Variation of soil and plant characteristics among old world bluestem species

doi:10.4236/as.2011.23046

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Vol.2, No.3, 347-356 (2011)

doi:10.4236/as.2011.23046

Agricultural Scienc es

Variation of soil and plant characteristics among old

world bluestem species

Ted M. Zobeck1*, Vivien G. Allen2, Jenny Jo Cox1,2, Dirk Philipp3

1USDA-ARS, Cropping Systems Research Lab, Lubbock, TX, USA;

2Department of Plant and Soil Science, Texas Tech University, Lubbock, TX, USA;

3 University of Arkansas, Fayetteville, AR, USA; *Corresponding Author: ted.zobeck@ars.usda.gov

Received 21 March 2011; revised 29 May 2011; accepted 29 July 2011.

ABSTRACT

Old world bluestems (Bothriochloa spp.) have

been successfully introduced as grasses for

livestock forage in the semiarid Texas High

Plains. Questions remain, ho wever, on effects o f

these grasses on soil resources. We tested the

hypothesis that differences in grass species

produce differences in soil properties important

to crop growth and useful in selecting the op-

timum species for the Southern High Plains of

Texas. Three old world bluestem (Bothriochloa)

species [C.E. Hubbard ‘Caucasian’, B. cau-

casica (Trin.) ; ‘WW Spar’, B. ischaemum (L.)

Keng. var ischaemum (Hack.); and S.T. Blake

‘WW-B Dahl’, B. bladhii (Retz)] were grown in a

randomized complete block design, with three

replications, for nine years on a clay loam soil

near Lubbock, Texas. Soil samples were col-

lected in the ni nth year to determine soil t exture,

wet aggregate stability, bulk density (BD), soil

organic carbon (SOC), particulate organic car-

bon (POC), and soil strength as measured by

the cone pentrometer. The grass species dif-

fered in their above-ground biomass and below-

ground root production. In the ninth year of

production, Bothriochloa caucasica and B.

bladhii produced about twice the above-ground

biomass with about 25% fewer roots than B.

ischaemum. Soils where B. caucasica was

grown had the highest BD (1.36 mg m–3) and B.

ischaemum had the lowest (1.31 mg m–3). The

soil in which B. ischaemum was growing had a

lower BD, greatest root biomass, organic matter

content, and aggregate stability suggesting

superior soil quality for agricultural production.

The species B. bladhii, however, often exhibited

soil properties that were similar to both other

species tested. Since Bothriochloa bladhii had

superior or similar soil properties for plant

growth among the species tested and has been

shown to be higher in forage quality, animal

performance, and carrying capacity than the

other species, it appears to be the best choice

among these three species to optimize both

animal performance and desirable soil proper-

ties.

Keywords: Old World Bluestem; Soil Quality;

Grasses; Soil Organic Ca r bon

1. INTRODUCTION

Old world bluestems (Bothriochloa spp.) have been

among the more successful grasses for livestock feed in

the semiarid Texas High Plains [1]. Research has sug-

gested that the nutritive value of old world bluestems is

influenced by species, environmental conditions, man-

agement, and physiographic location [1-3]. The Southern

High Plains is a semiarid region, often subject to ex-

tended periods with little rainfall, producing drought

stress in plants. Studies of B. caucasica and B. ischae-

mum comparing growth characteristics during periods of

abundant and restricted water availability suggested that

B. caucasica has a lower drought performance potential

relative to B. ischaemum [4-6]. Bothriochloa caucasica

and B. ischaemum have had a relatively long history on

the Southern High Plains, compared with the relatively

recently released B. bladii [7]. Recent studies comparing

the three species have shown that, in general, B. bladii

provided greater mineral concentrations to grazing ani-

mals than either B. caucasica or B. ischaemum [8]. In

addition, B. bladii has been recently used in an inte-

grated crop-livestock system and shown to provide at

least or above average levels of animal performance and

profitability [9]. Questions, however, remain on the ef-

fect of the grasses on the soil resources.

Grasses or other forages have distinct effects on soil

T. M. Zobeck et al. / Agricultural Sciences 2 (2011) 347-356

348

physical and chemical properties. Grasses and forages

provide organic matter to the soil as the roots and sur-

face plant matter die and decompose. Organic matter in

soil improves soil structure as well as the ability of soil

to hold nutrients and water [10]. Soils under continuous

plant cover tend to have a greater amount of associated

C than tilled or bare soils [11]. Plots with grasses had

significantly greater organic matter and favorable physi-

cal conditions than weedy plots [12]. Tillage of crops

often greatly reduces soil organic carbon (SOC),

whereas no-till systems help maintain and even increase

SOC [13-15].

