Plant Root Effects On Soil Erodibility, Splash Detachment
Transcript Of Plant Root Effects On Soil Erodibility, Splash Detachment
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PLANT ROOT EFFECTS ON SOIL ERODIBILITY, SPLASH DETACHMENT,
SOIL STRENGTH, AND AGGREGATE STABILITY
F. Ghidey, E. E. Alberts
ABSTRACTT.he influence of dead roots on soil erodibility, splash detachment, and aggregate stability was studied in the laboratory using a rainfall simulator on a Mexico silt loam (fine, montmorillnitic, mesic, Udollic Ochraqualf). Soil was collected from four cropping treatments including alfalfa, Canada bluegrass, corn, and soybeans. Rainfall of 64 mm h-l intensity was applied for 1 h during the first day. On the second day, a 30-min run of constant intensity (64 mm h-l) was appliedwhichwasfollowedbyfour l5-min stormsat intensitiesof 25, 100,50, and 75mm h-l. Dead rootmassand dead root length in the 0- to 0.l5-m depth from the perennial crops (alfalfa and bluegrass) were much higher than those from annual row crops (corn and soybean). There was almost afive-fold difference in root mass and root length between alfalfa and soybeans. The study showed that dead roots did not affect runoff, but had significant effect (p <0.05) on soil loss and sediment concentrations. However, the differences in soil loss and sediment concentrations were small relative to the differences in dead root mass and dead root length. lnterrill erodibility (K) decreased as dead root mass and dead root length increased. There were exponential relationships between Ki and dead root mass, and Ki and dead root length. Dead roots had significant effects (p <0.05) on soil shear strength, aggregate index, and dispersion ratio. Soil shear strength and aggregate index from alfalfa and Canada bluegrass were approximately 20 and 50%, respectively, higher than those from corn and soybean. Dispersion ratios from alfalfa and bluegrass were about 30% lower than those from corn and soybean. There was no significant difference (p <0.05) in soil splash among the crops. Splash detachment was highest during the initiall 0 min of the simulation and then decreased exponentially. Keywords. Runoff, Soil loss, Sediment concentration, lnterrill erosion, Soil properties.
The rill-interrill erosion concept facilitates basic erosion mechanics and erosion modeling studies (Foster and Meyer, 1975; Lane et aI., 1987). Rills are areas where flow concentrates in narrow channels a few centimeters wide because of natural
topographical features, soil roughness, or tillage marks and tracks. Erosion from areas between the rills is defined as interrill erosion. Interrill erosion is affected by many factors including rainfall intensity (Meyer, 1981; Park et aI., 1983), infiltration and runoff (Bradford et aI., 1987), slope (Lattanzi et al., 1974; Singer and Blackard, 1982; Watson and Laflen, 1986; Meyer and Harmon, 1989), and residue cover (Lattanzi et aI., 1974). Interrill erosion is also affected by soil properties including soil texture, organic-matter content, aggregate stability, and residual effects of crops and management practices.
Several research studies have been conducted to evaluate cropping effects on erosion under natural rainfall conditions. Laflen and Moldenhauer (1979) found that annual soil loss from corn following soybean was higher
Article was submitted for publication in August 1996; reviewed and approved for publication by the Soil & Water Div. of ASAE in December 1996.
The authors are Fessehaie Ghidey, ASAE Member Engineer, Senior Research Scientist, Dept. of Biological and Agricultural Engineering, Univ. of Missouri, Columbia, Mo.; E. Eugene Alberts, ASAE Member Engineer, Research Leader and Soil Scientist, USDA-Agric. Research Service, Cropping Systems and Water Quality Research Unit, Columbia, Mo. Corresponding author: F. Ghidey, Biological and Agricultural Engineering Dept., Univ. of Missouri, Columbia, MO 65211; tel.: (573) 882-1145; fax: (573) 882-1115; e-mail:.
than that from corn following corn. Most of the annual difference occurred during the rough fallow and rapid growth periods: Alberts et al. (1985) did not find any difference in soil loss during the seedbed period between continuous soybean and continuous corn that were conventionally tilled. Field-scale rainfall simulation has also been used to evaluate the effect of prior cropping on soil loss. Results have ranged from those that have found a prior cropping effect (Oschwald and Siemens, 1976) to those that have not found an effect (Laflen and Colvin, 1981; Colvin and Laflen, 1981).
Erosion losses due to cropping effects could be a combination of many factors including prior cropping effects on the soil, the amount of residue incorporated by tillage, canopy and residue cover, and live and dead root biomass. To more carefully isolate the influence of each factor on soil detachment, a more controlled laboratory study is needed whereby all casual factors other than that related to the study are eliminated. Ghidey and Alberts (1994) studied the effects of cropping system and antecedent water content on interrill soil erodibility in the laboratory. They used disturbed soil which was sieved through a 9-mm sieve to remove residues and clods. Thus, the influence of cropping systems on factors such as microbial population, decomposed residue and root masses, and dead root mass on soil resistance to sealing and detachment by raindrops were not specifically measured.
Numerous research had been conducted to evaluate the
effects of prior cropping systems, organic matter, and residue on aggregate size, aggregate stability, and soil erodibility (Alberts and Wendt, 1985; Bathke and Blake, 1984; Fahad et aI., 1982; McCracken, 1984; Chaney and
Transactions of the ASAE VOL. 40(1):129-135 @ 1997 American Society of Agricultural Engineers 0001-2351/97/4001-0129 129
Swift, 1984). Gantzer et al. (1987) also studied the effects
of soybean and corn residues on soil strength and splash detachment. However, very limited information is available on the effects of dead roots on soil erodibility, splash
detachment, and aggregate stability. The objective of this study was to evaluate the influence
of dead roots on soil erodibility, splash detachment, and
aggregate stability. Mathematical relationships were also developed that can be used in erosion models to predict the effect of dead root mass and dead root length on soil
erodibility parameters.
MATERIALS AND METHODS
The study was conducted in a laboratory using soil boxes and a rainfall simulator. The soil boxes were 100 cm
long and 30 cm wide. Soil depth was 10 cm overlaying 5 cm of sand. One end wall of these boxes was fitted with a
V-shaped collector to collect and concentrate runoff into a continuous stream. Two perforated tubes in the bottom of each box allowed for air venting and drainage.
