Carbon and Nitrogen Distribution in Aggregates from Cultivatedand Native Grassland Soils

C. A. Cambardella* and E. T. Elliott

ABSTRACTLong-term cultivation of grassland soils reduces soil organic C and

N content and has been associated with a deterioration in the aggre-gate structure of the soil. This study examined the effects of barefallow (moldboard plow), stubble mulch fallow (subtill), and no-tillfallow management on aggregate size distribution and aggregate or-ganic C and N contents compared with a native (virgin) grassland soil.Aggregate size fractions were separated by wet sieving and the pro-portion of soil was quantified for each aggregate size class. Mineral-associated (silt and clay) organic matter was isolated by dispersingaggregates in sodium hexametaphosphate and removing the sand andparticulate organic matter (POM) by passing the dispersed aggregatesthrough a 53-/un sieve. The POM fraction is composed primarily ofpartially decomposed root fragments and has an average C/N ratio ofabout 16. A large proportion of the total soil dry weight (50-60%)was isolated in the small macroaggregate (250-2000 fan) size class.The native grassland soil was more stable than the cultivated soilswhen slaked, and the no-till soil was more stable than the stubblemulch and bare fallow soil when slaked. Reduced tillage managementis effective at increasing the proportion of macroaggregates and resultsin the accumulation of wheat (Triticum aestivum L.) derived POMwithin the aggregate structure compared with bare fallow soil. It haspreviously been shown that the POM fraction accounts for the ma-jority of the soil organic matter (SOM) initially lost as a result ofcultivation of grassland soils. The data reported in this study relatesthe loss of structural stability from cultivation to losses of organic Cand N from the POM fraction.

NATIVE GRASSLAND SOILS are generally highlystructured and rich in SOM. Cultivation reduces

SOM content (Jenny, 1941; Haas et al., 1957) andresults in a deterioration of the aggregate structure(Chancy and Swift, 1984). The POM fraction, whichis composed primarily of partially decomposed rootfragments, accounts for the majority of the SOM ini-tially lost as a result of cultivation of grassland soils(Cambardella and Elliott, 1992). Reductions in aggre-gate stability after cultivation are most pronounced insoil macroaggregates, while the stability of soil mi-croaggregates remains unchanged (Tisdall and Oades,1982; Oades, 1984). As macroaggregates disintegratewith tillage, the proportion of microaggregates in-creases, since microaggregates are not destroyed bycultivation (Tisdall and Oades, 1980; Elliott, 1986).Microaggregates have a lower organic matter concen-tration than macroaggregates (Dormaar, 1983) and thisorganic matter is less labile than that associated withmacroaggregates (Elliott, 1986; Gupta and Germida,1988). The disintegration of macroaggregates withcultivation into nutrient-poor microaggregates and thesubsequent release of plant-available nutrients may beone explanation for the observed pattern of lower or-

C.A. Cambardella, USDA-ARS, National Soil Tilth Lab., 2150Pammel Drive, Ames, IA 50011; and E.T. Elliott, Natural Re-source Ecology Lab., Colorado State Univ., Fort Collins, CO80523. Received 30 June 1992. "Corresponding author.

Published in Soil Sci. Soc. Am. J. 57:1071-1076 (1993).

ganic matter contents and reduced nutrient-supplyingefficiencies in cultivated soils when compared withgrassland soils (Elliott, 1986).

Sustainable production has become a key issue inthe management of cropping systems. Reduced tillageand no-till management have been initiated to mitigatesome of the detrimental effects of intensive cultiva-tion, such as reduction of soil organic matter and theconcomitant increase in erosion and decrease in soilfertility (Fenster and Peterson, 1979). Little infor-mation exists in the literature on the effect of reducedtillage or no-till management on the nutrient contentsof soil aggregates and their associated organic matter.This study examined the effects of bare fallow (mold-board plow), stubble mulch fallow (subtill), and no-till management on aggregate size distributions andaggregate organic C and N contents compared withnative grassland.