The particulate organic matter (POM) fraction of soils

is comprised of large particles of organic matter (250 -

2000 µm) that exists as free organic matter or that is

encrusted with soil particles, which in turn offers protec-

tion from decomposition [16]. The carbon content of

POM is often referred to as particulate organic carbon

(POC). Particulate organic carbon seems to be more sen-

sitive to changes in management practices than total

SOC [17,18] and has been found to be higher in native

sods than cropped soils [17,19]. Increasing POC has

been associated with improved soil quality for plant

growth [20] and related to other soil and plant properties

such as nutrient mineralization, vegetation type, and

tillage practices [18].

Grass sods and forage crops have been shown to be

effective in improving soil structural stability [21].

Grasses have fibrous root systems provide both chemical

and physical benefits to aggregate stability through pro-

duction of chemical binding agents such as carbonates as

well as physically binding roots to the soil particles [16,

22,23]. Grass root growth increases aggregation through

the mesh-like network of root hairs and fungal hyphae

[24]. These root systems are beneficial even after the

plant dies, providing organic matter to the soil, and

leaving spaces for water infiltration. In addition, organic

matter on the surface of a soil may prevent crusting, in-

creasing water infiltration rates. Water infiltration was

found to be higher on a grass-covered soil than on a

tilled soil [25]. However, the effect of root biomass on

aggregate stability is not always clear. Soils with grass

swards have been shown to provide greater aggregate

stability than when in crops such as corn (Zea mays L.)

and soybean [Glycine max (L.) Merr.] [26]. Nevertheless,

root biomass has also been shown to have little or no

effect on aggregate stability [27].

Root proliferation affects bulk density (BD) and soil

strength, as well. In densely compacted soils, roots grow

into cracks in the hard layers [28], and BD is decreased

over time by the expansion of roots. For example, the

BD of soils with perennial grass cover for 15 years de-

creased more than soils in annual cropping over the

same time period [29]. Bulk density also differs with

depth [30]. The bulk density in the top 5 cm of a peren-

nial grass-covered soil was significantly less than at

depths below 5 cm [31]. The resistance of soil to pene-

tration by a cone-tipped rod, known as the penetration

resistance, is a measure of soil strength and is often re-

lated to bulk density. The penetration resistance was

greater for grass-covered soil than cultivated [32] or bare,

uncultivated soil [33].

Recent studies on the relative benefits of three old

world bluestem species have explored several qualities

of these grasses, including influence of irrigation on

production, water use efficiency, forage nutritive value,

and morphology [1,8,34]. We investigated the effects of

these different grass species on selected soil properties in

the Southern High Plains of Texas. Characteristics ex-

plored were aboveground plant biomass, root biomass,

soil surface texture and moisture content, total SOC and

POC content, soil strength, bulk density, water infiltra-

tion rate, and large (<8 mm diameter) and small (1 to 2

mm diameter) wet aggregate stability. We tested the hy-

pothesis that differences in old world bluestem grass

species produce differences in these surface soil proper-

ties important to crop growth.

2. MATERIALS AND METHODS

2.1. Study Site

This work was part of a larger irrigation study testing

the effects of four water irrigation treatments (dryland,

low, medium, and high) on three old world bluestem

(Bothriochloa) species (C.E. Hubbard ‘Caucasian’, B.

caucasica [Trin.]; ‘WW Spar’, B. ischaemum [L.] Keng.

var ishaemum [Hack.]; and S.T. Blake ‘WW-B Dahl’, B.

bladhii [Retz]) (Philipp et al., 2005). The site was lo-

cated in northeast Lubbock County, Texas (101˚47′ W;

33˚45′ N; 993 m elevation). The area has a semiarid cli-

mate with a mean annual precipitation of 470 mm and an

average air temperature of 15.5˚C.

Each species was replicated three times in a random-

ized complete block design. The grasses were estab-

lished in 1996 and were used in a lamb (Ovis aris) graz-

ing experiment from 1998 to 2000 [35] prior to the irri-

gation study. Prior to 1996 the site was disked and

planted to various forage species. Grazing ended in Sep-

tember 2000. The irrigation study was conducted for

three years, 2001 through 2003 [7]. After termination of

the irrigation study in 2003, no further irrigation or fer-

tilizer was applied. Plots were mowed once in mid-

summer each year and above-ground biomass removed

as hay in 2004. All plots were mowed in late winter to

remove residual herbage before spring growth. Samples

used in the current study were collected in the summer

T. M. Zobeck et al. / Agricultural Sciences 2 (2011) 347-356

349349

of 2005, two years after the end of the irrigation study.

