(1997) radiocarbon dating for determination of soil … · · 2016-01-02radiocarbon dating for...

Radiocarbon Dating for Determination of Soil Organic Matter Pool Sizes and DynamicsE. A. Paul,* R. F. Follett, S. W. Leavitt, A. Halvorson, G. A. Peterson, and D. J. Lyon

ABSTRACTThe size and turnover rate of the resistant soil organic matter

(SOM) fractions were measured by 14C dating and 13cy2C measure-ments. This involved soils archived in 1948, and recent samples, froma series of long-term sites in the North American Great Plains. Arevaluation of C dates obtained in the 1960s expanded the studyscope. The 14C ages of surface soils were modern in some native sitesand near modern in the low, moist areas of the landscape. They weremuch older at the catena summits. The "C ages were not related tolatitude although this strongly influenced the total SOM content.Cultivation resulted in lower C contents and increased the 14C age byan average of 900 yr. The 10- to 20-cm depths from both cultivatedand native sites were 1200 yr older than the O- to 10-cm depth. The90- to 120-cm depth of a cultivated site at 7015 yr before present (BP)was 6000 yr older than the surface. The nonhydrolyzable C of thisdepth dated 9035 yr BP. The residue of 6 M HC1 hydrolysis comprised23 to 70% of the total soil C and was, on the average, 1500 yr older.The percentage of nonhydrolyzable C and its 14C age analyticallyidentify the amount and turnover rate of the old resistant soil C.

SOIL ORGANIC MATTER is the primary storehouse of soilC and the source of available N and to some extent

P, S, and minor elements of agricultural, forest, andgrassland soils (Townsend et al., 1996). It reflects thevegetation, soil biota, climate, parent material, time tosteady state, and natural and human disturbances. ThusSOM acts as a signature, as well as a controller, ofecosystem functioning (Paul, 1984). It is one of the pri-mary factors that affect soil quality, water infiltration,erosion resistance, tilth, and adsorption and degradationof pollutants (Doran et al., 1994). Microbial decomposi-tion of SOM at times when there is no concurrent uptakeof nutrients by plants can lead to extensive losses ofnutrients, and pollution of both groundwater and theatmosphere. The dynamics of SOM must be known forits proper management.

The recent interest in SOM as a source-sink in globalC budgets results from the assessment that 25% of theradiative climate forcing in global change is attributableto agriculture (Tans et al., 1990; Schimel, 1995). Further,SOM is a major component of the terrestrial sink neces-sary to balance present global CO2 budgets (Lugo, 1992;Lal et al., 1995). The concept that SOM contains frac-tions with varying decomposition rates predates theavailability of tracers. In the 1930s, German workerstalked of Nahr or nutritive humus. The first tracer uti-lized involved 13C, followed by 15N and 14C (Colemanand Fry, 1991; Knowles and Blackburn, 1993). Jansson

E. A. Paul, Crop and Soil Sciences, Michigan State Univ., East Lansing,MI; R.F. Follett, USDA-ARS, Ft. Collins, CO; S.W. Leavitt, Lab. ofTree Ring Research, Univ. of Arizona, Tucson, AZ; A. Halvorson,USDA-ARS, Mandan, ND; G.A. Peterson, Dep. of Soil and CropSciences, Colorado State Univ., Ft. Collins, CO; and D.J. Lyon, Pan-handle Research and Extension Center, Univ. of Nebraska, Scotts-bluff, NE. Received 15 Dec. 1995. * Corresponding author ([email protected]).

Published in Soil Sci. Soc. Am. J. 61:1058-1067 (1997).

(1958), working with 15N added to soils, conceptualizedan active fraction and estimated it to have a turnovertime of 15 to 20 yr.

Bombardment of atmospheric N2 with cosmic raysproduces 14C with a half-life of 5568 yr. This and the14C artificially produced by thermonuclear (H) bombtesting, started in the 1950s, is oxidized to 14CO2 andassimilated by plants. Plant residues are transformedinto SOM with little isotopic discrimination. As 14C-dating techniques that measure these very low levels of14C became available, they were used to date buried soilhorizons in archaeological work and occasional surfacesoils. The 14C-dating nomenclature assigns samples with14C contents equivalent to 95% of a 1950 oxalic acidstandard as modern. Counts are expressed as percentagemodern C (pmC). Higher counts can come from bomb14C or by contamination from other 14C sources (Hark-ness et al.,1991). Ages are expressed as BP.

The first 14C age determined for a North Americangrassland soil was that of Simonson (1959). He reporteda soil from North Dakota to have an age of 1300 yr.Other early 14C ages included Swedish and Americanpodzols at 50 to 370 yr BP (Broecker and Olson, 1959;Tamm and Ostlund, 1960). Broecker and Olson (1960)suggested that 14C dating, especially bomb 14C, shouldbe useful for the investigation of soil characteristics.Paul et al. (1964) demonstrated that 14C dating togetherwith organic-C fractionation could be used to determinethe dynamics of SOM. Their work, with Canadian grass-land soils, showed that the mean residence time (MRT)of soils was dependent on soil type, landscape position,soil depth, length of cultivation, vegetation, and soilparticulates (Campbell et al., 1967a; Martel and Paul,1974a; Anderson and Paul, 1984).

