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PLANT ROOT EFFECTS ON SOIL ERODIBILITY, SPLASH DETACHMENT,

SOIL STRENGTH, AND AGGREGATE STABILITY

F. Ghidey, E. E. Alberts

ABSTRACT.The influence of dead roots on soil erodibility, splash detachment, and aggregate stability was studied in thelaboratory using a rainfall simulator on a Mexico silt loam (fine, montmorillnitic, mesic, Udollic Ochraqualf). Soil wascollected from four cropping treatments including alfalfa, Canada bluegrass, corn, and soybeans. Rainfall of 64 mm h-lintensity was applied for 1 h during the first day. On the second day, a 30-min run of constant intensity (64 mm h-l) wasappliedwhichwasfollowedbyfour l5-min stormsat intensitiesof 25, 100,50, and 75mm h-l. Deadrootmassanddeadroot length in the 0- to 0.l5-m depth from the perennial crops (alfalfa and bluegrass) were much higher than those fromannual row crops (corn and soybean). There was almost afive-fold difference in root mass and root length between alfalfaand soybeans. The study showed that dead roots did not affect runoff, but had significant effect (p

Swift, 1984). Gantzer et al. (1987) also studied the effectsof soybean and corn residues on soil strength and splashdetachment. However, very limited information is availableon the effects of dead roots on soil erodibility, splashdetachment, and aggregate stability.

The objective of this study was to evaluate the influenceof dead roots on soil erodibility, splash detachment, andaggregate stability. Mathematical relationships were alsodeveloped that can be used in erosion models to predict theeffect of dead root mass and dead root length on soilerodibility parameters.

MATERIALS AND METHODSThe study was conducted in a laboratory using soil

boxes and a rainfall simulator. The soil boxes were 100 cmlong and 30 cm wide. Soil depth was 10 cm overlaying5 cm of sand. One end wall of these boxes was fitted with aV-shaped collector to collect and concentrate runoff into acontinuous stream. Two perforated tubes in the bottom ofeach box allowed for air venting and drainage.

Soil was collected from plots located at the Universityof Missouri Midwest Claypan Experimental Farm nearKingdom City, Missouri. The soil was a Mexico silt loam(fine, montrnorillnitic, mesic, Udollic Ochraqualf) withsand, silt, and clay contents of 5, 69, and 26%, respectively.Soil was collected from four cropping treatments selectedto give a wide range in dead root parameters includingalfalfa, canada bluegrass, continuous corn, and continuoussoybeans. Four replications of each treatment wereimposed in 1982 on 3-m wide x 27-m long plots. Soil wascollected in the fall of 1988. Prior to soil collection, surfaceresidue biomass was removed from the sample area beforethe 0- to 0.15-m layer was tilled with a rototiller. About100 kg of soil was collected from each of the 16 plots, airdried, and sieved through a screen with 9-mm openings.

About 500 g of soil was taken from each plot sample forroot mass and root length analysis. Roots were separatedfrom the soil using a hydropneumatic elutriation system(Smucker et aI., 1982). Root length was determined usingthe line intersect technique (Newman, 1966).

Soil subsamples were taken from each plot sample todetermine the aggregate stability of the soil. The stabilityof air dry 2- to 1-mm aggregates was determined by wetsieving (Kemper and Rosenau, 1986), without vaporwetting prior to immersion. Aggregate Index (AI) wascalculated using the equation:

AI = WSAWSA + WUA

(1)

where WSA is the weight of stable aggregates, and WUA isthe weight of unstable aggregates.

Soil resistance to slaking and dispersion was alsoevaluated using Middleton's dispersion ratio andMiddleton's dispersion ratio as modified by Olson et aI.,1962. The dispersion ratio gives an index of stability of soilaggregates in water. The Olson et al. (1962) definition ofdispersion ratio is:

DR20= < 20 11mundispersed X 100< 20 11mdispersed

(2)

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Middleton definition of DR is:

DR50 = < 50 11mundispersed x 100< 50 11mdispersed

(3)

In brief, the dispersion ratio is defined as the ratio of themass of undispersed soil particles (either

Sf = 1.05 - 0.85 exp[-4sin(6)1 (5)

where e is the slope angle.

The average erosion rates obtained from the 15-minvariable intensity storms were used to evaluate equation 4.To fit equation 4 to the data using a linear relationship,both the erosion rate (D) and rainfall intensity (I) data weretransformed into logarithms. The transformed data werethen plotted and the resultant intercept and slope valueswere used to predict the Kj and b values of equation 4,respectively.

Interrill erosion has been approximately proportional tothe square of rainfall intensity and the slope factor (Meyerand Harmon, 1984; Watson and Laflen, 1986; Ghidey andAlberts, 1994):

Di =KiI2Sf' (6)

Recent studies has shown that interrill erosion can be

well expressed in terms of rainfall intensity and runoff rate(Ghidey and Alberts, 1994),

Di =Ki I R Sf (7)

where R is the runoff rate (m s-l). Erosion and runoff ratesfrom variable intensity runs were also used to evaluateequation 7.

RESULTS AND DISCUSSIONROOT PARAMETERS

Differences in dead root mass and dead root lengthamong the four crops were highly significant (p

bluegrass, corn, and soybeans, respectively. There was adefinite trend when K; values were plotted against the deadroot parameters. As dead root mass and dead root lengthincreased, K; decreased (fig. 1). The relationships betweenK; and dead root mass, and K; and dead root length, usingequation 6 were best expressed exponentially.

The relationship between K;dead root mass was:

Ki =3.55 e-0.71 RTM

r2 =0.63 (8)

where RTM is dead root mass in kg m-2.

The relationship between Kj and dead root length was:

Ki =3.62 e-0.029 RTL

r2 =0.59 (9)

where RTL is dead root length in km m-2.

The above relationships were detennined using deadroot mass and dead root length values measured from eachplot. For each cropping treatment, soil samples werecollected from four plots. When the mean values were used

,.E

14

12

. AlfalfaIZI Bluegrass

Deem~ SoybeanIi)

~ 10.9CCD 8E.r:.0

-m 8C.r:.II>as 4a.C/J

2

02-an Height 6-cm Height 1o-an Height Total

Figure 2-Soil splash collected 2-, 6-, and to-cm heights above the soilsurface.

SPLASH DETACHMENT

Splash detachment at the 2-, 6-, and lO-cm heights, andtotal splash from the three heights measured during the firstday constant intensity run are shown in figure 2. Therewere no significant differences (p

4

Alfalfac

Bgrass6Com

0Soybean*

e cII!

6.C

" 6~

0

~

00 20 7030 40

Time (minutes)

10 50 60

Figure 4-Total splash collected in IO-min intervals from 2-, 6-, and IO-cm heights above soil surface.

correlation between splash detachment and shear strength,since splash detachment was not measured during thevariable intensity runs.

SUMMARY AND CONCLUSIONSThe effect of dead roots on runoff, soil erodibility,

splash detachment, and aggregate stability were studied inthe laboratory. We found that:

I. Dead roots had no effect on runoff but significantlyinfluenced (p

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