Morphologic and hydrologic distinctions between shallow and deep podzolized carbon in the southeastern United States Coastal Plain

Geoderma 361 (2020) 114007

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Morphologic and hydrologic distinctions between shallow and deep podzolized carbon in the southeastern United States Coastal Plain

T

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Allan R. Bacon , Yaslin N. Gonzalez1, Kayci R. Anderson Soil and Water Sciences Department, University of Florida, PO Box 110290, Gainesville, FL 32611, United States

A R T I C LE I N FO

A B S T R A C T

Handling Editor: Ingrid Kögel-Knabner

Podzolization imparts soils and landforms of the southeastern United States Coastal Plain with two distinct pools of organic carbon (C), referred to as shallow podzolized C (SPC) and deep podzolized C (DPC). We evaluate podzolization across two catenas, and across the state of Florida, to establish that SPC and DPC are morphologically and hydrologically distinct. We demonstrate that SPC lightens downward (i.e. a Bh horizon with a darker top than bottom) and accumulates below low chroma material, while DPC darkens downward and accumulates below higher chroma material. We interpret these distinct morphologies to be manifestations of separate podzolization processes that are controlled by unique components of soil hydrology. Our analyses support existing notions that SPC is a product of vadose zone hydrology; formed within a pronounced eluvial-illuvial system where C, metals (predominately aluminum), and clay-sized particles are mobilized and immobilized together when near surface saturation thresholds are met. Our work however also identifies DPC to be a product of phreatic zone hydrology; part of an eluvial-illuvial system where C is translocated without metals or clays and immobilized when subsoil saturation thresholds are met. Recognizing the morphologic and hydrologic differences between SPC and DPC not only provides insights into interactions between soil hydrology and terrestrial C cycling in low-relief landforms, but also by incorporating these differences into field-based assessments soil scientists can improve their interpretations and communication of the hydropedology and podzolization.

1. Introduction Podzolization is a diverse and complex edaphic phenomenon whereby organic carbon (C) is mobilized in surface soils, hydrologically transported to the subsoil, and then immobilized with/by reactive metals. Podzolization usually produces a conspicuous, organic C enriched subsoil horizon that is described as a Bh horizon following most contemporary conventions and is central to soil classification (Food and Agriculture Organization, 2015; Soil Survey Staff, 2015, 2012). Podzolization is underpinned by a wide variety of soil processes and properties, including organo-metal complexation; pH; Eh; surface adsorption; microbial degradation of organic matter; filtering; and mineral weathering, that operate individually or in concert to translocate and immobilize soil C (see Sauer et al. (2007) and Lundström et al. (2000) for detailed mechanistic overviews). Depending on hydrology, podzolization can redistribute organic C vertically within a soil profile and/or horizontally across landforms (Bourgault et al., 2015; Mattson and Lönnemark, 1939; Tamm, 1950). Subsequently, accurate and detailed representations of podzolization not only inform pedological

interpretations, they also provide insights into terrestrial C cycling and soil hydrology. In the southeastern United States Coastal Plain more than 6 million ha potentially contain podzolized C; roughly 4 million ha with podzolized C in the top two meters of soil (Soil Survey Staff, 2017a) and roughly 2 million ha where podzolized C resides below two meters and is not included in standard soil survey assessments (Gonzalez et al., 2018). In this region podzolization occurs most often in relatively poorly drained soils (Fig. S1), subsequently most reactive iron (Fe) has been removed and the reactive metals participating in podzolization are almost entirely aluminum (Al). Podzolization in the Coastal Plain produces two distinct subsoil organic C pools, hereafter referred to as shallow podzolized C (SPC) and deep podzolized C (DPC). SPC and DPC are most obvious, and easily distinguished from one another, when they exist in the same soil profile (grey triangles in Fig. 1a, b, and d). The earliest contemplations of SPC and DPC in this region probably occurred in the mid-20th century when the observational depth of soil survey expanded to 60 in. (152 cm) to expose soil profiles comprised of A-E-Bh-E′(or Bw)-B′h sequences (Soil Science Division Staff, 2017). In

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Corresponding author. E-mail address: allan.bacon@ufl.edu (A.R. Bacon). 1 Florida Department of Environmental Protection, 2600 Blair Stone Road, Tallahassee, Florida 32399, United States. https://doi.org/10.1016/j.geoderma.2019.114007 Received 24 July 2019; Received in revised form 8 October 2019; Accepted 9 October 2019 0016-7061/ © 2019 Published by Elsevier B.V.

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Fig. 1. Depictions of shallow podzolized carbon and more continuous deep podzolized carbon in Coastal Plain landscapes modified from Watts (1998) (a) and Daniels et al. (1999) (b). Grey triangles in a and b identify soil profiles that would be described with a Bh and a B′h horizon, black triangles identity soil profiles that would be described with only a Bh horizon. (c) Shallow podzolized carbon along a transition from wetlands (background) to uplands (foreground) from Banik et al. (2014). (d) Shallow and deep podzolized carbon (Bh and B′h horizon respectively) in an excavated north central Florida landform. (f) Shallow and deep podzolized carbon (Bh and B′h horizon respectively) in a Coastal Plain soil near Green Cove, Virginia (photo credit: Lee Daniels). (f) Horizon samples from profile BC3 in the current study. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the region (Bolivar, 2000; Daniels et al., 1975; Garman et al., 1981; Gaston et al., 1990; Harris et al., 2005; Leigh, 2007; Phillips et al., 1996; Stone et al., 1993). Focused investigations of SPC and DPC in the Coastal Plain are rare, but they do suggest that differences between these podzolized C pools

this scenario the Bh and B′h horizons constitute SPC and DPC respectively. Today, official soil series descriptions identify 17 soil series that that can contain both SPC and DPC (i.e. a Bh and a B΄h horizon; Gonzalez et al., 2018; Soil Survey Staff, 2017b) and such profiles are periodically reported in peer-reviewed and technical literature across 2

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uncertainty also surrounds field based interpretations of soils that contain only a single Bh horizon (black triangles in Fig. 1a and b). Currently, if DPC and SPC are not concomitantly observed one does not know which podzolized C pool is encountered without further chemical analyses or extensive excavations to evaluate lateral continuity. Stone et al. (1993) summarized two meter deep soil assessments in Florida (n = 224) to show that Bh and B’h horizons exist anywhere from 10 to 180 cm and 75 to 200 cm respectively in the soil, demonstrating that depth alone cannot distinguish a singular Bh horizon as SPC or DPC. Because SPC and DPC represent distinct pools of podzolized C that cycle C over different timescales (Bolivar, 2000), have distinct geomorphic distributions (Gonzalez et al., 2018), and likely respond to different components of hydrology (Gonzalez et al., 2018; Harris, 2000), the inability to distinguish them during field-based assessments identifies a major knowledge gap in the pedological understanding and communication of Coastal Plain soils. At the same time, closing this knowledge gap represents an opportunity for pedology to improve regional representations of podzolization and ultimately provide insights into soil, water, and C interactions. The primary objective of this work is to improve field-based assessments of podzolization by identifying morphologic differences between SPC and DPC. Because the morphologic expression of podzolization often reflects hydrology (Lopes-Mazzetto et al., 2018), we also aim to interpret DPC morphology to provide insights into DPC hydropedology. To accomplish these goals we first present and discuss soil morphology and biogeochemistry across two catenas in north central Florida, which were identified to contain both SPC and DPC by reconnaissance, and then we analyze the regional morphology of podzolization across the entire state of Florida.

