Climatic Effects on Soil Organic Matter Composition in the Great Plains

Climatic Effects on Soil Organic Matter Composition in the Great PlainsW. Amelung,* K.W. Flach, and W. Zech

ABSTRACTTo examine temperature and moisture effects on the chemical com-

position of soil organic matter (SOM), six soil profiles were studiedfrom a climosequence in the native prairie of the Great Plains of theUSA. After the removal of plant debris, the vertical distributions(0-60 cm) of soil organic carbon (SOC), N, lignin, and cellulosicand noncellulosic polysaccharides were determined. Samples of thetopsoil horizons were additionally characterized by means of 13C and'H-Nuclear Magnetic Resonance (NMR) spectroscopy. As annual tem-perature increased from 7 to 23°C at sites with <500 mm precipitationper year, the amount of polysaccharides decreased from 605 to 422g kg-' SOC in the topsoil (0-15 cm) and from 516 to 278 g kg-1

SOC in the subsoil (30-40 cm). Polysaccharide contents increasedwith increasing precipitation, reaching 726 g kg ~' SOC in the top 15 cmand 876 g kg"' SOC at 50-cm soil depth (830 mm annual precipitation,12.6°C annual temperature). At all sites, the content of lignin-derivedphenols per kilogram SOC decreased with increasing soil depth. Poly-saccharides decreased much less. It is suggested that polysaccharideswere resynthesized during SOM alteration within the soil profiles andthereafter preferentially to lignin protected from decay in the mineralsoil. The NMR spectra suggested that both polysaccharides and alkylcompounds reflected climatic influences on the SOM composition ofthe Great Plains.

THE SOC is a major component of biochemical cyclesof nutrient elements such as N, P, and S, and its

quantity and quality both reflect and control primaryproductivity (Burke et al., 1989). Understanding theprocesses that control SOC dynamics as influenced byclimate is essential for sustainable land use. Such under-standing is particularly important for the Great Plains

W. Amelung and W. Zech, Dep. of Soil Science, Univ. Bayreuth, Germany95440; and K.W. Flach, El Macero Drive 4044, Davis, CA 95616.Received 24 July 1995. "Corresponding author (wulf.amelung®uni-bay reuth. de).

Published in Soil Sci. Soc. Am. J. 61:115-123 (1997).

where the impact of global warming may be greater thanin most other parts of temperate North America (Mitchellet al., 1990).

Jenny (1941) described relationships between the totalN and SOM contents as a function of mean annualtemperature or precipitation; the warmer and drier asite, the less SOM it contains. The influence of climateon the chemical structure of humus was first suggestedby Bracewell et al. (1976). Nuclear magnetic resonancespectrometry studies by Arshad and Schnitzer (1989)suggested that the aromaticity of humic acids in Kenyansoils may be negatively correlated with mean annualprecipitation. For various forest subsoils, Zech et al.(1989) reported decreasing aromaticity of SOM with anincreasing precipitation/temperature ratio. Similar stud-ies have not yet been made for soils under prairie vegeta-tion such as those of the Great Plains. Here, precipitationdetermines annual plant primary production, whereastemperature should affect primarily C turnover rates(Parton et al., 1987; Sala et al., 1988). Differences inturnover times are thought to be responsible for changesin SOM properties with depth in forest soils (Kogel-Knabner et al., 1988). Therefore, SOM composition atvarious soil depths must be known to accurately assessthe effects of climate on SOC dynamics.

It is known that different C pools control SOC dynam-ics in mineral soils (Tiessen et al., 1984; Parton et al.,1987). According to Skjemstad et al. (1986), stabilizationof SOC is the essential factor that controls SOM decom-

Abbreviations: SOM, soil organic matter; SOC, soil organic carbon;NMR, nuclear magnetic resonance; CP/MAS, cross polarization magicangle spinning; MAT, mean annual temperature; MAP, mean annualprecipitation; VSC, vanillyl, syringyl, and cinnamyl phenolic CuO oxida-tion products; NCPS, noncellulosic polysaccharides; LNQ, VSC-lignin/NCPS quotient; PC, principal components. *,** Significant at the 0.05and 0.01 probability levels, respectively.

116 SOIL SCI. SOC. AM. J., VOL. 61, JANUARY-FEBRUARY 1997

position, the chemical resistance of SOC to mineraliza-tion being less important. However, the degree to whichstabilizing processes are important for soils of the prairieis unknown. McDaniel and Munn (1985) suggested thatstabilizing influences of clay minerals are more importantin warmer climates than in cooler ones. This suggeststhat climate and texture may interact to affect SOMproperties. Therefore, the objectives of this study wereto (i) examine whether temperature and precipitationaffect properties of steppe SOM, and (ii) identify possibleprocesses that stabilize SOM compounds in soils of theprairie.

