Soil memory on a toposequence scale: Phytolith and isotopic evidence of paleoenvironmental changes in the Atlantic Forest, Brazil

ABSTRACT

Soils can preserve long-term records of environmental change, providing valuable insights into past vegetation, climate, and landscape changes. This study evaluates soil memory at the toposequence scale using phytolith and carbon-isotope evidence of paleoenvironmental change in Atlantic Forest highlands of southeastern Brazil (Espírito Santo State). Soil profiles from two toposequences were described and sampled along slope positions and characterized through morphological, physical, and chemical analyses, combined with δ¹³C measurements and AMS ¹⁴C dating of soil organic matter. Phytoliths were extracted, identified, and used to calculate paleoenvironmental indices and define stratigraphic zones. Soils are organic-matter-rich, reflecting cool, humid, high-elevation conditions, and their morphology and degree of development vary with topography, drainage, and colluvial inputs. Integrating phytolith assemblages with δ¹³C and ¹⁴C results indicates that these profiles are polygenetic and preserve multiple phases of pedogenesis and landscape reworking. Four main environmental phases were identified. Phase I (earlier than ~11,315 cal BP) reflects relatively warm and dry conditions with open vegetation and a stronger C4 grass signal. Phase II (~11,315 cal BP) is recorded as a buried paleosol at the footslope and indicates a shift to cooler and wetter conditions with increased woody cover and dominant C3 inputs. Phase III (prior to ~1,837 cal BP) documents renewed drying and vegetation opening during the late Holocene. Phase IV (from ~1,837 to ~1,698 cal BP) marks the onset of the modern Cwb highland climate regime, initially cooler/wetter and later approaching present conditions, under which umbric A horizons likely developed. Overall, the results demonstrate that slope position controls the preservation of paleoenvironmental signals and that phytolith and isotopic proxies provide complementary evidence for reconstructing vegetation–climate dynamics archived in tropical highland soils.

Keywords
pedogenesis; vegetation dynamics; highland soils; quaternary climate

INTRODUCTION

Soils are long-lived natural archives that integrate biological inputs, geochemical transformations, and geomorphic dynamics through time. As a result, soil profiles can preserve evidence of the environmental conditions under which they formed and of subsequent shifts in soil-forming factors. This notion has been conceptualized as “soil memory”, i.e., the capacity of soils to retain and express information about past processes and states within their internal organization and properties (Targulian and Sokolov, 1978; Targulian and Goryachkin, 2001, 2004; Targulian et al., 2018). In contrast to many sedimentary archives – often characterized by strong temporal resolution but limited spatial specificity – soil memory is formed in situ and therefore tends to provide high spatial resolution, reflecting local interactions among climate, organisms, relief, parent material, and time (Targulian and Goryachkin, 2004).

Because soil continues to form and transform, new features may develop over earlier ones, producing composite or polygenetic records. Interpreting such records requires linking internal soil characteristics (morphology, chemistry, mineralogy) to landscape context, particularly where relief promotes contrasts in drainage, stability, erosion, and sediment redistribution. Toposequences provide an explicit framework for evaluating these controls by examining how soils and horizons vary laterally along slope positions within a single geomorphic unit (Alves et al., 2024). In this perspective, a toposequence does not simply describe spatial variability: it allows assessment of how pedogenesis and morphodynamics coevolve and how soil memory is preserved, erased, or overprinted downslope.

Among the many proxies used in studying soil memory, phytoliths (silica phytoliths) have become especially useful in tropical and subtropical settings. Phytoliths (biogenic opal; SiO₂·nH₂O) are microscopic silica bodies precipitated within or around plant tissues and may persist in soils long after plant decay (Piperno, 2006; Pearsall, 2016). Because specific morphotypes can be linked to particular plant families and subfamilies, assemblages preserved in soils can provide direct evidence of vegetation composition and structure, supporting paleoenvironmental reconstruction. These records are often particularly informative in organic-matter-rich soils, where preservation conditions can favor the retention of phytolith assemblages through time, such as in highland environments of southeastern Brazil (Silva Neto et al., 2024). When interpreted together with isotopic information from soil organic matter, phytolith data can strengthen inferences about shifts between C₃- and C₄-dominated vegetation and associated climatic changes.

The highlands of southeastern Brazil encompass environmentally heterogeneous landscapes, including Atlantic Forest formations and associated high-elevation mosaics, many of which are protected within conservation units. These settings combine strong topographic gradients with distinct temperature and moisture regimes that favor the development of soils with high organic matter contents and, consequently, high potential to store paleoenvironmental signals. Despite this potential, paleoenvironmental studies in Espírito Santo have been concentrated mainly in northern areas (Buso Junior et al., 2013a, 2013b; Castro et al., 2013; França et al., 2013, 2015, 2016; Lorente et al., 2014, 2018; Lorente, 2015; Calegari et al., 2015, 2017), whereas fewer investigations have targeted the mountainous region, particularly those explicitly connecting paleoenvironmental proxies to soil characterization, stratigraphy, and genesis along slopes.

This study examined soil memory at the toposequence scale in the Atlantic Forest highlands of southeastern Brazil (Espírito Santo State) by integrating phytolith assemblages with carbon-isotopic composition (δ¹³C) and radiocarbon dating (¹⁴C) of soil organic matter. Using a multiproxy approach applied to soil profiles from two toposequences, we aimed to identify and correlate paleoenvironmental phases recorded along the slope and to evaluate how relief-mediated processes influence the preservation and expression of these records within soil profiles.

MATERIALS AND METHODS

Study area

For this study, we selected a toposequence comprising four soil profiles located in the Atlantic Forest highlands of southeastern Brazil, in Espírito Santo State (20° 09’ 47.26” S, 40° 55, 52.34” W) (Figure 1). The parent material is predominantly felsic, consisting mainly of granitic rocks. These rocks are of Upper Proterozoic age and, mineralogically, are composed primarily of microcline, plagioclase, and quartz, with biotite and amphibole as the main mafic minerals, in addition to minor accessory phases (Cordani et al., 1982; Scardua, 2000).

Figure 1
Location of the study area in the highlands of the Atlantic Forest, Southeastern Brazil.

The relief is mountainous and highly dissected, with altitudinal belts shaped by fluvial incision along lithological and structural weaknesses. Regional geology, combined with tectonic influences and a predominantly humid climate, promotes strong spatial variability in soil formation and favors the development of organic-matter-rich soils at higher elevations. These highland environments play an important role as biodiversity refugia and for watershed protection within the Atlantic Forest biome. Vegetation is predominantly Montane Dense Ombrophilous Forest, typically occurring between 500 and 1,500 m a.s.l., with a tree canopy reaching up to ~25 m and a high abundance of epiphytes and lianas. The climate is classified as tropical highland (Cwb; Köppen–Geiger), characterized by mild temperatures, with a mean annual temperature of 18.7 °C and an annual precipitation of approximately 1,300 mm.

Soil sampling, physical and chemical analyses

Soil profiles were described and sampled following the Manual de Descrição e Coleta de Solos em Campo (Anjos et al., 2025). Samples for routine physical and chemical characterization were collected by genetic horizons, and additional samples for phytolith and isotopic analyses (δ¹³C and ¹⁴C) were collected at 0.10-m depth intervals throughout each profile.

Physical and chemical analyses were performed according to Teixeira et al. (2017), including pH measured in water (soil:water ratio 1:2.5); Ca²⁺, Mg²⁺, and Al³⁺, available P, exchangeable K⁺ and Na⁺, potential acidity (H+Al), and extractable H⁺; all analytical procedures are detailed in Teixeira et al. (2017). Total organic carbon (C) was determined using a CHN elemental analyzer.

For soils with predominantly organic constitution, additional analyses were conducted for classification purposes, including determination of organic matter and mineral material contents by combustion, soil bulk density (BD) and organic matter density (OMD), minimum residue (MR), and the pyrophosphate index (PI) based on Munsell color standards; fiber content and the von Post decomposition scale were also used to classify organic horizons as fibric, hemic, or sapric, following Lynn et al. (1974) and Fontana et al. (2017).

Sulfuric acid digestion was applied to air-dried fine earth (ADFE) to quantify SiO₂, Fe₂O₃, Al₂O₃, and TiO₂ by boiling samples in a 1:1 H₂SO₄ solution followed by cooling and filtration (Teixeira et al., 2017). Total elemental composition was determined by X-ray fluorescence (XRF) using a Philips Magix Pro (PW-2440) instrument, and the Ki and Kr weathering indices were calculated from sulfuric acid digestion and XRF results using equations 1 and 2, respectively.

