Genesis of pseudo-sand structure in Oxisols from Brazil – A review

Martinez, Pedro; Souza, Ivan F.

doi:10.1016/j.geodrs.2020.e00292

Cited by 20 publications

(21 citation statements)

References 59 publications

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“…535It is likely that the high amount of pedogenic oxides usually measured in microaggregates (Doetterl et al, 2015a) and the low amount of POM measured in our study overall suggests that predominantly mineral-complexed SOC accumulated within the isolated aggregates. This is supported by studies showing that microaggregate sized particles in deeply weathered tropical soils are rich in Fe and Al concretions (Cooper et al, 2005;Zotarelli et al, 2005;Denef et al 2007;Martinez and Souza, 2019). In the light of this finding, aggregation is an important means 540 to promote the complexation of C with minerals (Martinez and Souza, 2019), but also the result of the tendency of pedogenic oxides to form stable aggregates (Doetterl et al, 2015a), which lends additional protection of soil C.…”

Section: Soil C Stabilization Driven By Soil Chemistry and Parent Matmentioning

confidence: 76%

“…While some studies in tropical regions have shown that variation in clay content explains SOC stocks in kaolinitic soils (Quesada et al, 2020), others have shown that SOC stabilization is not affected by clay quantity, but instead by the clay mineralogy (Bruun et al, 2010). The most important identified stabilization mechanisms in kaolinitic tropical soils are mineral-organic associations with short range 90 ordered (SRO) pedogenic oxides (Kleber et al, 2005;Bruun et al, 2010;Martinez and Souza, 2019) which stabilize 47 % to 63 % of the bulk SOC stocks in tropical forests (Kramer and Chadwick, 2018). Hence, differences in mineralogy affect a number of key soil fertility parameters and also the way C is stabilized onto minerals, ultimately impacting the interplay between mineral reactivity, microbial community structures and nutrient dynamics (Six et al, 2002;Denef and Six, 2005;Doetterl et al, 2018).…”

mentioning

confidence: 99%

“…Hence, differences in mineralogy affect a number of key soil fertility parameters and also the way C is stabilized onto minerals, ultimately impacting the interplay between mineral reactivity, microbial community structures and nutrient dynamics (Six et al, 2002;Denef and Six, 2005;Doetterl et al, 2018). For example, SOC stabilization by mineral-95 organic complexes in tropical soils are highly efficient as they appear in parallel and within highly stable soil aggregates and pseudosand structures (Martinez and Souza, 2019;Quesada et al, 2020). Furthermore, kaolinitic soils can form aggregates rapidly independent from biological processes due to electrostatic interactions between 1:1 clay minerals and oxides.…”

mentioning

confidence: 99%

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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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Section: Soil C Stabilization Driven By Soil Chemistry and Parent Matmentioning

confidence: 76%

mentioning

confidence: 99%

mentioning

confidence: 99%

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

Reichenbach¹,

Fiener²,

Garland³

et al. 2021

Preprint

View full text Add to dashboard Cite

show abstract

“…While some studies in tropical regions have shown that variation in clay content explains SOC stocks in kaolinitic soils (Quesada et al, 2020), others have shown that SOC stabilization is not affected by clay quantity but instead by the clay mineralogy (Bruun et al, 2010). The most im-M. Reichenbach et al: The role of geochemistry in organic carbon stabilization in tropical forest soils 455 portant identified stabilization mechanisms in kaolinitic tropical soils are mineral-organic associations with short-rangeordered (SRO) pedogenic oxides (Kleber et al, 2005;Bruun et al, 2010;Martinez and Souza, 2020), which stabilize 47 % to 63 % of the bulk SOC stocks in tropical forests (Kramer and Chadwick, 2018). Hence, differences in mineralogy affect a number of key soil fertility parameters and also the way C is stabilized onto minerals, ultimately impacting the interplay between mineral reactivity, microbial community structures and nutrient dynamics (Six et al, 2002;Denef and Six, 2005;Doetterl et al, 2018).…”

