DIVISION S-6—SOIL & WATER MANAGEMENT& CONSERVATION

Soil Structure and Organic Matter: I. Distribution of Aggregate-Size Classesand Aggregate-Associated Carbon

J. Six,* K. Paustian, E. T. Elliott, and C. Combrink

ABSTRACTCultivation reduces soil C content and changes the distribution

and stability of soil aggregates. We investigated the effect of cultivationintensity on aggregate distribution and aggregate C in three soilsdominated by 2:1 clay mineralogy and one soil characterized by amixed (2:1 and 1:1) mineralogy. Each site had native vegetation (NV),no-tillage (NT), and conventional tillage (CT) treatments. Slaked(i.e., air-dried and fast-rewetted) and capillary rewetted soils wereseparated into four aggregate-size classes (<53, 53-250, 250-2000,and >2000 ixm) by wet sieving. In rewetted soils, the proportion ofmacroaggregates accounted for 85% of the dry soil weight and wassimilar across management treatments. In contrast, aggregate distribu-tion from slaked soils increasingly shifted toward more microaggreg-ates and fewer macroaggregates with increasing cultivation intensity.In soils dominated by 2:1 clay mineralogy, the C content of macroag-gregates was 1.65 times greater compared to microaggregates. Theseobservations support an aggregate hierarchy in which microaggregatesare bound together into macroaggregates by organic binding agentsin 2:1 clay-dominated soils. In the soil with mixed mineralogy, aggre-gate C did not increase with increasing aggregate size. At all sites,rewetted macro- and microaggregate C and slaked microaggregate Cdiffered in the order NV > NT > CT. In contrast, slaked macroaggre-gate C concentration was similar across management treatments, ex-cept in the soil with mixed clay mineralogy. We conclude that increas-ing cultivation intensity leads to a loss of C-rich macroaggregatesand an increase of C-depleted microaggregates in soils that expressaggregate hierarchy.

TISDALL AND CADES (1982) presented a conceptualmodel for aggregate hierarchy that described how

primary mineral particles are bound together with bac-terial, fungal, and plant debris into microaggregates.These microaggregates, in turn, are bound together intomacroaggregates by transient binding agents (i.e., mi-crobial- and plant-derived polysaccharides) and tempo-rary binding agents (roots and fungal hyphae). Threeconsequences of this aggregate hierarchy are: (i) a grad-ual breakdown of macroaggregates into microaggreg-ates before they dissociate into primary particles, asincreasing dispersive energy is applied to soil (Oadesand Waters, 1991), (ii) an increase in C concentrationwith increasing aggregate-size class because large aggre-gate-size classes are composed of small aggregate-sizeclasses plus organic binding agents (Elliott, 1986), and

Natural Resource Ecology Lab., Colorado State Univ., Fort Collins,CO 80523. Received 21 Dec. 1998. *Corresponding author (johan®nrel.colostate.edu).

Published in Soil Sci. Soc. Am. J. 64:681-689 (2000).

(iii) younger and more labile organic matter is containedin macroaggregates than in microaggregates (Elliott,1986; Puget et al, 1995; Jastrow et al., 1996).

Oades and Waters (1991) tested the aggregate hierar-chy theory in different soils by applying a range of treat-ments to disaggregate soils. They concluded that aggre-gate hierarchy existed in two Alfisols and a Mollisolbecause organic materials were the major binding agentsfor aggregate formation and stabilization in these soils.In contrast, they found that an Oxisol did not expressany hierarchical aggregate structure, probably becauseoxides, rather than organic materials, were the dominantstabilizing agents. Elliott (1986) observed more organicmatter associated with macroaggregates than with mi-croaggregates in a temperate grassland soil. He alsofound that organic matter associated with macroaggre-gates was more labile than organic matter associatedwith microaggregates. Jastrow et al. (1996) reportedgreater amounts of recently incorporated organic matteras aggregates became larger, supporting the idea thatmicroaggregates are bound together by young organicmatter into larger macroaggregates.

