Carbon Storage in Florida SpodosolsE. L. Stone,* W. G. Harris, R. B. Brown, and R. J. Kuehl

ABSTRACTSpodosols are widely distributed in Florida and occupy about 27%

of the state's land area. Our objective was to estimate total organiccarbon (OC) in the 3.66 x 106 ha of Spodosols in Florida, and topartition that OC by horizon (Bh vs. other components) and by depth(above and below 1 m). Data employed were: (i) areas of the 61 Spo-dosol series mapped in the 48 (of 67) Florida counties having soilsurvey reports published since 1960, and (ii) profile descriptions andsoil characterization data for horizons from 244 Spodosol pedons rep-resenting 55 series. Individual profile data were averaged by threedifferent procedures: (i) gross averages of all profiles and for Humodsand Aquods separately, (ii) averages of suborder means weighted byrespective total areas of each, and (Hi) averages of series means weightedby areas of series-named map units in the 48 mapped counties. Or-ganic C concentrations (mean ± SD) in A, E, Bh, and B'h horizonswere 19 ± 20, 1.9 ± 2.1, 17 ± 12, and 8.5 ± 5.0 g kg-'. Gross OCdensities (mean ± SE) of profiles to 1 m and the complete sampledprofiles were 10.4 ± 0.8 and 18.3 ± 0.8 kg m-2. About one-half ofthe latter was in the combined Bh + B'h horizons. Weighting bysuborder and series areas increased these numbers slightly. Aquodshad significantly higher OC densities than Humods in all depths. Es-timated series-weighted total OC in Florida Spodosols is 809 Tg, ofwhich 431 Tg resides in Bh + B'h horizons.

CONCERN WITH RISING LEVELS of atmospheric CO2has directed attention to the stores of OC in soils,

and their roles as sources or sinks (Post et al., 1982;Schlesinger, 1984, 1985, 1990; U.S. EnvironmentalProtection Agency, 1991; Jenkinson et al., 1991). Thedifficulties in estimating the global pool with any ac-curacy are manifest. Most attempts have entailed com-piling analytical data from the literature or other sourcesto arrive at average values for large-order ecosystems,life zones, or other geographic units that can be ag-gregated to a global total. Schlesinger's reviews (1984,1985) indicate the relatively small numbers of profilesor area means represented, as well as the implied as-sumptions about OC distribution.

A major source of information about soil C in theUSA is the data from profiles sampled by, or in con-junction with the National Cooperative Soil Survey.Apparently, this is only now being exploited (Gal-braith et al., 1991; Kern, 1991). In Florida, the SoilCharacterization Program of the Soil Science Depart-ment of the University of Florida, collaborating withthe USDA-SCS Soil Survey, has described and ana-lyzed more than 1200 pedons representing 285 soilseries. The results have been published in seven de-partmental reports from 1974 to 1990. Only the mostrecent of these (Sodek et al., 1990) is cited here as areference for the sampling and analytical procedures.

Spodosols are widely distributed in Florida and oc-cupy approximately 27% of the land area. Of the fivemajor orders in the state, Spodosols are among the

Soil Science Dep. Univ. of Florida, Gainesville, FL 32611-0290.Florida Agric. Exp. Stn. Journal Series no. R-02384. Received 7May 1992. *Corresponding author.

Published in Soil Sci. Soc. Am. J. 57:179-182 (1993).

least affected by agricultural use and also provide themost conspicuous demonstration of OC distributiondeep within the soil.

Accordingly, our objective was to estimate total OCin the 3.66 x 106 ha of Spodosols in Florida, usingexisting characterization data and the mapped distri-butions of Spodosols. The results also illustrate oc-currence of OC below the 1.0-m depth commonly usedwhen sampling soils for such purposes.

MATERIALS AND METHODSTwo sources provided the primary data for this study: (i)

area summaries of the 61 Spodosol series mapped in the 48counties that published soil surveys since 1960, two-thirds ofwhich were published after 1980; and (ii) descriptions of 244pedons, representing 55 Spodosol series, including determi-nations of Db and OC for 1860 horizon samples.

Profile locations were usually selected by soil survey per-sonnel in the course of ongoing surveys and reviews. Presentvegetation at these locations included cultivated crops, pas-tures, natural forests, and pine plantations. Twenty-three per-cent of the profiles had Ap horizons that indicated present orpast tillage. An additional number had, at some time, beencleared for pasture without tillage, although some have sincereverted to forest. O horizons were present in all unburnedprofiles under forest but were not sampled.