MATERIALS AND METHODSSoils

Soils were selected along temperature and precipitation tran-sects across the Great Plains. Vegetation and soil attributesother than SOC were kept as constant as possible. We sampledsites with native prairie vegetation near the wet and dry ex-tremes of cold, warm, and hot temperature zones (Table 1).Soils and vegetation at the sites had been characterized pre-viously by USDA-SCS (1994). The vegetation species at eachsite resembled the potential grassland vegetation of respectiveland resource areas (USDA-SCS, 1981) with the exception ofSite 5(h,d). Buffalograss [Buchloedactyloides (Nutt.) Engelm.]was present at all sites that had <510 mm precipitation; Stipaand Agropyron species such as porcupine grass (5. sparteaTrin.), western wheatgrass [A. smythii (Rybd.) Gould], andbearded wheatgrass [A. subsecundus (Link) A. Love & D.Love] grew at both Northern Sites l(c,d) and 2(c,m), whereaspricklypair (Opuntia vulgaris Miller) and sagebrush (Artenisio)occurred at Site l(c,d), only. At Site 3(w,d) and 4(w,m),side-oats grama [Bouteloua curtipendula (Michaux) Torrey]was common. Big and little bluestem, (Agropyron gerardiiVitman) and [A. scoparium (Michaux) Nash], respectively, aswell as indian grass [Sorghastrum nutans (L.) Nash] joinedthe native grassland species at Sites 4(w,m) and 6(h,m) whichreceive >800 mm precipitation. At Site 5(h,d), brush andcactee had invaded, probably because of recent heavy grazing;mesquite [Prosopis juliflora (Sw.) DC.] and pricklypair grewup to 2 m, for instance. Aboveground annual plant productionas measured by USDA-SCS (1994) ranged from 161 to 268

g m~2 at the sites with MAP <510 mm and from 447 to 491g m~2 at Sites 4(w,m) and 6(h,m), respectively, with MAP>800 mm. Calculations performed according to Sala et al.(1988) yielded similar estimates: annual plant productionranged from 130 to 304 g m~2 at MAP <510 mm and from463 to 469 g m~2 at MAP >800 mm. The soils at all of thesites developed from clayey to loamy sediments, mainly shaleand siltstone. Smectites were the dominant clay minerals. Soilacidity values ranged from pH(CaCl2) 5.9 to 7.4 in the topsoil,with depth pH(CaCl2) decreased in soil at Site 4(w,m) andincreased at all other sites by as much as one unit. We collectedsamples by horizons to a depth of 60 cm from each pedon.In addition, we collected two composite samples (0-15 cm),each from 20 sub-sites, within 1 mi of the primary samplesite.

Sample PreparationsAll samples were air dried and sieved (<2 mm). As fresh

plant material influences SOM composition, we shook thesamples in water, waited 1 h, and removed floatable plantresidues from the water surface. After repeating this proceduretwice, we freeze-dried the samples. Macroscopic roots thatremained were picked out with forceps. The samples werethen ground for chemical analyses.

Chemical AnalysesTotal C and N were determined by dry combustion on a

Carlo Erba (Milano, Italy) ANA 1500 C/N-Analyzer. OrganicC was determined by a modified wet-oxidation method usinga redox electrode (Ingold) to identify the titration point forthe quantification of remaining CrjO?" (Nelson and Sommers,1982). The CaCO3 was estimated by (CTOTAL - SOC)100/12.

Amount and degree of oxidative decomposition of ligninwere estimated from lignin parameters using a modified alkalineCuO oxidation procedure at 170°C for 2 h (Hedges and Ertel,1982). We used C-18 (Baker) as filling material for solid stateextraction of the phenols. Phenolic oxidation products weredissolved and derivatized with a 1:1 mixture of pyridine andN,O-bis(trimethylsilyl)trifluoroacetamide (Fluka), separatedby capillary gas chromatography (HP Ultra 2 ftised silicacolumn) and detected by a flame ionization detector. Ethylvanil-lin was added as internal standard to the sample prior to alkalineextraction, and phenylacetic acid was used as a second internal

Table 1. Soils under study [climate, and particle-size distribution (0-30 cm), from USDA-SCS, 1994].Site identification and corresponding soil family Series Latitude, longitude MATt

°rMAP*

mm

Clay Siltn,

Sand Remarks

Fine-textured sites (main trancects)

l(c,d) Fine, montmorillonitic Borollic Camborthids2(c,m) Fine, montmorillonitic Typic Argiustolls3(w,d) Fine, mixed, thermic Pachic Paleustolls4(w,m) Fine-silly, mixed, mesic Typic Argiudolls5(h,d) Fine-loamy, mixed, hyperthermic Aridic Ustochrepts6(h,m) Fine-loamy, carbonatic, thermic Typic Calciustolls

ZatovilleBlackpipe-4HollisterElmontMcAllenTopsey

46°44°33°39°27°31°

16'N,OO'N,12'N,04'N,57'N,10'N,

106°103°100°95°98°97°

14'W47'W13'W36'W54'W58'W

6.56.8

17.712.623.420.0

274497507839446829

393837302928

503945483945

111218223827

cold-drycold-moistwarm-drywarm-moisthot-dryhot-moist

Coarse-textured sites

7(c,d) Fine-silty, mixed Borollic Haplargids8(c,m) Fine-loamy, mixed Typic Argiustolls9(w,d) Fine-loamy, mixed, mesic Aridic Paleustolls10(h,d) Fine-loamy, mixed, hyperthermic Ustollic Haplargids