The SOM of a 1889 Rothamsted, England, sample,dated in 1970, was 1000 yr BP in age (Jenkinson andRayner, 1977). This was much greater than would beexpected from calculations based on rates of SOM accu-mulation and plant residue inputs. Jenkinson andRayner (1977) combined 14C dating with studies involv-ing added 14C residues and knowledge of plant residueinputs in long-term experiments to develop a realisticmodel of C turnover. Subsequent models (van Veenand Paul, 1981; Parton et al., 1987; Paustian et al., 1992)utilized the available 14C dates to characterize SOMdynamics. Three SOM pools, decaying according tofirst-order reaction kinetics, have been shown to bestdescribe C dynamics (Parton et al., 1993; Paul et al.,1995). Interpretations have also been made using twopools (Hsieh, 1993; Harrison et al., 1993).

Campbell et al. (1967b) used 14C dating to measurethe 14C age of the fulvic, humic, and humin fractions ofSOM. These were interpreted relative to turnover times

Abbreviations: SOM, soil organic matter; BP, before present; MRT,mean residence time; TAMS, tandem accelerator mass spectrometer;FACE, free atmosphere carbon enrichment experiment; IRMS, iso-tope ratio mass spectrometer.

1058

Published July, 1997

PAUL ET AL.: SOIL ORGANIC MATTER DETERMINATION BY RADIOCARBON DATING 1059

by expressing the 14C ages as MRT. Humic acids werethe oldest materials, especially when humics associatedwith the metal ions in soil were isolated. Fulvics weremuch younger and humins were intermediate in age.Martel and Paul (1974a) dated nine fractions of theSOM of a grassland (Sceptre Association, Aridic Bor-oll). The light fraction obtained by flotation in a heavyliquid contained much of the bomb 14C; its removalchanged the soil 14C age from 350 to 500 yr BP. Hydroly-sis with 6 M HQ produced a nonhydrolyzable residuethat dated 1760 yr BP. Further extractions of this residuewith NaOH resulted in only small increases in the 14Cage of the remaining residue to 1965 yr.

Cultivation increased the 14C age of a northern grass-land soil (Oxbow Association, Udic Boroll) from 250to 295 yr BP after 15 yr of cultivation. The age increasedto 710 yr BP after 60 yr of cultivation. Hydrolysis with6 M HQ was used to fractionate the organic soil compo-nents. One would expect that, as the soil lost its C duringcultivation, the percentage of nonhydrolyzable C wouldincrease. However, the nonhydrolyzable C from boththe native and the cultivated soil represented approxi-mately one-half of the soil C (Martel and Paul, 1974b).A Udic Haploboroll soil (Indian Head), had a 14C ageof 1910 yr in 1963 and 1515 yr in 1978. This showed theeffect of bomb 14C incorporation (Anderson and Paul,1984). The residue of hydrolysis of the 1963 sampledated 2820 yr and that of the 1978 sample dated 2095yr. Humic acids were the next oldest, dating 2455 yr inthe 1963 and 1720 yr in the 1978 samples. The SOM ofthe coarse clay fraction of another Mollisol at 1255 yrwas shown to be the oldest of the particulate fractionsisolated from that soil. This was not as old as the humicacids (1425 yr). The Canadian soils were developed dur-ing the 12000 to 14000 yr since their glaciation. The14C ages of the pedogenically much older soils found inunglaciated regions of North America are not known.

Trumbore (1993) and Trumbore et al. (1996) con-firmed the observations of Anderson and Paul (1984)that acid hydrolysis was effective in differentiating oldC of temperate soils. They did not find this to be truefor tropical sites. Acid hydrolysis does not solubilize thelignin, phenols, and some cellulose of plant residues.Hydrolysis left 42% of the C of wheat (Triticum aesti-vum L.) straw and 34% of the C of maize (Zea maysL.) residues undissolved (Follett et al., 1997). It also hasdifferential effects on soil fulvics and humics (Schnitzerand Khan, 1972). Although most of the plant residuecan be removed by density flotation and microscopicpicking prior to hydrolysis, experiments with 14C-depleted CO2 in the Arizona Free-Air CO2 Enrichment(FACE) study showed incorporation of plant-residue Cinto the acid-resistant fraction after 2 yr exposure to14C-depleted CO2 (Leavitt et al., 1994).

The pool sizes and fluxes of SOM are best interpretedrelative to ecosystem dynamics by using soils from well-characterized long-term sites with a known history ofmanagement and known residue inputs (Paul et al.,1995). Such sites are available in a range of grasslandsoils from the Canadian Prairies south to Texas (Fig.1). Haas et al. (1957) characterized the C and N levels,

plant yields, and climate of many of these sites. Theyarchived samples of both native and cultivated soil. Paulet al. (1997) have provided more recent documentationfor many of these sites. The availability of tandem accel-erator mass spectrometers (TAMS) that analyze milli-gram-sized samples makes it possible to utilize archivedsamples by 14C dating. Present samples from those sites,under the same management, make the archived sam-ples especially valuable. This is the first of a series ofrelated studies that use acid hydrolysis, 14C dating, 13C/12C ratios, and extended incubation of soils from long-term plots to obtain analytically derived estimates ofthe pools and fluxes of soil organic C. Here we reportresults on 14C ages for a range of sites in the NorthAmerican grasslands that include samples to depth.Acid hydrolysis was used to determine both the sizeand 14C age of the old resistant fraction.

METHODSArchived soil samples, collected in 1947-1949 from Akron,

CO; Mandan, ND; Hayes, KS; and Big Springs and Dallas,TX (Haas et al., 1957) were obtained from the Northern GreatPlains Research Center in Mandan, ND. Field collection ofsoils from Akron, CO (Halvorson et al, 1997) and Sidney,NE (Peterson et al., 1994) research sites (native grassland vs.long-term winter wheat-fallow rotation) were accomplishedby use of a hydraulic corer using 3.5- and 3.8-cm-diam. tubes,respectively. Each sample was a composite of three soil coresper replicate for each treatment. Soil cores were collectedalong the length of each plot. In addition, soil samples froma replicated, uncultivated sod-plot treatment, with a grass-species shift to predominantly cool-season species, were ob-tained at Sidney, NE. The Lethbridge site represented samplescollected by Campbell et al. (1967a,b).