Soil was collected from plots located at the University of Missouri Midwest Claypan Experimental Farm near Kingdom City, Missouri. The soil was a Mexico silt loam (fine, montrnorillnitic, mesic, Udollic Ochraqualf) with sand, silt, and clay contents of 5, 69, and 26%, respectively. Soil was collected from four cropping treatments selected to give a wide range in dead root parameters including alfalfa, canada bluegrass, continuous corn, and continuous soybeans. Four replications of each treatment were imposed in 1982 on 3-m wide x 27-m long plots. Soil was collected in the fall of 1988. Prior to soil collection, surface residue biomass was removed from the sample area before the 0- to 0.15-m layer was tilled with a rototiller. About 100 kg of soil was collected from each of the 16 plots, air dried, and sieved through a screen with 9-mm openings.
About 500 g of soil was taken from each plot sample for root mass and root length analysis. Roots were separated from the soil using a hydropneumatic elutriation system (Smucker et aI., 1982). Root length was determined using the line intersect technique (Newman, 1966).
Soil subsamples were taken from each plot sample to determine the aggregate stability of the soil. The stability of air dry 2- to 1-mm aggregates was determined by wet sieving (Kemper and Rosenau, 1986), without vapor wetting prior to immersion. Aggregate Index (AI) was calculated using the equation:
AI = WSA
(1)
WSA + WUA
where WSA is the weight of stable aggregates, and WUA is the weight of unstable aggregates.
Soil resistance to slaking and dispersion was also evaluated using Middleton's dispersion ratio and Middleton's dispersion ratio as modified by Olson et aI., 1962. The dispersion ratio gives an index of stability of soil aggregates in water. The Olson et al. (1962) definition of dispersion ratio is:
DR20= < 20 11mundispersed X 100 (2)
< 20 11mdispersed
Middleton definition of DR is:
DR50 = < 50 11mundispersed x 100 (3)
< 50 11mdispersed
In brief, the dispersion ratio is defined as the ratio of the mass of undispersed soil particles (either <50 or <20 11m) to the mass dispersed after adding the soil to a graduated cylinder and inverting end-over-end several times. The <50 11mand <20 /lm dispersed fractions were measured
using the pipette method. The soil was firmly packed in the soil boxes to an
average bulk density of 1.07 g cm-3 using a vibrational packing device. The soil box was then placed in a supporting stand at 4% slope. Rain was applied with a multiple intensity rainfall simulator, similar to that described by Meyer and Harmon (1979), with a single 80150 v-jet nozzle. Mean waterdrop diameter was 3.0 mm falling from a height of 2.5 m. To each box, rainfall of
64 mm h-l intensity was applied for 1 h during the first day. On the second day, a 30-min run of constant intensity (64 mm h-l) was applied. On day 2, the 30-min constant intensity event was followed by four 15-min storms at intensities of 25, 50, 75, and 100 mm h-I. During each
rainstorm, surface runoff was measured and sampled for sediment analysis. After runoff began, samples were collected at 2 and 3 min intervals in the first 5 min, and at 5-min intervals thereafter.
Soil splash samples were collected during the initial 60-min constant intensity (64 mm h-I) run. Splash boxes,
each with a 0.2-cm high x 2.5-cm wide rectangular opening, were mounted at 2-, 6-, and IO-cm heights above the soil surface on each side of the box. Splash boxes were changed quickly at 10-min intervals during the event. Soil
splash was then quantitatively transferred from each splash compartment to glass dishes for oven drying and weighing on an analytical balance.
Four soil cores (56 mm x 33 mm high) were inserted into the soil prior to rainfall simulation. At the end of the
variable intensity sequence, the soil cores were carefully removed for measurement of soil strength and bulk density.
The samples were rewet with a 0.1 mole/L CaCI2-MgCI2 solution and allowed to equilibrate for 12 h. About 5 h
before the strength measurement, the samples were transferred to a tension table maintained at a I kPa soil
water potential. Soil shear strength was determined with a Swedish fall cone device (60 g cone) (AI-Durrah and Bradford, 1981). Four measurements were made on each
core and averaged before additional analysis was performed. Soil bulk density of each core was determined by standard methods.
Interrill detachment was described as being proportional to the power of rainfall intensity and slope factor (Meyer, 1981),
Di =Ki Ib Sf
(4)
where Di is the interrill erosion rate (kg m-2 s-l), Kj is the interrill erodibility parameter of the soil (kg s m-4), I is the rainfall intensity (m s-I), b is the exponent related to soil
clay content(MeyerandHarmon,1984), and Sf is the slope factor defined by Liebenow et al. (1990).
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TRANSACTIONS OF TIlE ASAE
Sf = 1.05 - 0.85 exp[-4sin(6)1
(5)
where e is the slope angle.
The average erosion rates obtained from the 15-min variable 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 were transformed into logarithms. The transformed data were then plotted and the resultant intercept and slope values were used to predict the Kj and b values of equation 4,
respectively. Interrill erosion has been approximately proportional to
the square of rainfall intensity and the slope factor (Meyer and Harmon, 1984; Watson and Laflen, 1986; Ghidey and Alberts, 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 rates from variable intensity runs were also used to evaluate equation 7.
RESULTS AND DISCUSSION
ROOT PARAMETERS
Differences in dead root mass and dead root length among the four crops were highly significant (p <0.05) (table 1). Dead root mass and dead root length in the 0- to 0.15-m depth from the perennial crops (alfalfa and canada bluegrass) were much higher than those from annual row crops (corn and soybeans). There was almost a five fold difference in root mass and root length between alfalfa and soybeans. Root length per unit weight of dry roots was 22.0, 32.0, 28.7, and 26.1 kIn/kg for alfalfa, Canada bluegrass, corn, and soybeans, respectively.
DAY 1, CONSTANT RAINFALL INTENSITY RUN
Runoff, soil loss, and sediment concentrations for alfalfa, canada bluegrass, corn, and soybeans on the first day of simulation (60-min constant intensity run at 64 mm h-l) are given in table 2. Runoff and soil loss were expected to be lower from the perennial crops than those from the annual row crops. As expected, runoff, soil loss, and sediment concentration from alfalfa were significantly lower (p <0.05) than from corn or soybean. However, runoff, soil loss, and sediment concentration from canada
Table 2. Mean runoff, soil loss, and sediment concentrations* measured during the first day constant intensity run
Crop
Runoff (nun)
Soil Loss (g min-1 m-2)
Sed. Concent. (Mg kg-I)
Alfalfa
Bluegrass Com
Soybean
53.3b 58.1a 57.1a 55.8ab
12.3b 16.0a 17.8a 14.9ab
13,800b 16,500a 18,600a 16,OOOab
* Values containing the same letter are not significantly different at 5% level.
bluegrass were significantly higher (p <0.05) than those from alfalfa and were similar to those of corn and soybean.
