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

Abstract

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.

A multi‐proxy study of Umbric Ferralsols genesis in a high‐mountain environment in Rio de Janeiro state

“…Despite the position of the profiles in the landscape, predominantly in the middle third of the slope and terrain slope (Table 1), the colluvial process has not been identified as the main factor responsible for umbric horizon formation and thickening (Fontana et al, 2017; Martinez et al, 2022; Nehren et al, 2016); this is evident from the analysis of the morphological and physical properties of the profiles. In contrast, a small variation in the fine sand/coarse sand ratio was only identified in P1, which suggests the occurrence of lithological discontinuity.…”

“…Ferralsols are soils typical of tropical and subtropical humid zones that occur over an extensive area in South America and Africa, frequently associated with stable geomorphic surfaces (IUSS Working Group WRB, 2014; Martinez et al, 2022). The morphological, physical, and chemical properties of Ferralsols indicate a high degree of weathering due to the expressive action of the pedogenic process of ferralitization (Buol et al, 2003; IUSS Working Group WRB, 2014; Kämpf & Curi, 2012).…”

“…Under tropical and subtropical climate conditions and/or in the Brazil highlands, Ferralsols can present an umbric horizon (from the Latin umbra , ‘shadow’, meaning dark), characterized by a thick mineral horizon (thickness > 100 cm), dark colours (value and chroma ≤ 3, wet), low exchangeable cation contents, and expressive soil organic matter (SOM) accumulation (Calegari et al, 2013; IUSS Working Group WRB, 2014; Kämpf & Curi, 2012; Marques et al, 2011; Martinez et al, 2022). The Umbric Ferralsols are important because they contribute significantly to the stocks of total C in the pedon, compared to other Ferralsols (Araujo et al, 2017; Batjes, 2014; Martinez et al, 2022).…”

“…Factor II: The high accumulation of SOM associated with paleoenvironmental conditions indicating changes of dry to humid climate . Using δ 13 C values of the SOM it is possible to identify changes in vegetation type, in that plants can be separated into C3, C4, and CAM according to their mechanisms of photosynthesizing CO 2 (Martinez et al, 2022). Several studies (Araujo et al, 2017; Calegari et al, 2013; Martinez et al, 2022; Silva & Vidal‐Torrado, 1999) revealed that the development of the umbric horizon is associated with paleoenvironmental conditions and involved mild temperatures (<18°C) and high humidity.…”

“…Using δ 13 C values of the SOM it is possible to identify changes in vegetation type, in that plants can be separated into C3, C4, and CAM according to their mechanisms of photosynthesizing CO 2 (Martinez et al, 2022). Several studies (Araujo et al, 2017; Calegari et al, 2013; Martinez et al, 2022; Silva & Vidal‐Torrado, 1999) revealed that the development of the umbric horizon is associated with paleoenvironmental conditions and involved mild temperatures (<18°C) and high humidity. In effect, these conditions enable the development of vegetation that favours a greater input, accumulation of SOM, and formation of hyper‐thick umbric horizons, in some conditions >100 cm (Marques et al, 2011).…”