Aggregate hierarchy theory has been used by manyauthors to explain the correlation between a reductionin aggregation and loss of soil organic matter (SOM)with cultivation (Elliott, 1986; Cambardella and Elliott,1993; Beare et al., 1994). A breakdown of macroaggre-gates results in a release of labile SOM (Elliott, 1986)and its increased availability for microbial decomposi-tion. The increased microbial activity depletes SOM,which eventually leads to lower microbial biomass andactivity and consequently a lower production of micro-bial-derived binding agents and a loss of aggregation(Jastrow, 1996; Six et al., 1998).

Reduced aggregation (and subsequent lower levelsof SOM) in conventional tillage (CT) vs. no-tillage (NT)(Paustian et al., 1999) is a result of several indirecteffects on aggregation. Tillage brings subsurface soil tothe surface where it is then exposed to wet-dry andfreeze-thaw cycles and subjected to raindrop impact(Beare et al., 1994; Paustian et al., 1997), thereby in-creasing the susceptibility of aggregates to disruption(Willis, 1955; Hadas, 1990; Edwards, 1991). Plowingchanges the soil conditions (e.g., temperature, moisture,and aeration) and increases the decomposition rates

Abbreviations: CT, conventional tillage; IPOM, intra-aggregate par-ticulate organic matter; LF, light fraction; NV, native vegetation; NT,no-tillage; POM, particulate organic matter; SOM, soil organic matter.

681

682 SOIL SCI. SOC. AM. J., VOL. 64, MARCH-APRIL 2000

Table 1. General characteristics of the agricultural experiment field sites.

Soil classificationSoil seriesTextureMineralogy!MAT (°C)tMAP (mm)§Crop rotationPrior vegetationYear of establishment

Sidney, NE

Pachic HaplustollDurocloamillite, chlorite8.5440winter wheat-fallowshortgrass prairie1969

Wooster, OH

Typic FragiudalfWoostersilt loamchlorite, illite9.1905continuous corngrass meadow1962

KBS, MI

Typic HapludalfKalamazoo and Oshtemosandy loamchlorite, illite9.2920corn-soybean-winter wheatgrassland1986

Lexington, KY

Typic PaleudalfMaurysilty clay loamvermiculite, kaolinite, illite13.11127continuous corn (84 kg N)blucgrass pasture1971

t Dominant clay minerals in order of dominance. Data adopted from Six et al. (1999b).t MAT = mean annual air temperature.§ MAP = mean annual precipitation.

of litter (Rovira and Greacen, 1957; Cambardella andElliott, 1993). In NT, residues accumulate at the surfacewhere the litter decomposition rate is slowed due todrier conditions and reduced contact between soil mi-croorganisms and litter (Salinas-Garcia et al., 1997). Fi-nally, the proportion of the microbial biomass composedof total fungi (Frey et al., 1999) and mycorrhizal fungi(O'Halloran et al., 1986) is generally higher in NT com-pared to CT and it has been observed that fungi (espe-cially mycorrhizal) contribute to macroaggregate forma-tion and stabilization (Tisdall and Oades, 1982).

The objectives of this study were to (i) test the validityof the aggregate hierarchy theory over a range of soilsand (ii) study the affect of increased cultivation intensityon aggregation and aggregate-associated C.

MATERIALS AND METHODSSampling

Soils from four long-term agricultural field experiments(Table 1) were sampled at two depths (0-5 cm and 5-20 cm)in November 1995. The four sites are located in Sidney, NE(41°14'N, 103°00'W), Wooster, OH (40°48'N, 82°00'W), W.K. Kellogg Biological Station (KBS), MI (42°24'N, 85°24'W),and Lexington, KY (38°07'N, 84°29'W). All four sites haveNV, NT, and CT treatments with either three or four repli-cates. At Sidney, all management treatments were establisheddirectly from the native shortgrass prairie. At KBS, NT and CTplots were under long-term cultivation before establishment,whereas the adjacent NV plot was in grass vegetation on aformer woodlot that had never been cultivated. At Wooster,the NV plot was in nearby forest, but the NT and CT plots wereestablished in a 6-yr-old grass meadow that was cultivated formany years prior to establishment. At Lexington, the experi-mental plots were initially under long-term cultivation, butfor 50 yr prior to treatment establishment were in bluegrasspasture. The Lexington soil is characterized by a mixed-claymineralogy dominated by kaolinite (1:1 layer type) and ver-miculite (2:1 layer type). In addition, a 2 to 16 times higherconcentration of amorphous and poorly crystalline Fe- andAl-oxides was reported for the Lexington soil compared tothe Sidney, Wooster, and KBS soils (Six et al., 1999b). TheSidney, KBS, and Wooster soils have only 2:1 minerals, pre-dominantly illite and chlorite (Six et al., 1999b).