Due to experimental limitations, only the medium water

treatment plots were used in our study. The amount of

water applied in the medium water treatment during the

irrigation study was 66% replacement of potential

evapotranspiration (PET) [36] minus precipitation. Irri-

gation was applied through a surface drip irrigation sys-

tem.

Each plot was 10 m by 15 m. The soil series was

Pullman sandy clay loam (fine, mixed, superactive,

thermic Torrertic Paleustoll) on a nearly level (0% - 1%

slope) land surface. This soil is a very deep, well drained,

slowly permeable soil formed in loamy sediments and

occupies approximately 1.3 million ha (3.3 million ac) in

the Southern High Plains of Texas and Oklahoma [37].

During the irrigation study (2001 to 2003), plots were

fertilized equally to meet all soil test recommendations

such that N and other nutrients were not limited. Details

of precipitation, irrigation and fertilizer applied and ap-

plication methods are described in Philipp et al. [1]. No

water or nutrients were applied in 2004 and 2005. Pre-

cipitation during these two years was 807 mm for 2004

and 277 mm through mid-August 2005.

2.2. Soil and Plant Sampling and Analyses

Soil samples were collected from depths of 0 - 5 cm

and 5 - 15 cm to determine soil texture, wet aggregate

stability, BD, SOC, POC in June 2005. Soil samples

were screened first through a 2-mm sieve and larger

pieces of plant material were removed.

2.2.1. Soil Physical Properties

Soil texture was determined on sieved samples using a

Beckman-Coulter LS230 [38] after dispersing the soil in

5 g·L–1 sodium hexametaphosphate by shaking overnight

in a reciprocating shaker. Bulk density was determined

using the soil core method [39]. Samples for BD (two

samples from each replication) were collected using a

2-cm diameter push probe with care taken to ensure no

compaction occurred during sampling. Soil BD was used

to calculate SOC mass on an area basis. In-situ soil

strength was measured to a depth of 0.3 m with an

automated Bush penetrometer [40] (Soil Penetrometer

SP1000, Findlay Irvine Ltd.) and observed at 1-cm in-

tervals. The cone-shaped penetrometer tip used for these

penetration resistance (PR) measurements had a 30˚ an-

gle and 12.6 mm diameter base. Fifteen PR measure-

ments were randomly inserted in each plot. Statistical

analyses of the soil strength tests were performed on the

sum of the PR observations to a depth of 15 cm. In addi-

tion, the PR was averaged for the 0 - 5 cm and 5 - 15 cm

depths in order to compare with other properties also

observed at these depths. Gravimetric soil water content

was measured at the time of PR readings for the depths 0

- 5, 5 - 10, 10 - 15, and 15 - 30 cm to determine interac-

tions of water content and PR readings. Water infiltration

was determined using a double-ring infiltrometer at two

locations in each replication [41]. Wet aggregate stability

was measured on duplicate subsamples of aggregates

smaller than 8.0 mm in diameter to determine water-

stable aggregate size distribution by wet sieving accord-

ing the procedure of Kemper and Rosenau [42] and ex-

pressed as the mean weight diameter (MWD). Duplicate

subsamples of aggregates 1.0 to 2.0 mm in diameter

were sieved from a portion of the bulk soil. Wet aggre-

gate stability was also measured on this 1.0 to 2.0 mm in

diameter aggregates according to Kemper [43] and ex-

pressed as the percentage of aggregates (WAS2).

2.2.2. Soil Chemical Properties

The chemical properties observed in this study con-

sisted of SOC and POC as measured as a concentration

(mass per unit mass) by soil depth and mass per unit area.

The measurements made on a mass per unit area basis

were determined by summing the total amount of each

constituent to a depth of 15 cm. Sieved soil samples

were used to determine POC according to the method of

Gregorich and Ellert [44]. Twenty-five-gram subsamples

of sieved samples were first dispersed in 100 mL of 5

g·L–1 sodium hexametaphosphate by shaking overnight

in a reciprocating shaker. The dispersed soil that was

retained on a 53 µm sieve was dried overnight at 60˚C

before analysis for organic C. These samples, along with

twenty-five-gram subsamples of sieved samples used to

determine SOC, were ground overnight in a roller-mill

(consisting of round bottles with three metal rods inside

to pulverize the samples) to a size passing a 150 µm

screen and analyzed for C content with an Elementar

Vario Macro C-N analyzer (Elementar Americas, Inc.,

Mt. Laurel, NJ).