are more substantive than simply their relative positions in a soil profile. Regional analysis of all reported Bh and B′h containing Coastal Plain soils shows that SPC and DPC exist on average 40 and 130 cm below the soil surface respectively, that the thickness of SPC and DPC average 25 and > 60 cm respectively, that the mass of C stabilized as DPC is on average at least 30% more than that of SPC in a given soil profile, and that deposition/burial processes do not explain the coincident existence of SPC and DPC in a soil profile (Gonzalez et al., 2018). This regional analysis also reveals that the geomorphic distribution of soils with DPC is distinct compared to that of soils with only SPC; the former is largely restricted to relatively high elevation relict shoreline features (beach and dune ridges and seaward facing scarps) while the latter tends to reside at lower elevations (in swales between relict shoreline features). Bolivar (2000) evaluated SPC and DPC across a Coastal Plain catena to show that the majority of SPC (55–63%) is operationally defined as fulvic acids while the majority of DPC (56–69%) is operationally defined as humic acids. Bolivar’s work also provides the only radiocarbon constraints of SPC (1.8 kyr BP average age) and underlying DPC (14.5 kyr BP average age), indicating that these two pools cycle over appreciably different timescales. Another distinction between SPC and DPC, which can be inferred from differences in their lateral continuity (Fig. 1a and b), is that they accumulate in response to different components of soil hydrology (Gonzalez et al., 2018; Harris, 2000). SPC is extensively studied (Banik et al., 2014; Garman et al., 1981; Harris et al., 1995; Harris and Hollien, 2000, 1999; Harris and Rischar, 2012; Tan et al., 1999) and well-established to be a product of vadose zone hydrology, specifically the duration and frequency of surface saturation events. This previous work, particularly an elegant series of column manipulations (Harris and Rischar, 2012; Harris and Hollien, 2000; Harris et al., 1995), shows that SPC accumulates when near surface saturation events exceed a duration threshold required to destabilize grain coatings in eluvial horizons and facilitate the mobilization of reactive metals (almost entirely Al) with C. In addition to duration thresholds, these works also show that SPC’s morphologic expression is positively correlated to the frequency of sufficiently long surface saturation events. More specifically, SPC will be morphologically well-expressed (a thick and deep Bh horizon; Bh horizon subdivisions that lighten downward; a heavily eluviated and thick E horizon; Fig. 1c background, 0–57 cm in Fig. 1d) when surface saturation events of sufficient duration occur frequently. As the frequency of these surface saturation events diminish, across topographic and hydrologic gradients, so too does the morphological expression of SPC (the Bh thins, lightens, loses horizon subdivisions, and moves closer to the soil surface as the E horizon “lenses out”; Fig. 1c foreground). Subsequently, as a product of vadose zone hydrology, SPC is a laterally discontinuous soil feature across Coastal Plain landforms (Fig. 1a–c; Banik et al., 2014; Tan et al., 1999; Gaston et al., 1990; Garman et al., 1981). In contrast to SPC, DPC is observed and interpreted to be a more continuous soil feature that transgresses Coastal Plain landforms (Fig. 1a, b, and d; Gonzalez et al., 2018; Bolivar, 2000; Harris, 2000; Daniels et al., 1999; Watts, 1998; Gaston et al., 1990). Although the hydropedology of DPC is presently poorly constrained, such lateral continuity suggests that the near surface saturation thresholds influencing SPC have little effect on DPC. The only observations of DPC hydrology that we know of come from Bolivar (2000) who observed soil water table levels across a catena in north Florida with SPC and DPC for one year. These data reveal that DPC was permanently saturated at all landscape positions except for the catena summit where it was saturated for ten months. As these observations occurred during a strong La Niña cycle (annual precipitation totaled only 0.95 m; 0.35 m less than the annual average) they not only support the notion that DPC is minimally influenced by surface saturation thresholds in the vadose zone, they also indicate that DPC is near permanently saturated and may be a product of phreatic zone hydrology. In addition to uncertainty regarding the hydropedology of DPC,

2. Methods 2.1. Catena investigations – sites and sampling We sampled soil profiles across two catenas in North Florida (Fig. 2a). Both catenas receive approximately 1.3 m of precipitation annually with most rainfall occurring in the summer months. The climate at both catenas is humid subtropical with summer high and winter low temperatures averaging approximately 33° and 4 °C respectively. Soils along both catenas formed in parent material that has been subaerially exposed since the early Pleistocene and are described as sandy marine sediments (Soil Survey Staff, 2017b). One catena is located in northeast Suwannee County (30°18′9.86″ N, 82°54′23.32″ W) in a physiographic region referred to as the Northern Highlands. There, we established a 120 m long transect perpendicular to a subtle south facing slope (< 3%). We excavated and sampled profiles at 0 (SC1), 36 (SC2), 60 (SC3), 90 (SC4), and 120 m (SC5) along the transect (Fig. 2a). At the time of sampling the field was fallow and supported naturally recruited grasses and forbs for cattle grazing. Previously the field supported a pine plantation (Pinus elliottii, prior to 2011) and row crops of corn and rye (Zea sp., Secale sp., 2011–2015). Following United States Department of Agriculture (USDA) conventions, soils in this field are mapped (Soil Survey Staff, 2017a) as Oxyaquic Alorthods (Hurricane series), Aeric Alaquods (Leon series), Grossarenic Paleudults (Albany series) and Aquic Quartzipsamments (Chipley series). The second catena is located in southeast Baker County (30°11′51.27″N, 82° 4′5.54″W) on the western slope of Trail Ridge, a prominent relict shoreline feature. There, a 400 m long transect was established perpendicular to a subtle west facing slope (< 3%). These soils have supported at least two successive rotations of pine plantations (Pinus elliottii and Pinus taeda). We excavated and sampled profiles at 0 (BC1), 100 (BC2), 400 (BC3) along the transect (Fig. 1). Following USDA conventions (Soil Survey Staff, 2017a), soils in this forest are mapped as Grossarenic Alaquods (Pottsburg series) and Aeric Alaquods (Leon series). 3

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Fig. 2. (a) Digital elevation model of north central Florida and the profile sampling locations across the Suwannee Co. (SC) and Baker Co. (BC) transects. Contour line units of are meters. (b) The location of 300 Spodosol profiles subset from the Florida State Soil Characterization Database for our regional analysis.

estimates of coarse silt (20–50 µm), very fine sand (50–100 µm), and total sand (50–2000 µm) to evaluate depositional discontinuities in each profile by calculating uniformity index (UI; modified from Cremeens and Mokma, 1986; Tsai and Chen, 2000) as:

We excavated soil profiles with either an Edelman combination auger (AMS Inc. American Falls, ID) or a 2 cm diameter environmentalist’s sub-soil probe (Clements Associates Inc. Newton, IA). We encountered saturated conditions in all profiles, and stopped sampling each profile where soil moisture resulted in sluffing so severe that clean uncontaminated material could no longer be collected. We separated excavated profiles into individual genetic horizons (n = 74 total samples) and estimated Munsell color (Cleland, 2010) of each horizon under natural light after moistening with water if needed. We air-dried and then sieved samples to two mm prior to chemical and physical characterization.

(CSi + VFSi )÷(Si − VFSi ) ⎞−1 UIi = ⎛ ⎝ (CSi − 1 + VFSi − 1 )÷(Si − 1 − VFSi − 1 ) ⎠ ⎜

⎟

(1)

where UIi in the uniformity index of sample i; CSi, VFSi, and Si are the concentrations of coarse silt, very fine sand, and total sand respectively in sample i; and CSi-1, VFSi-1, and Si-1 are the concentrations of coarse silt, very fine sand, and total sand in the sample immediately overlying sample i. We estimated total Al, Fe, titanium (Ti), and zirconium (Zr) with a Shimadzu EDX-7000 x-ray florescence (XRF) spectrophotometer (Shimadzu Corp. Kyoto, Japan). Prior to XRF analyses we pulverized a subsample of each sample with agate mortar and pestle, and loaded the pulverized material into polypropylene sample cups with mylar bottoms. We measured net intensities of Al (ka peak at 1.487 keV) at an xray voltage of 15 kV, and measured net intensities of Fe (ka peak at 6.400 keV), Ti (ka peak at 4.509 keV), and Zr (ka peak at 15.748 keV) with an x-ray voltage of 50 kV. We also analyzed powdered standards (USGS AGV-2, USGS SGR-1b, USGS W-2a, and NIST 2781) to generate calibration curves. The coefficient of determination of these calibration regressions was 0.98, 1.00, 0.93, and 0.98 for Al, Fe, Ti, and Zr respectively. The standard error of the calibration regressions was 0.66, 0.19, 0.09, and 0.002% (m/m) for Al, Fe, Ti, and Zr respectively. We analyzed all pulverized subsamples and standards for 180 s. We estimated total C by dry combustion at 900 °C with a Shimadzu total organic carbon analyzer (TOC-VCPH) connected to a solid sample introduction module (SSM-5000, Shimadzu Corp. Kyoto, Japan). We made C calibration curves using solid dextrose. Depending on the organic matter concentration of each sample we analyzed between 0.1 and 0.5 g of pulverized soil. As soil pH ranges from 3.4 to 5.3 across all of our samples we assume that all soil C is organic.