Eq. 1 $\mathrm{Ki}={\displaystyle \frac{1.70\times \%{\mathrm{SiO}}_2}{\%{\mathrm{Al}}_2{\mathrm{O}}_3}}$

Eq. 2 $\mathrm{Kr}=\frac{1.70\times \%{\mathrm{SiO}}_2}{\%{\mathrm{Al}}_2{\mathrm{O}}_3+\left(0.64\times \%{\mathrm{Fe}}_2{\mathrm{O}}_3\right)}$

Colluviation and lithological discontinuities

Colluviation and potential lithological discontinuities along the toposequence were assessed using the uniformity value (UV) and the ratio of total titanium (Ti) to zirconium (Zr). The Ti and Zr are widely used in soil genesis studies as indicators of lithological discontinuities because they are considered relatively immobile in most soil environments, due to the high stability of their mineral forms and their very limited solubility (Milnes and Fitzpatrick, 1989; Taboada et al., 2006). To determine UV (Schaetzl, 1998; Tsai and Chen, 2000), the sand fraction was mechanically sieved into five size classes according to the Soil Survey Staff (2014) particle-size classification: very fine sand (VFS; 0.05–0.10 mm), fine sand (FS; 0.10–0.25 mm), medium sand (MS; 0.25–0.50 mm), coarse sand (CS; 0.50–1.00 mm), and very coarse sand (VCS; 1.00–2.00 mm). The UV was calculated using the following equation 3.

Eq. 3 $\mathrm{UV}=\frac{\left[\left(\%\mathrm{silt}-\%\mathrm{VFS}\right)/\left(\%\mathrm{sand}-\%\mathrm{VSF}\right)\right]\mathrm{upper}\mathrm{horizon}}{\left[\left(\%\mathrm{silt}-\%\mathrm{VFS}\right)/\left(\%\mathrm{sand}-\mathrm{VSF}\right)\right]\mathrm{lower}\mathrm{horizon}}$

Isotopic Analysis of δ¹³C and ¹⁴C Dating

Stable carbon isotopes (δ¹³C) were determined at 0.10-m depth intervals in each soil profile using elemental analyzers coupled to isotope-ratio mass spectrometers (ANCA SL 2020, Europa Scientific; Delta 5 Advantage, Thermo) at the Stable Isotope Laboratory of the Center for Nuclear Energy in Agriculture (CENA–USP). Isotopic composition is reported in δ notation relative to the VPDB (Vienna Pee Dee Belemnite) standard and expressed in per mil (‰). Analytical precision was ±0.2 ‰ (1σ). The δ¹³C values were calculated according to equation 4.

Eq. 4 ${\mathrm{\delta }}_{\mathrm{sample}}\left(\%\circ \right)=\left(\frac{{\mathrm{R}}_{\mathrm{sample}}}{{\mathrm{R}}_{\mathrm{PDB}}}\right)\times 1000$

in which: R is the ¹³C/¹²C ratio for the carbon isotopic ratio.

Bulk soil was used for ¹⁴C dating of soil organic matter (SOM) (Campbell et al., 1967; Lehmann and Kleber, 2015). For stratigraphic correlation along the toposequence, only the deepest horizon with high organic matter content (C ≥40 g kg^-1) in each profile was selected for dating. Samples were pretreated using the ABA protocol (acid–base–acid) following Pessenda et al. (1996). After pretreatment, the samples were submitted for AMS ¹⁴C dating at the Center for Applied Isotope Studies (CAIS) at the University of Georgia, USA. Radiocarbon ages were calibrated and are reported as calibrated years before present (cal BP; 2σ), using the calibration approach of Reimer et al. (2013).

Soil phytolith analysis

Soil phytoliths were extracted following Piperno (2006) and Calegari et al. (2013a). For each sample, at least 200 phytolith morphotypes with taxonomic and/or environmental significance were identified on microscope slides under 400 × magnification using a Zeiss Axioskop 40 optical microscope, and nomenclature followed the International Code for Phytolith Nomenclature (ICPN) (Madella et al., 2005). Phytolith concentration was also determined to enable comparisons among samples regardless of pedogenic or colluvial inputs and post-depositional disturbance (Albert et al., 1999, 2000, 2003; Karkanas et al., 2002; Cabanes et al., 2009).

Based on soil phytolith assemblages, the following indices were calculated for paleoenvironmental interpretation: (i) the humidity/aridity index (Iph, %), which reflects moisture conditions during phytolith production and is calculated as the ratio of Chloridoideae phytoliths (saddle morphotype) to the sum of Chloridoideae (saddle) and Panicoideae (bilobate and cross) morphotypes, multiplied by 100 (Diester-Haass et al., 1973; Twiss, 1992); (ii) the climatic index (Ic), used as a temperature proxy and calculated as the ratio of Pooideae phytoliths to the sum of Pooideae (rondel and trapeziform short), Chloridoideae (saddle), and Panicoideae (bilobate and polylobate) morphotypes, multiplied by 100 (Twiss, 1992); (iii) the tree cover index (D/P), which estimates woody cover and has been shown to be particularly robust in tropical settings, calculated as the ratio of eudicot phytoliths (globular psilate and globular rugose) to the sum of Poaceae phytoliths (Pooideae, Chloridoideae, Panicoideae, trichomes, and bulliforms) (Alexandre et al., 1997; Delhon, 2005); and (iv) the bulliform index (IB), which may indicate periods of water stress in grasses and indirectly increased aridity, calculated as the ratio of bulliform morphotypes (cuneiform bulliform) to the sum of short cells, bulliforms, and acicular hair cells, multiplied by 100 (Bremond et al., 2005).

Statistical analyses

Soil phytolith data were analyzed using multivariate statistical approaches. For each soil profile, stratigraphically constrained cluster analysis (CONISS; total sum of squares method) was applied to define phytolith zones using TiliaGraph/CONISS (Grimm, 1987). In addition, principal component analysis (PCA) was performed in correlation mode on transposed data matrices using C2 (Juggins, 2007). To support inter-profile comparisons and stratigraphic correlation along the toposequence, phytolith zones were subsequently compared among profiles to identify corresponding assemblage shifts and potential correlative horizons. The abundance of taphonomized phytoliths was included as an explanatory variable, enabling ecological interpretation by summarizing assemblage composition through patterns of covariation among samples.

RESULTS

Soil morphology and classification

Variations along the toposequence highlight the influence of relief on soil formation (Figure 2). At the shoulder position, profile P1 is a shallow organic soil characterized by unsaturated O horizons (folic horizon) extending to 0.69 m depth, with variable vegetal fiber content. Organic material accumulates directly over mineral horizons that still retain features of the granitic parent material (Pacheco et al., 2018; Silva Neto et al., 2020). Accordingly, P1 was classified as an Organossolo Fólico Sáprico típico (Folic Histosol - IUSS Working Group WRB, 2022). At the upper third of the slope, profile P2 is thicker (≥1.30 m) and predominantly mineral compared with P1. It comprises a thin organic layer (0.00–0.15 m) over a thick, dark A horizon with high organic matter content, meeting the criteria for an umbric horizon. The subsurface horizon qualifies as cambic because it does not meet the diagnostic criteria for subsurface horizons associated with stronger alteration or illuviation; this likely reflects the steeper gradient at this slope position. Profile P2 was classified as a Cambisolo Hístico Distrófico típico (Folic Cambisol - IUSS Working Group WRB, 2022).

Figure 2
Schematic diagram of the toposequence in the highlands of the Atlantic Forest, Southeastern Brazil.

At the middle third of the slope, profile P3 was also classified as Organossolo Fólico Sáprico típico (Folic Histosol - IUSS Working Group WRB, 2022). The profile is dominated by organic horizons to ~0.60 m depth, underlain by an organic-carbon-rich A horizon, followed by a cambic horizon and a C horizon lacking pedogenic structure and preserving rock fabric and easily weatherable minerals inherited from the parent material. Further downslope, profile P4 is substantially thicker (≥2.20 m) and displays morphological features indicative of greater pedogenic and stratigraphic complexity. Dark subsurface horizons, together with the footslope setting, suggest the presence of a buried A horizon associated with colluvial deposition. In this profile, two buried organic horizons (2Ob1, 0.46–0.72 m; 2Ob2, 0.72–0.85 m) were identified based on their darker colors and stronger aggregate development, consistent with organic material accumulation. The surface horizon meets the criteria for an umbric horizon; however, its morphology indicates that it represents relatively recent colluvial material overlying an older soil. Below this surface layer, lighter colors, textural contrasts, and weakly developed structure point to an incipient stage of pedogenesis, with aggregate formation likely enhanced by root activity (Table 1).