2.2.3. Plant Biomass and Roots

Plant biomass was determined by clipping a 1-m2 plot

in each replication of each species in August 2005. Plant

biomass samples were dried at 60˚C to a constant weight.

Root mass was determined from six core samples col-

lected in each grass species and replication with a 5-cm

diameter soil probe to a depth of 15 cm.

2.2.4. Statistical Analyses

Statistical analyses were performed using procedures

of SAS version 9.1 [45]. Analyses of variance (ANOVA)

were performed with Proc Mixed using a randomized

complete block with three replications as the model.

Grass species was considered the main effect and the

blocks were considered as a random effect. When depth

T. M. Zobeck et al. / Agricultural Sciences 2 (2011) 347-356

350

was evaluated, the statistical design was a split-plot with

depth as a subplot. Statistical differences were evaluated

at the P ≤ 0.05 level, with the exception of SOC concen-

tration which was evaluated at the P ≤ 0.10 level, as

discussed later.

Openly accessible at

3. RESULTS

3.1. Soil Physical Properties

Sand and clay content of the surface revealed no sig-

nificant differences among grass species; however, there

were differences among depths (Table 1), but no interac-

tions among grass species and depths. The surface 0 - 5

cm contained more sand and less clay than the 5 - 15 cm

layer. Among all grass species, the surface had 21.6%

clay and 54.4% sand and the second layer had 26.5%

clay and 48.3% sand. Despite significant differences in

the amount of clay and sand particles between depths,

the texture for both depths was a sandy clay loam.

Although the surface 0 to 30 cm total soil water con-

tent, taken immediately prior to PR measurements, was

not significantly different among grass species (Table 1),

there was a slight difference in water content with depth.

There were no interactions among grass species and

depths. The surface 0 - 5 cm had significantly higher

water content of about 30.5% while the other depthswere

not significantly different and averaged about 23.0% soil

water (Figure 1).

Soil strength measurements, as determined by PR,

identify zones of resistance to an applied force. Figure 1

shows how PR differed among grass species and among

depths within grass species, but the effect of species did

not depend on depth (Ta ble 1). The lowest strength was

Figure 1. Soil water content and penetration resistance by

grass species and depth.

Table 1. Differences among old world bluestem grass species for selected soil and plant properties.

B. caucasica B. bladhii B. ischaemum

Source of Variation Standard Standard Standard

Property Depth (D) Treatment (T)DxTMean Error Mean Error Mean Error

Clay Content (%) <0.0001 0.3 0.6 24 0.7 24.9 0.7 23.3 0.7

Sand Content (%) 0.0004 0.3 0.6 51.3 1.1 50.2 1.1 52.6 1.1

Above Ground Biomass (kg) N/A 0.0007 N/A4402.0a211.6 4148.8a211.6 2265.3b 211.6

Root Biomass (kf·m–3) N/A 0.003 N/A10.2b 1.06 11.6b 1.06 15.5a 1.06

Soil Water Content (%) <0.0001 0.72 0.8 24.8 0.84 24.8 0.84 25.6 0.84

Penetration Resistance (Mpa) N/A 0.04 N/A10.6b 0.5 11.3ab 0.5 12.7a 0.5

Bulk Density (mg·m–3) <0.0001 0.03 0.8 1.36a 0.01 1.33ab 0.01 1.31b 0.01

MWD (mm) <0.0001 <0.0001 0.045.16 0.11 5.04 0.1 5.67 0.11

WAS2 (%) 0.02 0.02 0.1 49.5b 2.88 51.4ab 2.9 55.8a 2.87

SOC Concentration (g·kg–1) <0.0001 0.03 0.0514.3 0.6 15.7 0.6 16.5 0.6

SOC Mass (MG·ha-1) N/A 0.09 N/A26.7b 0.55 27.8ab 0.55 28.8a 0.55

POC Concentration (g·kg–1) <0.0001 0.005 0.025.89 0.37 8.06 0.37 7.95 0.37

POC Mass (MG·ha–1) N/A 0.04 N/A9.85b 0.63 12.54a 0.63 12.5a 0.63

Notes: MWD-mean weight diameter; WAS2-wet aggregate stability for aggregates 1.0 - 2.0 mm diameter; SOC-soil organic carbon; POC-particulate organic

matter carbon; C and POC mass (MG/ha) represent the total for the 0 - 15 cm depth; Mean clay and sand content, SOC concentration, MWD, WAS2, bulk density,

and root mass were measured through a depth of 0 - 15 cm (0 - 5 and 5 - 15 cm depth increments). Soil water content was measured through a depth of 0 - 30 cm