Nominal sampling depth was 2 m; the actual mean depthwas 2.01 ± 0.22 m (SD) with a range from 0.84 to 3.04 m.Depths < 2.0 m occurred when backhoe pits encountered bed-rock, unweathered clays, or unmanageable water tables. Depths>2.0 m resulted from occasional sampling of horizons thatcontinued below that depth or originated near it. Only 7% ofthe sampled profiles extended below 2.20 m.

Coarse fragments were essentially lacking in all profiles.Bulk density was determined from core samples from all ho-rizons (Sodek et al., 1990). A few missing values were re-placed by the means of all other horizons having the samedesignations. Organic C was determined by a slight modifi-cation'of the Walkley-Black procedure (Method no. 6Alc,Soil Survey Staff, 1984). Analytical values <0.1 g kg-1 weretreated as zero.

The mass of OC in each horizon was calculated from itsthickness, Db, and OC concentration. These values were summedin three ways: (i) for the entire profile sampled; (ii) for onlythe combined Bh and B'h (referred to as Bh) horizons or por-tion thereof sampled in each profile (excluding transitional sub-horizons); and (Hi) for the portions of the profile above andbelow 1.00 m. For the latter, OC in the horizon occurring at1.00 m was divided in proportion to depth above and below1.00 m.

Individual profile data were averaged by three different pro-cedures for comparison: (i) gross averages of the 244 profilesand for the two suborders (Humods and Aquods) separately;(ii) averages of the suborder means weighted in proportion totheir respective total areas; and (iii) series-weighted averages,calculated by weighting mean OC mass for each series by thetotal area of its series-named mapping units in the 48 mappedcounties.

The third procedure entailed several assumptions, notablythat the profile means did, in fact, describe the unknown meansof their respective cartographic units. The areas of complexesAbbreviations: ANOVA, analysis of variance; Db bulk density;OC, organic carbon; SD, standard deviation; SE, standard errorof the mean.

179

180 SOIL SCI. SOC. AM. J., VOL. 57, JANUARY-FEBRUARY 1993

or undifferentiated units were divided according to the per-centages indicated in the mapping unit descriptions. Six series,for which the mapping units totalled only 0.5% of the entireSpodosol area, were not represented by characterization pro-files. Default values for these were constructed through com-parison of their horizonation, texture, depth, and color withthose of related series. Twenty-five series were represented byonly one or two profiles each, and the remaining 30 series bythree to 14 profiles each. Map units for four widespread series(Immokalee, Myakka, Pomona, and Smyrna), each with 10 to13 sampled profiles, together made up 48% of the total Spo-dosol area. Coefficients of variation for the four series meansvaried from 42 to 84%.

Total area of the 48 mapped counties amounted to 71.1%of the entire state land area. This percentage was used to ex-pand the summed mapped areas of Aquods, Humods, and allSpodosols, with their respective total OC contents, to approx-imations for the entire state.

Values of the variously aggregated profile data for the twosuborders and for all Spodosols were subjected to one-wayANOVAs. The frequency distributions of all populations werestrongly skewed, with a few very high values, as also notedby Post et al. (1982). Accordingly, the nonparametric Wilcox-on's rank sum test was used to determine significance of dif-ferences in the various comparisons made. All statistical analysesused SAS Institute (1985) procedures.

RESULTSThe 244 sampled profiles were widely distributed among

the counties where Spodosols occur (Fig. 1). Mean OCconcentrations for selected horizons are given in Table1. The mean concentration of Ap horizons scarcely dif-fered from that for undifferentiated A horizons, but bothwere lower than that for horizons designated as Al (Ta-ble 1). Probably all of the latter were under establishedforest. Means of A and Bh horizons were generally sim-ilar, whereas E horizons were an order of magnitudelower. B'h horizons were fewer but their mean concen-trations ranged from about one-half to the same as theBh.

Table 1. Organic C concentration (meanof selected horizons.

± standard deviation)

Horizon

AAlApEElE2BhBhlBh2Bh3B'hB'hlB'h2

Samples

no.1355359

12013613769

17217137211515

Organic C

gkg-1

19 ± 2027 ± 3117 ± 141.9 ± 2.13.3 ± 2.51.5 ± 1.4

17 ± 1217 ± 1214 ± 911 ± 88.5 ± 5.06.8 ± 5.0

12 ± 12

Depth to the upper surface of the Bh horizon (Fig. 2)exceeded 1.0 m in about one-fourth of the population.By inference, this depth also indicates thickness of Ehorizons beneath 10- to 30-cm-thick A horizons.