Table 1
Soil morphological properties in a toposequence in the highlands of the Atlantic Forest, Southeastern Brazil

Hor(1)	Layer(2)	Munsell Collor		Soil Structure(3)	Rupture resistance(4)	Horizon transition	Soil Texture
		moist	dry
	m
P1 – Organossolo Fólico Sáprico típico (Folic Histosol - IUSS Working Group – WRB, 2022)
Oo	0.00–0.09	10YR 3/3	10YR 4/2	sl., vf./f., gr.	v. friable	smooth/diffuse	Organic
Od1	0.09–0.21	10YR 3/1	10YR 4/1	mo., m./co., gr.	v. friable	smooth/diffuse	Organic
Od2	0.21–0.37	10YR 3/1	10YR 4/1	sl., f./m., gr.	v. friable	smooth/diffuse	Organic
Od3	0.37–0.52	10YR 2/1	10YR 3/1	mo., m./co., gr.	v. friable	smooth/diffuse	Organic
Od4	0.52–0.69	10YR 2/1	10YR 2/1	mo., m./co., gr.	v. friable	smooth/abrupt	Organic
BC1	0.69–0.78	10YR 4/2	10YR 6/1	sl., f., abk.	friable	smooth/clear	Sandy clay loam
BC2	0.78–0.91	10YR 4/1	10YR 6/1	sl., f., abk.	friable	-	Sandy clay loam
P2 – Cambissolo Hístico Distrófico típico (Folic Cambisol - IUSS Working Group – WRB, 2022)
Oo1	0.00–0.05	10YR 3/3	10YR 4/3	sl., vf./f., gr.	v. friable	smooth/gradual	Organic
Oo2	0.05–0.15	10YR 3/3	10YR 4/2	sl., vf./f., gr.	v. friable	smooth/diffuse	Organic
A1	0.15–0.31	10YR 3/2	10YR 4/3	mo., m., gr.	v. friable	smooth/diffuse	Sandy loam
A2	0.31–0.49	10YR 3/4	10YR 4/3	sl., f., gr.	v. friable	smooth/diffuse	Sandy loam
A3	0.49–0.57	10YR 4/1	10YR 4/2	mo., f./m., gr.	friable	smooth/clear	Sandy loam
BA	0.57–0.76	10YR 5/2	10YR 6/1	mo., f., sbk.	friable	smooth/clear	Sandy clay loam
Bi1	0.76–0.86	10YR 5/4	10YR 7/4	mo., f./m., abk.	friable	smooth/clear	Sandy clay loam
Bi2	0.86–1.05	10YR 5/8	10YR 7/4	mo., f., abk.	v. friable	smooth/diffuse	Sandy clay loam
Bi3	1.05–1.20	10YR 5/8	10YR 7/8	mo., f., abk.	v. friable	smooth/gradual	Sandy clay loam
C	1.20–1.32+	10YR 6/3	10YR 7/8	gs.	loose	-	Loam
P3 – Organossolo Fólico Sáprico típico (Folic Histosol - IUSS Working Group – WRB, 2022)
Oo	0.00–0.09	10YR 3/3	10YR 4/2	sl., vf./f., gr.	v. friable	smooth/gradual	Organic
Od1	0.09–0.21	10YR 3/2	10YR 4/1	mo., m./co., gr.	v. friable	smooth/diffuse	Organic
Od2	0.21–0.32	10YR 3/2	10YR 4/1	sl., f./m., gr.	v. friable	smooth/diffuse	Organic
Od3	0.32–0.50	10YR 3/1	10YR 3/3	mo., m/co., gr.	v. friable	smooth/diffuse	Organic
Od4	0.50–0.62	10YR 3/2	10YR 3/3	mo., m/co., gr.	friable	smooth/diffuse	Organic
OA	0.62–0.79	10YR 3/5	10YR 4/4	mo., m., gr.	friable	smooth/diffuse	Sandy loam
A	0.79–1.02	10YR 4/2	10YR 5/3	mo., f./m., gr.	v. friable	smooth/gradual	Sandy loam
BA	1.02–1.19	10YR 4/3	10YR 5/3	mo., f., sbk.	v. friable	smooth/gradual	Sandy loam
Bi	1.19–1.43	10YR 5/3	10YR 7/6	sl., f., sbk.	friable	smooth/clear	Sandy loam
C	1.43–1.62+	10YR 6/3	10YR 7/6	gs.	loose	-	Sandy loam
P4 – Latossolo Amarelo Distrófico espesso-húmico (Umbric Ferralsol - IUSS Working Group – WRB, 2022) * Paleosol
A1	0.00–0.09	10YR 4/2	10YR 4/3	sl., vf./f., gr.	v. friable	smooth/gradual	Sandy loam
A2	0.09–0.32	10YR 4/2	10YR 4/3	mo., m./co., gr.	v. friable	smooth/diffuse	Sandy loam
A3	0.32–0.46	10YR 3/3	10YR 4/1	sl., f./m., gr.	v. friable	smooth/gradual	Sandy loam
2Ob1	0.46–0.72	10YR 3/1	10YR 3/3	st., m/co., sbk.	friable	smooth/diffuse	Organic
2Ob2	0.72–0.85	10YR 3/1	10YR 3/3	mo., m/co., gr.	friable	smooth/clear	Organic
2Ab	0.85–0.94	10YR 3/3	10YR 4/1	mo., m., gr.	v. friable	smooth/diffuse	Sandy clay loam
2ABb	0.94–1.12	10YR 4/2	10YR 5/2	mo., f./m., gr.	v. friable	smooth/gradual	Sandy clay loam
2Bwb1	1.12–1.36	10YR 4/3	10YR 5/3	mo., f., sbk.	friable	smooth/diffuse	Sandy clay loam
2Bwb2	1.36–1.84	10YR 4/4	10YR 5/6	sl., f., sbk.	friable	smooth/diffuse	Sandy clay loam
2C	1.84–2.20+	10YR 6/6	10YR 6/6	gs.	loose	-	Sandy loam

• ⁽¹⁾
  Hor: soil horizon;
• ⁽²⁾
  Depth: horizon depth;
• ⁽³⁾
  sl: slight; mo: moderate; st: strong; vf: very fine; f: fine; m: medium; abk: angular blocks; sbk: subangular block; gr: granular; single grain (sg).
• ⁽⁴⁾
  Rupture resistance.

Profiles classified as Organossolos (Histosols - IUSS Working Group – WRB, 2022) showed a marked decrease in fiber content with depth, consistent with progressive decomposition of plant residues and the strong link between surface vegetation inputs and organic horizon composition (Table 2). Although the local climate is cold and humid, hydromorphic conditions favor organic matter accumulation and the development of thick organic horizons. However, the convex slope positions of these profiles promote better drainage and greater susceptibility to erosion, which can limit net organic matter accumulation. Other properties displayed similar depth trends in both Histosols, including increases in organic matter density (OMd), minimum residue (MR), and the proportion of mineral material (MM) with depth (Table 2).

Table 2
Organic soil properties in a toposequence in the highlands of the Atlantic Forest, Southeastern Brazil

Hor(1)	Layer(2)	URF	RF	Pi	Bd	OMd	MR	GM	OM	MM	von Post
											Index	Class
	m	-- % --			---- Mg m-3 ---		m m-1	--------- % --------
P1 – Organossolo Fólico Sáprico típico (Folic Histosol - IUSS Working Group – WRB, 2022)
Oo	0.00–0.09	39	27	4	0.41	0.26	0.10	41.5	62.8	37.2	H6	Hemic
Od1	0.09–0.21	31	15	3	0.62	0.36	0.17	35.7	57.8	42.2	H7	Sapric
Od2	0.21–0.37	21	14	2	0.70	0.37	0.22	23.4	52.5	47.5	H7	Sapric
Od3	0.37–0.52	15	12	2	0.84	0.38	0.31	21.1	44.7	55.3	H8	Sapric
Od4	0.52–0.69	16	8	1	0.96	0.41	0.37	17.1	42.8	57.2	H8	Sapric
P3 – Organossolo Fólico Sáprico típico (Folic Histosol - IUSS Working Group – WRB, 2022)
Oo	0.00–0.09	49	37	4	0.43	0.26	0.11	48.1	60.4	39.6	H6	Hemic
Od1	0.09–0.21	32	16	3	0.62	0.34	0.19	34.2	54.7	45.3	H7	Sapric
Od2	0.21–0.32	36	16	2	0.62	0.29	0.22	22.8	47.3	52.7	H7	Sapric
Od3	0.32–0.50	28	15	2	0.63	0.23	0.26	18.1	37.2	62.8	H8	Sapric
Od4	0.50–0.62	21	11	1	0.67	0.26	0.27	17.1	38.7	61.3	H8	Sapric

• URF: Unrubbed Fibers; RF: Rubbed Fibers; Pi: Pyrophosphate Index; Bd: Bulk Density; OMd: Organic Matter Density; MR: Minimum Residue; GM: Gravimetric Moisture; OM: Organic Matter; MM: Mineral Material.