2.2. Catena investigations – soil analyses We estimated soil pH with a continuous flow electrode after a 15 min extraction with 0.01 M calcium chloride (5 ± 0.05 g soil: 10 mL 0.01 M CaCl2). We estimated 1 N sodium fluoride extractable pH (pHNaF) in all “organic-rich” samples (A, Bh, and B’h horizons) by adding 50 mL 1 N NaF to 1 ± 0.05 g soil, continuously stirring the slurry for two minutes, and then measuring pH with a continuous flow electrode. As NaF displaces hydroxide ions from soil exchange complexes pH-NaF is positively correlated to, and commonly used as an indicator of, the amount of amorphous material in the soil exchange complex (Soil Survey Staff, 2014). We estimated particle size distributions with a Beckman Coulter LS13320 MW laser diffraction particle size analyzer connected to an aqueous liquid module following the subsampling, dispersion, and analytical protocols detailed in Pachon et al. (2019). Briefly, we dispersed each sample overnight with 10 mL of 5% sodium hexametaphosphate without oxidative pretreatments for organic matter, measured each sample for 1 min at a pump speed of 7.2 L min−1, and analyzed light scattering patterns with the Mie theory after setting the suspension, imaginary, and real refractive indices to 1.332, 0.2, and 1.53 respectively. The mass of each analyzed subsample analyzed ranged from 1.0 to 2.0 g and obscuration ranged from 7 to 13% across all particle size analyses. We utilized volumetric concentration 4

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where ΔValue,i is the vertical variation of Munsell value in horizon i, Valuebottom,i is the Munsell value in the deepest subdivision of horizon i, and Valuetop,i is the Munsell value in the shallowest subdivision of horizon i.. Subsequently, positive or negative ΔValue,i estimates identify podzolized C containing horizons that lighten or darken with depth respectively. We calculate the vertical variation of organic C concentration (ΔOC) through podzolized C (again, the 199 Bh horizons and the 16 B′h horizons that are comprised of horizons subdivisions) as follows:

2.3. Regional analysis – database description The Florida State Soil Characterization (FSSC) database contains taxonomic, morphologic, physical, and chemical data for more than 8000 genetic soil horizons in more than 1200 soil profiles across the state of Florida (NRCS, 2007). All profiles were sampled and described to approximately 2 m under the auspices of the state soil survey. Included in the database are estimates of organic C concentration following acid dichromate digestion (Walkley and Black, 1934), Munsell color (Cleland, 2010), and soil texture by the pipette method after treatment with hydrogen peroxide and sodium hexametaphosphate (Soil Survey Staff, 2014).

ΔOC , i = OCbottom, i − OCtop, i

2.4. Regional analysis – database analyses We subset all soil profiles classified as Spodosols (n = 300 profiles, Fig. 1b) from the FSSC. These Spodosol profiles contain 300 observations of Bh horizons and 46 observations of B′h horizons. 66% (n = 199) of the Bh horizons and 35% (n = 16) of the B′h horizons are comprised of horizon subdivisions (for example, a Bh horizon reported as a Bh1-Bh2-Bh3 sequence) for a total of 603 individual soil samples with podzolized C. Previous analyses of these samples confirms that the reported Bh and B′h horizons are indeed products of podzolization, rather than buried surface horizons (Gonzalez et al., 2018). Across these 346 podzolized C observations, 63% reside in poorly drained soil, 20% in poorly drained soil, 9% in moderately well drained soil, 5% in very poorly drained soil, 1% in well drained soil, and 1% in excessively and somewhat excessively drained soil; a relative distribution of drainage classes that is representative of Spodosol drainage classes across the broader southeastern United Coastal Plain (Fig. S1a and b). Following USDA conventions for describing horizon boundaries (Soil Survey Staff, 2012), 66% of the observations are described as having “wavy” horizon topography, 12% as having “smooth” topography, 8% as having “irregular” or tongued topography, 1% as having “broken” topography, and 13% do not have accompanying horizon topography records (Fig. S2a). Although ortstein can accumulate in Coastal Plain Spodosols (Lee et al., 1988a,b), field based horizon descriptions of the 603 individual Bh and B′h horizons in the database note such cementation only three times (Table S1). We classified all 300 Bh horizons into one of five depth categories: “very shallow” when its top is reported < 50 cm deep (n = 79), “shallow” when its top is reported 50–74 cm deep (n = 103), “arenic” when its top is reported 75–124 cm deep (n = 67), “grossarenic1” when its top is reported 125–149 cm deep (n = 23), and “grossarenic2” when its top is reported > 150 cm deep (n = 28). We grouped the 46 B′h horizons together into a separate category regardless of depth (the depth to the top of B΄h horizons averages 140 cm, ranging from 79 to 206 cm, across this database). For each Bh and B′h horizon we report the following properties as they are reported in the FSSC database: chromaabv (the Munsell chroma of the soil horizon immediately overlying podzolized C), chromamax (the maximum Munsell chroma of all horizons overlying podzolized C), valuetop (the Munsell value of the shallowest horizon subdivision in a given Bh or B ′h horizon), OCtop (the organic C concentration of the shallowest horizon subdivision in a given Bh or B ′h horizon), and Claytop (the concentration of clay-sized particles in the shallowest horizon subdivision of a given Bh or B ′h horizon). For valuetop, OCtop, and Claytop we analyze the shallowest horizon subdivision of a given Bh or B ΄h horizon to ensure that the properties of podzolized C observation are counted only once despite the fact that some have multiple horizon subdivisions. We calculate the vertical variation of Munsell value (ΔValue) through podzolized C (the 199 Bh horizons and the 16 B′h horizons that are comprised of horizons subdivisions) as follows:

ΔValue, i = Valuebottom, i − Valuetop, i

(3)

where ΔOC,i is the vertical variation of organic C concentration in horizon i, OCbottom,i is the estimate of organic C concentration in the deepest subdivision of horizon i, and OCtop,i is the estimate of organic C concentration in the shallowest subdivision of horizon i. Subsequently, positive or negative ΔOC,i estimates identify podzolized C containing horizons that have increasing or decreasing organic C concentrations with depth respectively. We calculate the vertical variation in clay concentration (ΔClay) between podzolized C (all Bh and B′h horizons) and the immediately overlying soil horizon as follows:

ΔClay = Claytop − Clayabove

(4)

where Claytop is the concentration of clay-sized particles in the shallowest horizon subdivision of a Bh or B′h horizon and Clayabove is the concentration of clay-sized particles in the immediately overlying soil horizon. Subsequently, positive or negative Δclay estimates identify podzolized C that has higher or lower clay concentrations respectively than the overlying soil material. We conduct one-way analysis of variance on the aforementioned soil properties with podzolized C depth category (“very shallow”, “shallow”, “arenic”, “grossarenic1”, “grossarenic2”, and “B΄h”) as the independent variable and conduct post-hoc Tukey's Honest Significant Difference tests to estimate p-values for statistically significant differences between the categories. We conduct all statistical analyses in the base environment of R and consider p-values less than 0.05 to be statistically significant. Given the large sample size included in these statistical evaluations, dependent variables are not transformed prior to statistical tests and quantile-quantile plots indicate that model residuals are approximately normally distributed. 3. Results and discussion 3.1. Podzolized carbon across two catenas Soil profiles across the Suwannee Co. and Baker Co. transects typify the acidic and coarse-textured nature of soils with podzolized C in the southeastern United States Coastal Plain (Tables 1 and 2; Harris, 2000). Soil pH averages 4.7 in Suwannee Co., 4.6 in Baker Co., and does not exceed 5.3 in any sample. Concentration of sand sized particles average 90 and 93% (v/v) in Suwannee and Baker Co. respectively, and the concentration of clay-sized particles exceeds 4% in only 5 of the 74 sampled soil horizons. Occurrence of these relatively clay rich horizons coincide with fluctuations in total Ti to total Zr ratios and relatively high UI in a profile, and are thus interpreted to be products of depositional discontinuities (Schaetzl and Anderson, 2005). Across both transects, SPC is laterally discontinuous (Tables 1 and 2). In Suwannee Co., SPC is observed in profiles SC1, SC2, and SC3 at depths of 25–62, 15–40, and 37–64 cm respectively and, following USDA conventions for describing soil profiles (Soil Survey Staff, 2012) is described as a Bh horizon. SPC conspicuously resides below an E horizon in profile SC1. In profiles SC2 and SC3 the existence of SPC immediately subjacent to A horizons (i.e. A-Bh sequences) is revealed by the fact that Bh horizons are enriched in total Al (by more than 1%, roughly two-fold) and amorphous material in the soil exchange complex (pH-NaF elevated by at least 0.44 units) relative to A horizons (Fig.

(2) 5

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Table 1 General properties of soil profiles excavated in Suwannee County. Profile

Silt (%v/v) 15.5 4.6 7.3 5.7 6.1 4.5 4.7

Sand (%v/v) 82.6 94.4 90.8 92.4 90.7 93.1 92.9

Ti:Zr

UI‡

3.36 4.39 4.44 4.53 4.30 4.46 4.63

Clay (%v/v) 1.9 1.1 1.9 1.9 3.3 2.3 2.4

6 3 5 3 7 6 8

– 0.07 0.04 0.02 0.01 0.01 0.07

10YR 1/1 10YR 2/2 10YR 6/2 7.5YR 4/2 7.5YR 2.5/2

-§ 4.64 4.90 4.95 4.85

1.4 3.0 2.1 2.6 5.4

10.2 11.0 5.7 5.7 7.5

88.3 86.0 92.2 91.6 87.1

6 4 5 5 11

– 0.06 0.08 0.03 0.16

0–37 37–64 64–130 130–193 193–202 202–267 267–290

2.5Y 2.5/1 10YR 4/2 10YR 6/3 2.5Y 7/1 7.5YR 5/2 10YR 3/2 5YR 2/1

4.92 4.65 4.88 5.10 5.01 4.89 4.56

1.5 2.5 2.1 1.1 2.4 2.7 3.5

10.4 9.2 5.6 3.3 5.1 5.4 6.5

88.1 88.3 92.4 95.6 92.5 91.9 90.0

4 5 4 4 6 9 10

– 0.17 0.06 0.05 0.02 0.00 0.02

A1 A2 A3 A4 E Bhpp1 Bhpp2 Bhpp3 Bhpp4 2Bhpp5 2Bhpp6

0–24 24–44 44–70 70–85 85–154 154–177 177–195 195–201 201–217 217–224 224–260

10YR 2/1 10YR 3/2 2.5Y 3/2 10YR 4/2 10YR 5/2 7.5YR 3/1 10YR 6/1 7.5YR 3/2 7.5YR 2.5/1 7.5YR 3/2 7.5YR 1/1

4.87 4.40 4.44 4.55 4.86 4.82 4.91 4.88 4.81 4.83 4.52

1.8 2.5 2.8 1.9 1.9 1.1 1.0 1.7 3.0 9.0 3.0

11.6 10.4 8.7 7.0 5.9 5.8 3.8 4.9 7.5 16.5 9.8

86.6 87.1 88.5 91.1 92.2 93.1 95.2 93.4 89.5 74.4 87.2

6 6 5 5 4 3 3 5 9 16 15

– 0.09 0.01 0.05 0.03 0.03 0.03 0.00 0.06 0.19 0.07

A1 A2 A3 E1 E2 E3 Bhpp1 Bhpp2 Bhpp3 Bhpp4 2Bhpp5 2Bhpp6 2Bhpp7

0–32 32–50 50–68 68–76 76–90 90–156 156–161 161–170 170–213 213–228 228–235 235–243 243–265

10YR 2/1 10YR 4/3 10YR 4/2 10YR 5/2 10YR 6/4 2.5Y 6/3 10YR 6/3 10YR 5/3 10YR 3/2 10YR 4/2 10YR 4/2 10YR 2/1 10YR 1/1

4.60 4.49 4.45 4.65 4.82 4.89 4.85 4.90 4.94 4.86 4.65 5.15 4.59

1.9 2.3 2.2 1.7 2.1 1.9 1.6 1.7 1.4 1.2 8.2 6.6 2.7

11.5 7.9 8.4 6.1 5.3 3.9 3.0 3.4 3.9 3.9 19.0 10.4 9.4

86.6 89.7 89.4 92.2 92.6 94.2 95.4 94.9 94.7 94.9 72.7 83.0 87.9

4 5 6 4 4 4 4 6 5 7 21 30 22

– 0.13 0.01 0.06 0.02 0.05 0.01 0.01 0.04 0.00 0.30 0.11 0.05

Horizon (USDA)† A E Bh1 Bh2 E' B'h1 B'h2

Horizon (proposed) A E Bhvv1 Bhvv2 E' Bhpp1 Bhpp2

Depth (cm) 0–13 13–25 25–40 40–62 62–105 105–130 130–185

Munsel Color

soil pH

10YR 2/1 10YR 4/2 10YR 3/2 10YR 4/2 10YR 6/2 7.5YR 5/2 7.5YR 2.5/2

SC2

A Bh E B'h1 2B'h2

A Bhvv E Bhpp1 2Bhpp2

0–15 15–40 40–120 120–138 138–185

SC3

A Bh E1 E2 B'h1 B'h2 B'h3

A Bhvv E1 E2 Bhpp1 Bhpp2 Bhpp3

SC4

A1 A2 A3 A4 E Bh1 Bh2 Bh3 Bh4 2Bh5 2Bh6

SC5

A1 A2 A3 E1 E2 E3 Bh1 Bh2 Bh3 Bh4 2Bh5 2Bh6 2Bh7

SC1

† ‡ §

Horizon descriptions following Soil Survey Staff (2012). UI = Uniformity Index, not calculated in the absence of particle size data in a horizon or in overlying horizon. Not Determined.