DAY 2, CONSTANT RAINFALL INTENSITY RUN
On the second day 30-min run at 64 mm h-l intensity there was no significant difference (p <0.05) in runoff among the crops, but there were significant differences (p <0.05) in soil loss and sediment concentrations (table 3). Soil loss and sediment concentration were lower from
crops with higher dead root mass and dead root length, but the differences were small when compared to the differences in root mass and length. For instance, dead root mass and dead root length for alfalfa were approximately 5 times higher than those for soybeans, but the differences in soil loss and sediment concentrations between alfalfa and
soybeans were only 17 and 16%, respectively
INTERRILL ERODIBILITY, Kj
Erosion rates measured from the 15-min variable
intensity (25, 50, 75, and 100 mm h-l) storms were used to evaluate the Kj and b values given in equation 4. Kj values (x 10-6) for alfalfa, canada bluegrass, corn, and soybeans were 2.66, 2.52, 3.04, and 3.26 kg s m-4, respectively. The Kj values from perennial crops were significantly lower (p <0.05) than those from the annual row crops. The mean exponent, b, values were 2.12, 1.95, 2.14, and 2.13 for
alfalfa, canada bluegrass, corn, and soybeans, respectively. There were no significant difference (p <0.05) in the exponent, b, values among the crops, and they were all close to 2.0 which agrees with previous studies that found interrill erosion to be proportional to the square of rainfall intensity (Meyer and Harmon, 1984; Watson and Laflen, 1986; Ghidey and Alberts, 1994).
The differences in Kj values among the crops were small relative to the differences in root mass and root length. For instance, Kj value for soybeans was less than 20% higher than that for alfalfa, whereas the dead root mass and dead
root length for alfalfa were four to five times higher than those of soybeans. The mean Kj values (xlO--6) when computed from equation 6 (intensity square relationship) were 2.66, 2.56, 3.02, and 3.32 kg s m-4 for alfalfa, canada
Table I. Mean dead root mass and dead root length values* for alfalfa, Canada bluegrass, corn, and soybean
Crop
Alfalfa Bluegrass Com Soybean
Root Mass (g m-2)
495a 384a 150b 92b
Root Length (m m-2)
1O,858a 12,289a 4,258b 2,364c
* Values containing the same letter are not significantly different at 5% level.
Table 3. Mean runoff, soil loss, and sediment measured during the second day constant
Runoff
Soil Loss
Crop
(mm)
(g min-I m-2)
Alfalfa
Bluegrass Com
Soybean
27.3a 28.6a 28.2a 28.2a
12.4b 13.5b 15.7a 15.0a
concentrations* intensity run
Sed. Concent. (Mg kg-I)
13,400c 14,1 OObc 16,600a 16,OOOab
* Values containing the same letter are not significantly different at 5% level.
VOL. 40(1):129-135
131
bluegrass, corn, and soybeans, respectively. There was a definite trend when K; values were plotted against the dead
root parameters. As dead root mass and dead root length increased, K; decreased (fig. 1). The relationships between K; and dead root mass, and K; and dead root length, using
equation 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 dead root mass and dead root length values measured from each plot. For each cropping treatment, soil samples were collected from four plots. When the mean values were used
5
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'8 a=U2 "E .!! .E
1
L...
0
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O~
OA
O~
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. .
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(b)
Figure l-RelatioWihip dead root length (b).
between Kj and dead root mass (a), and !\j and
to define the relationships, better relationships were found
= between K; and dead root mass (r2 0.83), and K; and dead = root length (r2 0.92).
Interrill erodibility values were also computed using equation 7. The mean Kj values (x lO-6) for alfalfa, canada bluegrass, com and soybeans were 2.95, 2.81, 3.22, and 3.46 kg s m-4, respectively. These values were slightly higher than the ones computed using equation 6, however, relationships similar to the ones expressed in equations 8 and 9 were observed when dead root mass and dead root length values were plotted against the Kj values computed from equation 7.
SOIL PARAMETERS
Bulk Deusity and Soil Shear Strength. There were no significant differences (p <0.05) in bulk density among the crops measured in the soil within 50-mm x 33-mm high cores collected after the variable intensity sequence. Initial mean bulk density values were 1.06, ] .06, 1.07, and ] .07 mg m-3 for alfalfa, Canada bluegrass, corn, and soybean, respectively. At the end of the experiment, mean bulk density for alfalfa, Canada bluegrass, corn, and soybean were 1.10, 1.12, 1.]], and 1.1 0 mg m-3, respectively. Dead root mass and dead root length had no significant (p <0.05) effect on the final bulk density.
The effect of dead roots on the shear strength of the soil were significant (p <0.05) (table 4). At the end of the experiment, soil shear strength was higher from soil samples with greater amounts of dead root mass and length. Mean soil shear strength for alfalfa and bluegrass were approximately. 22% higher than those for corn and soybeans.
DISPERSION RATIO AND AGGREGATE INDEX
The effects of dead root parameters on aggregate stability such as aggregate index and dispersion ratio were significant (p <0.05) (table 4). As dead root mass and dead root length increased, aggregate index increased and the dispersion ratios (DR20 and DR50) decreased. The aggregate index values for alfalfa and Canada bluegrass were twice those for corn and soybeans. DR20 and DR50 for alfalfa and bluegrass were approximately 40 and 35% lower than those for corn and soybeans, respectively.
There was no correlation between runoff and aggregate index or dispersion ratio, but there was a definite trend between soil loss and aggregate index, and soil loss and dispersion ratio. Soil loss decreased as aggregate index increased and dispersion ratio decreased. Our results support previous research findings that lower aggregate index and higher dispersionratio values indicateincreasedsusceptibility of the soil to erosion (Kemperand Rosenau, 1986).
Table 4. Mean soil strength, aggregate index (AI), dispersion ratio as defined by Olson (DR20), and dispersion ratio as defined by Middleston (DR %)*
Crop
Soil Strength (kPa)
AI
DR20
DR50
Alfalfa Bluegrass Com Soybean
5. 14a 5.16a 4.22b 4.23b
0.49a 0.48a 0.21b 0.27b
6.7b 7.0b 12.2a 11.1a
18.1b 16.0b 25.4a
26.9a
* Values containing the same letter are not significantly different at 5% level.