Soil physical and chemical properties

Soil texture ranged from loam to sandy clay loam and sandy loam, consistent with the granitic parent material of the studied soils (Table 3). Profile P4 showed pronounced textural contrasts across the buried horizons, including a sharp decrease in coarse sand (from 562 g kg⁻¹ in horizon A3 to 220 g kg⁻¹ in horizon 2Ob1) together with increases in fine sand, silt, and clay. This shift supports the occurrence of a lithological discontinuity related to colluvial deposition. Overall, the soils were strongly acidic (pH 4.3–5.3). The lowest pH values were observed in organic horizons, reflecting the strong influence of organic matter and a predominance of H⁺ in the exchange complex. Unlike mineral tropical soils, in which Al³⁺ often dominates acidity, acidity in organic soils is largely associated with organic acids (Ebeling et al., 2008, 2011; Valladares et al., 2007, 2008; Pérez et al., 2009; Ebeling et al., 2013; Soares et al., 2015). In addition, the high organic matter content in Histosols promotes complexation of Al³⁺ by carboxylic and phenolic functional groups, which can reduce Al³⁺ toxicity to plants (Valladares et al., 2008; Silva et al., 2013).

Table 3
Physical and chemical properties in a toposequence in the highlands of the Atlantic Forest, Southeastern Brazil

Hor.	CS	FS	Sil	Clay	pH(H2O)	SB	Al3+	H+	CEC	BS	m	P	C
	‒‒‒‒‒ g kg-1 ‒‒‒‒‒					‒‒‒‒‒ cmolc kg-1 ‒‒‒‒‒				‒‒ % ‒‒		mg kg-1	g kg-1
P1 – Organossolo Fólico Sáprico típico (Folic Histosol - IUSS Working Group – WRB, 2022)
Oo	319	319	159	203	4.39	2.7	2.0	30.6	33.6	3	67	2	173.3
Od1	318	271	163	248	4.34	2.1	2.9	23.3	26.7	2	83	1	198.1
Od2	315	262	184	239	4.51	1.9	2.6	22.5	25.6	2	84	1	190.6
Od3	258	271	241	230	4.46	1.9	2.8	19.9	23.2	2	86	1	187.4
Od4	235	262	277	226	4.70	1.7	3.4	24.6	28.4	1	91	1	203.1
BC1	363	150	285	202	4.95	1.3	1.7	21.2	23.2	1	84	1	72.7
BC2	380	185	287	148	4.98	1.1	1.7	20.3	22.3	1	83	1	76.2
P2 – Cambissolo Hístico Distrófico típico (Folic Cambisol - IUSS Working Group – WRB, 2022)
Oo1	333	259	230	178	4.98	0.7	1.6	8.9	11.2	6	69	2	166.7
Oo2	355	286	210	149	4.13	0.4	1.3	7.6	9.3	5	75	1	151.9
A1	355	258	260	127	4.23	0.4	1.0	5.4	6.8	6	70	1	125.8
A2	333	260	248	159	4.73	0.4	0.9	4.3	5.6	8	67	1	128.9
A3	328	241	259	172	4.90	0.3	0.9	4.3	5.5	5	75	1	125.7
BA	321	218	258	203	5.00	0.4	0.7	4.5	5.6	8	62	1	75.3
Bi1	275	212	279	234	4.98	0.3	0.8	5.1	6.2	6	70	1	56.5
Bi2	277	226	253	244	4.96	0.4	0.8	3.9	5.1	8	66	1	52.8
Bi3	247	204	341	208	5.01	0.3	0.9	3.6	4.8	7	73	1	42.5
C	259	228	373	140	4.86	0.4	1.0	3.6	4.9	8	72	1	33.8
P3 – Organossolo Fólico Sáprico típico (Folic Histosol - IUSS Working Group – WRB, 2022)
Oo	275	374	149	202	4.28	1.8	2.5	21.0	25.3	7	59	2	206.5
Od1	270	330	159	241	4.41	1.5	2.3	19.4	23.2	6	61	2	224.9
Od2	263	309	184	244	4.37	0.9	2.4	13.4	16.7	6	72	1	232.7
Od3	244	299	201	256	4.22	0.9	2.9	15.6	19.5	5	75	1	181.5
Od4	230	292	217	261	4.23	0.8	1.9	10.8	13.5	6	71	1	157.0
A	341	279	199	181	4.95	0.6	1.7	12.3	14.6	4	74	1	125.5
AB	395	278	175	152	4.98	1.6	1.6	8.3	11.5	14	50	1	109.8
BA	391	258	189	162	4.32	0.5	1.3	4.3	6.1	8	73	1	60.1
Bi	409	243	204	144	4.59	0.3	1.2	3.7	5.2	6	79	1	37.4
C	336	218	287	159	4.69	0.30	1.0	4.1	5.4	6	77	1	24.6
P4 – Latossolo Amarelo Distrófico espesso-húmico (Umbric Ferralsol - IUSS Working Group – WRB, 2022) * Paleosol
A1	509	238	94	159	4.41	1.3	2.3	12.0	15.6	8	65	1	124.5
A2	556	236	63	145	4.37	1.2	1.8	9.6	12.6	10	60	2	127.6
A3	562	277	49	112	4.36	0.8	1.9	10.3	13.0	6	70	1	141.0
2Ob1	220	313	188	279	4.06	1.9	3.1	17.3	22.3	8	62	2	201.5
2Ob2	209	307	230	254	4.12	2.3	2.6	13.7	18.6	12	53	2	232.5
2Ab	204	348	201	247	4.36	1.5	1.6	9.3	12.5	12	51	1	131.2
2ABb	254	298	203	245	4.32	1.0	1.6	7.7	10.3	10	61	1	85.8
2Bwb1	255	307	159	279	4.64	0.6	1.3	5.7	7.6	8	70	1	63.0
2Bwb2	250	291	178	281	4.63	0.3	0.6	6.1	6.9	5	65	0	46.6
2C	313	292	236	159	4.42	0.3	0.9	2.4	3.6	8	75	0	32.7

• CS: Coarse Sand; FS: Fine Sand; Sil: Silt; SB: Sum of Bases; CEC: Cation Exchange Capacity; V%: Base Saturation; m%: Aluminum Saturation; C: Soil Carbon.

All studied soil profiles are dystrophic (V <50 %) and show low levels of exchangeable base cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺), consistent with the felsic (acid) parent material and the strongly leaching environment associated with humid climate conditions and forest vegetation. Slightly higher base status in surface horizons likely reflects nutrient cycling and biological uplift, with inputs from litterfall and shallow rooting partially offsetting leaching losses in the uppermost soil layers, as reported for comparable highland Atlantic Forest settings (Simas et al., 2005; Soares et al., 2016; Silva Neto et al., 2018a). Low nutrient availability combined with strong acidity can reduce microbial activity and slow organic matter decomposition, thereby favoring organic matter accumulation, particularly under the cool, wet conditions of the study area.

The Ki and Kr weathering indices indicate predominantly kaolinitic mineralogy in the studied soils (Ki and Kr >0.75) (Resende and Santana, 1988). In toposequence 1, SiO₂ contents increased both downslope (from upper to lower slope positions) and with depth within individual profiles (Table 4). This pattern is consistent with lateral redistribution and accumulation of materials transported from upslope positions, including fine mineral particles and organo-mineral constituents. In contrast, lower SiO₂ contents in surface horizons likely reflect preferential silica loss by leaching under humid conditions and more advanced weathering near the soil surface (Ibrahim and Lal, 2014). Given the overall sandy texture and the humid climate, silica depletion is enhanced, resulting in a relative enrichment of Al₂O₃ and Fe₂O₃ in the residual soil matrix.

