Grassland Management and Soil Bulk Density
Grassland Management and Soil Bulk Density Cheryl A. Murphy1, Bryan L. Foster1, Matthew E. Ramspott2, Kevin P. Price2
1. Department of Ecology and Evolutionary Biology, 2045 Haworth Hall, University of Kansas, Lawrence, Kansas 66045 (email: [email protected]) 2. Department of Geography, Kansas Applied Remote Sensing Program, University of Kansas, Lawrence, Kansas 66045.
Corresponding Author: Cheryl A. Murphy
1
Grasslands in Northeastern Kansas encompass several management regimes, all of which could impact soil properties. Differences in one such property, soil bulk density, could indicate differences in soil quality. Five regimes of privately managed grasslands (coolseason: hay or grazed, warm season: hay, grazed or Conservation Reserve Program (CRP)) in Douglas, Jefferson and Leavenworth Counties, Kansas, were sampled for soil bulk density in 2001 and 2002. Cool-season fields have been historically cultivated and recently seeded into Bromus inermis, while warm-season fields are either native prairie remnants or CRP. CRP fields have had warm-season native grasses seeded into them after extensive cultivation and soil erosion. Bulk density (dry soil weight/soil volume) cores were taken to a depth of 15 cm (volume = 76.01 cm3) and dried to constant weight (90ûC). Field averages significantly showed that CRP had the highest bulk density (0.90 and 0.74 g/cm3, 2001 and 2002, respectively, p < 0.001), cool-season fields had high to intermediate bulk density (0.80 and 0.72 g/cm3, 2001 and 2002, respectively) and warmseason native fields had the lowest bulk density (0.70 and 0.67 g/cm3, 2001 and 2002, respectively). Hay fields were not different from grazed in 2001, but were significantly lower than grazed in 2002. From soil surveys and field observations, CRP and coolseason fields corresponded to areas of higher erosion, in addition to being areas of historical cultivation. Thus, results reflect current management (hay vs. grazing), and the effects of historical land-use.
Key words: bulk density, CRP, grassland management, grazing, hay, land-use history, soil quality.
2
INTRODUCTION Land in Northeastern Kansas has been used for a variety of agricultural purposes since European settlement in the 1800’s, including cropland, pasture and haying. However, since the mid 1900’s, there has been a trend of decreasing the amount of land in cash crops and converting it back into grasslands (Jefferson County Soil Survey 1977). Several reasons brought about this trend: livestock has become more profitable, the bedrock is too close to the soil surface for plowing implements, the soils have too much clay content for cultivation, and there has been sufficient amounts of erosion to deplete soil quality.
These various agricultural changes, along with current practices, have likely had profound impacts on soil quality. Differences in soil quality could be detected by differences in soil bulk density. Bulk density is a measure of mass per unit volume of dry soil, which is essentially a measure of soil porosity in which higher amounts of soil pores gives lower bulk density values.
Several factors affect soil porosity including compaction, soil clay content and cultivation. As objects move over the soil (e.g. tractors, animals), it becomes compacted by the weight, which decreases porosity and, therefore, increases bulk density. In addition, when traffic occurs on wet soil, compaction effects become more pronounced. Clay particles have high amounts of pore space within them, when compared to other particles of soil, which allows for greater compaction. Soils with higher clay contents have higher potentials of compaction and increased bulk densities. Cultivation may
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actually increase soil porosity and lower bulk density over the short-term, however, over the long-term, cultivation will increase bulk density. Cultivation depletes organic matter and weakens soil structure, both of which lead to decreased soil porosity and increased bulk density (Brady and Weil 2002).
Our objective was to determine soil bulk density for five grassland management types and determine if there were differences among them. We hypothesized that the cultivated grasslands would have higher bulk densities than those that have never been plowed. In addition, we proposed that fields used currently for livestock would have higher bulk densities than those currently being used for hay.
METHODS We studied 32 private grasslands in Northeastern Kansas representative of five management regimes: 1) cool-season hay (C-H), 2) cool-season grazed (C-G), 3) warmseason native hay (W-NH), 4) warm-season native grazed (W-NG), and 5) Conservation Reserve Program (W-CRP). Cool-season fields were historically cultivated and later converted into grasslands in the mid 1900’s and are dominated by Bromus inermis (Brome). These fields obtain their highest productivity and seed dispersal during the cool part of the growing season (June) and are used for either hay or grazing.
Warm-season native grasslands were located on soils that have never been plowed and are native prairie remnants used for either hay or grazing. These fields are dominated by native grasses (Andropogon gerardii (big bluestem) and Schizachyrium scoparium (little
4
bluestem)) in which the highest productivity and seed dispersal occurs during the warm part of the growing season (late July to August).
The Conservation Reserve Program was established by Congress in the Food Security Act of 1985 (Title XII). This program provides participants an annual per acre rent and half the cost of creating a permanent cover with native vegetation (e.g. grasses) in exchange for retiring cropland that is highly erodible or environmentally sensitive for periods of 10 years. The main goals for creating this program was to reduce the amount of soil erosion on cropland, curb surplus production, support farmer income and improve the environmental quality of the land. During the first nine sign-ups for this program, 2.8 million acres were admitted in Kansas (from 1986-1990), which represented 9.9% of Kansas’ tillable cropland. During the next three sign-ups, another 83,000 acres were added, making Kansas the largest participant in the Central Great Plains (Diebel, Cable et al. 1993). CRP fields included in our study have been out of cultivation, at the most, since 1985. Native warm-season grasses (A. gerardii and S. scoparium) were seeded, which created recently cultivated, warm-season grasslands.
When selecting fields for the study, an effort was made to minimize differences in soil types by using fields in upland areas with minimal slopes and similar soil series. This helped eliminate potential confounding effects of differences in vegetation and soils between upland and lowland areas. Table 1 lists soil series for each field.