Forty-three percent of the 244 profiles had argillic ho-rizons within the depth of sampling. Such a horizon usu-ally precludes formation of distinct B'h horizons. B'hhorizons were present at or above 2.0 m in 13% of thetotal population (Fig. 2), however, and observation hasindicated numerous additional occurrences below sam-pling depth.

In all comparisons (Table 2), about 50% of the sam-

Curnulative20 40 60 80 100

50 -

.C-4-1

CL

Q

100 -

150 -

Fig. 1. Distribution of the 244 sampled pedons.

200 -

Fig. 2. Mean depth to the upper surface of the Bh and B'hhorizons in Florida Spodosols.

STONE ET AL.: CARBON STORAGE IN SPODOSOLS 181

Table 2. Mean (± standard error) organic C density estimatedby three procedures.t

Table 3. Estimated total organic C in Florida Spodosols (largelyabove 2-m depth).

Organic C densityAnalyticalprocedure

Bh + B'hSamples horizons

Profilesto 1.0 m

Sampledprofiles

nn ——— - ————— ko m~ 2 ———————————Unweighted

All SpodosolsAquodsHumods

24417371

Suborder-area weightedAll Spodosols 244

Series-area weightedAll Spodosols 244

9.2 ± 0.6 10.4 ± 0.8 18.3 ± 0.810.5 ± 0.7 12.6 ± 0.8 21.1 ± 1.06.1 ± 1.0 4.9 ± 0.3 11.6 ± 1.0

10.0 ±0.7 11.7 ± 0.6 20.0 ± 0.9

11.8 — 22.1t All comparisons between suborders and between depths above and

below are significant (P > 0.0001) by Wilcoxon's rank sum test.

pied-profile OC was contained in the Bh (Bh plus B'h)horizons. Humods averaged only about one-half as muchOC, for both the Bh horizons and sampled profiles, asAquods (Table 2). These differences are due in part toa greater mean OC concentration in all horizons desig-nated as Bh or B'h in Aquods, 15.4 ± 0.6 (SE) g kg-1,compared with Humods, 11.4 ± 0.8 g kg-1 (n - 396and 125, respectively). Better drainage of the solum pre-sumably accounts for lower OC concentration in Hu-mods but some proportion of the latter will have B'hhorizons well below the sampled depth.

The two suborders also differ in distribution of sam-pled-profile OC. In Humods, 50 ± 24% (SD) of the OCis below 1.00 m vs. 36 ± 22% in Aquods. Overall, only60% of the total is contained in the upper 1.0 m. (Thesevalues were calculated from individual profile data, notfrom the means of Table 2.)

Although Humods constituted 29% of the sampledprofiles they occupied only 11% of the area mapped asSpodosols. Such overrepresentation obviously woulddistort interpretation of an unweighted mean (Table 2)for all profiles. Accordingly, means for the two subor-ders weighted by their respective proportions of the totalSpodosol area yield a more realistic mean (Table 2). Thethird type of estimate, series-weighted by map unit area,yielded means for Bh only and sampled profiles thatwere, respectively, about 17 and 10% higher than thoseby suborder weighting (Table 2). The foregoing meanswere then converted to estimates of the total OC in allFlorida Spodosols (3.66 x 106 ha) for a mean samplingdepth of approximately 2.0 m. (Table 3).

DISCUSSIONThe OC values reported are based on the conventional

Walkley-Black conversion factor of 1.3 (indicating arecovery of 77% C). This factor was indeed found to beapproximately correct for 10 Florida soils in an earlycomparison with the standard dry combustion method(Dyal and Drosdoff, 1941). Oxidation of OC by chromicacid appeared to be complete for a single spodic horizonsample, however, indicating a factor of 1.0. Should thisfactor prove generally applicable to Coastal Plain spodichorizons, the indicated OC contents of Bh and B'h ho-rizons (Tables 1 and 2) would have to be reduced by23%. Likewise, values for sampled-profile OC would beapproximately 12% lower than reported in Tables 2 and3.

Organic CData treatment Bh + B'h Sampled profiles

UnweightedSuborder-weighted; totalSeries-weighted

338367431

— Tg ————————670752809

In principle, weighting mean C density for individualseries by their respective areas of occurrence should bethe most appropriate way of calculating area-wide totalsand means (Tables 2 and 3), but there are caveats. Thenumber of pedons sampled for many series was smalland not proportional to their mapping unit areas. Actualcomposition of the series-named cartographic units andthe degree to which their mean OC contents corre-sponded with those of the respective series profile meansis unknown. The latter caveat also applies to the subor-der-weighted means, but to a smaller extent. The subor-der-weighted means are obviously more appropriate thanthe unweighted, yet also allow calculation of standarderrors. Accordingly, we restrict further discussion to onlythese latter means.