VanstelBlackpipe-8AcuffTonio

46°44°34"27°

49'N,OO'N,33'N,57'N,

106°103°102°98°

14'W47'W13'W54'W

6.56.8

14.423.4

274497465446

22262125

45202622

33545353

cold-drycold-moistwarm-dryhot-dry

Sites for testing the stability of statistical analysis

ll(w,d) Fine-loamy, mixed, thermic Typic Paleustolls12(h,m) Fine, montmorillonitic, thermic Typic Haplustalfs

TillmanThurber

33c

32'48'N,100s

09'N, 97°36'W55'W

15.018.2

480829

2548

4635

3927

coarse texturefine texture

t MAT = mean annual temperature.t MAP = mean annual precipitation.

AMELUNG ET AL.: CLIMATIC EFFECTS ON SOM COMPOSITION IN THE GREAT PLAINS 117

standard added before derivatization to determine the recover-ies of ethylvanillin. These recoveries could be increased byadding 50 mg glucose to the sample.

Alkaline CuO oxidation releases phenols from reactive sitesof the ligninmacromolecule. Consequently, the sum of vanillyl,syringyl, and cinnamyl phenolic CuO oxidation products (VSC)gives a directly proportional, relative measure of the totallignin, i.e., the VSC-lignin. Absolute lignin contents cannotbe determined, because the contribution of VSC-lignin to totallignin remains unknown. The mass ratio of acid to aldehydefor the vanillyl and syringyl units, (ac/al)v,s, can be used todetermine the degree of oxidative decomposition of ligninwithin a sample, whereas selective losses of syringyl unitsduring lignin degradation are reflected by the mass ratio ofsyringyl to vanillyl units (S/V; Ertel and Hedges, 1984).

A two-step hydrolysis was performed for carbohydrate anal-ysis (Ziegler and Zech, 1991). The NCPS were hydrolyzedwith 1 M HC1 at 100°C for 5 h (Uzaki and Ishiwatari, 1983,modified), while cellulosic polysaccharides were digested bytreating the residue with 12 M H2SO4 (Cheshire and Mundie,1966). Carbohydrates were determined colorimetrically as de-scribed by Burney and Sieburth (1977). Although we equatedcellulosic polysaccharides with cellulose, this material mayalso include strongly sorbed microbial constituents that resistHC1 hydrolysis. The NCPS fraction may include both hemicel-luloses and microbial saccharides.

All analyses, including root removal, were performed induplicate.

Nuclear Magnetic Resonance MeasurementsFor liquid-state NMR measurements, SOC was initially

extracted with 1 M NaOH. Because the yields were low, werepeated the extraction with a 1:1 mixture of 0.1 M NaOHand Q.I M Na4P2O7. The combined extracts were then dialyzed,freeze-dried, and 100 to 150 mg of freeze-dried material wasdissolved in 0.24 M NaOD. The solution was transferred toa 10-mm NMR tube ( = 2.5 mL) for 13C measurement and toa 5-mm NMR tube ( = 0.5 mL) for 'H NMR measurement.Chemical shifts were given in ppm (Hz Mhz"1) relative to anexternal standard of TSP [3-(trimethylsilyl)propionic-2,2,3,3-d4 acid, sodium salt] in DaO. No internal standardwas used. For liquid-state 13C NMR spectra, we used a Bruker(Karlsruhe, Germany) AM 500 spectrometer: spectrometerfrequency, 125 MHz; inverse-gated decoupling; acquisitiontime, 0.33 s; delay time, 1.67 s; line-broadening factor, 100Hz. The same instrument was used for determination of solution'H NMR spectra: spectrometer frequency, 500 MHz; homonu-clear presaturation for solvent suppression; acquisition time,1.16 s; delay time, 1 s.

Samples for 13C CP/MAS NMR were treated with 0.1 MHC1 at room temperature to destroy carbonates and de-ashedwith 20% HF to improve resolution. A Bruker MSL 300 NMRspectrometer was used for determination of CP/MAS 13C NMR:spectrometer frequency, 75 MHz; rotation frequency, 4 kHz;contact time, 1.5 ms; acquisition time, 18 ms; delay time,5 s.

Assignment of resonances in the NMR spectra was con-ducted according to Kogel-Knabner et al. (1988). In 13C NMRspectra, alkyl C resonates at low chemical shifts (0-50 ppm);in particular, — (CHzVn: is seen at 30 to 35 ppm. Resonancesat 72 and 105 ppm are characteristic of carbohydrate-C. Signalsdue to aromatic C in lignin are found at =115, 130, and 150ppm, accompanied by methoxyl resonances at 55 ppm. Signalscentered at 175 ppm can be assigned to the carboxyl andcarbonyl C of lignin, proteins, hemicelluloses, and lipids, forexample.

