Aggregate Stability under Oak and Pine after Four Decades of Soil DevelopmentR. C. Graham,* J. O. Ervin, and H. B. Wood

ABSTRACTThe development of water-stable aggregates is an important soil

genesis process because it strongly influences important soil character-istics, including infiltration, aeration, and erodibility. We studied a41-yr-old biosequence of lysimeter soils at the San Dimas ExperimentalForest in southern California to assess water-stable aggregates as afunction of the imposed scrub oak (Quercus dumosa Nutt.) and Coulterpine (Pinus coulteriB. Don) communities. Significantly different aggre-gate stabilities developed in these initially identical and homogeneoussoils. Earthworms under the oak produced a 7-cm-thick A horizoncomposed almost entirely of worm casts. Casts were also depositedwithin the litter of the Oi horizon. The A horizon and the Oi horizonworm casts had aggregate stabilities near 90%, =35 g kg-' organicC, and abundant fungal hyphae, as observed by scanning electronmicroscopy. The I-cm-thick A horizon under pine contained no wormcasts and had 78% water-stable aggregates, 12.9 g kg"1 organic C,and abundant very fine roots and fungal hyphae. Subsoils under bothoak and pine had 43 to 51% water-stable aggregates, attributable toinorganic binding mechanisms since organic C contents were ^4 gkg"1, roots were relatively few, and no fungal hyphae were observed.After 41 yr of soil formation, aggregate stability was «15% greaterand the volume of stable aggregates was seven times larger underscrub oak than under Coulter pine.

THE DEVELOPMENT of structural aggregates is an im-portant part of soil formation, since aggregation

characteristics strongly influence soil behavior. Soil ag-gregates that retain their structural integrity when wethelp promote infiltration, aeration, seedling emergence,

R.C. Graham and J.O. Ervin, Dep. of Soil and Environmental Sciences,Univ. of California, Riverside, CA 92521-0424; and H.B. Wood, USDAForest Service, Pacific Southwest Exp. Stn., Riverside, CA 92507. Re-ceived 3 June 1994. "Corresponding author (graham@mail.ucr.edu).

Published in Soil Sci. Soc. Am. J. 59:1740-1744 (1995).

and root penetration, and reduce runoff and erosion(Troeh and Thompson, 1993). Thus, the nature of theaggregation developed early in soil formation may playan important role in establishing dominant soil processesand the direction of soil evolution.

Biotic factors have a major influence on the develop-ment of stable aggregates. Large aggregates (>0.25 mm)are stabilized by plant roots, fungal hyphae, or mesofau-nal fecal pellets (Tisdall and Oades, 1982; Oades, 1993),whereas the stability of microaggregates depends onpersistent organic binding agents (Tisdall and Oades,1982). Root exudates and moribund roots and root hairscontribute substantial amounts of organic matter to thesoil, while mesofauna, such as earthworms and ants,mix comminuted and digested plant organic matter fromthe surface into the mineral soil (Ross, 1989). Soil or-ganic matter is metabolized by a variety of microorgan-isms to produce polysaccharides that act to bind soilparticles into microaggregates (Oades, 1989). Becauseplants provide the major input of organic matter to soilsand the nature of the organic matter strongly affectsdecomposition processes and products (Ross, 1989), thekind of vegetation and associated mesofaunal and micro-bial decomposers should play an important role in thedevelopment of stable soil aggregates.

The large unconfined lysimeters at the San DimasExperimental Forest in southern California presented theopportunity to study the development of soil aggregatestability as a function of vegetation type. These lysimeterswere filled with homogeneous soil material and havebeen under near-monocultures of native woody plantssince 1946. Studies of the San Dimas lysimeter soilshave addressed morphologic development and clay redis-tribution (Graham and Wood, 1991), nutrient cycling

GRAHAM ET AL.: AGGREGATE STABILITY AFTER FOUR DECADES UNDER OAK AND PINE 1741

and mass balance of C, N, and exchangeable cations(Zinke, 1969, 1977; Ulery etal., 1995), and mica trans-formations (K.R. Tice, 1994, unpublished data). Ourobjective in this study was to assess the development ofwater-stable aggregates as a function of the imposed oakand pine vegetation on the lysimeter soils.

MATERIALS AND METHODSThe study site is at an elevation of 830 m in the San Gabriel

Mountains, 56 km northeast of Los Angeles, CA. The annualprecipitation averages 678 mm and ranges from 292 to 1224mm, mostly falling as rain between December and March(Dunn et al., 1988). Mean annual air temperature is 14.4°C,with summer temperatures often exceeding 38 °C and wintertemperatures rarely below -4°C (Dunn et al., 1988).