Table 4
Geochemical analyses (sulfuric acid digestion, X-ray fluorescence) in a toposequence in the highlands of the Atlantic Forest, Southeastern Brazil

Hor.	Sulfuric acid digestion							X-ray fluorescence
	SiO2	Al2O3	Fe2O3	TiO2	Ki	Kr	Al2O3/Fe2O3	SiO2	Al2O3	Fe2O3	TiO2	Ki	Kr	Al2O3/Fe2O3
	‒‒‒‒‒‒ % ‒‒‒‒‒‒							‒‒‒‒‒‒ %‒‒ vv
P1 – Organossolo Fólico Sáprico típico (Folic Histosol - IUSS Working Group – WRB, 2022)
Oo	13.8	14.2	9.1	1.2	1.65	1.17	1.56	15.03	14.23	12.46	3.77	1.80	1.15	1.14
Od1	14.6	14.9	8.1	1.3	1.67	1.24	1.84	15.53	16.35	13.43	3.78	1.61	1.06	1.22
Od2	14.8	13.2	7.1	2.2	1.91	1.42	1.86	15.63	16.31	14.55	3.26	1.63	1.04	1.12
Od3	15.1	13.7	7.1	1.8	1.87	1.41	1.93	17.81	15.76	15.21	3.00	1.92	1.19	1.04
Od4	14.9	11.4	8.5	1.8	2.22	1.51	1.34	16.58	12.24	13.18	3.12	2.30	1.37	0.93
BC1	15.8	18.6	9.4	2.5	1.44	1.09	1.98	16.83	20.12	14.59	3.39	1.42	0.97	1.38
BC2	14.6	18.1	9.6	1.7	1.37	1.02	1.89	18.72	20.63	14.77	3.04	1.54	1.06	1.40
P2 – Cambissolo Hístico Distrófico típico (Folic Cambisol - IUSS Working Group – WRB, 2022)
Oo1	17.2	12.4	8.4	2.0	2.36	1.65	1.48	19.43	14.03	12.68	2.80	2.35	1.49	1.11
Oo2	15.6	12.4	8.0	1.7	2.14	1.52	1.55	16.85	14.35	12.89	2.36	2.00	1.27	1.11
A1	13.8	14.2	9.1	1.9	1.65	1.17	1.56	15.99	15.16	12.39	3.06	1.79	1.18	1.22
A2	13.5	14.9	9.6	1.5	1.54	1.09	1.55	15.78	15.64	13.96	3.89	1.71	1.09	1.12
A3	13.1	12.1	12.4	1.8	1.84	1.11	0.98	14.42	17.02	14.07	3.99	1.44	0.94	1.21
BA	13.8	13.5	13.3	1.5	1.74	1.07	1.02	15.68	15.45	16.04	2.91	1.73	1.04	0.96
Bi1	14.9	19.4	12.1	1.6	1.31	0.93	1.60	13.61	20.63	14.10	2.22	1.12	0.78	1.46
Bi2	15.8	23.0	11.2	1.3	1.17	0.89	2.05	15.15	25.44	14.56	2.93	1.01	0.74	1.75
Bi3	17.1	20.5	11.3	1.6	1.42	1.05	1.81	19.49	23.49	14.68	1.91	1.41	1.01	1.60
C	17.7	21.0	10.9	1.4	1.43	1.08	1.93	20.27	24.32	14.62	1.73	1.42	1.02	1.66
P3 – Organossolo Fólico Sáprico típico (Folic Histosol - IUSS Working Group – WRB, 2022)
Oo	18.9	11.3	8.4	2.0	2.84	1.93	1.35	20.70	21.19	10.20	2.77	1.66	1.27	2.08
Od1	18.4	11.8	8.5	2.4	2.65	1.81	1.39	20.63	22.17	9.60	2.78	1.58	1.24	2.31
Od2	18.7	10.7	8.9	1.8	2.98	1.95	1.20	19.34	21.09	10.90	2.91	1.56	1.17	1.93
Od3	19.3	11.9	7.9	2.1	2.76	1.94	1.51	20.80	17.70	9.10	2.14	2.00	1.50	1.95
Od4	18.9	12.1	8.5	2.0	2.66	1.84	1.42	19.25	20.48	10.60	2.56	1.60	1.20	1.93
A	15.6	23.6	9.8	1.8	1.12	0.89	2.41	20.03	29.42	11.41	2.78	1.16	0.93	2.58
AB	16.9	25.1	9.9	1.8	1.14	0.91	2.54	18.14	29.47	11.89	3.38	1.05	0.83	2.48
BA	18.9	24.7	9.1	1.6	1.30	1.05	2.71	23.05	32.43	12.74	2.36	1.21	0.97	2.55
Bi	14.4	22.9	10.8	2.0	1.07	0.82	2.12	19.55	37.36	10.33	2.78	1.89	0.76	3.62
C	24.2	23.0	10.6	2.3	1.79	1.38	2.17	33.62	33.71	14.97	2.83	1.70	1.32	2.25
P4 – Latossolo Amarelo Distrófico espesso-húmico (Umbric Ferralsol - IUSS Working Group – WRB, 2022) * Paleosol
A1	15.4	14.1	11.2	1.3	1.86	1.23	1.26	18.60	20.74	15.87	2.10	1.53	1.03	1.31
A2	16.1	12.1	10.4	1.4	2.26	1.46	1.16	18.90	18.12	15.62	2.17	1.77	1.14	1.16
A3	16.9	15.1	10.7	1.6	1.90	1.31	1.41	17.58	19.15	13.96	1.96	1.56	1.07	1.37
2Ob1	20.6	19.6	6.1	2.4	1.79	1.49	3.21	23.80	24.70	9.14	2.74	1.64	1.33	2.70
2Ob2	19.1	19.8	6.7	2.3	1.64	1.35	2.96	24.25	26.48	10.20	2.66	1.56	1.25	2.60
2Ab	11.4	21.2	9.5	1.8	0.91	0.71	2.23	13.96	22.75	14.38	2.00	1.04	0.74	1.58
2ABb	10.7	22.4	10.7	1.5	0.81	0.62	2.09	14.83	26.95	14.12	2.02	0.94	0.70	1.91
2Bwb1	9.8	24.0	10.6	1.6	0.69	0.54	2.26	14.27	25.55	14.50	2.00	0.95	0.70	1.76
2Bwb2	10.3	24.1	11.5	1.5	0.73	0.56	2.10	19.47	31.56	15.38	2.02	1.05	0.80	2.05
2C	29.5	24.8	10.5	1.4	2.02	0.51	2.36	40.46	30.64	16.23	1.93	2.25	1.68	1.89

Results for the uniformity value (UV) and the Ti/Zr ratio are shown in figure 3. In the Histosols (P1 and P3), the largest changes in UV occurred at the transition from organic to mineral horizons, indicating shifts in particle-size composition across these boundaries. The most pronounced variations in both proxies, however, were observed in profile P4, which also displays clear morphological evidence of stratigraphic discontinuity. At the A3/2Ob1 boundary, UV decreased to 0.77, and the Ti/Zr ratio increased from 11.5 in A3 to 20.4 in 2Ob1. Together with morphological evidence (color, texture, and structure) and supporting chemical indicators (e.g., organic carbon and elemental ratios), these patterns corroborate lithological discontinuities associated with colluvial deposition in P4. The buried horizons likely represent material mobilized from upslope positions and redeposited by gravity over an older soil surface.

Figure 3
Indicators of lithological discontinuities in the studied profiles. UV: Uniformity Value; Ti/Zr: Ratio of TiO₂ to ZrO₂ content (X-ray fluorescence).

δ¹³C and ¹⁴C Dating

The δ¹³C profiles showed distinct depth patterns, with isotopic shifts exceeding 4 ‰ in nearly all profiles (Figure 4), indicating changes in plant community composition during soil formation. As a reference, C₃ plants typically exhibit δ¹³C values between -22 and -32 ‰ (mean ~-27 ‰), whereas C₄ plants range from -9 to -17 ‰ (mean ~-13 ‰) (Boutton, 1996; Boutton et al., 1998).

Figure 4
Isotopic results (δ¹³C) in the toposequence in the highlands of the Atlantic Forest, Southeastern Brazil.

In profile P1, δ¹³C values ranged from −19.12 ‰ (0.90 m) to −27.16 ‰ (0.60 m). Between 0.70 and 0.50 m, values became more depleted relative to underlying horizons, consistent with a stronger C₃ signal, followed by an enrichment of >4 ‰ from 0.40 m to the surface. Profile P2 displayed a smaller amplitude, ranging from −22.11 ‰ (0.20 m) to −25.21 ‰ (0.70 m), suggesting mixed C₃–C₄ inputs with dominance of C₃ vegetation. In profile P3, δ¹³C values ranged from −19.44 ‰ (1.50 m) to −28.21 ‰ (0.20 m). From the base of the profile (1.60 m) up to ~0.80 m, values were nearly uniform (−19.51 to −20.45 ‰; mean −20.20 ‰), indicating relatively stable inputs with a stronger C₄ component or mixed vegetation; above ~0.80 m, values progressively depleted toward the surface, consistent with increasing C₃ dominance. Finally, profile P4 (toposequence 1) showed the largest amplitude. From the base to ~0.70 m, δ¹³C values became strongly depleted (−16.32 ‰ at 2.20 m to −29.21 ‰ at 0.70 m), followed by enrichment toward the surface from 0.60 m upward (−27.55 ‰ at 0.60 m to −20.41 ‰ at 0.10 m).