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TRANSACfIONS OF THE ASAE
14
12 Ii) ~ 10 .9 C CD 8 E .r:. 0 -m 8 C .r:. II> as 4 a. C/J
2
. Alfalfa
IZI Bluegrass
Deem ~ Soybean
3
2-cm Height
~~ 02
0
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a
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A
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0
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6
:a
a
'Q.1
iii
A II
A
A
. . en
a
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0 2-an Height
6-cm Height
1o-an Height
Total
Figure 2-Soil splash collected 2-, 6-, and to-cm heights above the soil surface.
SPLASH DETACHMENT
Splash detachment at the 2-, 6-, and lO-cm heights, and total splash from the three heights measured during the first day constant intensity run are shown in figure 2. There were no significant differences (p <0.05) in the total detachment among the crops. The total detachment measured at the three heights were 12.1, 11.0, 11.8, and 12.3 g for alfalfa, Canada bluegrass, corn, and soybeans. There were also no significant differences (p <0.05) in splash detachment among the crops measured at 2-, 6-, and 10-cm heights. For each crop, about 60% of the splash was measured at the 2-cm height, 30% at the 6-cm height, and lO% at the lO-cm height.
Soil splash measured at lO-min time intervals during the one hour constant intensity run from the 2-, 6-, and lO-cm heights above the soil surface are given in figure 3. In all cases, splash detachment was highest during the initial 10-min of the simulation and decreased exponentially with time. The same trends were also observed when total
splash detachments (sum of soil splash measured at the three heights) were plotted against time (fig. 4).
The effects of root parameters were not observed on splash detachment as they were on soil strength, aggregate stability, dispersion ratio, and interrill erodibility. Soil splash is mainly due to the forces of falling raindrops breaking down aggregates. The kinetic energy of raindrops falling at terminal velocity is from one to two orders of magnitude greater than the kinetic energy associated with gently flowing water (Hudson, 1981). Root mass and root length might have stabilized the aggregates to reduce erosion by runoff; however, these aggregates may not be stable when subjected to the impact of raindrops. Although there was variability in soil erosion among the crops due to differences in dead root mass and length, the effect was not observed on splash detachment.
Previous studies showed that splash detachment was
highly correlated to soil shear strength (Cruse and Larson, 1977; AI-Durrah and Bradford, 1981; AI-Durrah and Bradford, 1982; Nearing and Bradford, 1985). In our study,
splash detachment from crops that resulted in greater shear strength was not significantly different (p <0.05) from
0
L
0
10
20
30
40
50
Time (minutes)
6-cm Height
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C'-IS 0.6 .9 .r::
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70
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20
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0
0
10
20
30
40
50
60
70
Time (minutes)
Figure 3-Splash coUected in to-min intervals at 2-, 6-, and to-em heights above soil surface.
crops with lower shear strength. Splash detachment was
measured durin$ the first day constant intensity dry run; whereas, the shear strength of the soil was measured at the
end of the experiment after the variable intensity runs. Thus, it is difficult to conclude whether there was a
VOL. 40(1):129-135
133
4
Alfcalfa
Bg6rass
f.9 3[ e .J:: 2
en .!2 a. en
c
0
6
6
*
0
Com 0
Soy*bean
c
~ 6
II!
.
"6
0
C
~
1
0
0
10
20
30
40
50
60
70
Time (minutes)
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 the variable intensity runs.
SUMMARY AND CONCLUSIONS The effect of dead roots on runoff, soil erodibility,
splash detachment, and aggregate stability were studied in the laboratory. We found that:
I. Dead roots had no effect on runoff but significantly influenced (p <0.05) soil loss and sediment concentrations. Soil loss and sediment
concentrations from annual row crops were significantly higher than those from perennial crops; however, the differences in soil loss among the crops were small relative to the differences in root mass and root length. 2. Interrill erodibility parameter, Kj, decreased as dead root mass and dead root length increased. There were exponential relationships between Kj and dead
root mass and ~ and dead root length.
3. Dead roots had no effect on the soil bulk density, but significantly influenced soil shear strength. Soil strength increased as root mass and root length increased.
4. Dead roots significantly (p <0.05) affected aggregate index and dispersion ratio. As the amount of root mass and root length increased, aggregate index increased, and dispersion ratio decreased.
5. The effects of dead roots were not observed on
splash detachment as they were on soil strength, aggregate index, and dispersion ratio. Splash detachment was highest during the initial 10-min of simulation and then decreased exponentially.
The results obtainedin this study have important implications for erosion scientists and modelers. The
relationships between interrill erodibility, Kj, and root mass or root length observed in this study can be used in the erosion models such as WEPP to adjust predicted Kj values to temporal changes in either root parameters.
REFERENCES
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TRANSACTIONS OF TIJE ASAE
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Kemper, W. D. and R. C. Rosenau. 1986. Aggregate stability and size distribution. In Methods of Soil Analysis, 2nd Ed., 425441, ed. A. Klute. Part I. Madison, Wis.: Am. Soc. Agron.
Laflen, J. M. and T. S. Colvin. 1981. Effect of crop residue on soil loss from continuous row cropping. Transactions of the ASAE 24(3):605-609.
Laflen,1. M. and W. C. Moldenhauer. 1979. Soil and water losses from com-soybean rotations. Soil Sci. Soc. Am. J. 43(6): 12131215.
Lane, L. J., G. R. FosterandA. D. Nicks. 1987. Use of fundamental erosion mechanics in erosion prediction. ASAE Paper No. 87-2540. St. Joseph, Mich.: ASAE.
Lattanzi, A. R., L. D. Meyer and M. F. Baumgardner. 1974. Influences of mulch rate and slope steepness on interrill erosion. Soil Sci. Soc. Am. J. 38(6):946-950.
Liebenow, A. M., W. J. Elliot, J. M. Laflen and K. D. Kohl. 1990. Interrill erodibility: Collection and analysis of data from cropland soils. Transactions of the ASAE 33(6): 1882-1888.
McCraken, D. V. 1984. Influence of com and soybean residue decomposition on soil aggregate wet stability. M.S. thesis. West Lafayette, Ind.: Purdue University.
Meyer, L. D. 1981. How rain intensity affects interrill erosion. Transactions of the ASAE 24(6):1472-1475.
Meyer, L. D. and W. C. Harmon. 1979. Multiple intensity rainfall simulator for erosion research on row side slopes. Transactions of the ASAE 22(1): 100-103.
Meyer, L. D. and W. C. Harmon. 1984. Susceptibility of agricultural soils to interrill erosion. Soil Sci. Soc. Am. J. 48(5): 1152-1157.
. 1989. How row-sideslope length and steepness affect sideslope erosion. Transactions of the ASAE 32(2):639-644.