Aggregate SeparationField-moist soil was gently broken to pass an 8-mm sieve

and air-dried. Aggregate separations and soil stability assess-ments were done by wet sieving. Two pretreatments wereapplied before wet sieving: (i) air-dried soil was rapidly im-mersed in water (slaked treatment) and (ii) air-dried soil was

capillary-rewetted before immersion in water (rewetted treat-ment). For capillary rewetting, dried soil was placed on filterpaper that was slowly moistened until a water content of1.05 times field capacity was reached. The volumetric watercontent of the soii at field capacity was determined for allindividual samples. A higher amount of disruptive energy oc-curs upon slaking because rapid wetting of dry soil leads toan entrapment of air and a buildup of air pressure within theaggregates (Kemper et al., 1985). Aggregates of lower stabilitydisrupt because they cannot withstand this pressure. In con-trast, soil rewetted to 1.05 times field capacity is at maximumstability (Hofman and De Leenheer, 1975).

The method used for aggregate-size separation was adaptedfrom Elliott (1986). Briefly a 100-g subsample (air-dried orcapillary-rewetted) was submerged for 5 min on a 2000-(jinisieve. Aggregates were separated by moving the sieve (byhand) up and down 3 cm with 50 repetitions during 2 min.The >2000-jxm aggregates were collected and sieving wasrepeated for the <2000-|xm fraction with the next smaller-sized sieve. This procedure was repeated for every sieve size(250 and 53 (xm). All aggregate fractions were oven-dried(50°C) and weighed. Sand content (>53 p-m) of the aggregateswas determined on a subsample of aggregates that were dis-persed with sodium hexametaphosphate (5 g L"1).

Free Light Fraction and Mineral-AssociatedFraction Analysis

The free light fraction (POM outside of the aggregates, orfree LF) and mineral-associated organic matter fraction wereisolated from the three largest aggregate-size classes accordingto Six et al. (1998). Briefly, free LF was isolated by densityflotation in 1.85 g cm"3 sodium polytungstate. The free LFprobably includes both LF outside of aggregates and someLF released from the aggregates upon submersion in sodiumpolytungstate. However, the dispersion of aggregates in so-dium polytungstate is minimal and therefore the released frac-tion was only a small proportion of the free LF. Sodium poly-tungstate was recycled according to Six et al. (1999c) to avoidcross- contamination of C and N between samples. After isola-tion of free LF, aggregates were dispersed in 5 g L~' sodiumhexametaphosphate by shaking for 18 h on a reciprocal shaker.Intra-aggregate (within aggregate) particulate organic matter(IPOM) was isolated by sieving. Aggregate-associated C andmineral-associated C were calculated by difference

aggregate-associated C = total aggregate C

- free LF C [1]

mineral-associated C = aggregate-associated C

- IPOM C [2]

where total aggregate C is total C measured in aggregates prior

SIX ET AL.: SOIL STRUCTURE AND ORGANIC MATTER, AGGREGATE-SIZE & CARBON 683

to free LF flotation. Aggregate-associated C and mineral-associated C were only determined for the slaked microaggreg-ates (53-250 (Jim) and small macroaggregates (250-2000 n,m).The yield of large macroaggregates (>2000 (xm) after slakingwas often too small to be analyzed. For the <53-(xm fraction,the LF yield was too small for C analysis and by definition(Cambardella and Elliott, 1992) there is no IPOM in particles<53 (xm.