MATERIALS AND METHODSField Sampling

Soil was collected from an experimental site located at theHigh Plains Agricultural Research Laboratory near Sidney, ME.The soil type is a Duroc loam (a fine-silty, mixed, mesic PachicHaplustoll) and its parent material is mixed loess and alluvium.The chemical properties of the soil are given in Table 1 (Fens-ter and Peterson, 1979). The site had never been cultivatedprior to 1969, when it was plowed and sown to winter wheat.Plow depth at sodbreaking was 20 cm. Three managementtreatments were established in addition to the control nativegrass plots: bare fallow (plowing), stubble mulch (subtill), andno-till. One wheat crop was removed every other year and thealternate year the ground was fallow. Fertilizer was not appliedto any of the plots. The bare fallow treatment was tilled to adepth of 10 to 15 cm using a moldboard plow in the spring ofthe fallow year followed by two to three cultivator or rotaryrod weeder operations. Stubble mulch fallow was cultivatedwith 0.9- and 1.5-m sweeps two to four times a year at a depthof 10 cm, followed by a rotary rod weeder. Weed controlduring fallow in the no-till treatment was accomplished withherbicides (Fenster and Peterson, 1979).

The experimental design was a randomized complete blockwith three field replicates. The three tillage treatments and thenative grassland control were randomly assigned plots withineach of the three field replicates. The entire design was re-peated in an adjacent area of the field so that the crop andfallow portions of the rotations were represented every yearinstead of every other year. Treatment plots within field rep-licates were 8.5 by 46.0 m (Fenster and Peterson, 1979). Wesampled the west rotation of the experiment, which had beenin fallow since the summer of 1989, in late July of 1990. Coreswere taken with an 8-cm-diam. steel coring bit to a depth of20 cm without removing surface residues. One core was pulledevery 4.5 m along the length of each plot, for a total of 10cores per plot. The exact horizontal location along the widthof the plot for each core was randomly located. To minimizeedge effects, a 0.50-m buffer zone was established along theedges of each plot. The soil was gently broken apart by handin the laboratory and passed through a 2.8-mm sieve while stillAbbreviations: POM, particulate organic matter; SOM, soil or-ganic matter.

1071

1072 SOIL SCI. SOC. AM. 3., VOL. 57, JULY-AUGUST 1993

Table 1. Chemical properties of the Duroc loam (0-1% slope)in 1969 (data from Fenster and Peterson, 1979).

Soil depthcm0-10

11-2021-30

PH

7.47.47.5

ctf.

2.331.550.92

NtIT

0.190.130.12

C/N

12.411.67.9

t Organic C.t Total Kjeldahl N.

moist. The large pieces of stubble and root that had passedthrough the sieve were removed by hand. The sieved soil wasdried overnight at 50 °C, after which the soil taken from eachplot was composited and stored at 4 °C . Bulk density wasestimated using the total weight and volume of all cores fromeach plot.

Laboratory MethodsA 100-g subsample of soil from each plot was wet sieved

through a series of three sieves to obtain four aggregate sizefractions: (i) >2000 fan (large macroaggregates), (ii) 250 to2000 fjLm (small macroaggregates), (iii) 53 to 250 /im (mi-croaggregates), and (iv) <53 yxm (silt- plus clay-size parti-cles). The initial moisture content of the soil prior to wet sievingcan alter the aggregate size distribution and partitioning of theorganic matter associated with the aggregate size fractions(Kemper and Rosenau, 1986). Soil was either capillary wettedovernight at 4 °C or left dry (slaked) prior to sieving in orderto obtain information on the distribution and relative stabilityof the aggregates. The capillary-wetted soils were consistentlybrought up to field capacity plus 5%, the moisture content atwhich maximum aggregate stability is attained for these soils.Six sieving replicates were completed for each plot for bothcapillary-wetted and slaked pretreatments. The prewetted ordry soil samples were suspended in room temperature wateron the largest sieve for 5 min before sieving. Aggregate dis-ruption was accomplished by moving the sieve 3 cm vertically50 times during a period of 2 min, being careful to break thesurface of the water with each stroke. Material remaining onthe sieve was backwashed into a round aluminum cake panand dried at 50 °C overnight in a forced air oven. Soil pluswater that passed through the sieve was poured onto the nextfiner sieve size and the process repeated (Elliott, 1986). Thesmallest fraction (silt plus clay) was centrifuged 10 min at 2500x g and the pellet backwashed to an aluminum cake pan anddried overnight at 50 °C. The dried aggregate size fractionswere weighed and stored in wide-mouth snap cap vials at roomtemperature.