Statistical AnalysisStatistical evaluation was carried out by principal axis com-

ponent analysis with subroutines adapted from Veldman(1967). In this study, we had to deal with two problems. First,the data of different horizons showed evidence of a spatialintercorrelation. This problem was solved by first using onlythe two data sets from the topsoil and subjacent subsoil (30-40 cm). The statistical analysis was then repeated with datafrom topsoils and subsoils at 40- to 50-cm soil depth. Thesecond potential problem arose from the small sample sizeemployed. In response, we supplemented the primary data setwith data from SOM analyses of six additional Great Plaintopsoils (Sites 7(c,d) to 12 (h,m), Table 1) and checked thestability of results by varying the input data randomly. Inputdata included N, NCPS, cellulosic saccharides, VSC-lignin(in g kg"1 SOC) as well as very fine sand, fine sand, clay +fine silt, water content at 15 bar, CEC and lime content (allin g kg'1 <2 mm soil ). Selection of the texture parameterswas also based on principal axis component analysis, whichindicated that clay + fine silt, very fine sand, and fine sandshowed the maximum loadings in the first three principalcomponents.

RESULTS AND DISCUSSIONOrganic Carbon and Nitrogen

Organic C contents generally decreased with profiledepth (Table 2). The N contents correlated strongly withthose of SOC. Thus, the SOC/N ratio remained nearlyconstant within each profile (Table 2). Local SOC/Nminima were found in the upper Bt-horizons, probablydue to the accumulation of leached N from the overlyinghorizons or due to inorganic, clay-bound NH/.

Lignin and PolysaccharidesIn all six profiles, VSC-lignin decreased drastically

with depth (Table 2), indicating pronounced degradationof lignin. This decomposition resulted in decreasing(VSC-lignin)/N quotients (Table 2), which may be abetter indicator of SOC dynamics than C/N ratios.

Alterations of the chemical structure of the vanillyland syringyl units during lignin decomposition oftenresult in increased mass ratios of acids to aldehydes (ac/al)v,s and decreased S/V quotients (Kogel-Knabner et al.,1988). In this study, both the mean (ac/al)v and mean(ac/al)s increased significantly in the subsoil (approxi-mately at 50-cm soil depth) relative to the O- to 15-cmdepth interval, whereas S/V decreased significantly (Ta-ble 3). Compared with previously reported data for forestsoils (Kogel-Knabner et al., 1988), the lower values for(ac/al)v and (ac/al)s observed in this study for grasslandsoils suggest a less pronounced oxidative side chainalteration. Selective losses of syringyl structural unitsby methoxyl demethylation, as indicated by decreasingS/V ratios with depth, were also small relative to thoseobserved for forest soils.

The amount of VSC-lignin in SOM of the topsoilschanged little with climate. According to Parton et al.(1987), the MAP correlated positively with the lignincontent of aboveground plant material but negativelywith that of belowground roots. In our samples, MAP


Table 2. Soil organic carbon (SOC), total N, noncellulosic polysaccharides (NCPS) and cellulosic polysaccharides (CPS), and lignin-derivedphenols (VSC) with depth for selected sites in the U.S. Great Plains.

Sample

Site l(c,d)ABwBkBky

Site 2(c,m)AlA2BtBCC

Site 3(w,d)AlA2BtlBt2Bt3Btk

Site 4(w,m)AlA2ABBtlBt2Bt3

Site 5(h,d)AlA2BklBk2Bk3BCk

Site 6(h,m)AlA2ABBwlBw2BCk

Depth

cm

0-22-16

16-4040-45

0-88-14

14-3131-4242-50

0-22-66-16

16-2727-4141-66

0-55-15

15-2929-4848-6060-65

0-44-16

16-2828-4444-5757-65

0-22-17

17-3636-5252-6464-69

SOC

-gkg'1

13.28.56.94.9

12.911.28.96.76.3

26.716.312.610.08.16.2

40.329.221.618.412.08.7

15.113.711.29.37.66.8

32.221.613.017.29.87.4

Nsoil —

1.51.11.00.7

1.31.21.10.80.7

2.31.61.41.00.80.6

3.32.72.01.61.20.9

1.41.31.00.90.70.6

2.92.41.11.70.80.6

SOC/N

8.87.76.97.0

9.99.38.18.49.0

11.710.29.0

10.010.110.4

12.210.810.811.510.09.7

10.810.511.210.210.811.3

11.19.0

11.810.112.212.4

NCPS

586441467478

572457519541364

567376424405250253

695643657659836805

498355388355260240

498569462447524469

CPS

- gkg-1 SOC -

60415038

6047525247

715534524728

976066364047

484333454138

785038344147

VSC

18.218.612.57.9

21.822.810.27.38.8

26.430.518.013.912.111.0

28.916.36.84.23.13.7

23.314.312.49.17.44.8

27.714.17.27.96.36.7

LNQt

31142.225.016.5

38.149.919.714.024.2

46.681.142.534.348.443.5

41.625.310.46.43.74.6

46.840.331.925.628.520

55.624.815.617.712.014.3

LCQt

——— 103 ——

303454268208

363485196132187

372555529267257393

2982721031177879

485333376202180126

355282189232154143

(VSQ/N

1611448955

216213

826180

309310162139122114

35217774483136

252150139937954

39712785807783

t LNQ = lignin/noncellulosic polysaccharide quotient (VSC/NCPS).t LCQ = lignin/cellulosic polysaccharide quotient (VSC/CPS).

explained only 10% (r2 = 0.1) of VSC-lignin variationsin the SOM of the O- to 15-cm depth interval.