Nearing, M. A. and J. M. Bradford. 1985. Single waterdrop splash detachment and mechanical properties of soils. Soil. Sci. Soc. Am. J. 49(3):547-551.
Newman, E. I. 1966. A method of estimating the total length of root in a sample. J. Appl. Ecology 3:139-145.
Olson, T. c., J. V. Mannering and C. B. Johnson. 1962. The erodibility of some Indiana soils. Proc. Indiana Academy Sci. 72:319-324.
Oschwald, W. R. and J. C. Siemens. 1976. Soil erosion after
soybeans. In World Soybean Research, 74-81, ed. L. D. Hill. Danville, Ill.: Interstate Printers & Publishers. Park, S. w., J. K. Mitchell and G. D. Bubenzer. 1983. Rainfall
characteristics and their relation to splash erosion. Transactions of the ASAE 26(4):795-804.
Singer, M. J. and J. Blackard. 1982. Slope angle-interrill soil loss relationships for slopes up to 50%. Soil Sci. Soc. Am. J. 46(6):1270-1273.
Smucker, A. J. M., S. L. McBurney andA. K. Srivastava. 1982. Quantitative separation of roots from compacted soil profiles by the hydro pneumatic elutriation system. Agron. J. 74:500503.
VOL. 40(1):129-135
135
PLANT ROOT EFFECTS ON SOIL ERODIBILITY, SPLASH DETACHMENT,
SOIL STRENGTH, AND AGGREGATE STABILITY
F. Ghidey, E. E. Alberts
ABSTRACTT.he influence of dead roots on soil erodibility, splash detachment, and aggregate stability was studied in the laboratory using a rainfall simulator on a Mexico silt loam (fine, montmorillnitic, mesic, Udollic Ochraqualf). Soil was collected from four cropping treatments including alfalfa, Canada bluegrass, corn, and soybeans. Rainfall of 64 mm h-l intensity was applied for 1 h during the first day. On the second day, a 30-min run of constant intensity (64 mm h-l) was appliedwhichwasfollowedbyfour l5-min stormsat intensitiesof 25, 100,50, and 75mm h-l. Dead rootmassand dead root length in the 0- to 0.l5-m depth from the perennial crops (alfalfa and bluegrass) were much higher than those from annual row crops (corn and soybean). There was almost afive-fold difference in root mass and root length between alfalfa and soybeans. The study showed that dead roots did not affect runoff, but had significant effect (p <0.05) on soil loss and sediment concentrations. However, the differences in soil loss and sediment concentrations were small relative to the differences in dead root mass and dead root length. lnterrill erodibility (K) decreased as dead root mass and dead root length increased. There were exponential relationships between Ki and dead root mass, and Ki and dead root length. Dead roots had significant effects (p <0.05) on soil shear strength, aggregate index, and dispersion ratio. Soil shear strength and aggregate index from alfalfa and Canada bluegrass were approximately 20 and 50%, respectively, higher than those from corn and soybean. Dispersion ratios from alfalfa and bluegrass were about 30% lower than those from corn and soybean. There was no significant difference (p <0.05) in soil splash among the crops. Splash detachment was highest during the initiall 0 min of the simulation and then decreased exponentially. Keywords. Runoff, Soil loss, Sediment concentration, lnterrill erosion, Soil properties.
The rill-interrill erosion concept facilitates basic erosion mechanics and erosion modeling studies (Foster and Meyer, 1975; Lane et aI., 1987). Rills are areas where flow concentrates in narrow channels a few centimeters wide because of natural
topographical features, soil roughness, or tillage marks and tracks. Erosion from areas between the rills is defined as interrill erosion. Interrill erosion is affected by many factors including rainfall intensity (Meyer, 1981; Park et aI., 1983), infiltration and runoff (Bradford et aI., 1987), slope (Lattanzi et al., 1974; Singer and Blackard, 1982; Watson and Laflen, 1986; Meyer and Harmon, 1989), and residue cover (Lattanzi et aI., 1974). Interrill erosion is also affected by soil properties including soil texture, organic-matter content, aggregate stability, and residual effects of crops and management practices.
Several research studies have been conducted to evaluate cropping effects on erosion under natural rainfall conditions. Laflen and Moldenhauer (1979) found that annual soil loss from corn following soybean was higher
Article was submitted for publication in August 1996; reviewed and approved for publication by the Soil & Water Div. of ASAE in December 1996.
The authors are Fessehaie Ghidey, ASAE Member Engineer, Senior Research Scientist, Dept. of Biological and Agricultural Engineering, Univ. of Missouri, Columbia, Mo.; E. Eugene Alberts, ASAE Member Engineer, Research Leader and Soil Scientist, USDA-Agric. Research Service, Cropping Systems and Water Quality Research Unit, Columbia, Mo. Corresponding author: F. Ghidey, Biological and Agricultural Engineering Dept., Univ. of Missouri, Columbia, MO 65211; tel.: (573) 882-1145; fax: (573) 882-1115; e-mail:
than that from corn following corn. Most of the annual difference occurred during the rough fallow and rapid growth periods: Alberts et al. (1985) did not find any difference in soil loss during the seedbed period between continuous soybean and continuous corn that were conventionally tilled. Field-scale rainfall simulation has also been used to evaluate the effect of prior cropping on soil loss. Results have ranged from those that have found a prior cropping effect (Oschwald and Siemens, 1976) to those that have not found an effect (Laflen and Colvin, 1981; Colvin and Laflen, 1981).
Erosion losses due to cropping effects could be a combination of many factors including prior cropping effects on the soil, the amount of residue incorporated by tillage, canopy and residue cover, and live and dead root biomass. To more carefully isolate the influence of each factor on soil detachment, a more controlled laboratory study is needed whereby all casual factors other than that related to the study are eliminated. Ghidey and Alberts (1994) studied the effects of cropping system and antecedent water content on interrill soil erodibility in the laboratory. They used disturbed soil which was sieved through a 9-mm sieve to remove residues and clods. Thus, the influence of cropping systems on factors such as microbial population, decomposed residue and root masses, and dead root mass on soil resistance to sealing and detachment by raindrops were not specifically measured.