Prior to chemical analysis, plant residue and roots that werelarger than =1 mm in length were removed by hand with aforceps from subsamples of the aggregate size fractions. Theaggregates were ground in a mortar and pestle and subsampledto determine total organic C (Snyder and Trofymow, 1984)and total Kjeldahl N (Nelson and Sommers, 1980) by wetoxidation. Total N was quantified using a Lachat flow-injec-tion system (Lachat Instruments, Milwaukee, WI).

The mineral-associated organic matter contained in each ag-gregate size fraction was separated by dispersing the aggregatesoil with hexametaphosphate and passing the dispersed sam-ples through a 53-/xm sieve (Cambardella and Elliott, 1992).The material remaining on the sieve consisted of sand andPOM; the soil slurry that passed through the sieve containedthe mineral-associated organic matter. Water in the slurry wasevaporated in a forced air oven at 50 °C and the dried samplewas ground with a mortar and pestle and analyzed for totalorganic C and total Kjeldahl N as described above. The sandcontent of each aggregate size fraction was determined byweighing the material that was retained on the sieve following

dispersion. Since there was a different amount of sand presentin each of the aggregate size fractions, some of which was notoriginally associated with the aggregates, it was necessary tocorrect the intact aggregate C and N data for sand in order tomake comparisons across aggregate size classes (Elliott et al.,1991).

Total interaggregate POM-C and POM-N were estimated bysuspending 1-g subsamples of whole soil taken from each ofthe tillage treatments in 15 mL of sodium polytungstate ad-justed to a density of 1.85 g/cm3 (Elliott et al., 1991). Thesuspended soil was evacuated for 10 min at —186 kPa toremove entrapped air from the soil pore space. The POM thatwas not protected within the aggregate structure of the soil(interaggregate POM) floated to the top of the heavy liquidafter sitting overnight at room temperature. The interaggregatePOM was removed by aspiration, washed several times withdeionized water, trapped on a glass fiber filter, and analyzedfor organic C and N as described above. Interaggregate POMC or N was expressed as the total amount of C or N in theinteraggregate POM per gram of original soil. Total POM Cand N were estimated as described by Cambardella and Elliott(1992). We calculated intraaggregate POM-C and N as thedifference between total and interaggregate POM C and N.

The data were analyzed as a split-plot design using the SASstatistical package for analysis of variance (ANOVA-GLM,SAS Institute, 1990). Tillage management was the main plottreatment and aggregate size class and pretreatment (capillarywetted or slaked) the split-plot treatments. Main and interactiveeffect means were compared using Tukey's honestly signifi-cant difference mean separation test with a 0.05 significancelevel (Steel and Torrie, 1980).

RESULTS AND DISCUSSIONAggregate Size Distributions

The small macroaggregates (250-2000 /Am) com-prised the highest percentage of the total soil for all man-agement treatments for capillary-wetted soils (Fig. la).No-till soil had significantly more material in the 250-to 2000-)iim size class relative to the other treatmentsalthough native grassland soil had more large macroag-gregates (>2000 fjaa). This suggests that no-till man-agement is effective at improving or maintaining themacroaggregate structure of these grassland soils com-pared with bare fallow and stubble mulch management.