Soil organic matter ¹⁴C ages for the studied profiles are presented in table 5. Overall, age increased downslope, with older material preserved toward the lower slope positions. In profile P2 (toposequence 1), the A3 horizon (0.49–0.57 m) yielded a modern age, likely reflecting sediment and organic matter turnover on the steeper slope segment, which promotes soil rejuvenation. In contrast, the 2Ob2 horizon (0.72–0.85 m) of profile P4 yielded an age close to the Pleistocene–Holocene transition (11,315 cal BP), supporting the interpretation of a buried, older organic layer.

Table 5
¹⁴C datings of soil carbon in a toposequence in the highlands of the Atlantic Forest, Southeastern Brazil

Profile	Horizon	Layer	2σ Calibrated age AP	Calibrated mean age AP
		m
P1T1	Od4	0.52–0.63	1.617 -1.743	1.698
P2T1	A3	0.49–0.57	-	Moderna
P3T1	Od4	0.50–0.62	1.736 - 1.885	1.837
P4T1	2Ob2	0.72–0.85	11.231 - 11.409	11.315

• AMS dating: CAIS (Center for Applied Isotope Studies, Universidade da Georgia, USA).

Soil phytoliths

Phytolith assemblages varied markedly with depth and among slope positions, while remaining consistently dominated by grasses throughout the toposequence (Figure 5). In all samples, Poaceae phytoliths were the most abundant, mainly represented by morphotypes attributed to Pooideae, Panicoideae, and Chloridoideae. Phytoliths associated with woody taxa (eudicotyledons) were also present but always at lower proportions, and notable contributions from Arecaceae and Cyperaceae were recorded in several samples.

Figure 5
Soil phytolith assemblage in the toposequence in the highlands of the Atlantic Forest, Southeastern Brazil.

The surface interval (0.00–0.10 m) captured the signal of the modern vegetation cover, which in the study area forms a mosaic of montane/upper montane dense ombrophilous forest interspersed with herbaceous–shrub vegetation. At this layer, non-diagnostic grass morphotypes—particularly elongates, hair cells, and bulliforms—were the most frequent components of the assemblages, comprising 18–49 % of counted phytoliths (mean 36.35 %) (Barboni et al., 2007). Despite the predominance of these generalized forms, diagnostically informative short-cell morphotypes were also abundant. Rondel and trapeziform short cells (14–27 %; mean 21.01 %) indicate an important contribution of Pooideae, which is commonly associated with cooler conditions and high-elevation tropical settings (Vogel et al., 1978; Tieszen et al., 1979). Bilobate and cross morphotypes (13–24 %; mean 18.59 %) indicate a substantial Panicoideae component, consistent with grasses adapted to warm, humid environments (Mulholland, 1989; Lu and Liu, 2001). Phytoliths produced by woody plants (including palms) accounted for 7–19 % of the surface assemblages (mean 14.91 %), indicating a persistent—though subordinate—arboreal signal in the uppermost soils.

Hierarchical clustering (CONISS) consistently distinguished three phytolith zones within each soil profile (Figure 5). In parallel, PCA summarized the main gradients in assemblage composition, with the first two components explaining ~70 % of the total variance (Figure 6). The first axis (PC1; 48.33 %) separated assemblages dominated by grass indicators from those with stronger woody and cool-adapted signals. Positive PC1 scores were associated with higher contributions of Panicoideae and Chloridoideae short cells, as well as common grass morphotypes (elongates, hair cells, and bulliforms) and taphonomized phytoliths, whereas negative scores were linked to Pooideae morphotypes and to woody-plant indicators, including eudicotyledon forms (globular and block) and Arecaceae (globular echinate), together with papillae. Accordingly, higher PC1 scores reflect more open vegetation and comparatively warmer conditions, while lower PC1 scores indicate increased woody cover and cooler conditions. The second axis (PC2; 17.54 %) captured a moisture-related gradient. Positive PC2 scores were mainly driven by Panicoideae and by morphotypes such as globular echinate, hair cells, elongates, and block, whereas negative scores were associated with higher frequencies of Chloridoideae and taphonomized phytoliths, as well as papillae, Pooideae, bulliforms, and globular forms. In this context, positive PC2 values are interpreted as indicating relatively wetter conditions (greater Panicoideae contribution), whereas negative values suggest comparatively drier conditions (greater Chloridoideae contribution).

Figure 6
Principal component analysis of the soil phytolith assemblage in the toposequence in the highlands of the Atlantic Forest, Southeastern Brazil.

Zone I represents the basal part of the profiles, encompassing horizons BC1–BC2 in P1 and the B–C horizons in P2, P3, and P4. Across all profiles, this zone is characterized by the highest proportions of taphonomized phytoliths (means: P1 = 14.2 %; P2 = 10.3 %; P3 = 9.9 %; P4 = 15.7 %), likely reflecting longer residence time and cumulative post-depositional alteration at depth. In P2, P3, and P4, the tree-cover index (D/P) indicates a lower woody component than at present (means: P2 = 14.2 %; P3 = 13.9 %; P4 = 7.86 %), whereas P1 shows comparatively higher D/P values (mean = 36.1 %), suggesting relatively more wooded conditions at that site. This pattern is broadly consistent with the δ¹³C signals, which point to mixed vegetation with a stronger C4 contribution during the period represented by the basal deposits. The ordination results reinforce this interpretation: samples from Zone I in P2, P3, and P4 plot toward positive PC1 values, consistent with a reduction in woody cover as defined by the PCA loadings.

Moisture-related indices also indicate conditions drier than those inferred for the modern surface assemblages. The values of Iph are relatively high in Zone I (means: P1 = 54.2 %; P2 = 31.2 %; P3 = 33.5 %; P4 = 54.2 %), and elevated bulliform index values in P2 (mean = 76.7 %), P3 (mean = 74.8 %), and P4 (mean = 49.5 %) are consistent with increased water stress in grasses. In the PCA, this moisture gradient is expressed by negative PC2 scores for Zone I samples in all profiles. Because bulliform morphotypes can be comparatively resistant to dissolution and mechanical breakdown (owing, in part, to lower specific surface area), their high abundance may also partly reflect preservation biases rather than climate alone; nonetheless, the convergence of Iph, PC2, and assemblage composition supports a relatively drier phase. In addition, Zone I shows the lowest Ic values (means: P2 = 20.6 %; P3 = 19.9 %; P4 = 41.2 %), suggesting comparatively warmer conditions during the formation of these basal assemblages.

Zone II encompasses horizons Od3–Od4 in P1, the lower portion of the A horizon and the transitional AB/BA horizons in P2 and P3, and the buried horizons in P4. In P4, the combined evidence from lithological discontinuity proxies (Ti/Zr and UV) and ¹⁴C dating of horizon 2Ob2 indicates a buried paleosol formed around the Pleistocene–Holocene transition (~11,000 cal BP). This stratigraphic marker coincides with a shift toward wetter conditions, which is clearly expressed by the phytolith assemblage in this profile: D/P and Ic values are high (means: D/P = 50.8 %; Ic = 51.3 %), whereas Iph and Bi are comparatively low (means: Iph = 24.4 %; Bi = 24.9 %), pointing to increased tree cover under cooler, wetter conditions. This interpretation is consistent with the strongly depleted δ¹³C signal indicative of C₃-dominated vegetation (≈ −29 ‰). Ordination results corroborate these patterns, with Zone II samples in P4 showing negative PC1 scores (greater woody contribution/cooler conditions) and less negative PC2 scores than Zone I samples, consistent with increased moisture. In addition, the sharp reduction in taphonomized phytoliths (mean 4.61 %) together with higher phytolith concentrations in the subsurface supports the interpretation of a buried and relatively well-preserved paleosol horizon.

In profiles P2 and P3, Zone II is characterized by a moderate woody signal (mean D/P = 30.2 %) under comparatively cooler conditions (mean Ic = 40.7 %), while assemblage composition suggests a more open vegetation structure than inferred for the P4 paleosol. Iph values increase in these profiles (mean = 48.1 %), although this is not mirrored by the bulliform index, which does not indicate a marked phase of water stress; taken together, these indices suggest that moisture conditions in Zone II differed among slope positions and that water-stress signals were not uniform across the toposequence. In profile P1, Zone II indicates an increase in woody components (mean D/P = 45.7 %) and a shift to cooler and wetter conditions relative to the modern surface assemblage (mean Ic = 70.2 %; Iph = 31.1 %; Bi = 11.4 %). This phase is consistent with more depleted δ¹³C values (≈ −27 ‰) and, based on the available chronology, likely occurred around 1,698 cal BP.