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In 2001, 10 fields (2 replications for each regime) were sampled within Douglas, Leavenworth and Jefferson Counties (Table 1). Within each field, six 20 x 20 m plots were established within an area of 160 x 90 m (total area for each field = 14,400 m2). For each plot, three soil samples were taken with a 36” tube sampler, for a total of 18 samples per field. Each core was taken to a depth of 15 cm, with densities based on the entire 15 cm core and a diameter of 2.54 cm.
In 2002, 30 fields were sampled in the same counties, establishing six replicates of each grassland regime (Fig. 1). Unlike the plot system in the previous year, transects were used within each field. Three parallel transects, each 50 m long and separated by 50 m, were positioned on the upland areas (total area for each field = 5,000 m2). Soil samples were taken at the 5, 25, and 45 m locations for each transect, generating nine soil samples for each field. Bulk density cores were taken in the same way as the previous year. Unlike 2001, however, soil cores were examined to determine if the core contained multiple horizons. Notes were taken if differences were noticed, but the soil remained as one core to be comparable to 2001 samples.
All soil samples were dried at 90°C until a constant weight. Bulk density was calculated as the dry soil weight (g) divided by the soil volume (76.01 cm3). Soil samples from 2002 were subsequently submitted to the Soil Testing Laboratory at Kansas State University for texture analysis.
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Analysis of variance (ANOVA) and least significant difference (LSD) tests in Statistix (Analytical Software 1998) were used to examine variation in bulk density among management regimes.
RESULTS In 2001, bulk density differed significantly among the regimes (F4, 174 = 35.61; p < 0.001; Fig. 2). Of the five management regimes, CRP fields had the highest bulk densities. Cool-season fields were significantly lower than CRP and warm-season native fields had the lowest bulk densities of all regimes. Within both cool- and warm native-season native grasslands, hay and grazed fields were similar.
As in 2001, bulk density varied significantly among the five regimes in 2002 (F4, 262 = 12.37, p < 0.001; Fig. 3). CRP and cool-season grazed fields were not significantly different and had the highest bulk densities. Cool-season hay and warm-season native grazed fields were not significantly different and significantly higher than warm-season native hay bulk densities. Further, within both cool- and warm-season native grasslands, hay fields had significantly lower bulk density than grazed fields.
Two-sample t-tests were conducted to compare fields between years. All fields were statistically similar between years, except for two. Field 10 (F17, 5 = 4.90, p = 0.044) and Field 12 (F16, 8 = 5.61, p = 0.009) had lower bulk densities in 2002 compared to 2001.
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Texture analysis showed significant differences in clay content among management regimes (Fig. 4). CRP contained the highest clay content, cool-season hay and grazed were similar and contained intermediate clay contents. Warm-season native grazed fields were significantly lower still, while warm-season native hay fields contained the lowest clay content.
DISCUSSION In our study, variation among fields in soil bulk density appears to reflect the impact of current (hay vs. grazing) and historical (cultivation) land-use practices on soil quality. The effects of current agricultural land-use is reflected by differences in bulk density among hay and grazed fields, independent of their presence in cool- or warm-season grasslands. Differences in bulk densities could arise through two reasons. First, grazing can exert a greater pressure onto soils through animal hooves (up to 200 kPa) than tractor wheel pressure (30-150 kPa), creating higher bulk densities (Proffitt, Bendotti et al. 1993). Second, grazing removes the cushioning affects of vegetation (Naeth, Pluth et al. 1990; Ford and Grace 1998), which creates larger areas of exposed bare ground and increases the potential of compaction. Bare ground also increases the potential for severe run-off and erosion during rain and wind events.
Effects of grazing on bulk density from the literature are variable. Several researchers have found that grazing increases bulk density (Bauer, Cole and Black 1987; Dormaar, Smoliak and Williams 1989; Naeth, Pluth et al. 1990; Villamil, Amiotti and Peinemann 1998; and Donkor, Gedir et al. 2002), while others have found no effect (Ford and Grace
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1998). In addition, if differences were seen, they were usually in the top five to 30 cm of the soil profile.
In our study, variable results were also found with no differences between hay and grazed fields in 2001, but higher bulk density for grazed fields in 2002. This discrepancy may have been due to larger sample sizes in 2002.
Soil texture may also impact bulk density (Bauer, Cole and Black 1987; Dormaar, Smoliak and Williams 1989) and our results show that clay content significantly differed among management regimes (Fig. 4). Interestingly, clay content patterns contain the same trend as soil bulk density patterns. Differences in clay content may be the result of each management regime having a different soil series. However, Table 1 shows that fields within a management regime have a variety of soil series, such that clay content does not differ between management regimes for both the A horizon (F4, 27 = 1.77, p = 0.165) and subsoil horizons (F4, 27 = 1.14, p = 0.360). This indicates that land managers were not choosing a priori which fields would receive a particular management.
We suggest that land-use history largely explains differences in clay content and soil bulk density among management regimes. From soil survey maps and personal observations (Table 1), erosion was detected in almost every cool-season and CRP field. The common thread with all of these fields is historical cultivation (personal communication with land owners), which caused the A horizon to erode and decreased the amount of soil organic
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matter. Subsequently, the B horizon was exposed, which is denser and has higher clay contents than the A horizon
CRP fields have been cultivated the longest and most recently, giving them the least amount of time for soil recovery and having the highest bulk density (Table 1). Coolseason hay and grazed fields have been out of cultivation for longer periods of time, enabling more soil recovery and having intermediate bulk densities. Since warm-season native fields have never been plowed, the soil has remained intact, erosion negligible and bulk densities low.
In general, bulk density values in 2001tended to be higher than in 2002 (Fig. 2 and 3). A possible reason for this trend could be the time of year in which sampling occurred. In 2001, soils were sampled in July, while in 2002, samples were taken in October and November. Results from the literature are indeterminate regarding intra-annual variation in bulk density. Naeth, Pluth et al. (1990) found that bulk densities were highest early in the season for grazed systems in Alberta, Canada. In contrast, Donkor, Gedir et al. (2002) found bulk density was lowest in the spring. Additionally, Dormaar, Smoliak and Williams (1989) found that densities did not vary over seasons or years in grazed and non-grazed fescue grasslands.