Numerous research had been conducted to evaluate the
effects of prior cropping systems, organic matter, and residue on aggregate size, aggregate stability, and soil erodibility (Alberts and Wendt, 1985; Bathke and Blake, 1984; Fahad et aI., 1982; McCracken, 1984; Chaney and
Transactions of the ASAE VOL. 40(1):129-135 @ 1997 American Society of Agricultural Engineers 0001-2351/97/4001-0129 129
Swift, 1984). Gantzer et al. (1987) also studied the effects
of soybean and corn residues on soil strength and splash detachment. However, very limited information is available on the effects of dead roots on soil erodibility, splash
detachment, and aggregate stability. The objective of this study was to evaluate the influence
of dead roots on soil erodibility, splash detachment, and
aggregate stability. Mathematical relationships were also developed that can be used in erosion models to predict the effect of dead root mass and dead root length on soil
erodibility parameters.
MATERIALS AND METHODS
The study was conducted in a laboratory using soil boxes and a rainfall simulator. The soil boxes were 100 cm
long and 30 cm wide. Soil depth was 10 cm overlaying 5 cm of sand. One end wall of these boxes was fitted with a
V-shaped collector to collect and concentrate runoff into a continuous stream. Two perforated tubes in the bottom of each box allowed for air venting and drainage.
Soil was collected from plots located at the University of Missouri Midwest Claypan Experimental Farm near Kingdom City, Missouri. The soil was a Mexico silt loam (fine, montrnorillnitic, mesic, Udollic Ochraqualf) with sand, silt, and clay contents of 5, 69, and 26%, respectively. Soil was collected from four cropping treatments selected to give a wide range in dead root parameters including alfalfa, canada bluegrass, continuous corn, and continuous soybeans. Four replications of each treatment were imposed in 1982 on 3-m wide x 27-m long plots. Soil was collected in the fall of 1988. Prior to soil collection, surface residue biomass was removed from the sample area before the 0- to 0.15-m layer was tilled with a rototiller. About 100 kg of soil was collected from each of the 16 plots, air dried, and sieved through a screen with 9-mm openings.
About 500 g of soil was taken from each plot sample for root mass and root length analysis. Roots were separated from the soil using a hydropneumatic elutriation system (Smucker et aI., 1982). Root length was determined using the line intersect technique (Newman, 1966).
Soil subsamples were taken from each plot sample to determine the aggregate stability of the soil. The stability of air dry 2- to 1-mm aggregates was determined by wet sieving (Kemper and Rosenau, 1986), without vapor wetting prior to immersion. Aggregate Index (AI) was calculated using the equation:
AI = WSA
(1)
WSA + WUA
where WSA is the weight of stable aggregates, and WUA is the weight of unstable aggregates.
Soil resistance to slaking and dispersion was also evaluated using Middleton's dispersion ratio and Middleton's dispersion ratio as modified by Olson et aI., 1962. The dispersion ratio gives an index of stability of soil aggregates in water. The Olson et al. (1962) definition of dispersion ratio is:
DR20= < 20 11mundispersed X 100 (2)
< 20 11mdispersed
Middleton definition of DR is:
DR50 = < 50 11mundispersed x 100 (3)
< 50 11mdispersed
In brief, the dispersion ratio is defined as the ratio of the mass of undispersed soil particles (either <50 or <20 11m) to the mass dispersed after adding the soil to a graduated cylinder and inverting end-over-end several times. The <50 11mand <20 /lm dispersed fractions were measured
using the pipette method. The soil was firmly packed in the soil boxes to an
average bulk density of 1.07 g cm-3 using a vibrational packing device. The soil box was then placed in a supporting stand at 4% slope. Rain was applied with a multiple intensity rainfall simulator, similar to that described by Meyer and Harmon (1979), with a single 80150 v-jet nozzle. Mean waterdrop diameter was 3.0 mm falling from a height of 2.5 m. To each box, rainfall of
64 mm h-l intensity was applied for 1 h during the first day. On the second day, a 30-min run of constant intensity (64 mm h-l) was applied. On day 2, the 30-min constant intensity event was followed by four 15-min storms at intensities of 25, 50, 75, and 100 mm h-I. During each
rainstorm, surface runoff was measured and sampled for sediment analysis. After runoff began, samples were collected at 2 and 3 min intervals in the first 5 min, and at 5-min intervals thereafter.
Soil splash samples were collected during the initial 60-min constant intensity (64 mm h-I) run. Splash boxes,
each with a 0.2-cm high x 2.5-cm wide rectangular opening, were mounted at 2-, 6-, and IO-cm heights above the soil surface on each side of the box. Splash boxes were changed quickly at 10-min intervals during the event. Soil
splash was then quantitatively transferred from each splash compartment to glass dishes for oven drying and weighing on an analytical balance.
Four soil cores (56 mm x 33 mm high) were inserted into the soil prior to rainfall simulation. At the end of the
variable intensity sequence, the soil cores were carefully removed for measurement of soil strength and bulk density.
The samples were rewet with a 0.1 mole/L CaCI2-MgCI2 solution and allowed to equilibrate for 12 h. About 5 h
before the strength measurement, the samples were transferred to a tension table maintained at a I kPa soil
water potential. Soil shear strength was determined with a Swedish fall cone device (60 g cone) (AI-Durrah and Bradford, 1981). Four measurements were made on each
core and averaged before additional analysis was performed. Soil bulk density of each core was determined by standard methods.
Interrill detachment was described as being proportional to the power of rainfall intensity and slope factor (Meyer, 1981),
Di =Ki Ib Sf
(4)
where Di is the interrill erosion rate (kg m-2 s-l), Kj is the interrill erodibility parameter of the soil (kg s m-4), I is the rainfall intensity (m s-I), b is the exponent related to soil
clay content(MeyerandHarmon,1984), and Sf is the slope factor defined by Liebenow et al. (1990).
130
TRANSACTIONS OF TIlE ASAE
Sf = 1.05 - 0.85 exp[-4sin(6)1
(5)
where e is the slope angle.
The average erosion rates obtained from the 15-min variable 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 were transformed into logarithms. The transformed data were then plotted and the resultant intercept and slope values were used to predict the Kj and b values of equation 4,
respectively. Interrill erosion has been approximately proportional to
the square of rainfall intensity and the slope factor (Meyer and Harmon, 1984; Watson and Laflen, 1986; Ghidey and Alberts, 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 rates from variable intensity runs were also used to evaluate equation 7.
RESULTS AND DISCUSSION
ROOT PARAMETERS
Differences in dead root mass and dead root length among the four crops were highly significant (p <0.05) (table 1). Dead root mass and dead root length in the 0- to 0.15-m depth from the perennial crops (alfalfa and canada bluegrass) were much higher than those from annual row crops (corn and soybeans). There was almost a five fold difference in root mass and root length between alfalfa and soybeans. Root length per unit weight of dry roots was 22.0, 32.0, 28.7, and 26.1 kIn/kg for alfalfa, Canada bluegrass, corn, and soybeans, respectively.