(0 - 5, 5 - 10, and 15 - 30 cm depth increments) and penetration resistance was totaled throughout the 0 - 15 cm depth. Significance levels for testing differences

among grass species (Treatment) and soil depth (Depth) are indicated by lower case letters. Means within the row with the same letter for SOC mass (MG/ha) are

not different among grass species (P ≤ 0.10). Means within the rows with the same letter for other comparisons among grass species are not different (P ≤ 0.05).

rties with no letter are not significantly different among grass species (P > 0.10) or have interactions of depth and grass species (DxT). Prope

T. M. Zobeck et al. / Agricultural Sciences 2 (2011) 347-356

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in the surface 0 - 5 cm and increased with soil depth as

soil water content decreased (Figure 1). We observed

significant differences soil strength among species when

PR was summed over the 30 cm depth for each species

and replication (Table 1 ). Bothriochloa ischaemum pro-

duced the greatest total resistance to the cone penetro-

meter and B. caucasica had the smallest total resistance.

The PR observed in B. bladhii was not significantly dif-

ferent from either of the other grass species (Table 1).

The mean PR was also determined for the 0 - 5 cm and 5

- 15 cm depths and PR by species. The surface 0-5 cm

soil strength in the grasses was, on average, about

one-fourth the strength found in the 5 - 15 cm layer. Av-

eraged over all species, PR was 6.6 Mpa·cm–1 (SE = 0.55)

in the surface 0 - 5 cm and 26.0 Mpa·cm–1 (SE = 0.55) in

the 5 - 15 cm depth.

Another measure of soil compaction, BD, differed.

among grass species and among depths within grass spe-

cies, but the effect of species did not depend on depth

(Table 1). Averaged over all species, BD was 1.25

Mg·m–3 (SE = 0.005) in the surface 0 - 5 cm and 1.41

Mg·m–3 (SE = 0.005) in the 5 – 15 cm depth. Bothri-

ochloa caucasica had the highest BD (1.36 Mg·m–3) and

B. ischaemum had the lowest (1.31 Mg·m–3). The bulk

density observed in B. bladhii was not significantly dif-

ferent from either of the other grass species (Table 1).

The mean infiltration rate was 0.97 cm·hr–1 (SE =

0.08). Water infiltration readings are very time consum-

ing and variable. Although we made two water infiltra-

tion readings on each replication, the double ring infil-

tration method used in this study still produced signifi-

cant variation. Given the variability of this method, sta-

tistical tests lacked power to be able to detect biologi-

cally significant differences.

Wet aggregate stability is a measure of the strength of

bonding of individual soil particles into larger aggre-

gates to withstand the force of disruption caused by agi-

tation in water. Two measures of wet aggregate stability

were tested; a measure of the stability of large aggre-

gates less than 8 mm diameter, MWD, and a measure of

the stability of smaller 1 to 2 mm diameter aggregates,

WAS2. These tests produced the same general result,

although WAS2 had no interaction between grass species

and depth (Table 1). In general, the plots with B. cau-

casica had the lowest wet aggregate stability and plots

with B. ischaemum had the greatest wet aggregate stabil-

ity (Tables 1 and 2). Plots with B. bladhii were not sig-

nificantly different from plots with either of the other

grass species. Often, within grass species, the surface 0 -

5 cm had a greater wet stability than the 5 - 15 cm depth.

Wet aggregate stability measured by WAS8 had the same

MWD with depth in plots with B. caucasica and the

same percent stability with depth in plots with B. is

Table 2. Differences by depth among old world bluestem grass

species for selected soil properties.