Composited soil samples from the Sterling, CO, site (twocores from each of two replications of the cultivated wheat-fallow system) were collected from summit, sideslope, andtoeslope positions using a hand probe. The 300-m-long cate-nary sequence at the Sterling site descends 5.5 m from thesummit position to the toeslope. The steepest part of thesequence occurs in the sideslope position. Water runs off bothsummit and sideslope positions when rainfall intensities ex-ceed soil infiltration rates. The toeslope catches most of thiswater and is more productive than either the summit or sides-lope positions. Soil samples from the Maricopa, AZ, enrichedCO2 site were obtained with two soil cores from each of tworeplicates of the ambient treatment of the cotton (Gossypiumhirsutm L.) field associated with the FACE experiment (Lea-vitt et al., 1994).

Soil locations are shown in Fig. 1; selected soil characteris-tics are given in Table 1. Plant-free soil and the nonhydrolyz-able C after hydrolysis with hot, 6 M HC1 were analyzedfor C content, 13C/12C, and 14C age. Soil samples were passedthrough a 2-mm sieve to remove large plant fragments andpebbles. A 5- to 20-g subsample of the soil was acidified with1 M HC1 to remove carbonates. After decanting the HC1, thesubsamples were immersed in a 1.2 g cm~3 NaCl solution andstirred. The heavier solids were allowed to settle and floatingplant fragments were skimmed off. This process was repeateduntil no more material floated to the surface. Subsamples wererinsed free of salt, dried, and pulverized with a mortar andpestle. Next, 1 to 5 g of the processed subsamples were pickedfree of recognizable plant and root fragments under 20 X mag-nification. Organic-C content and 13C were determined after

1060 SOIL SCI. SOC. AM. J., VOL. 61, JULY-AUGUST 1997

DUE

Fig. 1. The location of sampling sites in the North American grasslands.

combustion of 0.10 to 0.25 g of processed-soil subsamples inquartz tubes at 900°C in the presence of copper oxide, silverfoil, and copper turnings (Coleman and Fry, 1991). Carbonyield was determined manometrically from the CO2 combus-tion product, and CO2 was analyzed mass spectrometricallyto determine 8"C. Isotope ratio mass spectrometer (IRMS)precision is about 0.05%o for samples of the same gas. Sevenseparate combustions and analyses of one soil produced astandard deviation of 0.25%o in the 813C content and a precisionin C content of 0.03%.

Acid hydrolysis was performed using 1 to 3 g of soil refluxedin hot, 6 M HC1 for 18 h. The soluble materials were separatedby filtration followed by repeated evaporation to remove theHQ. The residue of hydrolysis was rinsed with deionized waterand dried. Soil and nonhydrolyzable fractions were combustedas described above. Soil C age, based on I4C activity, wasdetermined on a TAMS at the University of Arizona Accelera-tor Facility after reducing the CO2 to CO on hot zinc andsubsequently to graphite on hot iron. The accelerator measure-ments of 14C activity are expressed as percentage modern C(pmC), employing a 8I3C correction for isotope fractionation(Goh, 1991). The 14C content was converted into age (yearsBP) with A.D. 1950 being considered 100 pmC and O yr BP.

Organic-C content measurements were made using flota-tion procedures not normally used when determining totalorganic C in soil. Because of the need for comparison oforganic-C content measurements reported by Haas et al.

(1957) to our measurements for both archived and recentlycollected soil samples, we also utilized the following organic-C determination procedure for all samples. Total organic Cand S13C were determined on air-dried subsamples of the soilsby an automated CHN analyzer interfaced with an IRMS. Soilcarbonates were removed by addition of 100 mL of 0.03 MH3PO4 to 5 g soil and shaking for 1 h. The procedure wasrepeated until the pH of the soil solution remained within 0.2pH unit of that of the original acid solution. Regression ofthe manometer and CHN analyzer procedures for determina-tion of total organic C resulted in a correlation (R2) of 0.98.The slope of the linear regression line and its intercept werenot significantly different (P < 0.01) from 1.0 and 0.0, respec-tively. The reported organic-C contents are the averages forthe two procedures when data from both were available.

RESULTSThe loess soil site at Akron, CO, (Aridic Paleustoll)

has crop and soil management treatments initiated in1908. Comparison of the total C of the 1947 samples,as determined by Haas et al. (1957) and in two of ourlaboratories, shows good agreement (Table 2). Haas etal. (1957) reported that cultivation for 50 yr produceda 46% decrease in soil C in the O- to 15-cm depth andan 18% decrease in the 15- to 30-cm layer. The 1992


native site was very similar in soil C to the 1947 nativesample. The adjacent cultivated site after 90 yr of culti-vation had 35% less total organic C in the O- to 10-cmdepth and 21% less in both the 10- to 20- and 20- to30-cm depths.

There was a general decrease in the percentage ofnonhydrolyzable C with depth in the native Akron site.The deeper sampling of the cultivated site showed adecrease followed by an increase in the deepest layer.Sand content did not change with depth, but silt in-creased as the clay content decreased (Table 2). The14C age of the 1947 native sample was very similar to the1992 sample, especially when one considers the greaterdepth of the 1947 sample (Table 3). There is a problemin the interpretation of the 1947 cultivated, stored sam-ple in that the deeper horizon is younger in its 14C agethan the surface sample.