DAY 1, CONSTANT RAINFALL INTENSITY RUN
Runoff, soil loss, and sediment concentrations for alfalfa, canada bluegrass, corn, and soybeans on the first day of simulation (60-min constant intensity run at 64 mm h-l) are given in table 2. Runoff and soil loss were expected to be lower from the perennial crops than those from the annual row crops. As expected, runoff, soil loss, and sediment concentration from alfalfa were significantly lower (p <0.05) than from corn or soybean. However, runoff, soil loss, and sediment concentration from canada
Table 2. Mean runoff, soil loss, and sediment concentrations* measured during the first day constant intensity run
Crop
Runoff (nun)
Soil Loss (g min-1 m-2)
Sed. Concent. (Mg kg-I)
Alfalfa
Bluegrass Com
Soybean
53.3b 58.1a 57.1a 55.8ab
12.3b 16.0a 17.8a 14.9ab
13,800b 16,500a 18,600a 16,OOOab
* Values containing the same letter are not significantly different at 5% level.
bluegrass were significantly higher (p <0.05) than those from alfalfa and were similar to those of corn and soybean.
DAY 2, CONSTANT RAINFALL INTENSITY RUN
On the second day 30-min run at 64 mm h-l intensity there was no significant difference (p <0.05) in runoff among the crops, but there were significant differences (p <0.05) in soil loss and sediment concentrations (table 3). Soil loss and sediment concentration were lower from
crops with higher dead root mass and dead root length, but the differences were small when compared to the differences in root mass and length. For instance, dead root mass and dead root length for alfalfa were approximately 5 times higher than those for soybeans, but the differences in soil loss and sediment concentrations between alfalfa and
soybeans were only 17 and 16%, respectively
INTERRILL ERODIBILITY, Kj
Erosion rates measured from the 15-min variable
intensity (25, 50, 75, and 100 mm h-l) storms were used to evaluate the Kj and b values given in equation 4. Kj values (x 10-6) for alfalfa, canada bluegrass, corn, and soybeans were 2.66, 2.52, 3.04, and 3.26 kg s m-4, respectively. The Kj values from perennial crops were significantly lower (p <0.05) than those from the annual row crops. The mean exponent, b, values were 2.12, 1.95, 2.14, and 2.13 for
alfalfa, canada bluegrass, corn, and soybeans, respectively. There were no significant difference (p <0.05) in the exponent, b, values among the crops, and they were all close to 2.0 which agrees with previous studies that found interrill erosion to be proportional to the square of rainfall intensity (Meyer and Harmon, 1984; Watson and Laflen, 1986; Ghidey and Alberts, 1994).
The differences in Kj values among the crops were small relative to the differences in root mass and root length. For instance, Kj value for soybeans was less than 20% higher than that for alfalfa, whereas the dead root mass and dead
root length for alfalfa were four to five times higher than those of soybeans. The mean Kj values (xlO--6) when computed from equation 6 (intensity square relationship) were 2.66, 2.56, 3.02, and 3.32 kg s m-4 for alfalfa, canada
Table I. Mean dead root mass and dead root length values* for alfalfa, Canada bluegrass, corn, and soybean
Crop
Alfalfa Bluegrass Com Soybean
Root Mass (g m-2)
495a 384a 150b 92b
Root Length (m m-2)
1O,858a 12,289a 4,258b 2,364c
* Values containing the same letter are not significantly different at 5% level.
Table 3. Mean runoff, soil loss, and sediment measured during the second day constant
Runoff
Soil Loss
Crop
(mm)
(g min-I m-2)
Alfalfa
Bluegrass Com
Soybean
27.3a 28.6a 28.2a 28.2a
12.4b 13.5b 15.7a 15.0a
concentrations* intensity run
Sed. Concent. (Mg kg-I)
13,400c 14,1 OObc 16,600a 16,OOOab
* Values containing the same letter are not significantly different at 5% level.
VOL. 40(1):129-135
131
bluegrass, corn, and soybeans, respectively. There was a definite trend when K; values were plotted against the dead
root parameters. As dead root mass and dead root length increased, K; decreased (fig. 1). The relationships between K; and dead root mass, and K; and dead root length, using
equation 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 dead root mass and dead root length values measured from each plot. For each cropping treatment, soil samples were collected from four plots. When the mean values were used
5
,.
E
f; ~
. . . J:S. . . . . ~ . . ~3 . . . :is
'8 a=U2 "E .!! .E
1
L...
0
0.1
02
O~
OA
O~
Dead Root mass (kg m-2)
. .
0.6 0.7
(a)
5
't E
'b
. . . E ... . ~~3
:~is
'e6 ~2
S
. .
.....
.
1 0 3I 6I 9I 1I2 Dead Root l.sngIh (km m-2)
(b)
Figure l-RelatioWihip dead root length (b).
between Kj and dead root mass (a), and !\j and
to define the relationships, better relationships were found
= between K; and dead root mass (r2 0.83), and K; and dead = root length (r2 0.92).
Interrill erodibility values were also computed using equation 7. The mean Kj values (x lO-6) for alfalfa, canada bluegrass, com and soybeans were 2.95, 2.81, 3.22, and 3.46 kg s m-4, respectively. These values were slightly higher than the ones computed using equation 6, however, relationships similar to the ones expressed in equations 8 and 9 were observed when dead root mass and dead root length values were plotted against the Kj values computed from equation 7.
SOIL PARAMETERS
Bulk Deusity and Soil Shear Strength. There were no significant differences (p <0.05) in bulk density among the crops measured in the soil within 50-mm x 33-mm high cores collected after the variable intensity sequence. Initial mean bulk density values were 1.06, ] .06, 1.07, and ] .07 mg m-3 for alfalfa, Canada bluegrass, corn, and soybean, respectively. At the end of the experiment, mean bulk density for alfalfa, Canada bluegrass, corn, and soybean were 1.10, 1.12, 1.]], and 1.1 0 mg m-3, respectively. Dead root mass and dead root length had no significant (p <0.05) effect on the final bulk density.
The effect of dead roots on the shear strength of the soil were significant (p <0.05) (table 4). At the end of the experiment, soil shear strength was higher from soil samples with greater amounts of dead root mass and length. Mean soil shear strength for alfalfa and bluegrass were approximately. 22% higher than those for corn and soybeans.