The history of the lysimeter installation at the San DimasExperimental Forest is recorded by Colman and Hamilton(1947), Patric (1961a,b), Graham and Wood (1991), and Uleryet al. (1995). Our sites, the large unconfined lysimeters, consistof earthen-walled pits (5.3 by 5.3 m horizontally and 2.1 mdeep) that were filled in 1937 with a homogenized, sieved(<19-mm diam.) fine sandy loam (58% sand, 31% silt, 11%clay) derived on site from the weathering of diorite. Theclay fraction of the fill material contained biotite, vermiculite,interstratified biotite-vermiculite, and kaolinite (K.R. Tice,1994, unpublished data). The fill material was placed in thelysimeters in 7.5-cm-thick increments and each layer waschopped with a spade to minimize boundary effects with theunderlying layer.

The fill material was maintained free of vegetation and wasprotected from erosion by an excelsior mulch during a 3-yrsettling period. In 1940, overfill was removed, a 5% slopewas imposed on the surfaces, and an annual grass (Bromusmollis L.) was established. The grass cover was burned in1946 and was replaced by monocultures of native chaparralspecies and Coulter pine planted as seedlings. Each vegetationtype covered a 17 by 24 m area that included a single unconfinedlysimeter and buffer strips designed to eliminate edge effects.The monocultures were periodically weeded and, by the mid-1950s, were similar in size and density to natural stands (Patric,1961a,b). At the time of sampling in 1987, the scrub oakstand remained virtually pure, but scattered grass (Piptatherummiliaceum Coss.) and a few shrubs (mostly Ribes californicumvar. hesperium McClat.) grew under the pine (Milone, 1994).The two principal earthworm species associated with the oaklysimeter were Aporrectodea caliginosa Orley and A. trape-zoides Duges (Wood and James, 1993). The source of thesenonnative species is not known, but it is likely they wereintroduced when the lysimeters or nearby areas were planted.

In the summer of 1987, soils were described and sampledfrom pits as reported by Graham and Wood (1991). In thisstudy, we limited our investigations to the soils under scruboak (coarse-loamy, mixed, mesic Typic Xerorthent) andCoulter pine (coarse-loamy, mixed, mesic Typic Haploxeralf)since they showed the strongest morphologic differences. Be-cause the lysimeter soils occupy small areas and are irreplace-able experimental resources, destructive sampling was kept to aminimum. Samples from selected horizons of a single 1-m-deeppedon in each lysimeter were analyzed for this study. Thephysical and chemical properties of these pedons are describedin detail by Graham and Wood (1991) and Ulery et al. (1995).

Clods of soil fabric were collected intact from each sampledlayer. Bulk samples were air dried and sieved to remove coarsefragments (>2 mm). Particle-size distribution was determinedby the pipette method (Gee and Bauder, 1986). Bulk density

was determined on saran-coated clods or with the use of coresfor thin, fragile A horizons (Blake and Hartge, 1986). TotalC was determined by measuring CO2 evolved during drycombustion (Nelson and Sommers, 1982) and was assumedequivalent to organic C since the soil contains no other sourcesof C. The pH was measured in 1:1 soil/water suspensions.Exchangeable cations were extracted with 1.0 M NH4OAc atpH 7.0 (Thomas, 1982) and analyzed by atomic absorptionspectrophotometry.

Soil aggregate stability was determined by wet sieving(Kemper and Rosenau, 1986). Intact clods were broken toyield 2- to 5-mm aggregates. Five-gram samples of air-dryaggregates were placed into sample cups with 70-u.m screenbottoms and wetted to approximately — 33 J kg"' water potentialby aerosol misting. The samples were wet sieved in the cupsusing de-aired water, a stroke length of 15 mm, and a frequencyof 50 cycles rmV for 5 min. The retained aggregates wereoven dried, weighed, completely dispersed in a 0.05 M NaOHsolution, and resieved to collect the >70-um sand. All residueswere oven dried and weighed. Aggregation was calculated ona sand-free basis.

Aggregates were mounted on Al stubs using epoxy andcolloidal silver paint, sputter coated with Au/Pd, and examinedwith a scanning electron microscope.

RESULTSThe litter layer under the scrub oak consists of a

6-cm-thick horizon of loose, relatively fresh leaves andtwigs with earthworm casts mixed throughout. Beneaththis Oi horizon is a 7-cm-thick A horizon comprising95% worm casts (by visual estimate). The worm castsare aggregated to yield strong subangular blocky structurethat parts to strong fine granular structure of the individualcasts (Table 1). Common very fine and fine roots werefound in the A horizon. Below 7 cm, earthworm krotovinasand casts are a small component (1-5%) of the AC andC horizons to a depth of 35 cm, and none were foundbelow that depth. The C horizon is massive with rootsclassed as few very coarse and coarse and common fineand medium.

