Land use effects on soil carbon fraction
Biol Fertil Soils (2003) 38:386–392
DOI 10.1007/s00374-003-0652-z
ORIGINAL PAPER
Richard T. Conant · Johan Six · Keith Paustian
Land use effects on soil carbon fractions
in the southeastern United States.
I. Management-intensive versus extensive grazing
Received: 21 March 2003 / Accepted: 3 June 2003 / Published online: 29 July 2003
Springer-Verlag 2003
Abstract Changes in grassland management intended to
increase productivity can lead to sequestration of substantial amounts of atmospheric C in soils. Managementintensive grazing (MiG) can increase forage production in
mesic pastures, but potential impacts on soil C have not
been evaluated. We sampled four pastures (to 50 cm
depth) in Virginia, USA, under MiG and neighboring
pastures that were extensively grazed or hayed to evaluate
impacts of grazing management on total soil organic C
and N pools, and soil C fractions. Total organic soil C
averaged 8.4 Mg C ha–1 (22%) greater under MiG;
differences were significant at three of the four sites
examined while total soil N was greater for two sites.
Surface (0–10 cm) particulate organic matter (POM) C
increased at two sites; POM C for the entire depth
increment (0–50 cm) did not differ significantly between
grazing treatments at any of the sites. Mineral-associated
C was related to silt plus clay content and tended to be
greater under MiG. Neither soil C:N ratios, POM C, or
POM C:total C ratios were accurate indicators of
differences in total soil C between grazing treatments,
though differences in total soil C between treatments
attributable to changes in POM C (43%) were larger than
expected based on POM C as a percentage of total C
(24.5%). Soil C sequestration rates, estimated by calculating total organic soil C differences between treatments
(assuming they arose from changing grazing management
and can be achieved elsewhere) and dividing by duration
of treatment, averaged 0.41 Mg C ha–1 year–1 across the
four sites.
R. T. Conant ()) · J. Six · K. Paustian
Natural Resource Ecology Laboratory,
Colorado State University,
Fort Collins, CO 80523-1499, USA
e-mail: conant@nrel.colostate.edu
J. Six
Department of Agronomy and Range Science,
University of California,
Davis, CA 95616, USA
Keywords Carbon sequestration · Pasture · Grazing
management
Introduction
Land use and land changes are widely recognized as key
drivers of global C dynamics (Houghton et al. 1999;
Schimel 1995), but the role of grassland management has
only recently received attention as a substantial potential
C sink (Conant et al. 2001; Follett et al. 2001; Sampson et
al. 2000). A recent literature review concluded that a
variety of management practices including irrigation,
fertilization, sowing improved grass and legume species,
and improved grazing management all promote C
sequestration in grasslands (Conant et al. 2001). High
rates of C sequestration (0.1–3.0 Mg C ha–1 year–1)
coupled with large areas responsive to improved management suggest that 70 Tg C could be sequestered
annually in Annex I (developed) countries as a result of
changes in grassland management (Sampson et al. 2000).
This is more than twice what is likely to be sequestered as
a result of land use change (30 Tg C year–1; Watson et al.
2000).
Management-intensive grazing (i.e., MiG, or short
rotation grazing) is widely believed to increase grassland
forage production by ensuring more uniform forage
removal and allowing a recovery period (Gammon
1978). A recent review found that MiG in dry rangelands
does not influence forage production, but in more humid
regions, forage production increased by 20–30% (Holecheck et al. 1999). There is a long history of using MiG to
increase production (Haynes and Neal 1943; Hudson
1929) and in some areas, such as New Zealand and
Australia, short rotation grazing is widely used (Gifford et
al. 1992). Though pastureland is an important land
resource in the southeastern United States, covering more
than 12.6 Mha (12% of the total land area) supporting
6.5 million beef cattle and more than 990,000 dairy cows
(Census of Agriculture 1992) in 1992, the use of MiG in
the southeastern United States is not widespread (Conant
387
et al. 2002) and potential impacts of MiG adoption on soil
C have not been adequately researched.