None of the values, however, include allowance forO horizons. Long-unburned pine forest floors commonlycontain 1.0 to 2.0 kg OC m~2 (Heyward and Barnette,1936; Lee et al., 1983), although higher values are known.Furthermore, artificial drainage, agricultural use, and, atleast temporarily, intensive forestry practices (Gholz andFisher, 1987) would have reduced mean OC below itsvirgin state by some unknown degree.

Division of whole-profile OC estimates at the 1.00-mlevel demonstrates the large underestimate that wouldhave resulted from sampling only to this depth. In 25%of the profiles, a 1.0-m-deep sample would not havereached the upper boundary of the Bh horizon (Fig. 2)and, in 55% of the profiles, it would not have extendedto the lower Bh horizon boundary. Only occasionallywould the upper boundary of a B'h horizon have beendetected. Yet, in both Aquods and Humods, about one-half of the average total profile OC was in the combinedBh and B'h horizons.

The proportion of OC in the Bh horizon, as well asthe profile total, would have been greater if those profileslacking argillic horizons had been sampled to even greaterdepths. Many observations indicate that descriptions ofdeep Bh horizons in the North Carolina Coastal Plain(Daniels et al., 1975) apply also to many sandy soils inFlorida, including some now classified as Quartzipsam-ments because the spodic horizon begins below the 2.0-m depth. Morphological and chemical evidence (Yuan,1966), as well as a paucity of microfossils where ex-amined, indicate that deep spodic horizons have formedin place. Their age and relationships to past or presentvegetation and groundwater levels, however, are mostlyunknown, although Amick and Gelinas (1991) have em-phasized that, in the South Carolina Coastal Plain, watertables fluctuated throughout a range of about 1 to 4 mbelow present levels 2000 to 5000 yr ago.

Favorable circumstances for OC accumulation, com-bined with the nominal 2.0-m sampling depth, have re-sulted in a mean C density (approximately 20 kg m-2)

182 SOIL SCI. SOC. AM. J., VOL. 57, JANUARY-FEBRUARY 1993

that exceeds the estimates of Post et al. (1982) for 18 ofthe 22 life zones of the world. Only their estimates forwet and rain tundra, boreal rain forest, and tropical rainforest (22.2, 36.6, 25.6, and 21.0 kg m-2, respectively)are greater. Mean OC density in only the surface 1.00m of Florida Spodosols (11.7 kg m~2) (Table 2) alsoexceeds or is only slightly less than the estimates for allother of the above life zones.

Similarly, the sampled-profile Spodosol mean exceedsor approximately equals all of the ecosystem means inSchlesinger's (1984) compilation, excepting marsh andswamp (72.3 kg m~2) and montane tropical forest (28.7kg m-2). Some of the literature values drawn on bySchlesinger represent depths >1.0 m; the boreal forestmean also included some forested peats. Thus, apart fromthe above-noted exceptions, mean C density of the Flor-ida Spodosols is appreciably greater than estimated forother temperate and tropical forests, despite exclusion ofthe O horizon and unknown reductions in the Floridavalues due to previous land drainage and tillage.

If one accepts 1500 Pg as a plausible current estimateof the global soil OC pool on an area of 1.31 x 1014

m2 (Post et al., 1982) or 1.47 x 1014 m2 (Schlesinger,1984), then, by our estimates, Florida Spodosols alonewould contain about 0.05% of the global pool, on only0.028 or 0.025%, respectively, of the total area.

Disregarding Histosols, many of which have not beenstudied in detail (Schlesinger, 1985), present global es-timates of total OC in mineral soils probably are too low,if only because of limited sampling depth. Not only ac-cumulation of dissolved OC in spodic horizons (Danielset al., 1975; Holzhey et al.,. 1975), but burial by or inalluvium (Daniels et al., 1963) and other deposits,weathering of OC-containing sediments (Sainju and Kal-isz, 1990), and addition by deep roots (Weaver, 1926;Stone and Kalisz, 1991), all result in OC at depths > 1m over large areas.

ACKNOWLEDGMENTSWe thank V.W. Carlisle, personnel of the University of

Florida Soil Characterization Laboratory, and participating soilscientists of the USDA-SCS, who generated the data used here.