Date of sampling, combined with rainfall amounts for each year, could help explain lower bulk density in 2002. Several researchers have found that the amount of compaction is dependent (in part) on soil water content at the time of contact (Dormaar,
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Smoliak et al. 1989; Naeth, Pluth et al. 1990; Donkor, Gedir et al. 2002), with more compaction occurring under wetter conditions. Average rainfall for the area in 2001, January through July, was 749 mm (SD = 35.26) representing an above average rainfall for that time period (+ 174 mm (SD = 41.06), based on 30 year average of 974 mm (SD = 26.13)). In contrast, 2002 rainfall through September (right before soil samples were taken) was 593 mm (SD = 41.05), representing 155 mm less rainfall over a longer period of time (an additional two months, - 194 mm (SD = 36.37) away from the 30-year average) (Kansas State University, Weather Data Library 2003). The decrease in potential soil moisture in 2002 could have decreased the impact of grazing and tractor compaction and reduced bulk density. However, even though lower bulk densities occurred in 2002, the relative differences among management regimes were similar for each year.
In conclusion, our results suggest that variation in bulk density (soil quality) reflects both current (hay vs. grazing) and historical (cultivation) land-use practices. Agricultural practices such as grazing and cultivation can have long-lasting implications and the impacted soils can take decades to recover.
ACKNOWLEDGEMENTS We would like to thank all who were involved in collecting this data and aiding in fieldwork; Geoff Folker, Terri Hildebrand, Emily McGhee, Jarad Mellard, Ted Peterson and Andrea Repinsky. A special ‘Thank You’ goes to all the land owners that allowed access to their fields, without which, this study would not have been possible. We would
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also like to thank Stylianos Chatzimanolis……… and ……reviewers for helpful comments to improve this paper. Support for this research was provided by ………
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LITERATURE CITED Analytical Software. 1998. Statistix for Windows 2.0. Tallahassee, FL: Analytical Software
Bauer, A., Cole, C. V. and Black, A. L. 1987. Soil property comparisons in virgin grasslands between grazed and nongrazed management systems. Soil Science Society of America Journal. 51, p. 176-182.
Brady, N. C. and Weil, R. R. 2002. The Nature and Properties of Soils. Thirteenth Edition. Prentice Hall, Upper Saddle River, New Jersey.
Diebel, P. L., Cable T. T. and Cook, P. S. 1993. The future of Conservation Reserve Program land in Kansas: The landowner’s view. Report of Progress 690, Agricultural Experiment Station, Kansas State University, Manhattan.
Donkor, N. T., Gedir, J. V., Hudson, R. J., Bork, E. W., Chanasyk, D. S. and Naeth, M. A. 2002. Impacts of grazing systems on soil compaction and pasture production in Alberta. Canadian Journal of Soil Science 82, p. 1-8.
Dormaar, J. F., Smoliak, S. and Williams, W. D. May 1989. Vegetation and soil responses to short-duration grazing on fescue grasslands. Journal of Range Management 42, p. 252-256.
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Ford, M. A. and Grace, J. B. 1998. Effects of vertebrate herbivores on soil processes, plant biomass, litter accumulation and soil elevation changes in a coastal marsh. Journal of Ecology 86, p. 974-982.
Kansas State University. Research and Extension. Weather Data Library. http://www.oznet.ksu.edu/wdl (November 15, 2002)
Naeth, M. A., Pluth, D. J., Chanasyk, D. S., Bailey, A. W. and Fedkenheuer, A. W. May 1990. Soil compacting impacts of grazing in mixed prairie and fescue grassland ecosystems of Alberta. Canadian Journal of Soil Science 70, p. 157-167.
Proffitt, A. P., Bendotti, S., Howell, M. R. and Eastham, J. 1993. The effect of sheep trampling on soil physical properties and pasture growth for a red-brown earth. Australian Journal of Agricultural Research 44, p. 317-331.
Soil Survey of Douglas County, Kansas. July 1977. United States Department of Agriculture. Soil Conservation Service in cooperation with Kansas Agricultural Experiment Station.
Soil Survey of Jefferson County, Kansas. November 1977. United States Department of Agriculture. Soil Conservation Service in cooperation with Kansas Agricultural Experiment Station.
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Soil Survey of Leavenworth and Wyandotte Counties, Kansas. February 1977. United States Department of Agriculture. Soil Conservation Service with cooperation with Kansas Agricultural Experimental Station.
USDA-NRCS, SSURGO: Data Use Information report. January 1995. http://www.ncg.nrcs.usda.gov/pdf/ssurgo_bd.pdf (Appendix A, pp. 3750) (5 June 2000 by K. Apolzer)
Villamil, M. B., Amiotti, V. M. and Peinemann, N. July 2001. Soil degradation related to overgrazing in the semi-arid southern caldenal area of Argentina. Soil Science. 166, p. 441-452.
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Table 1: Soil Characteristics by Field Field Management Year in County No. Regime1 Study
Soil Series2
% Clay Content: A Horizon3
% Clay Content: Subsoil3
L-Use4
Soil Survey Notes5
1
W-CRP
2001, 2002
Douglas
Mc
33.5
44.23
1988
2
C-H
2001
Jefferson
Pc
34
42.43
1982
3
C-H
2001, 2002
Jefferson
Oc, Mc
33.5
44.29
Unkwn
Erosion
4
C-H
2002
Jefferson
Vc
31
29.5
1965
Erosion
5
W-NH
2001, 2002
Jefferson
Mo
33.5
44.37
NP
Rock outcrops
6
W-NG
2001, 2002
Jefferson
Mo
33.5
44.37
NP
7
C-G
2001, 2002
Jefferson
Mc
33.5
45.13
Unkwn
Erosion
8
C-G
2002
Jefferson
Vc
31
29.5
Unkwn
Erosion
9
W-NH
2001
Jefferson
Mo, Sc, Vc
30.99
36.31
NP
Vc = rock outcrops
10
W-NG
2001, 2002
Jefferson
Mo
33.5
44.37
NP
11
W-CRP
2001, 2002
Jefferson
Oc
33.5
43.45
1985
12
C-G
2001, 2002
Jefferson
Mo
33.5
44.37
Unkwn
13
C-H
2002
Jefferson
Sc
28.48
35.07
1992
Erosion
14
C-H
2002
Jefferson
Sc
28.48
35.07
1997
Erosion
15
W-NG
2002
Jefferson
Mc
33.5
45.13
NP
17
W-CRP
2002
Jefferson
Gb, Oc
32.5
40.55
Unkwn
18
C-H
2002
Jefferson
Vc
31
29.5
Unkwn
19
W-NH
2002
Jefferson
Pc
34
42.43
NP
20
C-G
2002
Jefferson
Pc
34
42.43
Unkwn
Observations6 Clayey
Some samples = clay Area has slopes
Clayey
Erosion
Some samples = clay Slopes = erosion? Clayey Erosion?