Short-term changes in soil C in response to changes in
grazing management are small relative to the amount of
total soil C and may be difficult to detect by measuring
bulk changes in soil C. Therefore, physical, chemical, and
biological soil C pools have been isolated in an attempt to
identify those soil C fractions likely to respond more
rapidly than total C to changes in management (e.g.,
Bardgett and McAlister 1999; Haynes 2000; Sparling
1992). It has been shown that particulate organic matter
(POM) C and N provide an early indication of changes in
C dynamics and total soil C under different agricultural
management practices (Cambardella and Elliott 1992; Six
et al. 1999; Wander and Bollero 1999). Likewise,
Franzluebbers et al. (2000) found 65% greater POM N
content but only a 34% greater total N in a grazed versus
hayed bermuda grass pasture and Burke et al. (1999)
reported significant differences in coarse POM C and N
(>500 mm) but no significant differences in total C and N
between grazed and ungrazed short grass steppe. Conversely, the amount of non-POM, or mineral-associated
(silt plus clay), C is largely constrained by soil mineral
surface area (Hassink and Whitmore 1997). Thus, POM C
may serve as a sensitive indicator of changes in soil C that
may not be detectable due to inherent soil C variability
and relatively small changes relative to total C pool size.
The purpose of this research was to assess impacts of
implementation of MiG on total organic soil C and N
pools and soil C fractions. We collected samples from
paired plots at four locations in Virginia for comparison
of total organic soil C and N, POM C, and mineralassociated C. Since grasslands allocate substantial portions of primary production to lower soil depths, we
evaluated differences in total organic soil C and N, and C
fractions for surface soils and depth increments to 50 cm.
We also evaluated the influence of soil texture on changes
in soil C and effects of grazing management on root C
stocks. Finally, we evaluated the regional potential for C
sequestration due to changes in grazing management.
Materials and methods
Comparative soil samples were collected from four MiG fields and
from four extensively grazed, or hayed, fields in Virginia, USA,
located either within the same farm or on a neighboring farm.
Slope, aspect, and soil series were uniform across comparative
sites, ensuring that land management was the primary factor
influencing soil C content. Farm names were derived from the
county in which they were located (Table 1). Louisa 1 (38.07N,
78.11W) consists of long term pasture sites on Nason silt loam soil
(Typic Hapludult) at two neighboring farms. The field sampled on
one farm was under MiG for 25 years preceding sampling while the
field on the other farm was extensively managed. Plots at Louisa 2
(38.01N, 78.17W) were both located on the same farm and soil
series (Poindexter loam; Typic Hapludalf), but were on fields
managed differently. One field had been under MiG for 25 years
and the other had been hayed since approximately 1950. Management intensive grazing was implemented 5 years prior to sampling
at a farm in Grayson County. A neighboring farm (in Carroll
County; 36.69N, 80.81W) with the same soil (Chester loam,
Table 1 Climatic (MAT mean annual temperature, MAP mean
annual precipitation) and edaphic characteristics for four comparative grazing sites in Virginia, United States
Site
MAT
(C)
MAP
(mm)
Soil series
Duration of MiGa
(years)
Louisa 1
Louisa 2
Grayson
Pulaski
14.3
14.3
13
12.3
992
992
1,157
1,087
Nason SiL
Poindexter L
Chester L
Lowell SiL
21
25
5
3
a
Managament-intensive grazing
Typic Hapludult) and comparable topography had been an extensively managed pasture since conversion from forest in the 1950s.
The Pulaski site (37.12N, 80.47W) consists of MiG (for the past
3 years) and extensively grazed fields on the same farm. Both fields
are underlain by Lowell silt loam soil (Typic Hapludalf) and had
been extensively grazed prior to implementation of MiG. Forage
species are actively managed at all sites and consisted of orchard
grass (Dactylis glomerata), which was the dominant forage cover at
all sites, with some Kentucky bluegrass (Poa pratensis) and white
clover (Trifolium repens).
Fields with different management were intensively sampled in
order to detect differences between sites and also to enable
measurement of changes in soil C over time with resampling in the
future. Our sampling scheme was based on that used by the
Canadian Prairie Soil Carbon Project (Ellert et al. 2001). Within
each field three “microsites”, each consisting of six regularly
aligned soil cores, were sampled in early spring of 1999. Microsites
were always oriented in the same direction, the location of the
northeastern-most core was measured using differential GPS, and a
relocatable Skotchmark EMS magnetic ball marker (3 M Corporation, Austin, Tex.) was buried at 1-m depth to enable future
relocation and resampling.