Zone III includes the near-surface horizons that represent the youngest part of the soil record: Oo, Od1, and Od2 in P1; Oo1, Oo2, and A1 in P2; the folic horizons in P3; and the surficial mantle overlying the buried paleosol in P4. This zone captures the transition toward modern vegetation conditions, which in the study area correspond to a mosaic of montane/upper montane dense ombrophilous forest interspersed with herbaceous–shrub patches. Relative to Zone II, the woody signal inferred from D/P shifts differently among slope positions. The D/P decreases in P1 (mean = 19.2 %) and P4 (mean = 25.3 %), whereas it increases in P2 (mean = 38.2 %) and P3 (mean = 44.9 %), indicating spatial heterogeneity in canopy cover during the most recent phase. These patterns are broadly consistent with the δ¹³C depth trends, which show shifts >4 ‰ in all profiles except P2, where δ¹³C changes are comparatively small (≈2 ‰ enrichment) relative to the preceding zone. The PCA moisture axis (PC2) shows predominantly positive scores in Zone III, consistent with the present humid highland climate (Cwb), except for P3. In P3, high Ic values (mean = 70.2 %) indicate cooler conditions, and the chronology from horizon Od4 (0.50–0.62 m) suggests that this cooler phase was established by ~1,837 cal BP.

DISCUSSION

Soil genesis and processes

The results highlight a strong topographic control on soil genesis along the studied toposequence. Changes in diagnostic horizons and horizon thickness, as well as color, texture, structure, and overall profile depth, track slope position, reflecting contrasts in drainage, material redistribution, and landscape stability. Although all profiles are nutrient-poor and strongly acidic, these constraints do not prevent the establishment of dense Atlantic Forest vegetation. In this setting, sustained leaching under a humid subtropical highland climate, combined with felsic parent materials (granites and gneisses), promotes intense weathering and the dominance of low-activity mineral phases typical of highly weathered tropical soils—especially 1:1 clay minerals and Fe/Al (oxyhydr)oxides with inherently low cation-exchange capacity. Therefore, nutrient retention and availability depend disproportionately on soil organic matter, whose carboxylic and phenolic functional groups complex metal cations in solution and provide most of the effective exchange sites; base retention is therefore largely governed by the dissociation and reactivity of these organic functional groups (Canellas et al., 1999).

Profiles classified as Folic Histosols (IUSS Working Group WRB, 2022) are dominated by organic horizons that show no clear hydromorphic features, indicating that organic matter accumulation is not primarily driven by persistent water saturation. Instead, these Histosols appear to form under well-drained conditions where low temperatures—typical of high-elevation environments—reduce microbial activity and slow litter decomposition, allowing organic horizons to thicken over time (Bispo et al., 2015; Soares et al., 2016; Silva Neto et al., 2018b, 2024). The higher fiber content in surface horizons is consistent with limited decomposition of recently added plant residues under cool, humid conditions, whereas the decrease in fibers with depth reflects progressive humification and increasing decomposition along the profile (Bispo et al., 2015; Soares et al., 2015).

The mineral soil profiles are characterized by organic-carbon-rich surface horizons that meet the criteria for an umbric horizon (IUSS Working Group WRB, 2022), corresponding to the “A húmico” diagnostic horizon in the Brazilian Soil Classification System (Santos et al., 2025). Soils with this diagnostic horizon are especially common in high-elevation landscapes of southern and southeastern Brazil and are frequently associated with Cambisols/Cambissolo and Ferralsols/Latossolos (Schaefer, 2013). Previous studies indicate that these humic A horizons are often polygenetic, reflecting multiple phases of soil formation and landscape dynamics, and therefore can preserve evidence of past environmental conditions (e.g. Silva and Vidal-Torrado, 1999; Muggler and Buurman, 2000; Calegari et al., 2013b). In sloping terrain, their thickness may be enhanced by repeated colluvial inputs, potentially involving material already enriched in organic matter prior to deposition. Colluviation has been widely recognized as a key mechanism contributing to the development and thickening of umbric horizons in these settings (Lepsch and Buol, 1986; Silva and Vidal-Torrado, 1999; Silva et al., 2007; Calegari, 2008). At the same time, in situ processes may reinforce this thickening, including intense bioturbation by soil fauna and the decomposition, translocation, and stabilization of humic substances within the profile (Boulet et al., 1995; Silva and Vidal-Torrado, 1999; Gouveia and Pessenda, 2000; Calegari, 2008).

Profile P4, located in the lower third of the slope, shows morphological features indicating greater pedogenic and stratigraphic complexity than the upslope profiles and supports its interpretation as a buried paleosol formed by colluvial deposition. Paleosols are soils that developed on former land surfaces and therefore record past landscape and environmental conditions (Ruhe, 1956, 1965; Yaalon, 1971). They are commonly classified as relict, buried, or exhumed (Ruhe, 1965, 1969): relict paleosols formed under past conditions and remained exposed; buried paleosols developed on an older surface that was later covered by sediments and thus experienced a shift to new surface conditions; and exhumed paleosols were previously buried but later re-exposed by erosion, after which a new phase of pedogenesis may overprint earlier features. Within this framework, P4 is best interpreted as a buried paleosol, with its deeper horizons representing an older soil surface subsequently mantled by colluvial materials.

A key difficulty in field recognition of paleosols is separating features produced by recent pedogenesis from those inherited from earlier phases of soil formation (Catt, 1990). For this reason, robust identification typically relies on converging lines of evidence from field morphology and laboratory proxies. Here, lithological discontinuity indicators—the uniformity value (UV) and the Ti/Zr ratio—were used to evaluate stratigraphic breaks associated with colluvial inputs (Cremeens and Mokma, 1986; Schaetzl, 1998; Tsai and Chen, 2000; Soil Survey Staff, 2014; Ahr et al., 2017). These proxies, together with contrasts in texture and organic carbon between the surficial mantle and underlying buried horizons, indicate that younger colluvial deposits covered older horizons in P4. Consistent with this interpretation, ¹⁴C dating of the 2Ob2 horizon (0.72–0.85 m) yields an age close to the Pleistocene–Holocene transition (11,315 cal BP), indicating that the buried soil record in P4 formed during the late Pleistocene/earliest Holocene.

Paleoenvironmental interpretation and correlation of toposequence records

Integrating phytolith assemblages with the carbon isotopic composition of soil organic matter revealed clear shifts in vegetation composition across the toposequence, allowing paleoenvironmental inferences about climate during soil formation and subsequent profile evolution. Stratigraphically constrained clustering of the phytolith data, interpreted alongside ¹⁴C ages, supports the recognition of four main environmental phases recorded within the studied toposequence.

Phase I (earlier than ~11,315 cal BP) is recorded in the basal portions of all profiles except P1 and represents the driest and warmest conditions inferred for the sequence relative to the present. The ¹⁴C age from P4T1 indicates that this phase predates the Holocene and is associated with a vegetation signal dominated by C4 grasses and by taxa adapted to warm, moisture-limited environments. Evidence for late-Pleistocene landscapes with more open vegetation under drier climates has been widely reported for southeastern Brazil, including records from Lagoa dos Olhos (MG) (Oliveira, 1992), Morro de Itapeva (SP) and Lago do Pires (MG) (Behling and Lichte, 1997), Serra dos Órgãos (Behling, 1995), and Serra da Bocaina (Behling and Safford, 2010). More broadly, as global cooling intensified throughout the Quaternary, conditions in many tropical regions became drier or seasonally water-limited during Northern Hemisphere glacial periods (Schaefer, 2013). In our profiles, Phase I is supported by elevated Iph (>30 %) and Bi (>40 %) values, consistent with increased aridity and grass water stress. While high bulliform abundances may be partially influenced by preservation bias (i.e., differential resistance of morphotypes in soils), their co-occurrence with Chloridoideae indicators strengthens the interpretation of reduced moisture availability. Bulliform phytoliths have been proposed as useful indicators of water stress in grasses, supporting this inference (Messager et al., 2010).

A second feature of Phase I is the high proportion of taphonomized phytoliths. Under reduced moisture availability during the late Pleistocene, physical denudation is expected to have been comparatively more important than chemical alteration, and sparse vegetation cover would have increased the exposure of soil material to erosion. In this context, sediment redistribution along slopes – potentially enhanced by episodic, high-energy runoff – could have accelerated slope reworking and promoted valley infilling, a scenario in which morphogenesis tends to outweigh pedogenesis (Bigarella et al., 1965). Beyond the longer residence time of phytoliths in deeper horizons, the abundance of damaged particles (e.g., broken and corroded forms) may therefore also reflect reworking and transport associated with erosional processes during this phase. Phytoliths are generally resistant to both physical and chemical degradation (Piperno, 2006), but their preservation state is strongly influenced by the physical and geochemical conditions of the depositional environment, which ultimately control dissolution, fragmentation, and post-depositional alteration.