Analyses of 14C by TAMS are restricted by high costsand limited accelerator accessibility. The 1947 and 1992native samples were analyzed in duplicate for surfacehorizons and singly for subsurface soils. Errors shownrepresent field variability where analyses were obtainedon more than a single composite sample. The 14C age(Table 3) of the total organic C of the native sites wasmodern in 1947 and 193 yr in 1992. Acid hydrolysisconfirmed that the 1947 sample, whose total C was 107pmC, was contaminated. The pmC of 153 calculated forthe acid hydrolyzable C would not be found in bomb-affected plant residues until 1970. The source of thiscontamination is not known. We had problems withcontamination of archived samples but not those col-lected in 1992.

The large increase in 14C age with depth, from 193 yrat O to 10 cm to 4068 yr BP at 30 to 45 cm, shows majorchanges in the factors controlling the deposition andturnover of the SOM within the soil profile. Similar ageswere found in the cultivated site; the total soil C reachedan age of 7000 yr BP at the 90- to 120-cm depth (Table3). Acid hydrolysis of soil from the native site increasedthe 14C age of the residue of hydrolysis by 1800 yr atthe surface and 3500 yr at the 30- to 45-cm depth. Nonhy-drolyzable C ages parallel those of the total C in thenative and cultivated sites. It is difficult to explain agesof 7000 yr in total C and 9000 yr for the 50% of the Cthat is nonhydrolyzable at soil depths that are still inthe crop rooting zone. The samples from the cultivatedAkron plots showed a wide range in the 14C age of thenonhydrolyzable C at the 10- to 20-cm depth. Two plots(205 and 307) with slightly different management histor-ies were used to obtain replication on this site. Plot 205had a 14C age of 3925 yr BP, Plot 307 dated at 7240 yr.

The inverse relationship between C content (Fig. 2A)and the 14C age of both the total soil C and the nonhydro-lyzable C (Fig. 2B) is reminiscent of data from an isotopedilution experiment where tracer contents and the levelof materials in the pool are inversely related. The ageprogression at lower depths does not reflect changes inthe soil texture. These soils become more silty at 90 to120 cm (Table 2) and very sandy at depths beyond whichthey were C dated. A decomposition rate constant k(yr"1) can be calculated if the 14C age is taken as the

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Table 2. Characteristics of the Akron, CO, site.

Soil treatment

Native 1992

Native 1947

Cultivated 1947

Cultivated 1992

Depth

cm0-10

10-2020-3030-450-15

15-300-15

15-300-10

10-2020-3030-6060-9090-120

TotalOrganic Ct organic Ct ± SE

B kp~'S K6

13.6 ± 1.459.5 ± 0.67.9 ± 0.75.9 ± 0.2

14.2 13.6 ± 0.567.9 8.25 ± 0.367.7 9.3fl6.5 8.1

8.9 ± 17.5 ± 0.46.2 ± 0.46.1 ± 0.63.5 ± 0.32.2 ± 0.2

Non-hydrolyzableC ± SE

55.9 ± 0.345.3U

44.5 ± 0.939.4 ± 2.954.4 ± 1.8

28.0fl_-

59.1 ± 2.554.2 ± 3.848.5 ± 1.849.9 ± 3.945.3 ± 0.451.1 ± 1.5

SandO/—————— /o —————

362929363329

Soil texture

Silt

314041394952

Clay

333130251819

t Data from Haas et al. (1957).* Data from this study.§ Interpolated from Halvorson et al. (1995).H Single-sample analyses—instrument replication only.

mean residence time and one assumes first-order kinet-ics. The value of k in first-order reactions at steadystate is 1/MRT. Figure 2C shows the decompositionrate constant of the nonhydrolyzable C to be relativelyuniform with depth. The total soil C shows a muchhigher decomposition rate at the surface. This reflectsthe effect of addition of materials with high decomposi-tion rates.

The Colorado catenary site, cultivated for 50 yr, hadorganic C contents that ranged from 6.9 g kg"1 in theO- to 10-cm layer of the summit to 9.8 g kg"1 in thesurface layer of the toeslope position (Table 4). Thiswas accompanied by a decrease in age from 1600 to 700yr BP. The aboveground biomass in this wheat-fallowsystem, since 1986, has averaged 5070,5540, and 6350 kgha"1 yr"1 for summit, sideslope, and toeslope positions,respectively. The straw that remained on the soil surfaceof the corresponding sites after grain harvest averaged3400, 3900, and 4300 kg ha"1 (Peterson et al., 1994).One would expect equivalent differences in root inputs.Differences in residue input affect the 14C age, but the

amount of residue cannot account for all of the agedifferences on these sites. Data from another catenarysite in Saskatchewan (Martel and Paul, 1974a) are shownfor comparison. The Saskatchewan site, cultivated for30 yr at the time of sampling, shows its northern location.It had three times as much C as the Colorado site, butno major C content changes along the catena. However,the 14C age decreased from 575 yr at the summit tomodern in the depression. This site shows the effects ofdiffering water-use efficiencies on the 13C content ofplants with a common C3 heritage. The upper slopes inthe catena had a 513C of -25.1%o. The change to -27.2%.at the lower, wetter sites is consistent with known effectsof plant water-use efficiency on the 813C content (Far-quhar et al., 1982). The 13C contents are used in thecalculation of the C dates and elsewhere (Follett et al.,1997) where mixtures of C3-C4 plants provide a powerfuladditional tracer.

The Sidney, NE, site (Table 5) represents a silty,mixed, mesic Pachic Haplustoll. Soil from adjacent na-tive pasture has an average C content of 21.0 g kg"1.