A Giddings hydraulic soil coring rig was used to collect 6.5-cm
diameter soil cores to a depth of 0.5 m. Soil samples were split into
four segments (0–10, 10–20, and 20–50 cm), returned to the
laboratory, and weighed. Surface litter and aboveground vegetation
were separated and quantified. Samples were passed through an
8-mm mesh sieve by gently breaking the soil along the plane of
least resistance; all visible root material was removed by handpicking during the 8-mm sieving. Soils were then air-dried and
composited by depth within each microsite (i.e., composites
consisted of soil from six individual cores). Bulk density was
calculated using volume of sample collected and the weight of soil
in the sample; sample weight of fresh samples was corrected for
soil moisture and root and rock content.
Composited samples were then sieved to pass a 2-mm sieve,
oven-dried at 60 for 72 h, and ground to fine powder using a ball
mill (Cianflone Scientific Instruments, Pittsburgh, Pa.). Particulate
organic matter and soil particle size distribution were determined
according to the method described by Cambardella and Elliott
(1992). Briefly, 30 g of 2-mm sieved soil was shaken overnight
(18-h) in 100 ml 0.5% hexametaphosphate solution to disperse the
soil. The dispersed soil was sieved through a 53-mm sieve and the
sand-sized organic material (i.e., POM) retained on the sieve was
thoroughly rinsed, transferred to aluminum pans, oven-dried
(50C), and weighed. The clay content was then determined by
the hydrometer method and silt content by difference. Soil C and N
concentration was determined for total soil, POM fraction, and root
and surface litter with a LECO CHN-1000 autoanalyzer (LECO
Corporation, St. Joseph, Mich.). Addition of strong acid to a subset
of samples indicated that carbonates were not present; thus soil C
hereafter is used for organic soil C. Mineral-associated C was
determined by difference between total C and POM C.
Planned-comparison analysis of variance with Scheffe’s means
comparison test was used to test for grazing treatment effects on
total soil C, N, root C, litter C, and C in POM and mineralassociated fractions at all four sites. Linear regression was used to
evaluate the importance of soil texture on changes in total soil C,
388
Fig. 1a–d Particulate organic
matter C (shaded portion of
bars) and total C (average with
95% confidence intervals) for
three depth increments (0–10,
10–20, and 20–50 cm) at four
sites in Virginia, United States.
Unhatched bars are management-intensively grazed (MiG)
pastures, hatched bars are extensively grazed, or hayed,
pastures (Ext.). Asterisks indicate significant (P
DOI 10.1007/s00374-003-0652-z
ORIGINAL PAPER
Richard T. Conant · Johan Six · Keith Paustian
Land use effects on soil carbon fractions
in the southeastern United States.
I. Management-intensive versus extensive grazing
Received: 21 March 2003 / Accepted: 3 June 2003 / Published online: 29 July 2003
Springer-Verlag 2003
Abstract Changes in grassland management intended to
increase productivity can lead to sequestration of substantial amounts of atmospheric C in soils. Managementintensive grazing (MiG) can increase forage production in
mesic pastures, but potential impacts on soil C have not
been evaluated. We sampled four pastures (to 50 cm
depth) in Virginia, USA, under MiG and neighboring
pastures that were extensively grazed or hayed to evaluate
impacts of grazing management on total soil organic C
and N pools, and soil C fractions. Total organic soil C
averaged 8.4 Mg C ha–1 (22%) greater under MiG;
differences were significant at three of the four sites
examined while total soil N was greater for two sites.
Surface (0–10 cm) particulate organic matter (POM) C
increased at two sites; POM C for the entire depth
increment (0–50 cm) did not differ significantly between
grazing treatments at any of the sites. Mineral-associated
C was related to silt plus clay content and tended to be
greater under MiG. Neither soil C:N ratios, POM C, or
POM C:total C ratios were accurate indicators of
differences in total soil C between grazing treatments,
though differences in total soil C between treatments
attributable to changes in POM C (43%) were larger than
expected based on POM C as a percentage of total C
(24.5%). Soil C sequestration rates, estimated by calculating total organic soil C differences between treatments
(assuming they arose from changing grazing management
and can be achieved elsewhere) and dividing by duration
of treatment, averaged 0.41 Mg C ha–1 year–1 across the
four sites.