Aggregate size distributions for slaked soils were dif-ferent than those obtained for capillary-wetted soils forall treatments (Fig. Ib). Slaking reduced the amount ofsoil in the large macroaggregate size class to near zeroin the cultivated treatments, and significantly reduced theamount of material in the small macroaggregate size classcompared with capillary-wetted soil for all the cultivatedtreatments (Fig. la and Ib). Slaking causes considerabledisruption of the soil structure as a result of internalforces that arise as water rapidly enters the soil porevolume. Capillary wetting slowly brings the soil to fieldcapacity, which allows trapped air to escape, with min-imal disruption of the soil structure. Macroaggregatesthat survive the disruptive forces of slaking have rela-tively higher structural stability when compared with ma-croaggregates that survive capillary wetting (Kemper andRosenau, 1986). The amount of soil in the small ma-croaggregate size class from the slaked no-till soil wasmore than that for the slaked stubble mulch or bare fal-low treatments (Fig. Ib). This suggests that no-till soilhas greater aggregate stability than stubble mulch or barefallow soil. The small macroaggregate size fraction in

CAMBARDELLA & ELLIOTT: CARBON AND NITROGEN IN AGGREGATES FROM CULTIVATED AND GRASSLAND SOILS 1073

Table 2. The effect of slaking on the percentage of sand in theaggregate size fractions.

Managementtreatment

Sand

>2000 fim 250-2000 fim 53-250 /un

Bare Fallow Stubble Mulch ' No-Till Math™ Grass

Management TreatmentFig. 1. The effect of management on the distribution of soil

within aggregate fractions for (a) capillary-wetted soil and(b) slaked soil. Values followed by the same uppercase letterwithin a management treatment and between aggregate sizeclasses are not significantly different at P > 0.05 accordingto Tukey's HSD mean separation test. Values followed bythe same lowercase letter within an aggregate size and betweenmanagement treatments are not significantly different at P> 0.05 according to Tukey's HSD mean separation test.Values followed by * in (b) are significantly different thancorresponding values in (a) at P > 0.05 according to Tukey'sHSD mean separation test.

the native grassland soil was not affected by slaking (Fig.la and Ib), suggesting that native grassland soils aremore stable than cultivated soils when slaked. These dataare in agreement with results reported by Elliott (1986).The microaggregate fraction increased significantly forall treatments with slaking, while only stubble mulch soilshowed an increase in the smallest size fraction (Fig. laand Ib).

Most of the material in all treatments remained in themicroaggregate fraction with slaking and was not re-duced to particles < 53 /urn, suggesting that microaggre-gates are more stable than macroaggregates. Thisobservation was also reported by Elliott (1986) and sup-ports the idea that macroaggregates are composed of mi-croaggregates (Tisdall and Oades, 1982) that are capableof resisting the disruptive forces of cultivation and slak-ing.

The macroaggregate size fraction had a smaller per-centage of sand than the microaggregate fraction for cap-illary-wetted soils, and there were no differences in thesand proportion within the aggregate size fractions acrosstreatments (Table 2). Slaking increased the sand contentof the cultivated soil macroaggregates and decreased theproportion of sand in the microaggregates (Table 2). Thesedata support the observation that slaking disrupts the ma-

Bare fallowcapillary-wettedslaked

Stubble mulchcapillary-wettedslaked

No-tillcapillary-wettedslaked

Native grasslandcapillary wettedslaked

38bt70a

32b74a

28b43a

30a31a

42b62a

37b49a

33a37a

34a34a

50a41b

45a37b

45a34b

44a38a

t Values within an aggregate size and within a management treatmentfollowed by the same letter are not significantly different at P >0.05according to Tukey's honestly significant difference mean separationtest.

croaggregate structure in the cultivated soils, leaving themacroaggregates depleted in soil and enriched in sand.The soil was redistributed into the microaggregate frac-tion, resulting in a smaller proportion of the microag-gregate fraction being composed of sand after slaking.