Grass SpeciesDepth (cm)SOC

(g/kg)

POC

(g/kg)

POC/SOC

Ratio

MWD

(mm)

B. caucasica0 - 5 18.0b 9.1b 0.50b 5.43bc

B. baldhii0 - 5 21.1ab 13.2a 0.63a 5.65ab

B. ischaemum0 - 5 22.0a 12.7a 0.57ab6.07a

B. caucasica5 - 15 10.6c 2.7c 0.25c 4.89cd

B. baldhii5 - 15 10.4c 2.9c 0.28c 4.44d

B. ischaemum5 - 15 10.9c 3.2c 0.29c 5.26bc

Notes: SOC-soil organic carbon, POC-particulate organic matter carbon,

MWD-mean weight diameter of wet sieved aggregates. Values with the

same letter within columns are not significantly different at the P = 0.05

level.

chaemum. Wet aggregate stability measured as MWD or

percent stability was significantly different by depth (P <

0.05) in plots with B. bladhi i (Table 2).

3.2. Soil Chemical Properties

Soil organic carbon and POC concentration (g·kg–1)

were each different between depths (Table 1). The

greatest concentrations of SOC and POC were found in

the surface 5 cm, with about twice the amount of SOC in

the surface as in the next 10 cm (Table 2). The mean

SOC concentration for the surface 5 cm was 20.3 g·kg–1

(SE = 0.46) and the 5 - 15 cm layer had 10.6 g kg-1·C

(SE = 0.46). The mean POC concentration for the sur-

face 5 cm was 11.7 g·kg–1 (SE = 0.20) and the 5 - 15 cm

layer had 2.9 g·kg–1 C (SE = 0.20). Soil with B. ischae-

mum had greater quantities of SOC and POC in the sur-

face 0 - 5 cm than soils with B. caucasica, with no dif-

ferences among species in the 5 - 15 cm depth (Table 2).

Soils with B. bladhii had the same SOC and POC con-

tent as both other species (Tables 1 and 2).

When SOC and POC contents were computed on a

mass per unit surface area basis, SOC content was dif-

ferent among grass species at the P = 0.1 level while

POC content was different among grass species at the P

= 0.05 level (Table 1). Bothriochloa ischaemum had the

greatest SOC and POC mass (MG·ha–1) and B. caucasica

had the least SOC mass. The SOC mass observed in soil

with B. bladhii was not different from either of the other

grass species. The POC mass observed in soil with B.

bladhii was not different from only B. ischaemum.

3.3. Plant Biomass and Roots

The above-ground plant biomass (herbage) of the

grasses differed among species (Table 1). The least

amount of herbage was produced by B. ischaemum,

which had about one half the amount of above-ground

biomass produced by B. caucasica and B. bladhii. Inter-

estingly, this is the inverse of the amount of root biomass

observed for each grass species. B. ischaemum had the

T. M. Zobeck et al. / Agricultural Sciences 2 (2011) 347-356

352

greatest total root biomass in the 0 to 15 cm depth,

whereas B. caucasica and B. bladhii had the least but

similar amounts of root biomass (Table 1)

4. DISCUSSION

Soil texture can have a significant impact on soil

properties that are important for crop growth. For exam-

ple, a sandy clay loam soil will hold more plant available

soil water than a sandy soil. In this study, although there

were no differences in the surface sandy clay loam soil

texture among grass species plots, we did observe statis-

tical differences in the amount of primary soil particles

such as clay content by soil depth (Tab l e 1 ). However,

the differences in clay and sand content among grass

species were not practically different, with an absolute

mean difference of only 1.2% and 2.4% for clay and

sand, respectively. The similarity in texture of all plots

suggests soil texture was not a factor in differences

found in the other variables observed in this study.

Although there were no differences in surface soil tex-

ture among grass species, we did observe differences in

plant growth. Bothriochloa caucasica and B. bladhii

produced about twice the above-ground biomass with

about 25% fewer roots than B. ischaemum. These results

are consistent with previous research conducted within

these plots showing lower aboveground biomass produc-

tion of B. ischaemum than either B. bladhii or B. cau-

casica [1,34]. Although these grasses had been grazed or

cut in the past, management was similar among these

grasses and did not explain differences in productivity.

The differences in plant properties seem to have contrib-

uted to the differences observed in soil properties.

The differences in root biomass among grass species

may have produced differences in soil BD. The grass

species plots with the higher soil bulk density (B. cau-

casica) also had the lowest root biomass. The BD values

of plots with B. bladhii were not different from BD of

the plots of the other grasses (Table 1). Lower BD in

plots with more roots was expected because the mass per

unit volume of roots is much lower than the mass per

unit volume of the mineral soil material. In addition,

when roots decompose they often leave pores that fur-

ther contribute to a reduction in BD.