Carbon, Nitrogen, and Isotope AnalysesIsotope and organic C and N analyses were performed

according to Six et al. (1998). The natural abundance 13Cmethodology for SOM studies was only done at the Sidneysite. The other locations did not have a single shift in thedominance of plant species with different metabolic pathways(C3 vs. C4) or archived soil samples from the beginning of theexperiment. At Sidney, the delta 13C values (8) were used tocalculate the proportion of wheat-derived C (/) in each organicmatter fraction:

f _ ^1 ~ Op

8W — So [3]

where 8, = 813C at time t, 8W = 813C of wheat straw (crop),80 = 813C of original grassland-derived SOM, and/ = fractionof wheat-derived C in the soil. The proportions of wheat-derived C vs. grassland-derived C (1 - /) provide a measureof the relative age of the organic matter in the different sizefractions. The proportions of crop-derived C are only pre-sented for the 0- to 5-cm layer because changes in the 13Csignature with depth confound interpretations at the 5- to20-cm depth. In addition, differences in C concentrations weremainly observed in the 0- to 5-cm layer.

Statistical AnalysesThe experimental field design was a randomized complete

block design for all sites. However, the NV were not replicatedwithin the experiments at Wooster, KBS, and Lexington andtherefore not included in the statistical analysis. Analysis ofvariance (ANOVA-GLM, SAS Institute, 1990) was performedwith multiple comparisons within a depth. Tillage treatmentwas the main factor in the model, with aggregate size andreplicate as secondary factors. Separation of means was testedusing Tukey's honestly significant difference at a level ofP < 0.05.

RESULTSWhole Soil Characteristics

Total organic C and N (0-20 cm) generally decreased(but not always significantly) in the order NV > NT >CT (Table 2). At all sites, significant differences in totalorganic C and N between treatments were observed inthe 0- to 5-cm depth, except at KBS where NT and CTwere not significantly different. Organic C and N levelswere on average 38% lower in CT compared to NT inthe 0- to 5-cm depth (Table 2). In contrast, there wereno significant differences observed in total C and N inthe 5- to 20-cm depth at any site. Bulk density wasnot significantly affected by tillage at any of the sites(Table 2).

Aggregate-Size DistributionAt all sites, rewetted aggregate-size distributions were

dominated by macroaggregates (250-2000 u,m and

>2000 (Jim), which on average accounted for 85% ofthe dry soil weight (Fig. 1). A nonsignificant higherproportion of large macroaggregates was found in NTcompared to NV and CT, especially at Sidney and KBS.Otherwise, there were no major differences amongtreatments in the rewetted aggregate distribution. Incontrast, the slaked aggregate-size distribution differedbetween management treatments at all sites (Fig. 1).Proportions of macroaggregates decreased in the orderNV > NT > CT, except that NV and NT were similar atWooster. The lack of differences in aggregation betweenNV and NT at Wooster (Fig. 1) was consistent withtheir similar values for total C (Table 1) and total POM(Six et al., 1999a). At all sites, there was a reduction inlarge macroaggregates and an increase in microaggreg-ates with slaking compared to rewetting. The proportionof silt and clay particles (<53 |a,m) increased from =0.05in rewetted samples to 0.15 in slaked samples and thisincrease was greatest in the Lexington soil (Fig. 1).

At Sidney and Wooster, tilled soils showed a substan-tial decrease in small and large macroaggregates con-comitant with an increase in microaggregates and asmall increase of silt and clay particles, compared withNT (Fig. 1). The small difference in slaked aggregatedistribution between NT and CT at KBS may be dueto the short duration (9 yr) of the experiment at KBS.At Lexington, CT had fewer large macroaggregates(1.4% vs. 18.0%), more microaggregates (37.3% vs.16.8%), and more silt and clay particles (15.5% vs. 9.3%)than in NT, but there was no difference in the proportionof small macroaggregates between tillage treatments.