R. T. Conant ()) · J. Six · K. Paustian
Natural Resource Ecology Laboratory,
Colorado State University,
Fort Collins, CO 80523-1499, USA
e-mail: conant@nrel.colostate.edu
J. Six
Department of Agronomy and Range Science,
University of California,
Davis, CA 95616, USA
Keywords Carbon sequestration · Pasture · Grazing
management
Introduction
Land use and land changes are widely recognized as key
drivers of global C dynamics (Houghton et al. 1999;
Schimel 1995), but the role of grassland management has
only recently received attention as a substantial potential
C sink (Conant et al. 2001; Follett et al. 2001; Sampson et
al. 2000). A recent literature review concluded that a
variety of management practices including irrigation,
fertilization, sowing improved grass and legume species,
and improved grazing management all promote C
sequestration in grasslands (Conant et al. 2001). High
rates of C sequestration (0.1–3.0 Mg C ha–1 year–1)
coupled with large areas responsive to improved management suggest that 70 Tg C could be sequestered
annually in Annex I (developed) countries as a result of
changes in grassland management (Sampson et al. 2000).
This is more than twice what is likely to be sequestered as
a result of land use change (30 Tg C year–1; Watson et al.
2000).
Management-intensive grazing (i.e., MiG, or short
rotation grazing) is widely believed to increase grassland
forage production by ensuring more uniform forage
removal and allowing a recovery period (Gammon
1978). A recent review found that MiG in dry rangelands
does not influence forage production, but in more humid
regions, forage production increased by 20–30% (Holecheck et al. 1999). There is a long history of using MiG to
increase production (Haynes and Neal 1943; Hudson
1929) and in some areas, such as New Zealand and
Australia, short rotation grazing is widely used (Gifford et
al. 1992). Though pastureland is an important land
resource in the southeastern United States, covering more
than 12.6 Mha (12% of the total land area) supporting
6.5 million beef cattle and more than 990,000 dairy cows
(Census of Agriculture 1992) in 1992, the use of MiG in
the southeastern United States is not widespread (Conant
387
et al. 2002) and potential impacts of MiG adoption on soil
C have not been adequately researched.
Short-term changes in soil C in response to changes in
grazing management are small relative to the amount of
total soil C and may be difficult to detect by measuring
bulk changes in soil C. Therefore, physical, chemical, and
biological soil C pools have been isolated in an attempt to
identify those soil C fractions likely to respond more
rapidly than total C to changes in management (e.g.,
Bardgett and McAlister 1999; Haynes 2000; Sparling
1992). It has been shown that particulate organic matter
(POM) C and N provide an early indication of changes in
C dynamics and total soil C under different agricultural
management practices (Cambardella and Elliott 1992; Six
et al. 1999; Wander and Bollero 1999). Likewise,
Franzluebbers et al. (2000) found 65% greater POM N
content but only a 34% greater total N in a grazed versus
hayed bermuda grass pasture and Burke et al. (1999)
reported significant differences in coarse POM C and N
(>500 mm) but no significant differences in total C and N
between grazed and ungrazed short grass steppe. Conversely, the amount of non-POM, or mineral-associated
(silt plus clay), C is largely constrained by soil mineral
surface area (Hassink and Whitmore 1997). Thus, POM C
may serve as a sensitive indicator of changes in soil C that
may not be detectable due to inherent soil C variability
and relatively small changes relative to total C pool size.
The purpose of this research was to assess impacts of
implementation of MiG on total organic soil C and N
pools and soil C fractions. We collected samples from
paired plots at four locations in Virginia for comparison
of total organic soil C and N, POM C, and mineralassociated C. Since grasslands allocate substantial portions of primary production to lower soil depths, we
evaluated differences in total organic soil C and N, and C
fractions for surface soils and depth increments to 50 cm.
We also evaluated the influence of soil texture on changes
in soil C and effects of grazing management on root C
stocks. Finally, we evaluated the regional potential for C
sequestration due to changes in grazing management.
Materials and methods
Comparative soil samples were collected from four MiG fields and
from four extensively grazed, or hayed, fields in Virginia, USA,
located either within the same farm or on a neighboring farm.