Suwannee and Baker Co. respectively (Tables 1 and 2). In contrast, the observed thickness of DPC ranges from 80–109 and from 127–365 cm in Suwannee and Baker Co. profiles respectively, although the bottom of DPC is not known in any of the eight soil profiles. Assuming bulk densities of 1.3 g cm−3 for A horizons and 1.5 g cm−3 for all other horizons (Gonzalez et al., 2018) we estimate that the observed portions of the Suwannee Co. and Baker Co. profiles contain between 93 and 186 and 195–482 Mg C ha−1 respectively. In Suwannee Co. anywhere from 0 to 24% of the total soil C exists as SPC (0–25 Mg C ha−1) and approximately 22–74% exists as DPC (20–80 Mg C ha−1). Similarly, in Baker Co. profiles anywhere from 0 to 16% of the total soil C exists as SPC and approximately 71–90% exists as DPC. In addition to lateral continuity, thickness, and C contents, SPC and DPC are morphologically distinguished from one another across both transects by vertical gradients in Munsell value (Tables 1 and 2, Figs. 1d, 3 and 4). Morphologically, SPC is observed with (SC1 and BC3) and without (SC2, SC3, and BC1) master horizon subdivision in both catenas. When master horizon subdivisions are present SPC lightens downward (with depth) as quantified by increased Munsell values. Such lightening is morphologically indicative of podzolized C that

S2a and b). In profiles SC4 and SC5, vertical gradients of total Al and pH-NaF through shallow soil horizons are muted, indicating that SPC does not accumulate at these higher landscape positions (Banik et al., 2014; Harris and Rischar, 2012). In Baker Co., across a more than three times longer transect (Fig. 2a), SPC conspicuously resides below E horizons in profiles BC1 (20–68 cm) and BC3 (28–57 cm) where it is described as a Bh horizon but is absent in profile BC2 (Table 2). In contrast to SPC, DPC is observed below one meter in all soil profiles and is a continuous subsoil feature across both transects (Tables 1 and 2). In Suwannee Co., again following USDA conventions (Soil Survey Staff, 2012), DPC is described as a B΄h horizon in SC1 (105–185 cm), SC2 (120–185 cm), and SC3 (193–290 cm) because it resides below a shallower Bh horizon, and described as a Bh horizon in SC4 (154–260 cm) and SC5 (156–265 cm) because an overlying Bh is absent. In Baker Co., DPC is described as a B΄h horizon in profiles BC1 (174–301 cm) and BC3 (120–485 cm) and described as a Bh horizon from 120 to > 415 cm in BC2. In agreement with previous work (Gonzalez et al., 2018; Harris, 2000), we find that DPC is thicker and larger in mass than SPC across both transects. SPC thickness ranges from 26–37 and from 29–48 cm in 6

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Table 2 General properties of soil profiles excavated in Baker County. Profile

Horizon

Horizon

Depth

(USDA)†

(proposed)

(cm)

BC1

E E&Bh Bh E'1 B'h1 B'h2 2B'h3 2B'h4 3B'h5 3B'h6

E E&Bhvv Bhvv E'1 Bhpp1 Bhpp2 2Bhpp3 2Bhpp4 3Bhpp5 3Bhpp6

0–13 13–20 20–68 68–174 174–190 190–201 201–213 213–220 220–295 295–301

7.5 YR 4/1 7.5 YR 3/1 7.5 YR 2.5/1 10 YR 6/2 7.5 YR 4/2 10 YR 2/1 10 YR 3/2 10 YR 2/2 5 YR 2.5/1 5 YR 2.5/2

BC2

A E1 E2 E3/B Bh1 Bh2 Bh3 Bh4 Bh5 2Bh6

A E1 E2 E3/B Bhpp1 Bhpp2 Bhpp3 Bhpp4 Bhpp5 2Bhpp6

0–16 16–47 47–77 77–120 120–184 184–218 218–247 247–299 299–369 369–415

BC3

A E Bh1 Bh2 Bh3 E' B'h1 B'h2 B'h3 B'h4 B'h5

A E Bhvv1 Bhvv2 Bhvv3 E' Bhpp1 Bhpp2 Bhpp3 Bhpp4 Bhpp5

0–20 20–28 28–37 37–41 41–57 57–120 120–148 148–166 166–215 215–400 400–485

† ‡ §

Munsel Color

soil pH

Ti:Zr

UI‡

96.5 94.6 90.9 94.3 96.3 95.3 75.3 86.7 82.7 68.6

5 4 6 6 5 7 22 20 8 7

– 0.04 0.06 0.11 0.04 0.09 0.56 0.05 0.14 0.13

6.4 6.1 6.2 5.3 4.0 2.7 2.5 4.0 3.3 4.2

92.4 91.8 91.0 92.8 95.2 96.5 96.6 94.0 95.2 94.9

6 4 6 5 7 5 6 8 7 13

– 0.02 0.01 0.01 0.05 0.02 0.02 0.05 0.03 0.07

5.1 3.7 8.1 -§ 6.1 3.9 2.6 2.1 2.2 0.6 1.0

94.1 95.3 90.3 -§ 92.6 94.6 95.4 96.6 96.8 98.9 98.2

5 5 3 6 4 4 6 5 5 4 5

– 0.03 0.07 – – 0.07 0.04 0.01 0.01 0.04 0.02

Clay

Silt

Sand

(%v/v)

(%v/v)

(%v/v)

3.63 3.72 3.94 4.04 4.49 4.66 4.57 4.56 4.52 -§

0.8 0.8 1.4 2.3 1.2 1.2 8.4 2.3 2.0 2.1

2.7 4.5 7.7 3.4 2.6 3.5 16.3 11.0 15.3 29.3

10 YR 4/2 10 YR 5/4 10 YR 5/6 10 YR 6/4 7.5 YR 3/2 7.5 YR 2.5/2 7.5 YR 2.5/1 7.5 YR 2/1 10 YR 2/2 10 YR 2/1

4.26 4.38 4.39 4.54 4.95 5.11 4.61 4.41 4.46 4.43

1.2 2.0 2.8 2.0 0.8 0.7 0.9 2.0 1.5 0.9

7.5 YR 3/1 7.5 YR 4/1 7.5 YR 3/2 5 YR 3/2 7.5 YR 4/3 10 YR 6/3 7.5 YR 3/2 7.5 YR 2.5/1 7.5 YR 2/1 7.5 YR 2.5/1 7.5 2/1

4.88 4.69 -§ -§ 4.84 4.94 5.04 5.28 5.14 5.02 4.24

0.8 1.0 1.6 -§ 1.3 1.6 2.0 1.3 1.0 0.5 0.8

Horizon descriptions followingSoil Survey Staff (2012). UI = Uniformity Index, not calculated in the absence of particle size data in a horizon or in overlying horizon. Not Determined.

3.2. Podzolized carbon across Florida

accumulates in response to the duration and frequency of surface saturation events (Banik et al., 2014; Harris, 2000), and it reflects a depth dependent decreases in organic C concentration (Figs. 3 and 4) that identifies shallow reaches of SPC as the predominant SPC accumulation zone. In contrast, all observations of DPC darken downward, as quantified by Munsell values that decrease vertically by as much as 5 and 3 units in the Suwannee and Baker Co. transects respectively. Such darkening reflects depth dependent increases in total C (by up to 30 and 11-fold in Suwannee Co. and Baker Co. respectively) that distinguishes lower reaches of DPC as the predominant DPC accumulation zone. SPC and DPC are also morphologically distinguished from one another across both transects by the morphology of overlying soil horizons, specifically Munsell chroma (Tables 1 and 2, Figs. 1d, 3 and 4). Chroma of soil material overlying our five SPC observations is restricted to either 1 (SC2, SC3, BC1, and BC3) or 2 (SC1), averaging 1.1. In contrast, the chroma of soil material above DPC (E′ horizons in SC1, BC1, and BC2; E horizons in SC2 and SC3; A and E horizons in SC4, SC5, and BC2) averages 2.1 and 3.5, reaching maximums of 4 and 6 in Suwannee and Baker Co. respectively. At the same time, estimates of pHNaF, total Al, and total Fe average 8.8 ± 0.1, 0.9 ± 0.2%, and 0.75 ± 0.02% respectively in relatively low chroma soil material overlying SPC and are significantly lower (p-values ≤0.01 from t-tests assuming unequal variance) than that of relatively high chroma material overlying DPC which average 9.4 ± 0.1, 2.9 ± 0.3%, and 0.84 ± 0.03% respectively. These morphological observations of overlying material indicate that SPC accumulates as part of a pronounced near surface eluvial-illuvial system, as previously suggested (Harris, 2000; Harris and Hollien, 1999, 2000), while DPC accumulates independently of this near surface eluvial-illuvial system.