Aggregate Carbon ConcentrationsIt is frequently observed that the major differences

in organic matter content between NT and CT soils arein the upper few centimeters of soil (Doran, 1987; Dicket al., 1997). Similarly, we found few differences in ag-gregate C among treatments at the 5- to 20-cm depth(data not shown). The only exception was KBS, wherethe NV treatment had substantially higher aggregate Cconcentrations at 5 to 20 cm compared to NT and CT(data not shown). This trend may be due to the long-term cultivation of NT and CT plots before establish-ment of the experiment. While not reported, trendsacross aggregate-size classes within treatments were thesame in the subsurface layer and surface layer. There-fore, further comparisons are made only for the 0- to5-cm layer.

In general, sand-free C concentrations of all rewettedaggregate-size classes differed in the order NV > NT> CT (Fig. 2). At KBS, NT and CT did not differsignificantly in this respect, which may again be a resultof the young age of the experiment. The apparent largedifferences between rewetted (and slaked) aggregate Cconcentrations in NV versus NT and CT at KBS isprobably also a result of the long-term cultivation ofthe NT and CT plots prior to establishment of the exper-iment. In contrast to the other sites, rewetted aggregateC concentrations at Wooster (the only site with forestvegetation) were not different between NV and NT,

684 SOIL SCI. SOC. AM. J., VOL. 64, MARCH-APRIL 2000

Table 2. Organic carbon, nitrogen, and bulk density in four agricultural experiment sites with native vegetation (NY), no-tillage (NT),and conventional tillage (CT) treatments.

Site

Sidney, NE

Wooster, OH

KBS, MI

Lexington, KY

Depth

cm0-55-200-200-55-200-200-55-200-200-55-200-20

NV

1437ta2653a4090a141811259140081540H140429442324fl27115036

Organic Ct

NT

— g C m~2 —1129b2299a3428ab1598a2209a3806a791a

1631a2422a1295a2448a3742a

CT

699c2208a2907b843b

2537a3380b#627a

1582a2209a662b

2463a3125b

NV

131a265a396a106194300125116241211277789

Organic N

NT

— g N m 2-108b258a366a150a196a346a68a

ISOa218a136a305a441a

CT

82c230a312a76b

219a294b46a

157a204a72b

275a348b

NV

0.82a1.12§a*

-1.021.19

-1.021.38

-1.021.37

-

Bulk density

NT

1.05b1.22b*

-1.19a1.30a

-1.62a1.69a

-1.07a1.26a-

CT

1.181)1.28b-

1.37a1.30a-

1.53a1.60a-

1.05a1.26a-

t For all sites, total organic C (0-20 cm) is adopted from Six et al. (1999a) and all data for Sidney is adopted from Six et al. (1998).t Tillage treatments within a site and depth followed by a different lowercase letter are significantly different (P < 0.05) according to Tnkey's HSD mean

separation test.§ Values within a site and treatment followed by * at the 5-20 cm depth are significantly different compared with corresponding values at 0-5 cm.K No statistics because only one replicate for this treatment.# Significance at the 10% level.

except for the microaggregates. This suggests that forestvegetation is not as beneficial as grassland vegetationfor the accumulation of aggregate C.

At Sidney, Wooster, and KBS (sites with 2:1 clay-dominated soils), slaked aggregate C content increasedwith increasing aggregate size within a managementtreatment (except NV at KBS), although the C contentof large macroaggregates was similar to that of smallmacroaggregates (Fig. 2). The C content of small macro-aggregates was on average 1.65 times greater comparedto microaggregates. This trend of greater C content insmall macroaggregates compared to microaggregates isalso apparent in the aggregate-associated C and min-eral-associated C of NT and CT (Table 3). Aggregate-associated C and mineral-associated C concentrationswere more similar across tillage treatments for slakedmacroaggregates compared to slaked microaggregates,in these 2:1 clay-dominated soils (Table 3). The C con-centrations of slaked microaggregates differed in theorder NV > NT > CT.

At Sidney, both grassland- and especially crop-de-rived aggregate C increased with increasing aggregatesize, except for the largest size which had equal or lowerconcentrations than the next-smaller size (Table 4). Inaddition, the percentage of crop-derived C increasedabout 32% and 38% with increasing aggregate size inNT and CT, respectively (Table 4). This observation isin agreement with Puget et al. (1995) and Jastrow etal. (1996) who also observed increasing proportions of"young" C with increasing aggregate size.