The amount of cellulosic polysaccharides in the top-soils averaged 53 g kg"1 SOC. This is much less thanwould be expected for fresh grass and root materials,which consist of =400 g cellulose kg"1 of tissue C(Molloy and Speir, 1977). Therefore, the data suggestthat cellulose deposited in plant residues is decomposedquickly and prior to extensive adsorption to the mineralmatrix.

Table 3. Average lignin signatures of six native topsoils (0-15cm) and the corresponding subsoil horizons (<= 50-60 cm) sam-pled from selected sites in the U.S. Great Plains.

Sample parameter

Mean topsoils(standard deviation)

Mean subsoils(standard deviation)

Difference

(ac/alXt

0.33(0.10)0.46

(0.09)0.13**

(ac/al),*

0.52(0.12)0.60

(0.14)0.08*

S/K§0.89

(0.12)0.79(0.17)

-0.10*

*, ** P < 0.05 and P < 0.001 significance; (-test via paired differences(Hartung, 1989).

t (ac/al)v = ratio of vanillic acid to vanillin.t (ac/al), = ratio of syringic acid to syringaldehyde.I SIV = ratio of syringyl to vanillyl structural units.

The NCPS constituted 240 to 840 g kg"1 SOC. Sincehemicelluloses are less stable than cellulose, it is reason-able to speculate that the NCPS fraction is mainly com-posed of microbial products as opposed to plant-derivedhemicelluloses. Assuming that polysaccharides consistmainly of — [CeHioOs],, and — [CsHgQt],, compounds withan average C content of =450 g kg"1 polysaccharides,C in NCPS represented 10.8 to 37.8% of the total SOC.

At a given MAP, polysaccharide contents per kilogramtopsoil SOC tended to decrease with increasing MAT(r = —0.45, not significant). Higher precipitationseemed to favor carbohydrate enrichment (r = 0.69, notsignificant). Combining both effects, we obtained a highcorrelation of total polysaccharide content per gram top-soil SOC with the MAP/MAT ratio (r = 0.86, P <0.05) (Fig. 1). Primary plant production, and thus theabsolute input of labile C, increased with increasingprecipitation (Sala et al., 1988; USDA-SCS, 1994). Wehave not found any data in the literature, however,indicating that different grass species of the North Ameri-can prairie might differ substantially with respect totheir carbohydrate contents. Assuming that the averagechemical composition of the grass vegetation is similar


800

-„„ 700 -.xanc/5Q2

600 -

500

r = 0.86

i 400 -O

I 300 -| 95 % confidence interval

10 20 30i

40 50 60 70 80

MAP/MAT [mm/°C]

Fig. 1. Total carbohydrate contents per kilogram soil organic carbon(SOC) of six native topsoils with different ratios of mean annualprecipitation (MAP) to mean annual temperature (MAT).

among sites, the relative proportion of plant-derivedcarbohydrates should remain constant under differentprecipitation regimes (temperature constant). Further-more, Parton et al. (1987) indicated that decompositionproceeds more quickly at moist sites than at dry ones.If true, then polysaccharides are lost rather than gainedin areas with higher MAP (temperature constant). Tobetter understand the processes involved, we reexaminedchanges in organic matter characterization with depth.

We found that the LNQ decreased with increasingprofile depth (Table 2). This suggests that the chemicallymore stable lignin compounds decomposed more rapidlythan did the polysaccharides, an observation that appar-ently contradicts decomposition patterns reported forforest litter. With increasing decomposition of beech(Fagus sylvatica L.) litter, Ziegler and Zech (1991) foundthat the LNQ increased despite microbial resynthesis ofNCPS during their incubation experiments. Values ofLNQ also increased with depth in organic layers of forestsoils and, thus, might have been indicative of advancedhumification (calculated from Kogel-Knabner et al.,1988). As the LNQ of our samples decreased with soildepth, we conclude that mechanisms must exist that leadto the stabilization of otherwise labile polysaccharidesso as to reduce their bioavailability (Baldock et al., 1992).Such a preferential stabilization of polysaccharides hasbeen reported by Tiessen et al. (1984) and Martin andHaider (1986).

In mineral soils, dissolved saccharides may diffuseinto pores where they are protected from microbial attack(Adu and Oades, 1978; Ladd et al., 1993). In addition,saccharides may become persistent when adsorbed onclay surfaces (Tisdall and Oades, 1982). Lignin, how-ever, does not have enough active sites to bind effectivelywith soil colloids or clay surfaces. It is, therefore, de-pleted relative to microbial-derived saccharides in thesilt- and clay-associated SOM fractions of grassland soilsof the tropical Savanna (Guggenberger et al., 1995) orthe Great Plains (Amelung, 1996, unpublished data).Lignin, though, is more stable than plant-derived sugarsin soil, but only the latter are incorporated into themicrobial biomass (Stott et al., 1983). Thus only carbohy-

drate structures are recycled in soil, whereas lignin de-rived phenols may be readily oxidized by cometabolicprocesses (Stott and Martin, 1990). Consequently, NCPSthat are reproduced by microbes during litter decomposi-tion may have longer residence times than lignin in soil.