The morphologic expression of B′h horizons across Florida serve to regionally benchmark DPC morphology because these horizons reside below SPC and therefore constitute known DPC observations (as opposed to a singularly observed Bh horizons that may be SPC or DPC). Across Florida, Munsell value in the top of B′h horizons averages 2.6 ± 0.1 (Valuetop, mean ± one standard error) while the vertical change in Munsell value vertically through B΄h horizons averages −0.3 ± 0.2 (ΔValue), identifying a tendency to darken downward (Fig. 5a and b). Such darkening reflects depth dependent increases in organic C which average 0.8 ± 0.1% in the top of B′h horizons (OCtop) and increases by an average of 0.4 ± 0.2% (ΔOC) vertically through B′h horizons (Fig. 5c and d). The chroma immediately overlying B′h horizons (Chromaabv) and the maximum chroma of all soil horizons overlying B′h horizons (Chromamax) average 2.7 ± 0.2 and 3.4 ± 0.2 respectively (Fig. 5e and f). This regional DPC morphology agrees well with our catena-based observations (Tables 1 and 2, Figs. 3 and 4) and the only other transect based investigation of SPC and DPC that we know of (Bolivar, 2000), further distinguishing that DPC darkens downwards (coincident with depth dependent total C increases) and accumulates below relatively high chroma (minimally eluviated) soil material. Regionally, Munsell value of singularly observed Bh horizons varies predictably by the depth of the Bh horizon. Valuetop of Bh horizons categorized as very shallow (< 50 cm), shallow (50–74 cm), and arenic (75–124 cm) average 2.4 ± 0.1, 2.4 ± 0.1, and 2.3 ± 0.1 respectively and are significantly (p < 0.05) higher than that of grossarenic1 (125–149 cm) and grossarenic2 (> 150 cm) Bh horizons which average 7

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Fig. 3. Munsell value, total carbon, Munsell chroma, 1 N sodium fluoride pH, and total iron in Suwannee Co. soil profiles. Grey shading identifies pools of podzolized carbon in each profile (Table 1).

−0.5 ± 0.2% respectively as C concentration tends to decrease with depth through these shallow Bh horizons. In contrast, ΔOC in grossarenic1 and grossarenic2 Bh horizons is positive, averaging 0.3 ± 0.1%, as organic C concentrations tend to increase with depth similar to DPC. In addition to Munsell value, Munsell chroma overlying singularly observed Bh horizons also varies predictably across the region by the depth of the Bh horizon (Fig. 5e and f). Chromaabv averages 1.3 ± 0.1 in very shallow, shallow, and arenic Bh horizons, while Chromamax averages 1.4 ± 0.1, 1.3 ± 0.1, and 1.4 ± 0.1 respectively as these near surface pools of podzolized C tend to accumulate below low chroma and heavily eluviated soil material. In contrast, Chromaabv and Chromamax in grossarenic1 (1.9 ± 0.2 and 2.6 ± 0.2 respectively) and grossarenic2 (2.0 ± 0.2 and 3.7 ± 0.4 respectively) Bh horizons is significantly higher and, much like DPC, indicates that deep Bh

2.9 ± 0.2 and 2.9 ± 0.1 respectively (Fig. 5a). ΔValue in very shallow, shallow, and arenic Bh horizons averages 0.6 ± 0.1, 0.6 ± 0.1, and 0.3 ± 0.1respectively, as these near surface Bh horizons tend to lighten downward. Such lightening is significantly different from grossarenic1 and grossarenic2 Bh horizons where ΔValue is negative, averaging −0.9 ± 0.2 and identifying downward darkening similar to DPC (Fig. 5b). Downward lightening of shallow Bh horizons and darkening of deep Bh horizons reflects significantly different depth dependent gradients in organic C concentration (Fig. 5c and d). OCtop in very shallow, shallow, and arenic Bh horizons average 1.7 ± 0.1, 1.9 ± 0.1, and 1.9 ± 0.2% respectively and is significantly higher, by nearly two-fold, than that of grossarenic1 (0.9 ± 0.3%) and grossarenic2 Bh horizons (0.9 ± 0.2%) as well as B΄h horizons. ΔOC in very shallow, shallow, and arenic Bh horizons averages −0.6 ± 0.1, −0.8 ± 0.1, and 8

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Fig. 4. Munsell value, total carbon, Munsell chroma, 1 N sodium fluoride pH, and total iron in Baker Co. soil profiles. Grey shading identifies pools of podzolized carbon in each profile (Table 2).

immediately overlying soil horizon (ΔClay, Fig. 5g and h). Claytop averages 3.9 ± 0.2, 4.0 ± 0.3, and 3.9 ± 0.3% (m/m) in very shallow, shallow, and arenic Bh horizons respectively. As estimates of ΔClay average 2.7 ± 0.2, 3.3 ± 0.2, and 3.1 ± 0.3% (m/m) in very shallow, shallow, and arenic Bh horizons, roughly 70 to 80% of the clay- sized phyllosilicates in these near surface Bh horizons are derived

horizons tend to accumulate below comparatively high chroma and less eluviated soil material. The tendency for shallow Bh horizons to accumulate as part of a pronounced near surface eluvial-illuvial system is further supported by our estimates of clay concentration in the top of podzolized C (Claytop) and the difference in clay concentration between podzolized C and the

Fig. 5. Regional properties of podzolization by depth category (VS = very shallow, < 50 cm; S = shallow, 50–74 cm; A = arenic, 75–124 cm; G1 = grossarenic1, 125–149 cm; G2 = grossarenic2, > 150 cm; and B΄h = reported as B΄h regardless of depth). Valuetop, ΔValue, OCtop, ΔOC, Chromaabv, Chromamax, Claytop, and ΔClay defined in Sections 2.4 (Eqs. (1)–(3)) and 3.2. Error bars identify one standard error, bars within a panel that do not share a letter are significantly different at p < 0.05. 9

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material with a Munsell chroma of three or less. In contrast, podzolized C across Florida meeting our DPC criteria have only a 55% probability of residing below soil material with a Munsell chroma of three or less.

from overlying horizons. Although clay concentration increases on the order of 3% are slight, it is worth noting that clay concentration increases required to constitute an argillic horizon below horizons with < 15% clay (as is the case for all soils evaluated here) is only 3% (Soil Survey Staff, 2015). In fact, 62% of these near surface Bh horizons meet the particle size requirement of an argillic horizon and, as previously suggested (Harris and Hollien, 2000, 1999), appreciable lessivage accompanies near surface podzolization. In contrast, Claytop and ΔClay average only 2.6 ± 0.4 and 1.3 ± 0.5% respectively in grossarenic1 and grossarenic2 Bh horizons and only 3.0 ± 0.4 and 0.9 ± 0.3% respectively in B΄h horizons. Across Florida, only 19% of these deep podzolized C pools meet the argillic particle size requirement, again indicating that they tend to accumulate under less eluviated soil material.