In contrast to the 2:1 clay-dominated soils, aggregateC (Fig. 2), aggregate-associated C, and mineral-associ-ated C (Table 3) concentrations for slaked aggregatesdid not differ between macro- and microaggregateswithin a management treatment in the Lexington soil(mixed-clay mineralogy). Feller et al. (1996) also ob-served similar C concentrations in different aggregate-size classes in a low-activity (1:1 clay) soil. Elliott et al.(1991) also found no significant differences in aggregate

C concentrations among aggregate-size classes in a Ulti-sol from the Amazon Basin of Peru.

DISCUSSIONIn soils where SOM is the major binding agent an

aggregate hierarchy has been observed (Tisdall andOades, 1982; Oades and Waters, 1991). SOM is expectedto be the primary binding agent in 2:1 clay-dominatedsoils because polyvalent-organic matter complexes formbridges between the negatively charged clay platelets.In contrast, SOM is not the only binding agent in oxide-and 1:1 clay-dominated soils. Electrostatic attractionsoccur between and among oxides and kaolinite plateletsdue to simultaneous existence of positive and negativecharges at field pH (Schofield and Samson, 1954; El-Swaify, 1980). Thus in those soils aggregate formationis partly induced by electrostatic interactions and aggre-gate hierarchy should be less pronounced (Oades andWaters, 1991).

The increased aggregate- and mineral-associated Ccontent of small macroaggregates vs. microaggregates(within treatment) at Sidney, Wooster, and KBS (Table3) indicates that both IPOM C and mineral-associated Care incorporated during formation of macroaggregates.This also suggests that IPOM C is a major C source formicrobial activity and thereby induces the binding ofclay- and silt-sized particles and microaggregates intomacroaggregates (Jastrow, 1996; Six et al., 1998,1999a)in these 2:1 clay-dominated soils. In addition, the simi-larity of aggregate-associated C concentrations of slakedmacroaggregates across management treatments indi-cates the stability of slaked macroaggregates is corre-lated to their C content (Elliott, 1986; Cambardella andElliott, 1993). The stability of microaggregates, in con-trast, does not seem to be correlated to C content, be-cause there is a difference in slaked microaggregate Ccontent among treatments at all sites (Table 3). Perhapsthe physical characteristics of microaggregates such as

SIX ET AL.: SOIL STRUCTURE AND ORGANIC MATTER, AGGREGATE-SIZE & CARBON 685

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aggregate size classFig. 1. Slaked and rewetted aggregate-size distributions in the surface layer (0-5 cm) of four long-term agricultural experiment sites (SID

Sidney, NE; WO = Wooster, OH; KBS = Kellogg Biological Station, MI; LX = Lexington, KY) with three management treatments (NVnative vegetation; NT = no-tillage; CT = conventional tillage). Bars are standard deviations.

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SIX ET AL.: SOIL STRUCTURE AND ORGANIC MATTER, AGGREGATE-SIZE & CARBON 687

Table 3. Aggregate-associated C and mineral-associated C concentrations for slaked microaggregates and small macroaggregates in fouragricultural experiment sites with native vegetation (NV), no-tillage (NT), and conventional tillage (CT) treatments (0-5 cm depth).

Aggregate-associated C Mineral-associated C

Site

Sidney, NE

Wooster, OH

KBS, MI

Lexington, KY

Aggregate-size class

53-250250-200053-250

250-200053-250

250-200053-250

250-2000

NV

37.37fa38.05a30.98*33.2764.52*47.5941.84*39.70

NT

24.95b33.83a22.16c32.73b15.52b22.61a25.76a26.34a

CTr> i -l A t.

19.27b*31.99a*11.07b*25.63a11.61a*15.08a*12.09a*14.56a*

NV

21 Ma26.57a25.74*27.5837.25*38.1227.21*30.70

NT

21.55b26.15a17.32b21.93a13.36b18.38a19.77a21.12a

CT

17.71b*25.96a9.36a*

19.45b10.44a12.11a*11.34a*12.44a*

t Values within a site and treatment followed by a different lowercase letter are significantly different (P < 0.05) according to Tukey's HSD meanseparation test.