Organic Carbon and Nitrogen ConcentrationsSand-free organic C and N concentrations for capil-

lary-wetted soil were equal to one another in the threelarger aggregate size classes and were always higher thanthe organic C and N concentration of the <53-/u,m frac-tion for all the treatments (Fig. 2a and 3a). There wereno significant differences in the sand-free organic C orN concentrations among the three cultivated treatmentsfor any of the aggregate size classes (Fig. 2a and 3a).Native grassland soil generally had significantly higherorganic C and N concentrations in the 250- to 2000- and53- to 250-/Am size classes compared with the cultivatedsoils (Fig. 2a and 3a). In slaked soils, the amount oforganic C and N in the 250- to 2000-ju.m size class wasequal for all the treatments, although native grasslandhad more organic C and N in the >2000-ju,m size thanthe cultivated soils (Fig. 2b and 3b).

The amount of organic C and N in an aggregate sizefraction is a function of how much soil is in that aggre-gate fraction and the concentration of C and N of thatsoil. The distribution of organic C (Fig. 4) and N in theaggregate fractions of cultivated and native grassland soilsappears to be primarily controlled by the amount of soilpresent in the aggregate size fraction (Fig. 1) and not bythe concentration of organic C and N in the aggregates(Fig. 2 and 3). This effect was especially pronounced inthe slaked soils, suggesting that the organic matter con-tent is related to aggregate stability in grassland soils.

Slaking increased the concentration of sand-free or-ganic C in the small macroaggregate size fraction for thecultivated soils (Fig. 2a and 2b). The sand-free organicN concentration in the small macroaggregate size frac-tion increased with slaking in the no-till and stubble mulchtreatments but not in the bare fallow treatment (Fig. 3aand 3b). Recall that slaking reduced the amount of soilin the small macroaggregate size fraction for the culti-

1074 SOIL SCI. SOC. AM. J., VOL. 57, JULY-AUGUST 1993

30,000 -

I '.

, 20,000 -

10,000

^

*g* Bare Fallow Stubble Mulch No-Till Native Grass

Management Treatment(• Not enough soil for analysis)

Fig. 2. The effect of management on sand-free aggregate Cconcentrations for (a) capillary-wetted soil and (b) slakedsoil. Values followed by the same uppercase letter within amanagement treatment and between aggregate size classesare not significantly different at P > 0.05 according to Tukey'sHSD mean separation test. Values followed by the samelowercase letter within an aggregate size and betweenmanagement treatments are not significantly different at P> 0.05 according to Tukey's HSD mean separation test.Values followed by * in (b) are significantly different thancorresponding values in (a) at P > 0.05 according to Tukey'sHSD mean separation test.

£f

4,000 -

3,000 -

2,000 -

1,000 -

(3)

0

4,000 -

_ 3,000 -'&ig 2,000 -

*" 1,000-

0

> * ? -rfif*

w

Bare Fallow Stubble Mulch No-Till Native Grass

Management Treatment< * Not enough soil for analysis)

Fig. 3. The effect of management on sand-free aggregate Nconcentrations for (a) capillary-wetted soil and (b) slakedsoil. Values followed by the same uppercase letter within amanagement treatment and between aggregate size classesare not significantly different atP > 0.05 according to Tukey'sHSD mean separation test. Values followed by the samelowercase letter within an aggregate size and betweenmanagement treatments are not significantly different at P> 0.05 according to Tukey's HSD mean separation test.Values followed by * in (b) are significantly different thancorresponding values in (a) ait P > 0.05 according to Tukey'sHSD mean separation test.

vated treatments (Fig. la and Ib). This suggests thatslaking destroys less-stable small macroaggregates in thecultivated soils, leaving behind more-stable small ma-croaggregates enriched in organic C and N, similar toobservations made by Elliott (1986).