3.4. Hydropedology of Coastal Plain podzolization As the southeastern United States Coastal Plain is a depositional landscape an obvious, and reasonable, first order hypothesis to explain the co-existence of SPC and DPC is deposition or burial (Bernal et al., 2016). Previous regional analysis of DPC however demonstrates that depositional discontinuities are not required for DPC accumulation (Gonzalez et al., 2018), and our catena observations show that DPC vertically transgresses such features (Tables 1 and 2). Subsequently, similar to Daniels et al. (1975), Harris (Harris, 2000), and Gonzalez et al., (2018), we interpret DPC to be a post-depositional product, and we build upon the existing hydropedologic understanding of SPC (Harris, 2000) to explain the formation, concurrent existence, and distinct morphology of both SPC and DPC (Fig. 6c–g). Our hydropedologic interpretations begin by recognizing that low relief, subsoil aquitards, and base level impart most Coastal Plain soils and landforms with free water for an appreciable portion, if not all, of the year (Fig. S4). Subsequently, the integrated probability of saturation in these coarse-textured, endosaturated environments not only increases with depth, it also has an inflection point that distinguishes the vadose zone-phreatic zone boundary (Fig. 6c; Bliss and Comerford, 2002; Daniels et al., 1975; Tan et al., 1999). In the vadose zone, saturation is relatively dynamic as the water table fluctuates in response to individual precipitation events and seasonal transpiration patterns. In the phreatic zone soil is more permanently saturated as the water table slowly fluctuates in response to base level and longer term precipitation patterns (annual, decadal, or even centurial). SPC is well documented and understood (Banik et al., 2014; Harris and Rischar, 2012; Harris, 2000; Harris and Hollien, 2000, 1999; Garman et al., 1981) to accumulate in the vadose zone, between the top and bottom of a rapidly fluctuating water table, in response to near surface saturation thresholds (Fig. 6c, black triangle). When these thresholds are not met (Fig. 6d) grain coatings in eluvial horizons remain intact, preventing reactive metals (predominately Al) and claysized particles from mobilizing with C and inhibiting SPC formation. When these thresholds are met (Fig. 6e and f), destabilized grain coatings allow reactive metals and clay-sized particles to mobilize with C and SPC formation is initiated. Episodic lowering of the fluctuating water table transports the mobilized constituents to illuvial horizons where they accumulate (Figs. 3–5 and S2). Initially, the depth at which

3.3. Two morphologic populations of podzolized carbon Our catena-based (Tables 1 and 2, Figs. 3 and 4) and regional (Fig. 5) analyses not only constrain the morphologic expression of DPC, they also show that singularly observed Bh horizons are more likely to express DPC morphology as their depth increases. These findings thus build upon previous work differentiating SPC and DPC by mass, lateral continuity, geomorphic distribution, and organic matter properties (Bolivar, 2000; Daniels et al., 1999; Gonzalez et al., 2018; Watts, 1998) and ultimately provide the opportunity to distinguish these pools of podzolized C during field-based soil assessments. We interpret soil horizons that accumulate podzolized C and either lighten downward (lower Munsell value at their top than their bottom) or neither lighten nor darken downward (same Munsell value at their top and bottom) to constitute SPC. Across Florida, the probability that such horizons will emerge in the upper one meter of a soil profile is 80–85% (Fig. 6a). In contrast, we interpret soil horizons that accumulate podzolized C and darken downward (have a higher Munsell value at their top than their bottom) to constitute DPC. Across Florida, the probability that such horizons will emerge in the upper one meter of a soil profile is only a 30% (Fig. 6a). It is important to acknowledge that this 30% probability is without doubt an overestimate because it is derived from a database that samples to only two meters (NRCS, 2007) and DPC accumulates below two meters in many Coastal Plain soils (Gonzalez et al., 2018; Harris et al., 2005). In addition to Munsell value, we propose that Munsell chroma overlying podzolized C can also inform field-based distinctions between SPC and DPC (Fig. 6b). Across Florida, there is an 85–90% probability that podzolized C meeting our SPC criteria will reside below soil

Fig. 6. The cumulative probability of the depth (a) and the maximum Munsell chroma overlying (b) podzolized carbon that lightens (positive ΔValue), neither lightens nor darkens (ΔValue = 0), and darkens (negative ΔValue) with depth (data in a and b from Bh and B΄h horizons in the FSSC database; NRCS, 2007). A generalized probability density function of soil saturation by depth (c) and four soil profiles with various expressions of podzolized carbon (d-g) that form in response to hydrologic thresholds in the vadose zone (black triangle in c) and the phreatic zone (grey triangle in c). Purple, orange, green, and yellow lines in panel c insert correspond to soil profiles in d, e, f, and g respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

10

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across our two catenas and the only other catena based observations that we know of (Bolivar, 2000) indicate that lateral variability in DPC concentration is inconsistently related to landform position and potentially quite small (Fig. S5). Subsequently, at present we lack evidence to support the notion that lateral C transport is central to DPC accumulation and interpret DPC to be predominately transported vertically through the soil profile. Our hydropedologic interpretation thus not only accounts for DPC’s distinct morphology at a variety of scales, it also provides mechanistic insights into how and why DPC exists below SPC and also in the absence of SPC (Fig. 6c–g).

SPC accumulates is dynamic and positively correlated to frequency with which near surface saturation thresholds are met (Fig. 6e and f). However, as suggested by Harris and Hollien (2000, 1999) and supported by our regional analysis (Fig. 5c and d), SPC cannot deepen in perpetuity because eventually lessivage clogs the soil fabric to establish SPC’s depth and promote its distinct morphology (downward lightening) by creating a fine textured upper boundary with increased surface area, water residence time, and potential for C accumulation (Fig. 6f and g). As this vadose zone process involves the translocation of reactive metals and clay-sized particles and C, we interpret SPC to be a manifestation of some combination of the chelate-complex (or “classical”) and the inorganic colloidal sol theories of podzolization (Sauer et al., 2007; Farmer et al., 1980; De Coninck, 1980). In contrast to SPC, we propose that DPC accumulates in the phreatic zone (Fig. 6c–g) following an altogether different podzolization process that is controlled by subsoil saturation thresholds (Fig. 6c, grey triangle). In addition to DPC’s depth (Fig. 6a) and Bolivar’s (2000) hydrologic observations (Section 1), our catena-based estimates of total Fe concentration (which either decrease with depth above DPC or remain consistently low in each soil profile; Figs. 4 and 5) support the notion that DPC accumulates in more permanently saturated soil reaches by positioning DPC below the point where stagnant soil water facilitates the reduction, mobilization, and loss of Fe. Our work also demonstrates that organic C translocation from the soil surface to DPC illuvial zones occurs without appreciable translocation of reactive metals (predominately Al) and clay-sized mineral particles (Figs. 5e–h and 6b); a translocation process that contrasts sharply to that of SPC and poorly represents the chelate-complex and inorganic colloidal sol theories of podzolization. Subsequently, as suggested by Harris (2000), we interpret DPC to be a manifestation of podzolization process presented by Farmer et al. (1983) whereby C is mobilized in the soil surface, transported to the phreatic zone by percolating water, and immobilized by in situ reactions with metals (predominately Al) in groundwater and/or on solid surfaces. A first order requirement of these in situ associations is that subsoil saturation be of sufficient duration for C to react with metals, and thus phreatic zone thresholds (Fig. 6c grey triangle) must be exceeded in order for DPC to accumulate. Freely drained soils (or freely drained portions of the soil profile) lack potential to accumulate DPC because subsoil saturation durations are insufficient to facilitate in situ C-metal associations. In more permanently saturated soils (or more permanently saturated portions of the soil profile) when subsoil saturation thresholds are met the potential to immobilize DPC emerges (Fig. 6d–g) and this phreatic zone phenomena will accumulate in response to C and metal activities in solution and on soil surfaces. Following our interpretation, DPC’s upper boundary delineates the shallowest soil depth where phreatic zone conditions are, or have been, conducive to DPC immobilization (Fig. 6c–g). Below this depth the potential to immobilize and accumulate organic C increases coincident with increased probability of saturation, thereby imparting extraordinarily thick portions of the deep soil profile with the hydrologic potential to accumulate DPC and promoting the downward darkening of DPC (Figs. 1d and f, 3,4, and 6; Tables 1 and 2). Along with such depth dependent saturation probabilities, periodic lowering of the vadose zone-phreatic zone boundary also promotes the downward darkening of DPC by exposing DPC’s upper reaches to aerobic conditions and C loss through microbial oxidation as has been observed for podzolized C following anthropogenic drainage improvements (Buurman et al., 2005; Buurman and Jongmans, 2005). At the same time, following our interpretation, the lateral continuity of DPC across coarsetextured, low-relief Coastal Plain landforms (Fig. 1a, b, and d; Tables 1 and 2; Gonzalez et al., 2018; Bolivar, 2000; Harris, 2000; Daniels et al., 1999; Watts, 1998; Gaston et al., 1990) reflects the fact that the phreatic zone similarly transgresses these landforms (Schoeneberger and Wysocki, 2005). Although podzolization has been documented to transport C laterally across certain landforms (Bourgault et al., 2015; Mattson and Lönnemark, 1939; Tamm, 1950), estimates of total C