Several samples = in road/mowed area Erosion: B horizon? Clayey
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Field No.
Management Regime1
Year in Study
County
Soil Series2
% Clay Content: A Horizon3
% Clay Content: Subsoil3
L-Use4
21
W-NH
2002
Leavenworth
Sh
25.5
31.85
NP
22
C-H
2002
Leavenworth
Pc
34
36.8
Unkwn
Erosion
23
W-CRP
2002
Jefferson
Vc
31
29.5
1985
Erosion
24
W-NG
2002
Jefferson
Pc
34
42.43
NP
25
W-NG
2002
Jefferson
Sc
28.48
35.07
NP
26
W-NH
2002
Jefferson
Sc
28.48
35.07
NP
27
C-H
2002
Leavenworth
Gb
31.5
37.75
Unkwn
28
W-NH
2002
Leavenworth
Sc
31.5
33.6
NP
29
W-CRP
2002
Douglas
Mc
33.5
44.23
Unkwn
30
W-NG
2002
Jefferson
Sc
28.48
35.07
NP
31
C-G
2002
Jefferson
Mc
33.5
45.13
Unkwn
Erosion
32
W-CRP
2002
Jefferson
Mc
33.5
45.13
1988
Erosion
33
W-NG
2002
Douglas
Sh
20.5
22.2
NP
Soil Survey Notes5
Observations6
Clayey; Erosion?
Rock outcrops
Erosion
Clayey
Clayey
1
Management regime: C-H = cool-season hay; C-G = cool-season grazed; W-NH = warm-season native hay; W-NG = warm-season native grazed; W-CRP = warm-season Conservation Reserve Program. 2 Soil Series that underlies each field as determined by soil surveys and digitized soil maps (USDA-NRCS 1995): Gb = Grundy silty clay loam; Mc = Martin silty clay loam; Mo = Martin-Oska silty clay loam; Oc = Oska silty clay loam; Pc = Pawnee clay loam; Sc = Shelby Pawnee complex (Jefferson Co.), Sharpsburg silty clay loam (Leavenworth Co.); Sh = Shelby loam (Leavenworth Co.), Sibleyville loam (Douglas Co.); Sw = Sogn-Vinland complex; Vc = Vinland complex. 3 A Horizon = surface layer content, Subsoil = depth weighted average; both are expressed as a percentage of the material < 2 mm in size (USDA-NRCS 1995). 4 L-Use = Year when current management regime was started; NP = never plowed; Unkwn = unknown land-use management initiation, but field was cultivated at some point in its history (Personal communication with land owners). 5 Soil Survey Notes = description of comments found in field area on the soil survey maps (Soil Survey of Douglas, Jefferson and Leavenworth, Counties, 1977). 6 Observations = any notes about the texture, position and other observations when taking the soil sample that could be relevant to interpreting bulk density data.
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FIGURE LEGEND Figure 1. Location of fields used in 2002 (black dots). The two fields located within the Kansas Ecological Reserves were not included in this analysis. Field 33 is located in the Western edge of Douglas County and is not shown. Picture courtesy of M. E. Ramspott.
Figure 2. Average soil bulk density (g/cm3) with standard deviation for each management (n = 2) in 2001: W-CRP = conservation reserve program; C-H = coolseason hay; C-G = cool-season grazed; W-NH = warm-season native hay; and W-NG = warm-season native grazed. CRP have been most recently cultivated, C-H and C-G cultivated historically and W-NH and W-NG have not been cultivated. Differing letters signify significant differences at α = 0.05 level using least significant difference tests (tvalue = 1.974).
Figure 3. Average soil bulk density (g/cm3) with standard deviation for each management (n = 6) in 2002: W-CRP = conservation reserve program; C-H = coolseason hay; C-G = cool-season grazed; W-NH = warm-season native hay; and W-NG = warm-season native grazed. CRP have been most recently cultivated, C-H and C-G cultivated historically and W-NH and W-NG have not been cultivated. Differing letters signify significant differences at α = 0.05 level using least significant difference tests (tvalue = 1.969).
Figure 4. Percent clay content and standard deviation for each management (n = 6) in 2002: W-CRP = conservation reserve program; C-H = cool-season hay; C-G = cool-
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season grazed; W-NH = warm-season native hay; and W-NG = warm-season native grazed. CRP have been most recently cultivated, C-H and C-G cultivated historically and W-NH and W-NG have never been cultivated. Differing letters signify significant differences at α = 0.05 level using least significant difference tests (t-value = 1.969).
19
Figure 1
20
Figure 2
1.0
Soil Bulk Density (g/cm3)
a b 0.9
b c
c
W-NH
W-NG
0.8
0.7
0.6
0.5 W-CRP
C-H
C-G
Management
21
Figure 3
Soil Bulk Density (g/cm3)
1.0
0.9
a 0.8
b
a b c
0.7
0.6
0.5 W-CRP
C-H
C-G
W-NH
W-NG
Management
22
Figure 4
40
a b
b c
% Clay
30
d 20
10
0 W-CRP
C-H
C-G
W-NH
W-NG
Management
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1. Department of Ecology and Evolutionary Biology, 2045 Haworth Hall, University of Kansas, Lawrence, Kansas 66045 (email: [email protected]) 2. Department of Geography, Kansas Applied Remote Sensing Program, University of Kansas, Lawrence, Kansas 66045.
