafic to felsic and a topographic gradient covering plateau, slope and valley positions. To do so we conducted a series of soil C fractionation experiments in combination with an analysis of the geochemical composition of soil and a sequential extraction of pedogenic oxides. Relationships between our target and predicting variables were investigated using a combination of regression analyses and dimension reduction. Here, we show that reactive secondary mineral phases drive SOC properties and stabilization mechanisms together with, and sometimes more strongly than, other mechanisms such as aggregation or C stabilization by clay content. Key mineral stabilization mechanisms for SOC were strongly related to soil geochemistry, differing across the study regions. These findings were independent of topography in the absence of detectable erosion processes. Instead, fluvial dynamics and changed hydrological conditions had a secondary control on SOC dynamics in valley positions, leading to higher SOC stocks there than at the non-valley positions. We also detected fossil organic carbon (FOC) at several sites, constituting up to 52.0 ± 13.2 % of total SOC stock in the C depleted subsoil. Interestingly, total SOC stocks for these soils did not exceed those of sites without FOC. Additionally, FOC decreased strongly towards more shallow soil depths, indicating decomposability of FOC by microbial communities under more fertile conditions. Regression analysis showed that variables affiliated with soil weathering, parent material geochemistry and soil fertility, together with soil depth, explained up to 75 % of the variability of SOC stocks and Δ14C. Furthermore the same variables explain 44 % of the variability in the relative abundance of C associated with microaggregates versus free silt and clay associated C fractions However, geochemical variables gained or retained importance for explaining SOC target variables when controlling for soil depth. We conclude that despite long-lasting weathering, geochemical properties of soil parent material leave a footprint in tropical soils that affects SOC stocks and mineral related C stabilization mechanisms. While identified stabilization mechanisms and controls are similar to less weathered soils in other climate zones, their relative importance is markedly different in the investigated tropical soils.

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“…For this estimate, we assumed that fossil organic C is free of 14 C due to the high age of parent material (Doetterl et al, 2021 in review;Schlüter, 2006). Further, we assumed that biogenic SOC in the mixed sediment sites had the same radiocarbon values (Fbio) with depth as the mean depth-specific radiocarbon content measured from plateau soils of the mafic and felsic regions (regions without fossil organic C) and that these values represent biogenic SOC from active biological cycling in plant-soils systems (Cerri et al, 1985;Kalks et al, 2020 in review). However, because rates of biogenic C cycling likely vary across sites, with potentially slower biogenic cycling in mixed sediment sites (see Discussion), this estimate is likely an upper bound on the fossil organic C contribution to these samples.…”

Section: Assessing Fossil Vs Biogenic Organic Carbonmentioning

confidence: 99%

Controls on heterotrophic soil respiration and carbon cycling in geochemically distinct African tropical forest soils

Bukombe¹,

Fiener²,

Hoyt³

et al. 2021

Preprint

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Abstract. Heterotrophic soil respiration is an important component of the global terrestrial carbon (C) cycle, driven by environmental factors acting from local to continental scales. For tropical Africa, these factors and their interactions remain largely unknown. Here, using samples collected along strong topographic and geochemical gradients in the East African Rift Valley, we study how soil chemistry and soil fertility, derived from the geochemical composition of soil parent material, can drive soil respiration even after many millennia of weathering and soil development. To address the drivers of soil respiration, we incubated soils from three regions with contrasting geochemistry (mafic, felsic, and mixed sedimentary) sampled along slope gradients. For three soil depths, we measured the potential maximum heterotrophic respiration under stable environmental conditions as well as the radiocarbon content (Δ14C) of the bulk soil and respired CO2. We found that soil microbial communities were able to mineralize C from fossil as well as other poor quality C sources under laboratory conditions representative of tropical topsoils. Furthermore, despite similarities in terms of climate, vegetation, and the size of soil C stocks, soil respiration showed distinct patterns with soil depth and parent material geochemistry. The topographic origin of our samples was not a main determinant of the observed respiration rates and Δ14C. In situ, however, soil hydrological conditions likely influence soil C stability by inhibiting decomposition in valley subsoils. Our study shows that soil fertility conditions are the main determinant of C stability in tropical forest soils. Further, in the presence of organic carbon sources of poor quality or the presence of strong mineral related C stabilization, microorganisms tend to discriminate against these sources in favor of more accessible forms of soil organic matter as energy sources, resulting in a slower rate of C cycling. Our results demonstrate that even in deeply weathered tropical soils, parent material has a long-lasting effect on soil chemistry that can influence and control microbial activity, the size of subsoil C stocks, and the turnover of C in soil. Soil parent material and its lasting control on soil chemistry need to be taken into account to understand and predict C stabilization and rates of C cycling in tropical forest soils.

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Geogenic organic carbon in terrestrial sediments and its contribution to total soil carbon

Cited by 3 publications

References 49 publications

Roots are key to increasing the mean residence time of organic carbon entering temperate agricultural soils

Roots are key to increasing the mean residence time of organic carbon entering temperate agricultural soils

The role of geochemistry in organic carbon stabilization in tropical rainforest soils

Controls on heterotrophic soil respiration and carbon cycling in geochemically distinct African tropical forest soils

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