The litter layer under pine consists of an Oil horizonof fresh pine needles (10-6 cm), an Oi2 of partiallydecomposed needles (6-4 cm), and an Oe of mostlydecomposed needles (4-0 cm). No earthworm casts werefound in the litter or in any part of the soil under thepine. Without the mixing activity of earthworms, an Ahorizon that is only 1 cm thick developed. It has moderatesubangular blocky structure (Table 1) and many veryfine and common fine roots. A weak Bt horizon withfew thin clay films, weak subangular blocky structure,and few roots formed within the 10- to 20-cm depth.The C horizon is massive and has few roots.

All of the soil materials have fine sandy loam textures,except the worm casts from the oak Oi horizon, whichhave a loam texture (Table 1). The finer texture hasbeen attributed to sorting during the ingestion of soilmaterial by earthworms (Graham and Wood, 1991), amechanism noted in other studies (Lee, 1985; Shipitaloand Protz, 1988). Bulk densities are high (1.7-1.8 gctrr3), except in A horizons, where they are <1.1 gcm-3 (Table 1).

Total (organic) C content was greatest (36 g kg"1) in

1742 SOIL SCI. SOC. AM. J., VOL. 59, NOVEMBER-DECEMBER 1995

Table 1. Physical and chemical properties of the samples.

Horizon DepthDrycolor Structure!

BulkSand Silt Clay density

TotalC

Exchangeable cations

pHt Ca Mg Na

g cm~ • cmolc kg"Oak

Oi§AC

ABtC

6-00-7

80-100

0-110-2080-100

10YR 5/310YR 5/37.5YR 5/4

10YR 4/47. SYR 5/47.5YR 5/4

3fgr (worm casts)3fsbk-"3fgr (worm casts)massive

2msbkImsbkmassive

48.2 36.054.3 32.157.4 32.4

Pine

60.8 30.557.6 30.457.7 31.8

15.713.610.2

8.811.910.5

nd0.921.71

1.081.761.80

36.034.8

1.9

12.94.02.6

5.65.85.7

4.95.96.1

12.9014.469.23

7.8010.639.31

7.026.766.73

4.226.116.50

0.070.050.18

0.050.050.21

0.400.330.11

0.280.140.11

11 = weak; 2 = moderate; 3 = strong; f = fine; m = medium; gr = granular; sbk = subangular blocky; -» = parting to.I pH values presented here are considered more accurate than those given by Graham and Wood (1991).§ Worm casts from the Oi horizon were analyzed.

the worm casts from the Oi horizon under oak (Table1), where earthworms mixed organic matter with mineralsoil material. Organic C content was only slightly lessin the A horizon under oak, which was essentially allearthworm casts. Under pine, organic C was consider-ably lower (13 g kg~') but still enriched compared withthe subsoils (<5 g kg"1; Table 1). The pH value for theA horizon under pine was lower (4.9) than those for thesubsoil samples, but the soil pH under oak varied littlewith depth (Table 1). Exchange sites were dominatedby Ca and Mg (Table 1).

Aggregate stability was best developed in the Oi hori-zon earthworm casts and A horizon material under oak,both of which had aggregates that were =89% waterstable (Fig. 1). The aggregates in the A horizon underpine were significantly less stable (78%) than those underoak. The aggregate stability of both A horizons wasmuch greater than that of the Bt horizon under pine(51%) or the C horizons (49% under pine, 43% underoak). The aggregate stabilities of these subsoil horizonswere not significantly different from each other.

Scanning electron micrographs of soil fabrics are pre-

100

CO

O)CDO)O)CO

•81/5

80

60

40

20

a a

Oak PineFig. 1. Water-stable aggregate contents of material from selected

horizons under scrub oak and Coulter pine. Material from the Oihorizon consisted of earthworm casts separated from the leaf litter.Error bars indicate one standard deviation from the mean. Similarletters above bars indicate no significant difference between meansat the 0.05 probability level as determined by Fisher's protectedleast significant difference test.

sented in Fig. 2. Earthworm casts in the Oi and Ahorizons under oak consist of globular units (micrographsshown for Oi horizon only; Fig. 2a) with silt and clayplatelets oriented to create relatively smooth surfaces(Fig. 2b). Numerous filaments wrap across the surface(Fig. 2b) and extend throughout the aggregate interiors(not shown). Filament width ranges from <1 to ~4 p,mwide, which includes the size range of actinomycetes onthe smaller end and fungal hyphae on the larger end(McLaren and Cameron, 1990). These observations areconsistent with the general observation that earthwormcasts are typically enriched in fungi and actinomycetes(Edwards and Lofty, 1977). The C horizon fabric fromunder the oak (Fig. 2c) shows individual mineral grainsranging from sand to clay size adhering to each otherto form a rough surface with numerous interparticlepores and no filaments (Fig. 2d). The A horizon fabricfrom under the pine also shows a rough, porous surfacewith a range of particle sizes (Fig. 2e); however, fila-ments extend throughout the aggregate (Fig. 2f), as inthe worm casts under oak.