Plant roots also add carbon to the soil matrix as dem-

onstrated in this study. The differences among plots in

root mass paralleled the differences found in SOC.

Bothriochloa ischaemum had the greatest root mass and

SOC and POC and B. caucasica had the least. This trend

is the inverse of the trend found in BD, with B. ischae-

mum having the lowest BD and B. caucasica the greatest.

This is consistent with many past studies where BD and

SOC were inversely related [30,46-48]. This apparently

anomalous result is explained by considering that roots

contribute organic material to the soil matrix, causing

SOC to increase, and also function to decrease the den-

sity as described above.

Soil resistance is strongly influenced by both soil wa-

ter and bulk density [49,50]. Figure 1 shows the pene-

tration resistance and soil water content measurements

by grass species for the 0 to 30 cm depth. The ANOVA

of penetration resistance in Table 1 was performed for

only the 0 to 15 cm depth to correspond to the depths

observed for bulk density. There were no differences in

mean soil water content among grass species, although

there were differences in soil water content by depth

(Table 1). In addition, since there were no interactions of

grass species with depth of soil water content, it was not

considered a factor in affecting the BD and soil strength

results.

The surface 0 - 5 cm had the greatest water content

and lowest BD and soil strength. Soil BD and PR are

quite variable but are often positively correlated [51];

however, B. ischaemum produced a higher PR than B.

caucasica, which was the inverse of the mean soil BD

observations. Rachman et al. [52] found a negative cor-

relation of soil strength with BD. Although they did not

use an in situ cone penetrometer test to measure soil

strength as in our study, their soil strength test used a

falling cone on intact soil cores and the results can be

considered analogous. They interpreted their results to

indicate that differences in soil strengths were related to

differences in aggregate stability. They and others have

concluded that soil management and cropping systems

that accumulate organic matter which, in turn, reduce the

soil’s vulnerability to slaking and dispersion on wetting,

increase the soil’s resistance to penetration by a dropped

cone [52,53].

This relation of soil strength and aggregate stability is

supported by our results of the wet aggregate stability

tests described in Tables 1 and 2. The tests showed that,

similar to the trends shown for PR, the aggregate stabil-

ity of B. ischaemum (WAS2) was significantly greater

than the stability of B. caucasica. The wet aggregate

stability (WAS2) in B. bladhii was not different from the

other grasses (Table 1). Additionally, the tests showed

that B. ischaemum broke down into larger aggregates

upon wetting (MWD in Table 2), suggesting greater

stability. Differences in wet aggregate stability with

depth shown in Ta b l e 2 are complicated but, in general

they show greater stability for the less dense and resis-

tant surface layer.

Many studies have shown a positive correlation of

SOC and aggregate stability [22,52,54,55]. In this study,

the trends in SOC and POC mass followed those found

for wet aggregate stability; B. ischaemum had a greater

SOC and POC mass than B. caucasica, and the SOC

T. M. Zobeck et al. / Agricultural Sciences 2 (2011) 347-356

353353

mass for B. bladhii was not different from either of the

other grasses (Table 1). As mentioned earlier, the greater

values for SOC and POC mass are attributed to the

greater root biomass found in the soil of the B. ischae-

mum treatment (Table 1). The greater root biomass pro-

duced more carbon and root structures and chemical

binding agents to improve the wet aggregate stability

found in the soil of the B. ischaemum treatment. Figure

2 illustrates the similar trends of wet aggregate stability,

SOC content, and root biomass.

Studies suggest that POC is more sensitive to changes

in management than changes in total SOC [17,18,56].

This is attributed to the faster turnover rate of POC

compared with SOC [17,30]. An Australian Vertisol lost

organic carbon upon conversion from pasture to crop-

land with 70% of the loss in SOC coming from POC

[57]. The POC/SOC ratio has been used to indicate dif-

ferences among locations of grass communities in the

Argentinean Pampa [30]. Noellemeyer et al. [30] found

a ratio of 0.54 for communities in the shade and 0.39 for

communities in the sun. All of our plots were in full sun.

We found differences in the POC/SOC ratio in the sur-

face 0 - 5 cm among bluestem species that paralleled the

findings of the individual SOC and POC results (Table

2). Bothriochloa caucasic had a POC/SOC ratio of 0.50

while B. bladhii had a significantly greater ratio of 0.63.