Table 3. The "C activity and "C age of the total soil organic matter and hydrolysis fractions of the Akron, CO site.

Soil

Native 1992

Native 1947

Cultivated 1947

Cultivated 1992

Depth

0-1010-2020-3030-450-15

15-300-15

15-300-10

10-2020-3030-6060-9090-120

Totalorganic C ± SE

97.6 ± 1.482.7 ± 1.374.2 ± 0.260.4 ± 4.0

107.5 ± 2.972.2182.4*87.8*

85.2 ± 2.774.5 ± 0.964.4 ± 2.152.8 ± 2.246.5 ± 1.641.8 ± 2.3

14C activity

NonhydrolyzableC ± SE

———— pmC —— ———78.0 ± 1.3

66.5$51.4 ± 1.439.0 ± 3.769.6 ± 0.8

57.61_-

66.7 ± 4.961.4*

53.4 ± 4.841.5 ± 3.837.0 ± 3.432.5 ± 2.5

HydrolyzableC

123989372

15378_-

11410675645451

Total orgC ± SE

193 ± 1181528 ± 1282398 ± 224068 -t 528

Modern2611$1560*1040*

1296 2622368 1043540 2655143 3336160 2857015 450

14C age


————— yr BP ———1994 ± 134

3260*5355 ± 2207600 ± 7752915 ± 85

4431*_-

3326 6165736 4725070 7307187 6588013 7429035 440

Hydrolyzablect

Bomb130625

2480Contain.

1990--

BombBomb2355354049055420

t Calculated by difference.* Single-sample analyses—instrument replication only.

PAUL ET AL.: SOIL ORGANIC MATTER DETERMINATION BY RADIOCARBON DATING

A) Soil Organic C B) Soil Age

1063

C) Decompositionrate constant

20-

40-

60-

80-

100-

120 H

0-

20-

40-

60-

80-

100-

120-

0-

20-

40-

60-

80-

100-

120 H

O Total C• Non-hydrolysable C

i I I I I I l I I I I I0 2 4 6 8 1 0 0 2 4 6 8 1 0 0 2 4 6

C C k g k a " 1 ) Age (years x 103)Fig. 2. The relationship of soil organic C content, 14C age and decomposition rate constant with depth in the Akron, CO, cultivated soil

1 I I I I IO 2 4 6 8 10

Decomposition rate constant (y x 10 )

This is 50% greater than the nearby, native Akron siteat 13.6 g kg"1 but 20% lower than a native, mixed prairiesite in Saskatchewan. The Sidney experimental plots,when established in 1970, were fenced. Three replicatedstrips of native vegetation remained as part of the rota-tion. These were not grazed nor did they undergo prairiefires. The vegetation in these strips changed to primarilya C3 sod during the 20 yr after fencing. The C contentin the O- to 10-cm depth increased from 23.3 g kg"1 in1970 (Fenster and Peterson, 1979) to 30 g kg"1 in 1992.One could argue that the sod collected wind-blown soilfrom adjacent cultivated strips, but eroded soil would belower in C content. The strong influence of unharvestedvegetation in the sod soils is shown by its 14C signatureof 112 vs. 105 pmC for the native. The 112 pmC of thetotal soil C represents the mixture of old material andmaterials >112 pmC derived from bomb-affected 14Ccontaining residues added between 1972 and 1992. TheO to 10 cm of the plow layer with less C than the native,

and only one-half that of the sod, had a 14C age of 490yr. The sod sites had 61% of its C present as nonhydro-lyzable residue with a pmC activity of 87.1 and a 14Cage of 1110 yr. The plowed soil had a hydrolyzabilitysimilar to that of sod but did not have the usual largeincrease in age for the nonhydrolyzable residue; its agewas 905 yr with a large error figure. The 10- to 20-cmlayer of the plowed soil dated 900 yr older than the 0-to 10-cm layer, even though one would expect mixingof layers in a cultivated soil. This reflects the shallowdepth of plowing (12 cm) at this site.

The wheat-fallow and wheat-hay-fallow rotations atLethbridge, Alberta, (Table 6) were sampled in 1962.The rotations at that time had been established for 30 yr.Carbon contents vary within the different entry points ofthe wheat-fallow rotation, showing the soil variabilityencountered in the field (Campbell, 1964). The 14C agesare the oldest obtained for unfractionated, surface soilsin either the earlier Canadian studies or this study. They

Table 4. The tracer-C signatures in the catenerary sequence of native soil at Sterling, CO, and Waldheim, Saskalchewan.t

Position

SummitSideslope

Toeslope

SummitSideslopeToeslope

Depression

Depth

cm

0-100-10

10-200-10

0-100-150-15

15-2222-520-18

22-3560-70

Total organicC ± SE 14C activity

gkg-'

6.9 ± 0.46.4t5.8*

9.8 ± 0.2

2421276.24.2

245.24.5

pmCSterling, CO

81.9 ± 0.786.3*72.6:|

91.2 ± 0.6Waldheim, Saskatchewan

_---_---

"C ageyr BP

1602 ± 681182*2571*

742 ± 53

575 ± 80270 ± 45216 ± 45635 ± 55930 ± 55Modern700 ± 55

4870 ± 60

UC ± SE0/00

-19.9 ± 0.06-20.5$-18.9*

-20.1 ± 0.02

_-25.0-25.1-26.4-29.2-27.2-27.9-

t The Martel and Paul (1974a) site at Waldheim, Saskatchewan, is classified in the Oxbow Association in the Canadian classification with the upper-slopesoils being classified as Udic Haploborolls.

t Analysis of only one field sample was obtained.