DISPERSION RATIO AND AGGREGATE INDEX
The effects of dead root parameters on aggregate stability such as aggregate index and dispersion ratio were significant (p <0.05) (table 4). As dead root mass and dead root length increased, aggregate index increased and the dispersion ratios (DR20 and DR50) decreased. The aggregate index values for alfalfa and Canada bluegrass were twice those for corn and soybeans. DR20 and DR50 for alfalfa and bluegrass were approximately 40 and 35% lower than those for corn and soybeans, respectively.
There was no correlation between runoff and aggregate index or dispersion ratio, but there was a definite trend between soil loss and aggregate index, and soil loss and dispersion ratio. Soil loss decreased as aggregate index increased and dispersion ratio decreased. Our results support previous research findings that lower aggregate index and higher dispersionratio values indicateincreasedsusceptibility of the soil to erosion (Kemperand Rosenau, 1986).
Table 4. Mean soil strength, aggregate index (AI), dispersion ratio as defined by Olson (DR20), and dispersion ratio as defined by Middleston (DR %)*
Crop
Soil Strength (kPa)
AI
DR20
DR50
Alfalfa Bluegrass Com Soybean
5. 14a 5.16a 4.22b 4.23b
0.49a 0.48a 0.21b 0.27b
6.7b 7.0b 12.2a 11.1a
18.1b 16.0b 25.4a
26.9a
* Values containing the same letter are not significantly different at 5% level.
132
TRANSACfIONS OF THE ASAE
14
12 Ii) ~ 10 .9 C CD 8 E .r:. 0 -m 8 C .r:. II> as 4 a. C/J
2
. Alfalfa
IZI Bluegrass
Deem ~ Soybean
3
2-cm Height
~~ 02
0
E
a
. I!!
A
A
0
. ..9r:: 0
6
:a
a
'Q.1
iii
A II
A
A
. . en
a
.
0 2-an Height
6-cm Height
1o-an Height
Total
Figure 2-Soil splash collected 2-, 6-, and to-cm heights above the soil surface.
SPLASH DETACHMENT
Splash detachment at the 2-, 6-, and lO-cm heights, and total splash from the three heights measured during the first day constant intensity run are shown in figure 2. There were no significant differences (p <0.05) in the total detachment among the crops. The total detachment measured at the three heights were 12.1, 11.0, 11.8, and 12.3 g for alfalfa, Canada bluegrass, corn, and soybeans. There were also no significant differences (p <0.05) in splash detachment among the crops measured at 2-, 6-, and 10-cm heights. For each crop, about 60% of the splash was measured at the 2-cm height, 30% at the 6-cm height, and lO% at the lO-cm height.
Soil splash measured at lO-min time intervals during the one hour constant intensity run from the 2-, 6-, and lO-cm heights above the soil surface are given in figure 3. In all cases, splash detachment was highest during the initial 10-min of the simulation and decreased exponentially with time. The same trends were also observed when total
splash detachments (sum of soil splash measured at the three heights) were plotted against time (fig. 4).
The effects of root parameters were not observed on splash detachment as they were on soil strength, aggregate stability, dispersion ratio, and interrill erodibility. Soil splash is mainly due to the forces of falling raindrops breaking down aggregates. The kinetic energy of raindrops falling at terminal velocity is from one to two orders of magnitude greater than the kinetic energy associated with gently flowing water (Hudson, 1981). Root mass and root length might have stabilized the aggregates to reduce erosion by runoff; however, these aggregates may not be stable when subjected to the impact of raindrops. Although there was variability in soil erosion among the crops due to differences in dead root mass and length, the effect was not observed on splash detachment.
Previous studies showed that splash detachment was
highly correlated to soil shear strength (Cruse and Larson, 1977; AI-Durrah and Bradford, 1981; AI-Durrah and Bradford, 1982; Nearing and Bradford, 1985). In our study,
splash detachment from crops that resulted in greater shear strength was not significantly different (p <0.05) from
0
L
0
10
20
30
40
50
Time (minutes)
6-cm Height
0.8
0 E
C'-IS 0.6 .9 .r::
:a 0.4 'Q.
en 0.2
a0
A
&
. . A !!
a
A
0
A
0 A
60
70
~~
~
0
0
10
20
30
40
50
Time (minutes)
0.5
1O-cm Height
- 0.4
UI
-CE'-IS0.3
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.r::
:a 0.2 'Q. UJ
tI
"
0
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a
. 4
A
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0
. a
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0.1
60
70
AI0IIIJIa
egl.1:.l88 Co0m
~
.
g
0
0
10
20
30
40
50
60
70
Time (minutes)
Figure 3-Splash coUected in to-min intervals at 2-, 6-, and to-em heights above soil surface.
crops with lower shear strength. Splash detachment was
measured durin$ the first day constant intensity dry run; whereas, the shear strength of the soil was measured at the
end of the experiment after the variable intensity runs. Thus, it is difficult to conclude whether there was a
VOL. 40(1):129-135
133
4
Alfcalfa
Bg6rass
f.9 3[ e .J:: 2
en .!2 a. en
c
0
6
6
*
0
Com 0
Soy*bean
c
~ 6
II!
.
"6
0
C
~
1
0
0
10
20
30
40
50
60
70
Time (minutes)
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 the variable intensity runs.
SUMMARY AND CONCLUSIONS The effect of dead roots on runoff, soil erodibility,
splash detachment, and aggregate stability were studied in the laboratory. We found that:
I. Dead roots had no effect on runoff but significantly influenced (p <0.05) soil loss and sediment concentrations. Soil loss and sediment
concentrations from annual row crops were significantly higher than those from perennial crops; however, the differences in soil loss among the crops were small relative to the differences in root mass and root length. 2. Interrill erodibility parameter, Kj, decreased as dead root mass and dead root length increased. There were exponential relationships between Kj and dead
root mass and ~ and dead root length.
3. Dead roots had no effect on the soil bulk density, but significantly influenced soil shear strength. Soil strength increased as root mass and root length increased.
4. Dead roots significantly (p <0.05) affected aggregate index and dispersion ratio. As the amount of root mass and root length increased, aggregate index increased, and dispersion ratio decreased.
5. The effects of dead roots were not observed on
splash detachment as they were on soil strength, aggregate index, and dispersion ratio. Splash detachment was highest during the initial 10-min of simulation and then decreased exponentially.
The results obtainedin this study have important implications for erosion scientists and modelers. The
relationships between interrill erodibility, Kj, and root mass or root length observed in this study can be used in the erosion models such as WEPP to adjust predicted Kj values to temporal changes in either root parameters.
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