South of South Dakota [Sites 3(w,d) to 6(h,m)], poly-saccharides tended to remain constant or increase withdepth at the wetter sites and decrease at the drier sites(Table 2). This resulted in significantly lower lignin/polysaccharide ratios (LNQ) at the lowest B horizonsand, thus, steeper LNQ gradients in the wet sites thanin the corresponding dry ones (Table 2). Therefore, itappears that stabilizing processes were more active inthe subsoils at the wetter sites. Our results can be recon-ciled with the findings of Tsutsuki and Kuwatsuka (1989)who found polysaccharide maxima in very old (14C age=30 000 yr) SOM of deep subsoils of Japanese Andisols.

Our reasoning for the increased stability of polysaccha-rides in more humid climates is as follows: (i) Plantsproduce more organic C under moist conditions (Sala etal., 1988). Microbes convert the litter to higher absoluteamounts of NCPS. Because microbes contribute activelyto stability of microaggregates (Cheshire, 1985; LynchandBragg, 1985; Haynes and Swift, 1990), microaggreg-ates are more stable in humid environments. Thus, theremay be a positive feedback-loop, i.e., polysaccharidescontribute to the stability of microaggregates and stablemicroaggregates contribute to the stability of their poly-saccharides.

(ii) At drier sites, there is less earthworm activity;for example, we found wormcasts only at the moist Sites4(w,m) and 6(h,m). Earthworm casts, however, showedan enrichment of both plant and microbial derived sugarscompared with surrounding savanna soils (Guggenbergeret al., 1995). In addition, Lavelle and Martin (1992)suggested that SOM may be physically protected fromdecay in the compost structure of the casts.

Nuclear Magnetic ResonanceThe quantitative reliability of solid state 13C CP/MAS

NMR spectroscopy was discussed controversially bySnape et al. (1989). Ernst et al. (1990, p. 189) statedthat, in general, intensities and lineshapes of spectraobtained with cross polarization tend to be unreliable.Therefore, changes of NMR signal intensities obtainedwith cross polarization should be interpreted cautiously.In contrary, liquid state NMR spectra were obtainedquantitatively.

Area measurements from liquid state 13C-NMR spectra(Fig. 2) indicated that the alkali-extracted organic matterwas composed of 25% paraffinic C (0-60 ppm), themajority of which (13% of total extracted organic matter)resonated at O to 35 ppm (aliphatic chains), 6% of theextracted SOM seemed to be present as branched alkylstructures or amino acid C (35-50 ppm), while the re-maining 5% of total signal intensity was attributed toOCHa-C as present in lignin (50-60 ppm). Both carbohy-drate structural C (60-110 ppm) and aryl-C (110-160ppm) averaged 29% of the total area intensity. Only aminor part (5% of total area intensity) of aryl-C, how-

120

Inverse gated decoupling13C liquid-state NMP spectra

SOIL SCI. SOC. AM. J., VOL. 61, JANUARY-FEBRUARY 1997

1H liquid-state NMP spectra

Site 2 (c,m)MAP = 497 mmMAT = 6.8" C

Site 3 (w,d)MAP = 500 mmMAT=17.7°C

Site 5 (h,d)MAP = 446 mmMAT = 23.4<>C

Site 4 (w,m)MAP = 839mmMAT =12.6° C

Site 6 (h,m)MAP = 829 mmMAT = 20.0°C

13C CP/MAS solid-stateNMP spectra

Carboxyl-C O-Alkyl-CI I I I,Aryl-C, 'Alkyl-C

O-Alkyl-H,mainly I

Alkyl-H

150 100PPM

150 100PPM

Chemical Shift

10.0 B.O 6.0 4.0PPM

2.0 0.0

Fig. 2. Nuclear magnetic resonance (NMR) spectroscopic characterization of the soil organic matter from topsoils (0-15 cm) of the Great Plains.The upper three spectra describe a transect of mean annual temperature (MAT) in the dry region of the prairie; the lower two spectra arefrom a transect in the moist region, which receives higher mean annual precipitation (MAP) (CP/MAS = cross polarization magic anglespinning).

ever, was substituted by O or N (140-160 ppm). Carbox-ylic-C (160-200 ppm) remained fairly constant at 17 +1 % of total area intensity. In 'H liquid state NMR spectra,neither phenolic H nor H from carboxylic functionalgroups can be detected because of chemical exchangereactions with the solvent. Higher area percentages are,therefore, obtained for H in alkyl (0-3 ppm; 50% oftotal area intensity) or O-alkyl structures (3-4.5 ppm;35% of total area intensity; Fig. 2).

There was no significant correlation of aryl-C withclimatic elements such as MAT, MAP, or the MAP/MAT ratio. As the relative signal intensities for car-boxyl-C (should increase if lignin is oxidized) and me-thoxyl-C (should decrease if lignin is mineralized) re-mained fairly constant for the SOM extracts investigated,small variations of aryl-C are unlikely to be caused byalterations of lignin. Liquid state 13C-NMR, therefore,

confirmed that dynamics of lignin were not related toclimate.