Corresponding Author: Cheryl A. Murphy
1
Grasslands in Northeastern Kansas encompass several management regimes, all of which could impact soil properties. Differences in one such property, soil bulk density, could indicate differences in soil quality. Five regimes of privately managed grasslands (coolseason: hay or grazed, warm season: hay, grazed or Conservation Reserve Program (CRP)) in Douglas, Jefferson and Leavenworth Counties, Kansas, were sampled for soil bulk density in 2001 and 2002. Cool-season fields have been historically cultivated and recently seeded into Bromus inermis, while warm-season fields are either native prairie remnants or CRP. CRP fields have had warm-season native grasses seeded into them after extensive cultivation and soil erosion. Bulk density (dry soil weight/soil volume) cores were taken to a depth of 15 cm (volume = 76.01 cm3) and dried to constant weight (90ûC). Field averages significantly showed that CRP had the highest bulk density (0.90 and 0.74 g/cm3, 2001 and 2002, respectively, p < 0.001), cool-season fields had high to intermediate bulk density (0.80 and 0.72 g/cm3, 2001 and 2002, respectively) and warmseason native fields had the lowest bulk density (0.70 and 0.67 g/cm3, 2001 and 2002, respectively). Hay fields were not different from grazed in 2001, but were significantly lower than grazed in 2002. From soil surveys and field observations, CRP and coolseason fields corresponded to areas of higher erosion, in addition to being areas of historical cultivation. Thus, results reflect current management (hay vs. grazing), and the effects of historical land-use.
Key words: bulk density, CRP, grassland management, grazing, hay, land-use history, soil quality.
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INTRODUCTION Land in Northeastern Kansas has been used for a variety of agricultural purposes since European settlement in the 1800’s, including cropland, pasture and haying. However, since the mid 1900’s, there has been a trend of decreasing the amount of land in cash crops and converting it back into grasslands (Jefferson County Soil Survey 1977). Several reasons brought about this trend: livestock has become more profitable, the bedrock is too close to the soil surface for plowing implements, the soils have too much clay content for cultivation, and there has been sufficient amounts of erosion to deplete soil quality.
These various agricultural changes, along with current practices, have likely had profound impacts on soil quality. Differences in soil quality could be detected by differences in soil bulk density. Bulk density is a measure of mass per unit volume of dry soil, which is essentially a measure of soil porosity in which higher amounts of soil pores gives lower bulk density values.
Several factors affect soil porosity including compaction, soil clay content and cultivation. As objects move over the soil (e.g. tractors, animals), it becomes compacted by the weight, which decreases porosity and, therefore, increases bulk density. In addition, when traffic occurs on wet soil, compaction effects become more pronounced. Clay particles have high amounts of pore space within them, when compared to other particles of soil, which allows for greater compaction. Soils with higher clay contents have higher potentials of compaction and increased bulk densities. Cultivation may
3
actually increase soil porosity and lower bulk density over the short-term, however, over the long-term, cultivation will increase bulk density. Cultivation depletes organic matter and weakens soil structure, both of which lead to decreased soil porosity and increased bulk density (Brady and Weil 2002).
Our objective was to determine soil bulk density for five grassland management types and determine if there were differences among them. We hypothesized that the cultivated grasslands would have higher bulk densities than those that have never been plowed. In addition, we proposed that fields used currently for livestock would have higher bulk densities than those currently being used for hay.
METHODS We studied 32 private grasslands in Northeastern Kansas representative of five management regimes: 1) cool-season hay (C-H), 2) cool-season grazed (C-G), 3) warmseason native hay (W-NH), 4) warm-season native grazed (W-NG), and 5) Conservation Reserve Program (W-CRP). Cool-season fields were historically cultivated and later converted into grasslands in the mid 1900’s and are dominated by Bromus inermis (Brome). These fields obtain their highest productivity and seed dispersal during the cool part of the growing season (June) and are used for either hay or grazing.
Warm-season native grasslands were located on soils that have never been plowed and are native prairie remnants used for either hay or grazing. These fields are dominated by native grasses (Andropogon gerardii (big bluestem) and Schizachyrium scoparium (little
4
bluestem)) in which the highest productivity and seed dispersal occurs during the warm part of the growing season (late July to August).
The Conservation Reserve Program was established by Congress in the Food Security Act of 1985 (Title XII). This program provides participants an annual per acre rent and half the cost of creating a permanent cover with native vegetation (e.g. grasses) in exchange for retiring cropland that is highly erodible or environmentally sensitive for periods of 10 years. The main goals for creating this program was to reduce the amount of soil erosion on cropland, curb surplus production, support farmer income and improve the environmental quality of the land. During the first nine sign-ups for this program, 2.8 million acres were admitted in Kansas (from 1986-1990), which represented 9.9% of Kansas’ tillable cropland. During the next three sign-ups, another 83,000 acres were added, making Kansas the largest participant in the Central Great Plains (Diebel, Cable et al. 1993). CRP fields included in our study have been out of cultivation, at the most, since 1985. Native warm-season grasses (A. gerardii and S. scoparium) were seeded, which created recently cultivated, warm-season grasslands.
When selecting fields for the study, an effort was made to minimize differences in soil types by using fields in upland areas with minimal slopes and similar soil series. This helped eliminate potential confounding effects of differences in vegetation and soils between upland and lowland areas. Table 1 lists soil series for each field.
5
In 2001, 10 fields (2 replications for each regime) were sampled within Douglas, Leavenworth and Jefferson Counties (Table 1). Within each field, six 20 x 20 m plots were established within an area of 160 x 90 m (total area for each field = 14,400 m2). For each plot, three soil samples were taken with a 36” tube sampler, for a total of 18 samples per field. Each core was taken to a depth of 15 cm, with densities based on the entire 15 cm core and a diameter of 2.54 cm.