Table 5. The organic C content and 14C signatures of the Sidney, NE, site.14C activity

Treatment

NativeSodPlow

Depth

cm0-100-100-10

10-20

Organic C ± SE

6 KS21.0 ± 0.8530.0 + 0.7114.0 ± 0.3412.2 ± 1.10

NonhydrolyzableC ± S E

%70t

61 + 362 + 1

Total organicC ± SE

105.5t112.0 ± 7.094.1 + 1.0

84.6t

Residue C ± SE

pmC— ————————94.4t

87.1 ± 6.789.6 ± 6.0


ModernModern

490 ± 1001338f

"C age


yrBP —————————462f

1110 ± 648905 ± 550


show large treatment effects. An average age of 1950 yrwas found in the wheat-fallow. In the wheat-hay-fallowcropping sequence, it was 1500 yr. Haas et al. (1957)found a significant relationship between the percentageof soil particles by weight that were <0.005 mm andthe total C and N of their soils. Our study contained alarge number of variables such as landscape position,management, and latitude. This, together with the lownumber of samples, did not allow us to obtain a relation-ship between 14C age and silt plus clay. Table 6 showsa 10% range in silt plus clay in the soils of the differententry points of the Lethbridge wheat-fallow rotation.If the results of three points can be believed, these showthat the 14C age was least at the lowest silt plus claycontent and highest at the high silt plus clay content.

The Mandan, ND, 1947 native site (Table 7) had Ccontents similar to those of the more northern Wald-heim sites in Saskatchewan. These occurred in a soilwhere silt plus clay accounted for only 12 to 18% of thetotal soil particles and were part of the reason for thepoor overall relationship between texture, soil C con-tent, and 14C age. The Mandan site had an age of 1200yr BP for the surface O to 15 cm and 2150 yr BP forthe 15- to 30-cm depth. Acid hydrolysis left a residueaccounting for 58% of the original SOM with an age of2245 yr. The 1947 cultivated sample showed an incon-gruity with depth. The surface and subsurface sampleswere similar in soil C and 14C age at both depths, beingvery similar to the 15- to 30-cm depth of the nativesite. Explanations for this could involve erosion of thesurface layer or the effects of deep plowing. Samplingthe Mandan native site in 1992 showed the surface tobe near modern. This would be expected from bomb-C effects. The 15- to 30-cm depth was unaffected; it wassimilar to the 1947 sample from the same site.

The 1947 Hays, KS, silty clay soil dated 645 yr BP atthe surface and 1215 yr BP for the 15- to 30-cm depth.This small difference between the surface and subsur-face relative to other sites was associated with one of

the smallest age differences between total soil and non-acid-hydrolyzable C (1055 yr). The Dalhart, TX, nativeloam to fine sandy loam showed the low C content foundin drier southern USA soils. Of special interest is the77% hydrolyzability of the soil. The surface soil at 930yr BP produced a 1380-yr difference for the nonhydro-lyzable C. This represents only 23% of the total C. The29% nonhydrolyzable C of the subsurface was 1330 yrolder than the total C of this soil.

The soil from Big Springs, TX, shows one problemassociated with 14C dating. Many laboratories utilize en-riched tracer 14C in experiments that range from cropresidues to pesticides. Soil samples from Big Springs,archived for 45 yr, were contaminated at 680 and 560pmC for the surface and subsurface, respectively. Thesource of this contamination is unknown. The samplesare now stored in a well-isolated archival site. The con-tamination was removed by acid hydrolysis of the sur-face soil to obtain a residue with a count of 72.4 pmC.This is equivalent to a 14C age of 2600 yr BP and indicatesthat the contaminant was associated with a protein orsugar rather than a ligniferous plant residue that wouldnot have been solubilized by hydrolysis. The subsurfacesoil was not hydrolyzed.

The data from Maricopa, AZ, (Table 7) come froma previously arid site that now is irrigated. This TypicTorrifluvent has a C content very similar to that ofnative Big Springs and Dalhart, TX, sites at 5 to 6 g Ckg~! soil. The 14C age of the residue of hydrolysis alsois similar at 2400 yr BP. This soil is now under irrigationwith relatively high substrate inputs; the percentage ofnonhydrolyzable C and the great age were unexpected.One would expect a more significant contribution fromthe active fraction. However, under irrigation the activefraction could rapidly decompose leaving only the oldresistant fraction. Other reasons could involve the flu-vial origin and the deposition of older C. The site, al-though having four replicates rather than two like theother sites, showed the highest field variability in 14C

Table 6. The effect of cultural practices on the 14C content of soil organic matter at Lethbridge, Alberta (from Campbell, 1964).

Crop sequence

Wheat-fallow-wheat

Wheat-Hay-Fallow-Wheat

Rotationentryt

Wheat yr2Fallow

Wheat yrlWheat yr2

Wheat yrl

Yield

kg ha '1030

15251293

1095

Depth

cm0-150-150-150-150-15-

Totalorganic C

gkg '1311101415-

"C age ± SEyr BP

2400 ± 801515 ± 951930 ± 701430 ± 751560 ± 75

Silt

2521232426

Clay

2822252727

Silt plusClay

5343485153

t Crop in place the year sampled.


Table 7. The 14C signature of historical and modern soil samples in the Great Plains.