The results of the wet-chemical analyses of polysaccha-rides generally supported compositional interpretationsderived from the intensities of O-alkyl-C signals in13C-CP/MAS and 13C solution NMR (60-110 ppm) or'H solution NMR (3.0-4.5 ppm) spectra (Fig. 2 and3). In general, carbohydrate contents as estimated fromO-alkyl-C signals intensities exceeded that of quantita-tive, wet-chemical saccharide analysis.

The spectra of Sites 2(c,m), 3(w,d), and 5(h,d), whichreceived similar amounts of MAP, indicate that the rela-tive proportions of alkyl-species (0-50 ppm, 13C NMR;0-3.0 ppm, 'H NMR) increased with higher temperatures(Fig. 2, upper three spectra). Whether this tendency alsoexisted at sites that had higher precipitation amounts(830 mm) is uncertain, because higher alkyl-C resonances


40

O 35OCO

30 -

S 25 -

20 -

15

• 13,

3C-solid-state-NMR(r=0,92**) [1]'c-liquid-state-NMR (r = 0.64, n.s.) [2]

H-liquid-state-NMR(r = 0.75*) [3]

20 25 30 35 40—T~

45 50

Signal intensity of O-alkyl structures derived from I3C and 'H-NMR[% of total intensity]

Fig. 3. Polysaccharide concentration of soil organic matter as relatedto area measurements from O-alkyl signals of I3C- and 'H-nuclearmagnectic resonance (NMR) (SOC = soil organic matter).

in the 13C CP/MAS NMR spectra of bulk soil samplesfrom Site 6(h,m) were not confirmed by solution 'H and13C NMR measurements (Fig. 2, lower two spectra). Thismay, however, be attributed to incomplete extraction ofSOC (rendering the sample poorly representative) or tothe low quantitative reliability of CP/MAS mentionedbefore. Subdividing the area of alkyl-C signals revealedthat alkyl structures resonating in the range of 35-50ppm (13C-NMR) and 1.9-3 ppm ('H-NMR) significantly(P < 0.05) and positively correlated with MAT (r =0.72* for 13C; r = 0.90* for 'H). This correlationremained significant even after canceling the most intensealkyl resonances in the spectra obtained from the extractof Site 5(h,d) (r = 0.80*, 13C; r = 0.99**, 'H).

An enrichment of alkyl structures at high temperaturesmight be explained by the following processes: (i) Duringaccelerated mineralization of carbohydrates in hotter cli-mates, more refractory fatty components may be pro-duced by microbes and selectively preserved in the clayfraction (Baldock et al., 1989, 1992; Schnitzer et al.,1988). (ii) Higher input of waxes due to cacti and bushesmay be the reason for additional, higher proportions ofalkyl resonances in the NMR spectra from Site 5(h,d).Also algae may produce refractory, branched alkyl struc-tures (Tegelaar et al., 1989). Little is known, however,about algal activities in steppe soils.

These data suggest that the relative abundance of al-kyl-C species and of polysaccharides may be the bestindicators of the effect of climate on the composition ofSOM.

Texture EffectsAccording to McDaniel and Munn (1985), stabilizing

effects of clay minerals should occur mainly at highertemperatures. We tested this effect by analyzing topsoilsamples from Sites 7(c,d), 8(c,m), 9(w,d), and 10(h,d)(Table 1), which have a similar climate and vegetationspecies but coarser texture than Sites l(c,d), 2(c,m),3(w,d), and 5(h,m).

200 150 100PPMChemical Shift

Fig. 4. The 13C cross polarization magic angle spinning nuclear mag-nectic resonance spectra of the soil organic matter from coarse-(solid lines) and fine-textured (dotted lines) soils of Sites (a) 8(c,m)and 2(c,m) and (b) 10(h,d) and 5(h,d) in O- to 15-cm soil depth.

In regions of low MAT, the area percentages of thespectra from coarse- and fine-textured sites differed by<3%. The similarity of these spectra suggests that factorsother than clay content control SOM composition. Ifclays preferentially adsorb alkyl compounds, then theSOM of soils with lower clay contents should containless alkyl structures than the SOM of finer textured sites.However, only the spectra from the topsoils of Site10(h,d) suggested the presence of lower proportions ofalkyl-C as indicated by a 6% lower relative signal inten-sity at 0-50 ppm (Fig. 4). This gives some support tothe hypothesis that enrichment of alkyl moieties in theclay fraction might be more important in hot than incool climates.