Slope, aspect, and soil series were uniform across comparative
sites, ensuring that land management was the primary factor
influencing soil C content. Farm names were derived from the
county in which they were located (Table 1). Louisa 1 (38.07N,
78.11W) consists of long term pasture sites on Nason silt loam soil
(Typic Hapludult) at two neighboring farms. The field sampled on
one farm was under MiG for 25 years preceding sampling while the
field on the other farm was extensively managed. Plots at Louisa 2
(38.01N, 78.17W) were both located on the same farm and soil
series (Poindexter loam; Typic Hapludalf), but were on fields
managed differently. One field had been under MiG for 25 years
and the other had been hayed since approximately 1950. Management intensive grazing was implemented 5 years prior to sampling
at a farm in Grayson County. A neighboring farm (in Carroll
County; 36.69N, 80.81W) with the same soil (Chester loam,
Table 1 Climatic (MAT mean annual temperature, MAP mean
annual precipitation) and edaphic characteristics for four comparative grazing sites in Virginia, United States
Site
MAT
(C)
MAP
(mm)
Soil series
Duration of MiGa
(years)
Louisa 1
Louisa 2
Grayson
Pulaski
14.3
14.3
13
12.3
992
992
1,157
1,087
Nason SiL
Poindexter L
Chester L
Lowell SiL
21
25
5
3
a
Managament-intensive grazing
Typic Hapludult) and comparable topography had been an extensively managed pasture since conversion from forest in the 1950s.
The Pulaski site (37.12N, 80.47W) consists of MiG (for the past
3 years) and extensively grazed fields on the same farm. Both fields
are underlain by Lowell silt loam soil (Typic Hapludalf) and had
been extensively grazed prior to implementation of MiG. Forage
species are actively managed at all sites and consisted of orchard
grass (Dactylis glomerata), which was the dominant forage cover at
all sites, with some Kentucky bluegrass (Poa pratensis) and white
clover (Trifolium repens).
Fields with different management were intensively sampled in
order to detect differences between sites and also to enable
measurement of changes in soil C over time with resampling in the
future. Our sampling scheme was based on that used by the
Canadian Prairie Soil Carbon Project (Ellert et al. 2001). Within
each field three “microsites”, each consisting of six regularly
aligned soil cores, were sampled in early spring of 1999. Microsites
were always oriented in the same direction, the location of the
northeastern-most core was measured using differential GPS, and a
relocatable Skotchmark EMS magnetic ball marker (3 M Corporation, Austin, Tex.) was buried at 1-m depth to enable future
relocation and resampling.
A Giddings hydraulic soil coring rig was used to collect 6.5-cm
diameter soil cores to a depth of 0.5 m. Soil samples were split into
four segments (0–10, 10–20, and 20–50 cm), returned to the
laboratory, and weighed. Surface litter and aboveground vegetation
were separated and quantified. Samples were passed through an
8-mm mesh sieve by gently breaking the soil along the plane of
least resistance; all visible root material was removed by handpicking during the 8-mm sieving. Soils were then air-dried and
composited by depth within each microsite (i.e., composites
consisted of soil from six individual cores). Bulk density was
calculated using volume of sample collected and the weight of soil
in the sample; sample weight of fresh samples was corrected for
soil moisture and root and rock content.
Composited samples were then sieved to pass a 2-mm sieve,
oven-dried at 60 for 72 h, and ground to fine powder using a ball
mill (Cianflone Scientific Instruments, Pittsburgh, Pa.). Particulate
organic matter and soil particle size distribution were determined
according to the method described by Cambardella and Elliott
(1992). Briefly, 30 g of 2-mm sieved soil was shaken overnight
(18-h) in 100 ml 0.5% hexametaphosphate solution to disperse the
soil. The dispersed soil was sieved through a 53-mm sieve and the
sand-sized organic material (i.e., POM) retained on the sieve was
thoroughly rinsed, transferred to aluminum pans, oven-dried
(50C), and weighed. The clay content was then determined by
the hydrometer method and silt content by difference. Soil C and N
concentration was determined for total soil, POM fraction, and root
and surface litter with a LECO CHN-1000 autoanalyzer (LECO
Corporation, St. Joseph, Mich.). Addition of strong acid to a subset
of samples indicated that carbonates were not present; thus soil C
hereafter is used for organic soil C. Mineral-associated C was
determined by difference between total C and POM C.
Planned-comparison analysis of variance with Scheffe’s means
comparison test was used to test for grazing treatment effects on
total soil C, N, root C, litter C, and C in POM and mineralassociated fractions at all four sites. Linear regression was used to
evaluate the importance of soil texture on changes in total soil C,
388
Fig. 1a–d Particulate organic
matter C (shaded portion of
bars) and total C (average with
95% confidence intervals) for
three depth increments (0–10,
10–20, and 20–50 cm) at four
sites in Virginia, United States.
Unhatched bars are management-intensively grazed (MiG)
pastures, hatched bars are extensively grazed, or hayed,
pastures (Ext.). Asterisks indicate significant (P