In 2002, 30 fields were sampled in the same counties, establishing six replicates of each grassland regime (Fig. 1). Unlike the plot system in the previous year, transects were used within each field. Three parallel transects, each 50 m long and separated by 50 m, were positioned on the upland areas (total area for each field = 5,000 m2). Soil samples were taken at the 5, 25, and 45 m locations for each transect, generating nine soil samples for each field. Bulk density cores were taken in the same way as the previous year. Unlike 2001, however, soil cores were examined to determine if the core contained multiple horizons. Notes were taken if differences were noticed, but the soil remained as one core to be comparable to 2001 samples.
All soil samples were dried at 90°C until a constant weight. Bulk density was calculated as the dry soil weight (g) divided by the soil volume (76.01 cm3). Soil samples from 2002 were subsequently submitted to the Soil Testing Laboratory at Kansas State University for texture analysis.
6
Analysis of variance (ANOVA) and least significant difference (LSD) tests in Statistix (Analytical Software 1998) were used to examine variation in bulk density among management regimes.
RESULTS In 2001, bulk density differed significantly among the regimes (F4, 174 = 35.61; p < 0.001; Fig. 2). Of the five management regimes, CRP fields had the highest bulk densities. Cool-season fields were significantly lower than CRP and warm-season native fields had the lowest bulk densities of all regimes. Within both cool- and warm native-season native grasslands, hay and grazed fields were similar.
As in 2001, bulk density varied significantly among the five regimes in 2002 (F4, 262 = 12.37, p < 0.001; Fig. 3). CRP and cool-season grazed fields were not significantly different and had the highest bulk densities. Cool-season hay and warm-season native grazed fields were not significantly different and significantly higher than warm-season native hay bulk densities. Further, within both cool- and warm-season native grasslands, hay fields had significantly lower bulk density than grazed fields.
Two-sample t-tests were conducted to compare fields between years. All fields were statistically similar between years, except for two. Field 10 (F17, 5 = 4.90, p = 0.044) and Field 12 (F16, 8 = 5.61, p = 0.009) had lower bulk densities in 2002 compared to 2001.
7
Texture analysis showed significant differences in clay content among management regimes (Fig. 4). CRP contained the highest clay content, cool-season hay and grazed were similar and contained intermediate clay contents. Warm-season native grazed fields were significantly lower still, while warm-season native hay fields contained the lowest clay content.
DISCUSSION In our study, variation among fields in soil bulk density appears to reflect the impact of current (hay vs. grazing) and historical (cultivation) land-use practices on soil quality. The effects of current agricultural land-use is reflected by differences in bulk density among hay and grazed fields, independent of their presence in cool- or warm-season grasslands. Differences in bulk densities could arise through two reasons. First, grazing can exert a greater pressure onto soils through animal hooves (up to 200 kPa) than tractor wheel pressure (30-150 kPa), creating higher bulk densities (Proffitt, Bendotti et al. 1993). Second, grazing removes the cushioning affects of vegetation (Naeth, Pluth et al. 1990; Ford and Grace 1998), which creates larger areas of exposed bare ground and increases the potential of compaction. Bare ground also increases the potential for severe run-off and erosion during rain and wind events.
Effects of grazing on bulk density from the literature are variable. Several researchers have found that grazing increases bulk density (Bauer, Cole and Black 1987; Dormaar, Smoliak and Williams 1989; Naeth, Pluth et al. 1990; Villamil, Amiotti and Peinemann 1998; and Donkor, Gedir et al. 2002), while others have found no effect (Ford and Grace
8
1998). In addition, if differences were seen, they were usually in the top five to 30 cm of the soil profile.
In our study, variable results were also found with no differences between hay and grazed fields in 2001, but higher bulk density for grazed fields in 2002. This discrepancy may have been due to larger sample sizes in 2002.
Soil texture may also impact bulk density (Bauer, Cole and Black 1987; Dormaar, Smoliak and Williams 1989) and our results show that clay content significantly differed among management regimes (Fig. 4). Interestingly, clay content patterns contain the same trend as soil bulk density patterns. Differences in clay content may be the result of each management regime having a different soil series. However, Table 1 shows that fields within a management regime have a variety of soil series, such that clay content does not differ between management regimes for both the A horizon (F4, 27 = 1.77, p = 0.165) and subsoil horizons (F4, 27 = 1.14, p = 0.360). This indicates that land managers were not choosing a priori which fields would receive a particular management.
We suggest that land-use history largely explains differences in clay content and soil bulk density among management regimes. From soil survey maps and personal observations (Table 1), erosion was detected in almost every cool-season and CRP field. The common thread with all of these fields is historical cultivation (personal communication with land owners), which caused the A horizon to erode and decreased the amount of soil organic
9
matter. Subsequently, the B horizon was exposed, which is denser and has higher clay contents than the A horizon
CRP fields have been cultivated the longest and most recently, giving them the least amount of time for soil recovery and having the highest bulk density (Table 1). Coolseason hay and grazed fields have been out of cultivation for longer periods of time, enabling more soil recovery and having intermediate bulk densities. Since warm-season native fields have never been plowed, the soil has remained intact, erosion negligible and bulk densities low.
In general, bulk density values in 2001tended to be higher than in 2002 (Fig. 2 and 3). A possible reason for this trend could be the time of year in which sampling occurred. In 2001, soils were sampled in July, while in 2002, samples were taken in October and November. Results from the literature are indeterminate regarding intra-annual variation in bulk density. Naeth, Pluth et al. (1990) found that bulk densities were highest early in the season for grazed systems in Alberta, Canada. In contrast, Donkor, Gedir et al. (2002) found bulk density was lowest in the spring. Additionally, Dormaar, Smoliak and Williams (1989) found that densities did not vary over seasons or years in grazed and non-grazed fescue grasslands.