DISCUSSIONOrganic materials are often largely responsible for

aggregate stability, but the subsoils contained <5 g kg"'organic C and still had 43 to 51 % water-stable aggregates.Regression equations with organic C as the sole variablepresented by Tisdall and Oades (1982) predict no aggre-gate stability for soils with such low organic C contents.Poor correlation may arise when inorganic substancesact as binding agents (Tisdall and Oades, 1982). Thescanning electron micrograph in Fig. 2d shows claycoating sand grains and bridging between grains. Similarfeatures were observed by Singer et al. (1992) for syn-thetic aggregates produced by wetting and drying mix-tures of clay and sand. The aggregate stabilities of thelysimeter subsoils are about three times greater thanvalues for aggregates of quartz sand with illite or kaolin-ite, but are within the range measured for aggregatesproduced using smectite (Singer et al., 1992). Thus,the stability of the subsoil aggregates, with a mica-vermiculite-kaolinite clay fraction among coarser quartz,feldspar, and mica grains (K.R. Tice, 1994, unpublished

GRAHAM ET AL.: AGGREGATE STABILITY AFTER FOUR DECADES UNDER OAK AND PINE 1743

Fig. 2. Scanning electron micrographs of soil fabrics from under oak and pine: (a) earthworm cast from Oi horizon under oak, (b) close-up ofcast surface, (c) C horizon fabric from beneath oak, (d) close-up of oak C horizon fabric, (e) A horizon fabric from beneath pine, (f) close-upof pine A horizon fabric.

data) may be largely due to inorganic binding mecha-nisms.

Surface soil materials from under the oak, consistingnearly entirely of worm casts, had the highest aggregatestabilities and organic C contents of the samples analyzed.Worm casts are enriched in organic C, since the earth-worms ingest plant debris and soil, producing an intimatemixture of fine mineral grains and decomposing, muci-lage-coated organic fragments that are excreted as casts(Shipitalo and Protz, 1989). Furthermore, microorgan-isms, particularly fungi, that are present in the excretedcasts and proliferate subsequently (Satchell, 1983), pro-duce extracellular polysaccharides that are effective bind-ing compounds (Tisdall and Oades, 1982). Fungal hyphaeon cast surfaces can physically enmesh and stabilizecasts, but stabilization is most strongly promoted bydrying (Marinissen and Dexter, 1990), which brings theorganic and mineral compounds into close association,promoting clay-polyvalent cation-organic matter link-ages (Shipitalo and Protz, 1989). The lysimeter soils,which have a xeric moisture regime, experience pro-nounced wetting and drying cycles, especially in thesurface horizons. The abundant fungal hyphae suggestthe production of polysaccharides, while the predomi-nance of Ca and Mg on exchange sites favors formation

of cation bridges between clay and organic matter. Thus,surface soil conditions under the oak are conducive todevelopment of highly stable aggregates.

Despite the absence of worm casts, the A horizonunder pine contained a high proportion of water-stableaggregates. Regression equations predict a low aggregatestability for soils with low organic C contents such asthis horizon, but the kind and disposition of the organicmatter is more important than the total amount (Tisdalland Oades, 1982). Polysaccharides excreted by fungi,microorganisms not viewed by scanning electron micros-copy, and plant roots may effectively aggregate soilmaterials via cation bridges (Tisdall and Oades, 1982;Dorioz et al., 1993). Furthermore, fungal hyphae, plantroots, and root hairs can promote aggregate stability byphysically entangling soil materials (Tisdall and Oades,1982; Dorioz et al., 1993). As a result of these variousmechanisms, the aggregate stability of A horizon materialunder pine approaches that of the worm cast materialunder oak.

Graham and Wood (1991) emphasized the degree ofearthworm activity, which varies with plant species, asa major determinant in the morphologic development ofthe four-decade-old biosequence soils of the San Dimaslysimeters. In this study, we have shown that the organic-

1744 SOIL SCI. SOC. AM. J., VOL. 59, NOVEMBER-DECEMBER 1995

influenced A horizons under both oak and pine havedeveloped aggregates that are much more water stablethan those in the subsoils, where inorganic binding pre-dominates. Earthworm activity in the A horizon underoak produced aggregates that were significantly morestable than those in the A horizon under pine, whereworms were absent. Within the 41-yr period of soilformation, the volume of water-stable aggregates formedunder scrub oak was seven times greater than that underCoulter pine, based on A horizon thicknesses.

ACKNOWLEDGMENTSThis research was supported in part by National Science

Foundation Grant no. EAR-9316378. The authors thankAmanda Tate, Heather Swift, and Matt Carfagno for laboratoryassistance and Thai's Winsome for her review of an earlierdraft of the paper.