* Indicates that there are significant differences (P < 0.05) between treatments within a site and an aggregate-size class according to Tukey's HSD meanseparation test.

* There are no statistics for this treatment because there were no replicates available.

lower porosity and higher bulk density (Oades and Wa-ters, 1991) are the main factors that confer resistanceto slaking rather than their C content.

The comparison of rewetted and slaked aggregatedistribution and the aggregate-associated C concentra-tions provides information on the degree of aggregatehierarchy exhibited by the different soils. The aggregatehierarchy theory seems applicable to the 2:1 clay-domi-nated soils at Sidney, Wooster, and KBS because of(i) small increases in silt and clay particles, but largeincreases in microaggregates, upon disruption of themacroaggregate (Fig. 1) (Elliott, 1986; Oades and Wa-ters, 1991), (ii) small differences in silt and clay propor-tion between NT and CT (Fig. 1) (Elliott, 1986), and (iii)increased aggregate-associated C concentrations withincreasing aggregate sizes in slaked soils (Table 3) (El-liott, 1986). Additional support for the aggregate hierar-chy at Sidney is provided by the 13C natural abundancedata (Table 4). The increase in proportions of young Cwith aggregate size indicates that microaggregates arebound together into larger macroaggregates by crop-derived C (Puget et al., 1995; Jastrow et al., 1996).

Elliott (1986) used the aggregate hierarchy theory asa basis to explain reduced SOM level in a stubble mulchagroecosystem compared to native sod. We apply thistheory to explain the decreasing SOM content in theorder NV > NT > CT at Sidney, Wooster, and KBS(soils that express aggregate hierarchy). However, NVat KBS had much higher slaked aggregate C contents

than NT and CT, probably a result of the difference inhistory of the NV vs. the NT and CT plots; thereforethe NV treatment is ignored in the discussion below.

Increasing cultivation intensity led to a loss of C-richmacroaggregates and an increase of C-depleted mi-croaggregates. There were no consistent significant dif-ferences in the proportions of rewetted macroaggre-gates (>250-|o,m fractions) among managementtreatments (Fig. 1), but the rewetted large and smallmacroaggregate C concentrations differed in the orderNV > NT > CT (Fig. 2). In contrast, the proportionsof slaked macroaggregates differed in the order NV >NT > CT (Fig. 1), but the slaked macroaggregate Cconcentrations were similar across management treat-ments (Fig. 2 and Table 3). These observations suggestthat the C lost with increasing cultivation intensity isresponsible for the higher proportions of stable macro-aggregates (i.e., slaked macroaggregates) in the orderNV > NT > CT; it is the C of binding agents that bindsindividual microaggregates into stable macroaggregates(Tisdall and Oades, 1982; Elliott, 1986). In our study,increasing cultivation intensity increased the proportionof slaked microaggregates, which were depleted in Ccompared to macroaggregates and increasingly depletedin C with increasing cultivation intensity (Fig. 2). There-fore we conclude that increasing cultivation intensityleads to a loss of C-rich macroaggregates and an increaseof C-depleted microaggregates in soils that express ag-gregate hierarchy. This shift results in a loss of total

Table 4. Grassland-derived aggregate C concentrations and crop-derived aggregate C proportions and concentrations in Sidney, NE, asdetermined by 13C natural abundance analysis (0-5 cm; slaked).

Treatment

No-tillage

Conventional tillage

Aggregate-size class

tun<5353-250*

250-2000>2000<5353-250

250-2000>2000

Grasslandt-derived C

————— g C kg ' sand16.57c21.01b25.97a19.05bc20.34b18.72a26.44aNA

Crop-derived C

2.20bS.OOb

12.49a14.85a0.30b1.69h

16.72aNA

Percentage crop-derived C

%12c19b32a44a0.4c8h

39aNA

t Values within a treatment followed by a different lowercase letter are significantly different (P < 0.05) according to Tukey's HSD mean separation test.t Grassland- and crop-derived C data for the 53-250 and 250-2000 |xin aggregate-size class are adopted from Six et al. (1999a).NA = not analyzed.