However, when the amount of C and N contained inthe POM that was trapped inside the small macroaggre-gates was subtracted from the sand-free C and N contentof the small macroaggregates, there were no significantdifferences in the organic C and N concentrations be-tween slaked and capillary-wetted small macroaggre-gates (Table 3 and 4). This suggests that the loss ofstructural stability in the small macroaggregates withslaking is related, either directly or indirectly, to the lossof POM in these soils.

The C/N ratios of bare fallow, stubble mulch, no-till,and native grassland intraaggregate POM are shown inTable 5 along with values previously reported for totalPOM for the four management treatments (Cambardellaand Elliott, 1992). For the native grassland soil, the C/N ratio of total POM did not differ from that of intraag-gregate POM. For the cultivated soils, however, the C/N ratio was higher for the intraaggregate POM than thetotal POM, with a trend of higher C/N ratios as the tillage

3,000

lo-Till Native Grass

Management TreatmentFig. 4. The effect of management on the amount of organic C

contained in the aggregate size classes for capillary-wettedsoil. Values followed by the same uppercase letter within amanagement treatment and between aggregate size classesare not significantly different at P > 0.05 according to Tukey'sHSD mean separation test. Values followed by the samelowercase letter within an aggregate size and betweenmanagement treatments are not significantly different at P> 0.05 according to Tukey's HSD mean separation test.

CAMBARDELLA & ELLIOTT: CARBON AND NITROGEN IN AGGREGATES FROM CULTIVATED AND GRASSLAND SOILS 1075

Table 3. The effect of slaking on the mineral-associated organicC concentration of aggregates derived from cultivated andnative grassland soil.

Table 5. The effect of cultivation on the C/N ratios of totaland intraaggregate particulate organic matter (POM).

Managementtreatment

Total C>2000/im 250-2000/un 53-250/tm <53/tm

Bare fallowcapillary-wettedslaked

Stubble mulchcapillary-wettedslaked

No-tillcapillary-wettedslaked

Native grasslandcapillary-wettedslaked

19t*

19

20*

21a21a

- mg C g-1 soil -

19a23a

22a21a

21a25a

23a23a

19a17a

20a19a

20a14a

21a20a

17a18a

16a18a

17a18a

18a19a

t Values within an aggregate size and within a management treatmentfollowed by the same letter are not significantly different at P > 0.05according to Tukey's honestly significant difference mean separationtest.

t Not enough soil for analysis.

intensity decreased (Table 5). Wheat residue has a C/Nratio of 80 (Buyanovsky and Wagner, 1987) and bluegrama grass [Bouteloua gracilis (Willd. ex Kunth) La-gasca ex Griffiths], one of the dominant grass species inthe native grassland community at our research site, hasa C/N ratio of 40 (Clark, 1977). These patterns suggestthat no-till and stubble mulch management are accu-mulating wheat-derived POM within the aggregate struc-ture of the soil, where it may be protected fromdecomposition. Plowing disrupts the aggregate structureand increases the availability of the wheat-derived POMthat was protected within the structure of the soil (Roviraand Greacen, 1957). Tillage also changes the local soilmicroclimate, which may also lead to increased decom-position of the wheat-derived POM. Stable C isotopedata indicated that only 13% of the total POM C in barefallow soil was wheat derived, while the other manage-ment treatments contained a larger proportion of wheat-derived POM (Cambardella and Elliott, 1992). There-

Table 4. The effect of slaking on the mineral-associated organicN concentration of aggregates derived from cultivated andnative grassland soil.