Date of sampling, combined with rainfall amounts for each year, could help explain lower bulk density in 2002. Several researchers have found that the amount of compaction is dependent (in part) on soil water content at the time of contact (Dormaar,
10
Smoliak et al. 1989; Naeth, Pluth et al. 1990; Donkor, Gedir et al. 2002), with more compaction occurring under wetter conditions. Average rainfall for the area in 2001, January through July, was 749 mm (SD = 35.26) representing an above average rainfall for that time period (+ 174 mm (SD = 41.06), based on 30 year average of 974 mm (SD = 26.13)). In contrast, 2002 rainfall through September (right before soil samples were taken) was 593 mm (SD = 41.05), representing 155 mm less rainfall over a longer period of time (an additional two months, - 194 mm (SD = 36.37) away from the 30-year average) (Kansas State University, Weather Data Library 2003). The decrease in potential soil moisture in 2002 could have decreased the impact of grazing and tractor compaction and reduced bulk density. However, even though lower bulk densities occurred in 2002, the relative differences among management regimes were similar for each year.
In conclusion, our results suggest that variation in bulk density (soil quality) reflects both current (hay vs. grazing) and historical (cultivation) land-use practices. Agricultural practices such as grazing and cultivation can have long-lasting implications and the impacted soils can take decades to recover.
ACKNOWLEDGEMENTS We would like to thank all who were involved in collecting this data and aiding in fieldwork; Geoff Folker, Terri Hildebrand, Emily McGhee, Jarad Mellard, Ted Peterson and Andrea Repinsky. A special ‘Thank You’ goes to all the land owners that allowed access to their fields, without which, this study would not have been possible. We would
11
also like to thank Stylianos Chatzimanolis……… and ……reviewers for helpful comments to improve this paper. Support for this research was provided by ………
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LITERATURE CITED Analytical Software. 1998. Statistix for Windows 2.0. Tallahassee, FL: Analytical Software
Bauer, A., Cole, C. V. and Black, A. L. 1987. Soil property comparisons in virgin grasslands between grazed and nongrazed management systems. Soil Science Society of America Journal. 51, p. 176-182.
Brady, N. C. and Weil, R. R. 2002. The Nature and Properties of Soils. Thirteenth Edition. Prentice Hall, Upper Saddle River, New Jersey.
Diebel, P. L., Cable T. T. and Cook, P. S. 1993. The future of Conservation Reserve Program land in Kansas: The landowner’s view. Report of Progress 690, Agricultural Experiment Station, Kansas State University, Manhattan.
Donkor, N. T., Gedir, J. V., Hudson, R. J., Bork, E. W., Chanasyk, D. S. and Naeth, M. A. 2002. Impacts of grazing systems on soil compaction and pasture production in Alberta. Canadian Journal of Soil Science 82, p. 1-8.
Dormaar, J. F., Smoliak, S. and Williams, W. D. May 1989. Vegetation and soil responses to short-duration grazing on fescue grasslands. Journal of Range Management 42, p. 252-256.
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Ford, M. A. and Grace, J. B. 1998. Effects of vertebrate herbivores on soil processes, plant biomass, litter accumulation and soil elevation changes in a coastal marsh. Journal of Ecology 86, p. 974-982.
Kansas State University. Research and Extension. Weather Data Library. http://www.oznet.ksu.edu/wdl (November 15, 2002)
Naeth, M. A., Pluth, D. J., Chanasyk, D. S., Bailey, A. W. and Fedkenheuer, A. W. May 1990. Soil compacting impacts of grazing in mixed prairie and fescue grassland ecosystems of Alberta. Canadian Journal of Soil Science 70, p. 157-167.
Proffitt, A. P., Bendotti, S., Howell, M. R. and Eastham, J. 1993. The effect of sheep trampling on soil physical properties and pasture growth for a red-brown earth. Australian Journal of Agricultural Research 44, p. 317-331.
Soil Survey of Douglas County, Kansas. July 1977. United States Department of Agriculture. Soil Conservation Service in cooperation with Kansas Agricultural Experiment Station.
Soil Survey of Jefferson County, Kansas. November 1977. United States Department of Agriculture. Soil Conservation Service in cooperation with Kansas Agricultural Experiment Station.
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Soil Survey of Leavenworth and Wyandotte Counties, Kansas. February 1977. United States Department of Agriculture. Soil Conservation Service with cooperation with Kansas Agricultural Experimental Station.
USDA-NRCS, SSURGO: Data Use Information report. January 1995. http://www.ncg.nrcs.usda.gov/pdf/ssurgo_bd.pdf (Appendix A, pp. 3750) (5 June 2000 by K. Apolzer)
Villamil, M. B., Amiotti, V. M. and Peinemann, N. July 2001. Soil degradation related to overgrazing in the semi-arid southern caldenal area of Argentina. Soil Science. 166, p. 441-452.
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Table 1: Soil Characteristics by Field Field Management Year in County No. Regime1 Study
Soil Series2
% Clay Content: A Horizon3
% Clay Content: Subsoil3
L-Use4
Soil Survey Notes5
1
W-CRP
2001, 2002
Douglas
Mc
33.5
44.23
1988
2
C-H
2001
Jefferson
Pc
34
42.43
1982
3
C-H
2001, 2002
Jefferson
Oc, Mc
33.5
44.29
Unkwn
Erosion
4
C-H
2002
Jefferson
Vc
31
29.5
1965
Erosion
5
W-NH
2001, 2002
Jefferson
Mo
33.5
44.37
NP
Rock outcrops
6
W-NG
2001, 2002
Jefferson
Mo
33.5
44.37
NP
7
C-G
2001, 2002
Jefferson
Mc
33.5
45.13
Unkwn
Erosion
8
C-G
2002
Jefferson
Vc
31
29.5
Unkwn
Erosion
9
W-NH
2001
Jefferson
Mo, Sc, Vc
30.99
36.31
NP
Vc = rock outcrops
10
W-NG
2001, 2002
Jefferson
Mo
33.5
44.37
NP
11
W-CRP
2001, 2002
Jefferson
Oc
33.5
43.45
1985
12
C-G
2001, 2002
Jefferson
Mo
33.5
44.37
Unkwn
13
C-H
2002
Jefferson
Sc
28.48
35.07
1992
Erosion
14
C-H
2002
Jefferson
Sc
28.48
35.07
1997
Erosion
15
W-NG
2002
Jefferson
Mc
33.5
45.13
NP
17
W-CRP
2002
Jefferson
Gb, Oc
32.5
40.55
Unkwn
18
C-H
2002
Jefferson
Vc
31
29.5
Unkwn
19
W-NH
2002
Jefferson
Pc
34
42.43
NP
20
C-G
2002
Jefferson
Pc
34
42.43
Unkwn
Observations6 Clayey
Some samples = clay Area has slopes
Clayey
Erosion
Some samples = clay Slopes = erosion? Clayey Erosion?