688 SOIL SCI. SOC. AM. J., VOL. 64, MARCH-APRIL 2000

organic C. The C lost was that which binds microaggre-gates into macroaggregates. These observations madeat Sidney, Wooster, and KBS extend Elliott's resultsfrom stubble mulch agroecosystems to NT and CT agroe-cosystems characterized by 2:1 clay-dominated soils.

The aggregate hierarchy theory seems to be less appli-cable to the Lexington soil (mixed mineralogy) becausewithin management treatments (i) similar total aggre-gate C, aggregate-associated C, and mineral-associatedC concentrations were observed across aggregate-sizeclasses (Table 3), (ii) the proportion of silt- and clay-sized particles increased the most from the rewetted toslaked aggregate distribution at Lexington (Fig. 1), and(iii) large macroaggregates broke up into silt and clayparticles and microaggregates with increasing cultiva-tion intensity, whereas the proportion of small macroag-gregates was about the same.

Beare et al. (1994) also observed only small differ-ences in aggregate distribution between NT and CTin a kaolinitic soil in Georgia. The largest differencebetween NT and CT was in the proportions of largemacroaggregates, which primarily fell apart into silt-and clay-sized particles. The proportions of small macro-aggregates were similar across tillage regimes (Beare etal., 1994). As previously mentioned, Feller et al. (1996)and Elliott et al. (1991) did not observe increased Cconcentrations with increasing aggregate size in 1:1 clay-dominated soils. The less-pronounced aggregate hierar-chy in the Lexington soil is probably a result of thepresence of Fe- and Al-oxides and kaolinite (1:1 clay)which contribute to soil stability through electrostaticinteractions (Oades and Waters, 1991). We concludethat the Lexington soil does not express as much aggre-gate hierarchy as the 2:1 clay-dominated soils becauseof the presence of oxides and 1:1 clays.

In capillary-wetted aggregates from a kaolinitic soil,Beare et al. (1994) observed a higher C content in mi-croaggregates than in macroaggregates. We observedthe same trend of increasing C content from large mac-roaggregates to microaggregates in rewetted soils atLexington. However, at the other sites no significantdifferences in C content between macroaggregates andmicroaggregates in rewetted soils within a managementtreatment were observed. Other authors also found nodifferences in misted or rewetted aggregate C contentswithin a treatment (Elliott, 1986; Cambardella and El-liott, 1993). The higher C content observed in nonslakedmicroaggregates may therefore be specific for soils with1:1 clay minerals.

CONCLUSIONSAggregate hierarchy was observed in soils from Sid-

ney, KBS, and Wooster, all dominated by 2:1 clay miner-alogies. The Lexington soil expressed less aggregate hi-erarchy, which may be due to the presence of oxidesand low-activity clays (kaolinite). There was a clear rela-tionship between loss of soil structure and loss of SOMin the soils that expressed aggregate hierarchy. Increas-ing cultivation intensity induced a loss of C-rich macro-

aggregates and a gain of C-depleted microaggregates,resulting in an overall loss of SOM C.

ACKNOWLEDGMENTSThanks are extended to Scott Pavey and Matt Nemeth for

their many hours of sieving, weighing, and C and N analysis.Dan Reuss's help during the laboratory work is greatly ap-preciated. We acknowledge the assistance of Drew Lyon(Univ. of Nebraska, Panhandle Research and Extension Cen-ter, Scottsbluff) at the Sidney site, Edmund Perfect and RobertBlevins (Univ. of Kentucky, Lexington) at the Lexington site,H.P. Collins and G.P. Robertson (Univ. of Michigan, W.K.Kellogg Biological Station, Hickory Corners) at the Kelloggsite, and W.A. Dick (Ohio State Univ., Wooster) at the Woos-ter site. Comments on the manuscript by three anonymousreviewers and the associate editor are acknowledged. Thisresearch was supported by grant DEB-9419854 from the Na-tional Science Foundation.

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