"C activity "C

Site

Mandan, ND

Mandan, ND

Mandan, ND

Hays, KS

Big Spring, TX

Dalhart, TX

Maricopa, AZ

Site andyear sampled

Native1947

Cultivated1947

Native1992

Native1947

Native

Native

Cultivated

Depth

cm0-15

15-300-15

15-300-15

15-300-15

15-300-15

15-300-15

15-300-30

Total C in hydrolysisorganic C residue

gkg '25.6 ± 1.117.3 ± 0.8

16.517.52817

23.2 ± 0.814.0 ± 0.65.9 ± 0.25.2 ± 0.26.0 ± 0.24.7 ± 0.4

5.8

%58 ± 3

__--_

,56 ± 0.2-

43 ± 1.4-

23t29 ± 11

54t

Total C ± SE Residue C ± SE

——————— pr-"86.2 ± 0.8

76.45f78.5 ± 0.577.4 ± 0.5

99.380.0

92.3 ± 0.186.0t

680560

89.1 ± 0.682.1 ± 1.292.8 ± 3.6

HIV. ————————————————— ——

75.6 ± 1.6__—-_

80.9 + 0.2-

72.4 ± 2.7-

75.069.6 ± 0.672.0 * 3.7


—————— yr1200 ± 75

2155t1945f2060t

561825

645 ± 51215 ± 55contam.contam.

931t1588 ± 120

400

age

Residue C ± SE

BP ————————2245 ± 175

-----

1700 ± 25-

2600 ± 300-

2314f2918 ± 68

2400


that we have encountered to date. Leveling of sites priorto irrigation can expose varying amounts of subsurfacesoil. These have very different ages than the surface.

DISCUSSIONThe data showing similar to possibly slightly increased

SOM levels in recently sampled sites relative to thosesampled in 1947 do not represent a large enough samplefor us to say that SOM levels have increased since 1947.They, however, point out that the cultivation-induced,drastic decrease in SOM levels that has mesmerized soilscientists for so long should be an issue of the past asnew steady-state levels are reached. The Sidney siteconverted from a grazed C4 to an ungrazed C3 sod actu-ally showed a SOM increase above native levels. Thisseries of sites with a large range of climate, landscape,and management effects was not designed to specificallymeasure clay effects. It is not surprising that the influ-ence of silt plus clay could not be ascertained. The effectof the north-south gradient on the content of total Cis demonstrated. The latitude effect is not noticeable insoil 14C ages.

The use of 14C-dating techniques made it possible tocharacterize the 14C age of a cross section of NorthAmerican Great Plains soils. In relating these ages toMRT, we realize that the soil has a continuum of differ-ent-aged materials (Goh, 1991). The interpretation of14C ages in the past led to the concept of a physicallyactive, relatively biologically inactive pool of C thatconstitutes approximately one-half of the soil C (Pauland van Veen, 1978; Paul, 1984). The 14C ages and resul-tant MRT were used to determine pool sizes and fluxesin the mathematical models of Jenkinson and Rayner(1977) and van Veen and Paul (1981). They were indi-rectly used in later models such as CENTURY (Partonet al., 1987,1993). Our data from a more extensive rangeof sites support the original concepts.

The 14C ages of surface soil represented some withhigh concentrations of bomb 14C (Sidney, NE, sod). The1676 yr BP of the summit of the Sterling, CO, catenashows the influence of erosion. The 2400 yr BP age ofthe wheat-fallow rotation at the Lethbridge, Alberta,site is the oldest age for the total C of surface horizonsin our studies. Depth effects were consistent and strik-

ing. The 10- to 20-cm depth was an average of 1200 yrolder than the O- to 10-cm depths of both cultivated andnative sites sampled in 1992. The archived samples fromthe 1947-1949 study of Haas et al. (1957) were obtainedbefore bomb 14C production; they had been influencedby the dilution of the atmospheric 14CO2 by industrialCO2 with no radioactivity in what is known as the Seusseffect (Goh, 1991). These soils showed a 1200 yr ageincrease between the O- to 15- and the 15- to 30-cmdepths. We must conclude that generally there cannothave been a large amount of mixing of surface andsubsurface soils by either soil fauna or by human activi-ties such as cultivation.

We do not have a good explanation for the 7000 yrBP age of the 90- to 120-cm depth of the Akron soil. The!4C age differences between the surface and subsurfacesamples from the other soils indicate that they are likelyto be similarly old at depth once they are 14C dated.Such differences with depth have been previously notedfor other geographical locations (Goh et al, 1984; Schar-penseel, 1993, 1994). The total SOM at depths of 30cm and deeper had 14C ages as old and older than thepaleosols collected from Colorado by Kelly and Yon-kers (1993). They attributed the 6000-yr age of the deepsoil samples to Holocene sand dune formation ratherthan to ongoing pedogenic processes. The Weld soilseries such as that at Akron is known to contain paleo-sols. The increase in the silt content and decrease inclay at depths below 60 cm at this site show a differencein soil particle deposition patterns. The 14C age, how-ever, increased uniformly at the various depths (Fig. 2).The possibility of a paleosolic profile is not an easyexplanation for our results. Old, inert soluble C diffusingdown the profile has been considered as a cause of theold age with depth. This, however, should be minimal inthe dry, high base exchange soils in this study. Inorganiccarbonates that occur at depth in grassland soils wereremoved prior to analysis. They should not affect theage of the SOM. The carbonates of the Maricopa, AZ,soil comprised 2.5 g CO3-C kg"1 with a 14C age of 2420 ±170 yr (Leavitt et al., 1994). These were similar in ageto the acid-insoluble C. Root CO2 fixation is known tooccur to some extent. The possibility that root fixationof C from carbonates at depth must be considered. How-


ever the 14C content of soil CO2 has been shown not todiffer greatly from that fixed by the aboveground plantparts (Broecker and Olson, 1959).