Section: Environmental and Geochemical Controls On Soc Dynamics In Tropical Forestsmentioning

confidence: 99%

“…Hence, differences in mineralogy affect a number of key soil fertility parameters and also the way C is stabilized onto minerals, ultimately impacting the interplay between mineral reactivity, microbial community structures and nutrient dynamics (Six et al, 2002;Denef and Six, 2005;Doetterl et al, 2018). For example, SOC stabilization by mineral-organic complexes in tropical soils is highly efficient as they appear in parallel and within highly stable soil aggregates and pseudosand structures (Martinez and Souza, 2020;Quesada et al, 2020). Furthermore, kaolinitic soils can form aggregates rapidly independent from biological processes due to electrostatic interactions between 1 : 1 clay minerals and oxides.…”

Section: Environmental and Geochemical Controls On Soc Dynamics In Tropical Forestsmentioning

confidence: 99%

The role of geochemistry in organic carbon stabilization against microbial decomposition in tropical rainforest soils

Reichenbach

Fiener

Garland

et al. 2021

SOIL

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Abstract. Stabilization of soil organic carbon (SOC) against microbial decomposition 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 changes in soil moisture conditions had a secondary control on SOC dynamics in valley positions, leading to higher SOC stocks there than at the non-valley positions. At several sites, we also detected fossil organic carbon (FOC), which is characterized by high C/N ratios and depletion of N. FOC constitutes 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 models, considering depth intervals of 0–10, 30–40 and 60–70 cm, 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 vs. 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 tropical soils investigated.

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Carbon stocks in umbric ferralsols driven by plant productivity and geomorphic processes, not by mineral protection

Martinez

Silva

Calegari

et al. 2021

Earth Surf Processes Landf

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Umbric ferralsols (UF) have organic carbon stocks within the first 1 m of soil profile depth that are nearly double the size (~19 kg C m−2) compared to the average carbon stocks of all other ferralsols (~10 kg C m−2). Here, we investigate processes that allow UF to gain and maintain such extraordinary carbon stocks. Using the International Soil Research and Information Center database, complemented with recently published datasets, we show that the carbon saturation capacity of the clay + silt fractions in UF (22.8 ± 5.7 g C kg−1 soil, n = 59) does not exceed the carbon saturation capacity in other ferralsols (23.3 ± 6.2 g C kg−1 soil, n = 165), thus eliminating a particularly retentive mineral matrix as a possible explanation. Rather, carbon stocks were correlated with the thickness of A horizons (106 ± 49 cm, n = 59 in umbric versus 35 ± 22 cm, n = 165 in non‐umbric ferralsols). We subsequently hypothesized that differences in A‐horizon thickness arose from two pedogenic scenarios: (i) superior carbon accrual resulting from a unique vegetation cover capable of allocating large quantities of carbon to belowground plant organs; and (ii) geomorphic processes that led to carbon burial. We evaluated the plausibility of these scenarios considering landscape history and associated carbon flux rates, hillslope position, as well as δ13C values and Ti/Zr ratios for subsets of ferralsol profiles studied by us in a region that encompasses latitudes from 26 to 7°S in eastern Brazil. We found that exceptional carbon stocks were most prevalent in the gradual transition zone (ecotone) between savanna and rainforest, likely resulting from erosion–deposition processes, historic changes of vegetation cover composed of grasses and arboreal elements, and active bioturbation. We suggest that attempts to sequester carbon in non‐umbric ferralsols will be most successful if they emulate or promote the biotic and abiotic processes prevalent in the savanna/rainforest ecotone.

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Genesis of pseudo-sand structure in Oxisols from Brazil – A review

Cited by 20 publications

References 59 publications

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

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

The role of geochemistry in organic carbon stabilization against microbial decomposition in tropical rainforest soils

Carbon stocks in umbric ferralsols driven by plant productivity and geomorphic processes, not by mineral protection

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