Phase II (~11,315 cal BP) is represented by the buried paleosol in the lower third of the slope (profile P4), with the dated paleohorizon marking the onset of the Holocene. The combined increase in the tree-cover index (D/P) and the climatic index (Ic), together with strongly depleted δ¹³C values, indicates a shift toward denser woody vegetation accompanied by C₃ grasses, consistent with cooler and wetter conditions than those inferred for Phase I. The high phytolith concentrations in this interval further suggest substantial plant input to the soil during paleosol development. A cool and wet early Holocene has been documented in multiple paleoecological records from southeastern Brazil (Behling, 2002). In the Serra do Caparaó region, Veríssimo et al. (2012) reported cool, wet conditions between 11,410 and 8,990 cal BP, with high-altitude grasslands dominating the upper elevations and forest vegetation occurring downslope. In the southern Serra do Espinhaço (MG), Horak-Terra et al. (2015) described very wet and cool conditions between ~10,000 and 7,360 cal BP associated with dense semideciduous and montane forests and humid rupestrian grasslands. Comparable evidence for early Holocene moisture increase and forest expansion has also been reported from Salitre (MG), where Araucaria forests occurred between 10,360 and 8,840 cal BP (Ledru, 1993), from Lago do Pires (MG), where gallery forests expanded between 9,980 and 8,180 cal BP (Behling et al., 1995), and from Lagoa Nova (MG), where semideciduous forest signals date to 9,540–8,220 cal BP (Behling, 2003).

Evidence for increased moisture toward the end of the Pleistocene is also reported from the mid–Paraíba do Sul Valley, where clay-rich and organic deposits in Bananal (São Paulo) have been dated to ~9,500 cal BP (Mello, 1997). In that region, slope–channel coupling is expressed by ramp–terrace systems, in which slope segments articulate with fluvial terraces, recording spatial and temporal discontinuities that are particularly relevant throughout the Holocene (Moura and Mello, 1991; Moura et al., 1991). In the study area, the establishment of denser vegetation on the lower slope during Phase II likely reflects not only a shift toward wetter conditions but also the greater availability of weathered material and redistributed fines in downslope settings. As rainfall increased in both volume and regularity, vegetation cover would have expanded and stabilized the surface, enhancing biological activity and favoring pedogenesis over morphogenesis (Bigarella et al., 1965). Under these wetter conditions, chemical weathering is also expected to have become more effective than physical weathering, reinforcing soil development. Accordingly, Phase II is interpreted as the formation of a paleosol at the Pleistocene–Holocene transition. More broadly, paleosols constitute key paleoenvironmental archives preserved within Quaternary deposits, particularly in tectonically stable continental regions such as Brazil (Suguio, 2010). They commonly occur as buried soils overlain by younger sediments and may exhibit variable organic matter contents and evidence of bioturbation, reflecting both the conditions of formation and subsequent burial and preservation processes (Retallack, 1988).

Phase III (earlier than ~1,837 cal BP) corresponds to a late-Holocene interval characterized by warmer and drier conditions in both study areas, constrained by ages around ~2,063–1,837 cal BP. In the stratigraphic framework defined by the phytolith zonation, this phase aligns with Zone II in profiles P3T1 and P1T2 and with Zone I in P3T2. Assemblages from this interval show an increased contribution of C4 grass indicators associated with lower moisture availability, particularly Chloridoideae morphotypes, together with a decline in Pooideae short cells. This shift is reflected in lower Ic values (<40 %) and is accompanied by phytolith indices and δ¹³C signals indicating more open vegetation, with reduced woody input (D/P <30 %) and a stronger C4 component. Regional paleoecological records support the occurrence of late-Holocene variability with alternating forest and high-altitude grassland cover in southeastern Brazil. In Caparaó Mountain Range (ES), Veríssimo et al. (2012) documented late-Holocene oscillations between forest and high-altitude grasslands, consistent with shifts in moisture availability. In Espinhaço Meridional Mountain Range (MG), Horak-Terra et al. (2015) reported drier conditions between ~2,200 and 1,600 cal BP in a peatland record, with vegetation dominated by Cerrado-type formations and evidence of moisture and temperature variability inferred from frequent stratigraphic changes.

Phase IV (from ~1,837 to ~1,698 cal BP) marks the establishment of the modern climate regime (Cwb; subtropical highland climate with dry winters and mild summers) during the late Holocene, in line with the regional synthesis of southern and southeastern Brazilian paleoclimates presented by Behling (2002). The onset of this phase is characterized by cooler and wetter conditions, which we treat as an initial sub-stage within the broader modern regime. Phytolith assemblages show an increased contribution of morphotypes typical of cool, high-elevation settings, particularly Pooideae, resulting in high Ic values (>60 %). At the same time, the tree-cover index indicates comparatively denser forest vegetation than today (D/P >40 %), accompanied by a stronger C3 signal in δ¹³C values, consistent with greater woody input and C3 grasses. These conditions would have favored the accumulation of plant residues in well-drained environments, initiating the development and thickening of organic horizons between ~1,837 and ~1,698 cal BP. Similar mechanisms have been reported for the genesis of Folic Histosols in high-altitude Atlantic Forest settings, where cool and humid conditions reduce biological activity and slow decomposition, promoting organic matter accumulation (Soares et al., 2016; Silva Neto et al., 2018b). In addition, the dystrophic status and strong acidity typical of these soils can further constrain microbial processing and humification pathways, reinforcing organic matter preservation (Santos et al., 2018).

Toward the end of Phase IV, the proxies indicate a shift to lower tree cover (D/P ≈ 30 %) and conditions that were slightly warmer (Ic ≈ 45) and wetter (Iph ≈ 20 % and Bi ≈ 13 %). Under this late-Holocene Cwb climate regime, the umbric A horizons identified in both toposequences likely developed, reflecting sustained biomass inputs and organic matter stabilization under cool, humid highland conditions. This interpretation is consistent with previous studies in southeastern Brazil highlighting the genesis and paleoenvironmental significance of umbric A horizons. Based on combined δ¹³C–¹⁴C and phytolith evidence, Calegari (2008) suggested that organic matter in Ferralsols (Latossolos) with umbric A horizons in this region has been incorporated since the late Holocene. Likewise, Marques (2009) argued that high biomass production and organic matter accumulation in these horizons are closely linked to Quaternary paleoenvironmental changes.

CONCLUSIONS

The studied highland soils are polygenetic and preserve a toposequence-scale “soil memory” shaped by pedogenesis, geomorphic reworking, and shifts in vegetation. By integrating soil morphology and stratigraphy with phytolith assemblages, δ¹³C, and ¹⁴C ages, we identified four main paleoenvironmental phases recorded in the profiles.

The basal record indicates a pre-Holocene interval characterized by open, grass-rich vegetation under relatively warm, dry conditions, with strong evidence of reworking and morphogenesis. This was followed by the development of a buried paleosol at the Pleistocene–Holocene transition, reflecting increased moisture, cooler conditions, and a stronger arboreal signal at the footslope. Late-Holocene phase documents renewed drying and vegetation opening, before a shift toward the modern Cwb climate regime, initially cooler/wetter and subsequently closer to present conditions, under which the umbric A horizons likely formed.

Overall, the results demonstrate that slope position controls both soil development and the preservation of paleoenvironmental signals, and that phytolith and isotopic proxies provide complementary evidence to reconstruct vegetation–climate changes archived in tropical highland soils.

• How to cite:
  Silva Neto EC, Anjos LHC, Schiavo JA, Pereira MG. Soil memory on a toposequence scale: Phytolith and isotopic evidence of paleoenvironmental changes in the Atlantic Forest, Brazil. Rev Bras Cienc Solo. 2026;50:e0250161. https://doi.org/10.36783/18069657rbcs20250161
• FUNDING
  This study was financed by National Council for Scientific and Technological Development (CNPq) (grant number 407835/2018-0), in part by the Coordination for Higher Education Staff Development- Brasil (CAPES) - Finance Code 001 and Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro (FAPERJ).

DATA AVAILABLE

The data will be provided upon request.

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Edited by

• Editor:
  José Miguel Reichert https://orcid.org/0000-0001-9943-2898 and Alberto Vasconcellos Indá Júnior https://orcid.org/0000-0001-5252-0313

Publication Dates

• Publication in this collection
  31 Aug 2026
• Date of issue
  2026

History

• Received
  06 Sept 2025
• Accepted
  23 Feb 2026