The dilution of old geological organic C in the parentmaterial with modern C must be considered. The closeinverse relation between the C content and 14C age withdepth as in Fig. 2 is reminiscent of an isotope dilutionexperiment. Small amounts of very old or dead C cangreatly influence the 14C age. One could envision a loesssoil such as the one at Akron, CO, receiving depositedmaterial with significant contents of old C. This wasfound to be the case in a lacustrine site studied by Haaset al. (1986). However, similar increases in age withdepth also were found in glacial till sites such as thoseof Saskatchewan (Martel and Paul, 1974b) and in otherMollisols (Scharpenseel, 1993). One must seek a pedo-logical-biological explanation with the interesting ques-tion of what the decomposition dynamics (CO2 evolu-tion) would be of the 7000-yr-old Akron, 90- to 120-cmsoil sample under laboratory conditions.

The residue of hydrolysis from the various soils con-tained 23 to 70% of the total organic soil C. Hydrolysisproduced an average age difference of 1400 yr in surfacesoils and 1700 yr in subsurface samples where a largerportion of the C appeared to be hydrolyzable. The Ccontent and 14C activity of the hydrolyzable fraction canbe calculated from the data for the whole soil and thenonhydrolyzable fractions. Acid hydrolysis removesproteins, nucleic acids, and polysacharides of soil(Schnitzer and Khan, 1972). It does not solubilize all ofthe cellulose of plant residues. Schnitzer and Khan(1972) measured the effects of 6 M HC1 on the character-istics of isolated humic and fulvic acids. Hydrolysis re-sulted in a weight loss of approximately 40% for bothfractions. The C content of humic acids remained un-changed at 57%; that of fulvics was 53% in the residuerelative to 49% in the total fulvics. This is related tothe observation that the H content and O-containingfunctional groups remained unchanged as a percentageof the residue in the humic acids, whereas the fulvics weredrastically decarboxylated with an evolution of CO2-C.

We conclude that acid hydrolysis, if interpreted rela-tive to its action on both plant residues and SOM, givesmeaningful pool size estimates for temperate soils. Someof our archived samples were suspected to be contami-nated. Hydrolysis helped verify the contamination. Italso provided the basis for determining the decomposi-tion rates of the soil-C pools. The great age of both thetotal soil and the nonhydrolyzable fraction at depth isnot yet fully explained. Martel and Paul (1974b) exhaus-tively treated the nonhydrolyzable residue with NaOHto peptize and solubilize humics. Hydrofluoric acid wasthen used to remove clays. This resulted in the defloccu-lation or solubilization of 93% of total soil C. The Cremaining was not as old as nonhydrolyzable C. TheNaOH extract of the 6 M HCL residue of hydrolysisgave the oldest fraction.

Soil depth, catenary position, and management allaffected the soil 14C age. The position in the catenawas especially notable. Lower slopes in the catenarysequences not only had lower 14C ages, they also hadmuch different 13C contents. The explanation for this is

not clear. Erosion of the upper slopes would exposeolder subsurface organic matter. The lower slopesshould contain some of the transported C but shouldalso contain a larger fraction of young, active material.The S13C values of plants become less negative as water-use efficiency of plants increases on upper slopes. Thissignature is reproduced in the SOM. The 813C valueswere essentially not different in the catenary sequencein Colorado. This catena did not contain the moist de-pressional site equivalent to that in Saskatchewan. Theeffect of the plant water-use efficiency in the lower,wetter sites could be offset by CH4 formation duringdecomposition under anaerobic conditions. The CH4evolved (Boulton, 1991) can have a signature (8) of-56%o. The C remaining in the soil would be morepositive than that of the incoming plant residues. Thatthis did not occur on these two sites gives an indicationthat CH4 formation is not a major process in these moistbut not often flooded areas.

Our results show the applicability of both 14C datingand soil hydrolysis (Leavitt et al., 1997) to analyticallyestablish the pool size and flux rates of the resistant,soil organic C. Plant-residue lignin and related com-pounds are known to be included in the nonhydrolyz-able residue; the nonhydrolyzable residue may overesti-mate the size of the old, resistant fraction. Sodiumhydroxide peptization could further separate these ma-terials. The material further removed from the residueby NaOH would be only slightly older than the residueof hydrolysis (Martel and Paul, 1974b). The resistantfractions are so old that, although of great importancefrom a structure, water and nutrient, and pesticide ad-sorption standpoint, they play a small role in nutrientcycling. Global C cycle questions must, however, con-sider both the size and the turnover rate of these olderfractions (Paul et al., 1995).

This study is the first of a series that will incorporate14C ages and 813C contents of soil fractions as affectedby parent material, vegetation, management, location,and depth. Also being conducted are long-term incuba-tions with measurement of the 813C of the evolved CO2on sites with a known signature of different 813C con-tents. This will allow us to model the data relative toplant residue inputs on sites with a long managementhistory. Leavitt et al. (1994) showed that 813C was moreconsistent and easier to measure than 14C ages. Carbon-14 dating gives us pool sizes and fluxes of the resistantfractions as well as an indication of the overall pedogeniccharacteristics; 813C and long-term incubation will mea-sure the dynamics of the more active fraction(s).

ACKNOWLEDGMENTSThis study was initiated while E.A. Paul was on sabbatical

leave with the Soil, Plant, and Nutrient Research of theUSDA-ARS in Fort Collins, CO. Financial assistance fromthe USDA-ARS is appreciated. The efforts of E.G. Pruessnerare especially acknowledged for her assistance with 813C andtotal organic C analyses, and for preparation of standards foruse by other analytical laboratories. D. Donohue, J. lull, andL.Tooling, among others at the NSF, AMS facility at the Uni-versity of Arizona, provided the TAMS analysis.

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