The POC/SOC ratio of plots with B. ischaemum was not

different from the ratios of the plots of the other grasses

(Ta ble 2 ). The SOC and POC values were much lower

for the 5 - 15 cm layer causing the POC/SOC ratios to

be about on half that of the surface 5 cm. The SOC in

the second layer was about on half that of the

Figure 2. Wet aggregate stability (1 - 2 mm diameter, %), soil

organic carbon (SOC) mass (MG·ha–1) and root biomass (kg·m–3)

by grass species. Means with the same letter, by data type, are

not significantly different at the P = 0.05 lever for wet aggre-

gate stability and root biomass and at the P = 0.10 level for

SOC mass.

surface while the POC of the second layer was from

about one quarter to one third that of the surface layer,

supporting a faster turnover rate for the POC in this

study.

Since SOC is a key attribute of soils that impacts ero-

sion control, soil structure, water infiltration, conserva-

tion of nutrients and other soil properties, the stratifica-

tion of SOC has been proposed as a simple diagnostic

tool to identify land management strategies that improve

soil properties [58,59]. In a study comparing plots under

long-term conventional and no-tillage, Franzluebbers [59]

found the stratification of SOC at shallow depths (the

ratio of the SOC at 0 - 3 cm depth divided by that of 6 -

12 cm depth) increased as water infiltration rate and wet

stability (as measured by the MWD) increased and was

inversely related to BD and PR. Thus, the SOC stratifi-

cation ratio (SOCSR) tended to increase with soil prop-

erties indicating increasing soil quality for plant growth.

In our study, the SOCSR was determined as the ratio of

the SOC at 0 - 5 cm depth divided by that of 5 - 15 cm

depth. Bothriochloa caucasic had a mean SOCSR of

1.70 (using data provided in Table 2) while B. bladhii

and B. ischaemum were the same, with SOCSRs of 2.03

and 2.02, respectively. These ratios are similar to

SOCSRs observed in a study of a silty clay loam soil

under no tillage management in south central Texas [58].

Although we did not discern differences in water infil-

tration among grass species in this study, the trends of

SOCSR with the MWD of the 0 - 5 cm depth, bulk den-

sity, and penetration resistance were similar to those

reported by Franzluebbers [59]. Bothriochloa caucasic

had the lowest SOCSR, suggesting a lower soil quality

than B. bladhii and B. ischaemum , which were the same.

5. SUMMARY AND CONCLUSIONS

Several Old World Bluestem grass species are com-

monly used in the Southern High Plains in forage man-

agement systems and as conservation reserve grasslands

species. Although studies have identified differences in

growth and production among these species, little is

known about how they affect soil properties. The results

of this study support the hypothesis that differences in

grass species produce differences in soil properties. The

grass species differed in their above-ground biomass and

below-ground root production. Bothriochloa caucasica

and B. bladhii produced about twice the above-ground

biomass with about 25% less root mass than B. ischae-

mum. This may help to explain observed differences in

plant persistence under dryland conditions. The greater

root biomass of the B. ischaemum also produced a lower

soil bulk density than the other species. Although we

expected that the density would affect soil strength, this

was not the case. Although the soil in which B. ischae-

T. M. Zobeck et al. / Agricultural Sciences 2 (2011) 347-356

354

mum was growing had a lower bulk density, it had the

greatest soil strength. The soil strength was greatest in B.

ischaemum, which also had the greatest root biomass,

organic matter content, and aggregate stability. These

three characteristics are related. The root biomass in-

creases the organic matter of the soil. The organic matter

provides energy for microbial populations and other soil

fauna which produce materials that bind soil particles

into aggregates. In addition, roots may provide addi-

tional particle aggregation through effects of root exu-

dates and very small root structures.

The species B. bladhii usually exhibited desirable soil

properties that were similar to both other species tested.

Since Bothriochloa bladhii also has been shown to be

higher in forage quality, animal performance, and carry-

ing capacity than B. ischa emum, it appears to be the best

choice among these three species to optimize both ani-

mal performance and desirable soil properties.

6. ACKNOWLEDGMENTS

The authors are indebted to Dean Holder (USDA ARS) and Deanna

Halfmann Faubian (formerly USDA ARS, currently USDA NRCS) for

technical assistance with field work and laboratory analyses.

7. NOTES

Mention of this or other proprietary products is for the convenience of

the readers only, and does not constitute endorsement or preferential

treatment of these products by USDA-ARS. USDA is an equal oppor-

tunity provider and employer.

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