Managementtreatment

Bare fallowcapillary-wettedslaked

Stubble mulchcapillary-wettedslaked

No-tillcapillary-wettedslaked

Native grasslandcapillary-wettedslaked

> 2000 fan

2.0tt

2.1t

2.0t

2.3a2.3a

Total250-2000 /tm

——— mg N g-

2.0a2.4a

2.2a2.3a

2.2a2.3a

2.4a2.4a

N53-250 ion

1 -QJ]

1.9a2.0a

2.2a2.1a

2.1a1.4a

2.3a2.1a

<53/un

1.9a2.0a

1.9a2.0a

1.9a2.0a

2.1a2.0a

t Values within an aggregate size and within a mangement treatmentfollowed by the same letter are not significantly different at P > 0.05according to Tukey's honestly significant difference mean separationtest.

t Not enough soil for analysis.

C/NManagement treatment Total POMf Intraaggregate POMBare fallowStubble mulchNo-tillNative sod

17 ±0.521±417±2

20±926±760 ±1317 ±0.4

t Data from Cambardella and Elliott (1992) from same site.$ Mean ± one standard deviation of replicate plots.

fore, bare fallow management appears to stimulate thedecomposition of wheat-derived POM, whereas wheat-derived POM may be protected inside macroaggregatesin stubble mulch and no-till soil.

Mineral-associated organic C and N concentrations forcapillary-wetted bare fallow small macroaggregates weregenerally lower than for capillary-wetted no-till and stub-ble mulch small macroaggregates (Fig. 5a and 5b). Wehypothesize that plowing during 20 yr of bare fallowmanagement not only destroyed the less-stable macroag-gregates that contain the intraaggregate POM, but alsodisrupted the structure of the more-stable macroaggre-gates. This hypothesis is supported by the observationthat total soil mineral-associated organic C and N was

30,000 n

» 20,000 -

! 10,000 -

(a)

3,000 i

Bare Fallow Stubble Mulch No-Till Native Grass

Management Treatment

Fig. 5. The effect of management on aggregate mineral-associated (a) organic C concentrations and (b) organic Nconcentrations for capillary-wetted soil. Values followed bythe same uppercase letter within a management treatmentand between aggregate size classes are not significantlydifferent at P > 0.05 according to Tukey's HSD meanseparation test. Values followed by the same lowercase letterwithin an aggregate size and between managementstreatments are not significantly different atP > 0.05 accordingto Tukey's HSD mean separation test.

1076 SOIL SCI. SOC. AM. J., VOL. 57, JULY-AUGUST 1993

reduced after 20 yr of bare fallow management of thesegrassland soils (Cambardella and Elliott, 1992).

CONCLUSIONSCultivation destroys the macroaggregate structure of

grassland soils with a concomitant reduction in soil or-ganic C and N. The reduction in total soil organic mattercontent has been related to losses of organic C and Nfrom the POM fraction (Tiessen and Stewart, 1983;Cambardella and Elliott, 1992). Therefore, the loss ofstructural stability in grassland soils that is associatedwith cultivation may be related, either directly or indi-rectly, to losses of organic C and N from the POM frac-tion. Organic C and N losses resulting from cultivationalso appear to occur in the more mineral-associated frac-tions from bare fallow macroaggregates.

Research presented here suggests that reduced tillagemanagement may ameliorate the detrimental effects ofintensive agricultural land use. No-till management ap-pears to be especially effective at maintaining or increas-ing aggregate stability and slowing the decompositionloss of newly incorporated wheat-derived POM C andN. Wood et al. (1991) reported that imposing no-tillmanagement on soils previously cultivated for >50 yrrapidly altered the depth distribution of organic C and Nand reduced losses of NOj —N to leaching. They alsoreported a more rapid accumulation of soil organic Ccompared with soil organic N under no-till management,reflecting inputs of high C/N ratio plant residues. Wheat-derived POM may be an important fraction in these tilledgrassland soils for providing nutrients for plant growth.Adoption of sustainable management practices, such asno-till, may promote the maintenance of this importantsoil organic matter fraction and enhance the fertility ofintensively managed soils.

ACKNOWLEDGMENTSFunding for this research was provided by NSF Grant no.

BSR-860591. The authors wish to thank Indy Burke, SteveCorak, and Tim Parkin for their critical review of this manu-script.