3.5. Field based descriptions of podzolization Utilizing current conventions (Soil Survey Staff, 2012) to describe the entire regolith of Coastal Plain landforms is known to be challenging (Harris et al., 2005). Our work, which recognizes two morphologically and hydrologically distinct podzolized C pools that may or may not exist together in the same profile or landform, identifies a particular opportunity to improve field-based soil descriptions and the communication of pedogenic processes in the region. Current USDA conventions to describe podzolized C within a soil profile do well to capture the presence and relative positions of SPC and DPC when both are encountered by employing the prime (′) notation (Tables 1 and 2; profiles SC1-SC3, BC1, and BC3). The intended use of the prime is however clear (“to indicate the reoccurrence of identical horizon descriptor(s)”; Soil Survey Staff, 2012), and while it indeed distinguishes SPC and DPC when simultaneously encountered, its ability to communicate Coastal Plain Podzolization in a comprehensive and process based manner is limited. First, it is technically incorrect to identify DPC with a prime when overlying SPC is absent. Subsequently, following current conventions, DPC will be inconsistently described across a same landform (sometimes a B΄h, sometimes a Bh; Tables 1 and 2) despite the fact that it is a continuous subsoil stratigraphic unit (Fig. 1a and b; Gonzalez et al., 2018; Bolivar, 2000; Harris, 2000; Daniels et al., 1999; Watts, 1998; Gaston et al., 1990). Such inconsistencies are not only confusing, they also fundamentally misrepresent lateral continuity of soils across landforms. Additionally, the prime notation does little to communicate processes because it is not intended to do so. Rather, distinguishing SPC and DPC with A-E-Bh-E′(or Bw)-B′h sequence descriptions are more of a clerical activity that accounts for reoccurring master horizons rather than a pedological interpretation of podzolization. One way to overcome the limitations associated with distinguishing SPC and DPC with the prime notation is to employ additional horizon suffixes. We suggest that Coastal Plain Bh horizons be described with a “vv” horizon suffix when they are interpreted to be the vadose zone phenomenon SPC (i.e. they do not darken with depth, are a horizontally transient landform feature, and accumulate as part of a pronounced eluvial-illuvial system that may include lessivage), and that Coastal Plain Bh horizons be described with a “pp” horizon suffix when they are interpreted to be the phreatic zone phenomenon DPC (i.e. they darken with depth, are laterally continuous across landforms, and reside below minimally eluviated material). Our suggested horizon suffixes not only communicate the distinct morphology and hydropedologic underpinnings of SPC and DPC, they will also more accurately represent the stratigraphic continuity that exists across Coastal Plain landforms (Tables 1 and 2). 4. Conclusions SPC and DPC have previously been distinguished in Coastal Plain soils and landforms by their thickness and mass, lateral continuity, geomorphic distribution, and organic matter properties (Bolivar, 2000; Daniels et al., 1999; Gonzalez et al., 2018; Watts, 1998). Here, by showing that these podzolized C pools can also have morphologically distinct expressions (Figs. 3–6), we provide a means by which 11

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singularly observed Bh horizons can be identified as SPC or DPC during field-based assessments. SPC is expected to lighten downward (lower Munsell value at its top than its bottom) or neither lighten nor darken downward (same Munsell value at it top and bottom) and accumulate below low chroma material; a morphologic expression reflecting hydrologic thresholds in the vadose zone and a podzolization process involving the mobilization, transport of organic C, metals (predominately Al), and clay-sized particles. In contrast, DPC is expected to darken downward (higher Munsell value at its top than its bottom) and accumulate below higher chroma material; a morphologic expression reflecting hydrologic thresholds in the phreatic zone and a podzolization process involving the mobilization and transport of organic C with few metals and clay-sized particles. Recognizing SPC and DPC as morphologically and hydrologically distinct phenomena not only provides the opportunity to better interpret and communicate podzolization processes on the Coastal Plain, it also provides insights regarding interactions between terrestrial water and C cycles in low-relief landforms.

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Acknowledgements This work would not be possible without the thousands of individuals within the USDA-NRCS who conduct field, laboratory, and administrative work required to create, disseminate, and manage regional soil survey efforts. We also thank Alex Finkral and Andy Norris from The Forestland Group and Matthew Simpson from Natural Resource Planning Services for access to the Baker Co. research site; Charles Barrett from the Suwannee Valley Agricultural Extension Center for access to the Suwannee Co. research site; Willie Harris and James Jawitz for their thoughts related to hydrology and podzolization; and Julip Pachon for reviewing our work. This work was supported by the University of Florida Soil and Water Sciences Department. Declarations of Competing Interest None Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.geoderma.2019.114007. References Banik, C., Harris, W.G., Ellis, R., Hurt, W., Balboa, S., 2014. Relations of iron, aluminum, and carbon along transitions from Udults to Aquods. Geoderma 226–227, 332–339. https://doi.org/10.1016/j.geoderma.2014.03.016. Bernal, B., McKinley, D.C., Hungate, B.A., White, P.M., Mozdzer, T.J., Megonigal, J.P., 2016. Limits to soil carbon stability; Deep, ancient soil carbon decomposition stimulated by new labile organic inputs. Soil Biol. Biochem. 98, 85–94. https://doi.org/ 10.1016/j.soilbio.2016.04.007. Bliss, C.M., Comerford, N.B., 2002. Forest harvesting influence on water table dynamics in a florida flatwoods landscape. Soil Sci. Soc. Am. J. 66, 1344–1349. https://doi.org/ 10.2136/sssaj2002.1344. Bolivar, J.G.P., 2000. The Genesis Of Carbon Sequestration In Subtropical Spodosols. University of Florida, Gainesville, Fl. http://ufdcimages.uflib.ufl.edu. Bourgault, R.R., Ross, D.S., Bailey, S.W., 2015. Chemical and morphological distinctions between vertical and lateral podzolization at hubbard brook. Soil Sci. Soc. Am. J. 79, 428–439. https://doi.org/10.2136/sssaj2014.05.0190. Buurman, P., Jongmans, A.G., 2005. Podzolisation and soil organic matter dynamics. Geoderma 125, 71–83. https://doi.org/10.1016/j.geoderma.2004.07.006. Buurman, P., Bergen, P.F.V., Jongmans, A.G., Meijer, E.L., Duran, B., Lagen, B.V., 2005. Spatial and temporal variation in podzol organic matter studied by pyrolysis-gas chromatography/mass spectrometry and micromorphology. Eur. J. Soil Sci. 56, 253–270. https://doi.org/10.1111/j.1365-2389.2004.00662.x. Cleland, T.M., 2010. A Practical Description of the Munsell Color System and Suggestions for Its Use 1937. Kessinger Publishing, LLC. Cremeens, D.L., Mokma, D.L., 1986. Argillic horizon expression and classification in the soils of two michigan hydrosequences 1. Soil Sci. Soc. Am. J. 50, 1002–1007. https:// doi.org/10.2136/sssaj1986.03615995005000040034x. Daniels, R.B., Gamble, E.E., Holzhey, C.S., 1975. Thick Bh horizons in the North Carolina Coastal Plain: I. Morphology and relation to texture and soil ground water 1. Soil Sci. Soc. Am. J. 39, 1177–1181. https://doi.org/10.2136/sssaj1975.

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