The lignin-derived phenols did not change significantlywith clay content. Noncellulosic polysaccharides weresignificantly (P < 0.05) higher at Site 9(w,d) (536 gkg^ SOC) than at Site 3(w,d) (450 g kg"1 SOC) and,similarly, they were higher at Site 10(h,d) (462 g kg"'SOC) than in Site 5(h,m) (372 g kg"1 SOC). Becausethis differs from the findings obtained with NMR spec-troscopy, which indicated similar O-alkyl-C resonancesin the 13C CP/MAS NMR spectra of coarse- and fine-textured sites, it remains to be shown whether the higherclay content may have inhibited hydrolysis of carbohy-drates (Tanaka et al., 1992). The same applies to cellu-losic saccharides, which were significantly (P < 0.05)enriched by a factor of 1.6 and 1.3 at Sites 7(c,d) and8(c,m), respectively, compared with the correspondingfine-textured sites.

Statistical AnalysisPrincipal axis component analysis was used to assess

whether changes observed for SOM species followedchanges in soil properties. Canceling input parametersrandomly did not change the intercorrelations. Combin-ing the data for topsoils and that for subsoils taken from~30- to 40-cm depth, we obtained four PC that explained81% of the variance:

1st PC (23%): pH, N, NCPS


2nd PC (21 %): fine sand, clay + fine silt, water con-tent at 15 bar

3rd PC (21%): very fine sand, CEC, CaCO3, (N)4th PC (16%): cellulosic saccharides, VSC-lignin,

(N)Using the data from topsoils and from horizons at

~ 40- to 50-cm depth yielded five principal componentsthat explained 88% of variance:

1st PC (20%): very fine sand, fine sand, clay + finesilt

2nd PC (20%): pH, NCPS3rd PC (20%): N, cellulosic saccharides, VSC-lignin4th PC (15%): CEC, CaCO35th PC (13%): water content at 15 bar, (CEC)The factor loadings, i.e., the correlation coefficients

of the variables with the particular PC they were assignedto, ranged from 0.69 to 0.96. Parameters in parenthesesalso loaded in the particular PC in question at a correlationcoefficient X).50 but <0.69. The assignment of vari-ables to principal components became clear only afterVARIMAX rotation, for which it is assumed that a simplestructure does not exist within the data. Including thedata from Sites ll(w,d) and 12(h,m) and varying theinput data randomly did not affect the data structure,i.e., we were able to assume that our small sample sizedid not create artifacts. The correlations varied amongsoil depths, e.g., N was explained by different PC.Nevertheless, pH and NCPS seemed to be controlled byone factor, whereas cellulosic saccharides and VSC-lignin were controlled by a different single factor. Wefound that values for CEC, CaCOs, water content at 15bar, and texture values were necessarily assigned to PCother than the PC that contained lignin and polysaccha-rides. Thus, changes in the SOM composition with profiledepth did not appear to depend on these soil properties.So we may summarize: (i) The soils were similar enoughto permit comparison of sites across different climaticregions, (ii) Variables not included in the statistical analy-sis, such as MAT and MAP, were responsible for varia-tions in SOC composition. Except for Site 5(h,d), grassspecies dominated the vegetation present at the sites. Wecould not find published reports that the different grassspecies encountered vary substantially with respect totheir carbohydrate contents or production, (iii) Valuesfor pH and NCPS loaded into the same PC but withdifferent signs, i.e., they were negatively correlated.The lower the pH, the greater the contribution of NCPSto SOC. Hence, there was no significant correlationbetween cellulosic saccharides or lignin and pH, indicat-ing that decreasing mineralization of NCPS was not dueto increasing acidity. We suggest that the correlationobserved between pH and NCPS is spurious. A higherwater supply favors accumulation of saccharides in soil,but it also induces leaching of cations and thus lowersthe pH. Partializing the effect of pH yielded a coefficientof 0.97** (P < 0.01) for the partial correlation betweentotal carbohydrate contents and the MAP/MAT ratio.This emphasizes the important effect of climate on SOCcomposition in topsoil apparent in Fig. 1. We suggest

that both pH and NCPS were controlled by precipitationand that there was no causal correlation between the twovariables.

CONCLUSIONSThe climate of a site influences the chemical composi-

tion of SOM. Yet, no reports could be found to indicatethat the differences in grass species distribution mayhave caused the observed differences in soil saccharidecontent. In addition, the soil properties of the sites sam-pled were sufficiently similar to discount the possibilitythat they significantly influenced the chemical composi-tion of SOM. We conclude that our results can be ex-plained by the following processes: (i) Poly saccharidesare not mineralized in preference to lignin in soil profiles.Thus, in addition to the recycling of carbohydrate struc-tures by microbes, there must be processes that protectless stable polysaccharides from decay. These processesseem to depend on climate, (ii) The warmer the climate,the more quickly microbes decompose SOM and the lesspolysaccharides persist. This is probably accompaniedby an enrichment of alkyl structures relative to otherfunctional groups, (iii) Moisture promotes the productionof polysaccharides by plants and microbes as well astheir stabilization. Thus, SOM from wet sites containsmore polysaccharides than does that found under drierconditions.

ACKNOWLEDGMENTSWe thank US National Resources Conservation Service for

support during field work. R. Malcolm is thanked for helpfuldiscussions and D.W. Nettleton for sample storage and trans-port. The research was supported by the Deutsche Forschungs-gemeinschaft (SFB 137, TP Cl; DFG Ze 154/22-1).


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Climatic Effects on Soil Organic Matter Composition in the Great Plains