Several samples = in road/mowed area Erosion: B horizon? Clayey
16
Field No.
Management Regime1
Year in Study
County
Soil Series2
% Clay Content: A Horizon3
% Clay Content: Subsoil3
L-Use4
21
W-NH
2002
Leavenworth
Sh
25.5
31.85
NP
22
C-H
2002
Leavenworth
Pc
34
36.8
Unkwn
Erosion
23
W-CRP
2002
Jefferson
Vc
31
29.5
1985
Erosion
24
W-NG
2002
Jefferson
Pc
34
42.43
NP
25
W-NG
2002
Jefferson
Sc
28.48
35.07
NP
26
W-NH
2002
Jefferson
Sc
28.48
35.07
NP
27
C-H
2002
Leavenworth
Gb
31.5
37.75
Unkwn
28
W-NH
2002
Leavenworth
Sc
31.5
33.6
NP
29
W-CRP
2002
Douglas
Mc
33.5
44.23
Unkwn
30
W-NG
2002
Jefferson
Sc
28.48
35.07
NP
31
C-G
2002
Jefferson
Mc
33.5
45.13
Unkwn
Erosion
32
W-CRP
2002
Jefferson
Mc
33.5
45.13
1988
Erosion
33
W-NG
2002
Douglas
Sh
20.5
22.2
NP
Soil Survey Notes5
Observations6
Clayey; Erosion?
Rock outcrops
Erosion
Clayey
Clayey
1
Management regime: C-H = cool-season hay; C-G = cool-season grazed; W-NH = warm-season native hay; W-NG = warm-season native grazed; W-CRP = warm-season Conservation Reserve Program. 2 Soil Series that underlies each field as determined by soil surveys and digitized soil maps (USDA-NRCS 1995): Gb = Grundy silty clay loam; Mc = Martin silty clay loam; Mo = Martin-Oska silty clay loam; Oc = Oska silty clay loam; Pc = Pawnee clay loam; Sc = Shelby Pawnee complex (Jefferson Co.), Sharpsburg silty clay loam (Leavenworth Co.); Sh = Shelby loam (Leavenworth Co.), Sibleyville loam (Douglas Co.); Sw = Sogn-Vinland complex; Vc = Vinland complex. 3 A Horizon = surface layer content, Subsoil = depth weighted average; both are expressed as a percentage of the material < 2 mm in size (USDA-NRCS 1995). 4 L-Use = Year when current management regime was started; NP = never plowed; Unkwn = unknown land-use management initiation, but field was cultivated at some point in its history (Personal communication with land owners). 5 Soil Survey Notes = description of comments found in field area on the soil survey maps (Soil Survey of Douglas, Jefferson and Leavenworth, Counties, 1977). 6 Observations = any notes about the texture, position and other observations when taking the soil sample that could be relevant to interpreting bulk density data.
17
FIGURE LEGEND Figure 1. Location of fields used in 2002 (black dots). The two fields located within the Kansas Ecological Reserves were not included in this analysis. Field 33 is located in the Western edge of Douglas County and is not shown. Picture courtesy of M. E. Ramspott.
Figure 2. Average soil bulk density (g/cm3) with standard deviation for each management (n = 2) in 2001: W-CRP = conservation reserve program; C-H = coolseason hay; C-G = cool-season grazed; W-NH = warm-season native hay; and W-NG = warm-season native grazed. CRP have been most recently cultivated, C-H and C-G cultivated historically and W-NH and W-NG have not been cultivated. Differing letters signify significant differences at α = 0.05 level using least significant difference tests (tvalue = 1.974).
Figure 3. Average soil bulk density (g/cm3) with standard deviation for each management (n = 6) in 2002: W-CRP = conservation reserve program; C-H = coolseason hay; C-G = cool-season grazed; W-NH = warm-season native hay; and W-NG = warm-season native grazed. CRP have been most recently cultivated, C-H and C-G cultivated historically and W-NH and W-NG have not been cultivated. Differing letters signify significant differences at α = 0.05 level using least significant difference tests (tvalue = 1.969).
Figure 4. Percent clay content and standard deviation for each management (n = 6) in 2002: W-CRP = conservation reserve program; C-H = cool-season hay; C-G = cool-
18
season grazed; W-NH = warm-season native hay; and W-NG = warm-season native grazed. CRP have been most recently cultivated, C-H and C-G cultivated historically and W-NH and W-NG have never been cultivated. Differing letters signify significant differences at α = 0.05 level using least significant difference tests (t-value = 1.969).
19
Figure 1
20
Figure 2
1.0
Soil Bulk Density (g/cm3)
a b 0.9
b c
c
W-NH
W-NG
0.8
0.7
0.6
0.5 W-CRP
C-H
C-G
Management
21
Figure 3
Soil Bulk Density (g/cm3)
1.0
0.9
a 0.8
b
a b c
0.7
0.6
0.5 W-CRP
C-H
C-G
W-NH
W-NG
Management
22
Figure 4
40
a b
b c
% Clay
30
d 20
10
0 W-CRP
C-H
C-G
W-NH
W-NG
Management
23
