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Oklahoma Carbon Sequestration Enhancement Act

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					Oklahoma Carbon Sequestration Enhancement Act



    An Assessment Report to the Oklahoma Legislature
             Relating to the Requirements of
    Oklahoma Law 27A O. S. 2001, Section 3-4-102.



   Prepared by the Oklahoma Conservation Commission
  With Assistance From the Carbon Sequestration Advisory
                        Committee



                     January 6, 2003
Acknowledgements:

The Oklahoma Conservation Commission, Oklahoma State University Department of
Plant, and Soil Sciences, and the USDA Natural Resources Conservation Service (NRCS)
developed this Phase One Assessment Report, with assistance from the Carbon
Sequestration Advisory Committee. Special thanks go the science subcommittee for
providing leadership and direction for the report and to Jeremy L. Seiger, Oklahoma State
University, and Jim Henley, NRCS for developing much of the data, charts, and maps for
the report.

Thanks also to F. Dwain Phillips for assembling the materials, editing and coordinating
the development of the final report.




                                                i
                                         Contents

An assessment report to the Oklahoma Legislature relating to the requirements of
Oklahoma Law 27A O.S. 2001, Section 3-4, Oklahoma Carbon Sequestration
Enhancement Act.
                                                                                   Page

Summary                                                                             1

Legislative Background                                                              4

Carbon Sequestration Advisory Committee Members                                     6

Phase I Objectives                                                                  8

Phase I Methods                                                                     8

Phase I Findings                                                                    10

Exhibit 1 -Oklahoma General Land Use Map                                            12

Exhibit 2 – Total Soil Organic Carbon (Pre-Settlement) Map                          13

Exhibit 3 – Acre Furrow Slice Soil Organic Carbon (Pre-Settlement) Map              13

Exhibit 4 – Oklahoma Eroded Soils Map                                               14

Methods Used to Derive Organic Carbon Levels in Oklahoma Soils                      15

References                                                                          18




                                              ii
                                           Summary

The emerging national and international interest in carbon sequestration, as a means of
helping offset carbon dioxide emissions, has generated many questions and the need for
information. The Oklahoma Legislature has requested information concerning how much
carbon was in the state’s soils originally before settlement, how much carbon is currently
stored in the soil and what is the potential for future storage of carbon in the state’s
croplands, grazing lands, and forestland.

The Oklahoma Conservation Commission, Oklahoma State University, Natural
Resources Conservation Service and the Carbon Sequestration Advisory Committee
has completed Phase One Assessment Study to begin addressing these questions. The
Phase II part of the study, if funded, would look more at detail approaches and methods
to assess the impacts of various agricultural management strategies on soil and vegetation
carbon sequestration.

Objectives for the Phase One Report:

    1. Use “Best Technology” to determine and display historic soil organic carbon
       amounts in Oklahoma soils;

    2. Determine the amount of soil organic carbon loss since settlement;

    3. Identify and display general land use information for Oklahoma;

    4. Develop estimates for soil organic carbon gains and losses through soil
       management practices.


Findings:
Historic Data –Soil Organic Carbon Amounts -Pre-Settlement:
Historic data for 150 different soil series, representing 60 percent of the state’s land area,
was used to estimate the soil organic carbon in Oklahoma soils prior to settlement.
Oklahoma State University provided this data to the USDA Natural Resources
Conservation Service Geographic Information System (GIS) Section who attached the
actual sample values to the representative soil map unit in the Oklahoma soil survey Map
Information Assembly Display System (MIADS). These values were also used to refine
the existing soil carbon organic matter values in the State Soil Survey National Soils
Information System (NASIS) database.

 Prior to settlement of Oklahoma it is estimated that there were 2,303,598,211 tons of
organic carbon in Oklahoma soils, with 555,400,560 tons of soil organic carbon in the
acre furrow slice (0-6 inches).


                                                 1
Historic Soil Organic Carbon Losses Since Settlement:
The weighted average of soil organic carbon in the acre furrow slice and the total soil for
arable lands was calculated to determine the amount of soil organic carbon loss from
agriculture. A carbon depletion of 38 percent was used to determine how much carbon
was lost in the plow layer. A carbon depletion of 2 percent (weighted average) was used
for the rest of the profile.

It is estimated that 84,414,303 tons of soil organic carbon has been lost to tillage in the
acre furrow slice; 19,180,846 tons of soil organic carbon lost to tillage below the acre
furrow slice; and 10,042,043 tons of organic carbon has been lost to accelerated erosion.
The total of soil organic carbon loss in the soil since settlement is 113,637,192 tons.

Soil Organic Carbon Gains and Losses Through Changes in Soil Management
Practices:

Soil carbon gains or losses for croplands were determined using the USDA-NRCS Soil
Conditioning Index worksheet. This worksheet evaluates the effect of crop residue,
tillage, and soil erosion to determine if a soil is increasing in soil organic carbon,
decreasing in soil organic carbon, or soil organic carbon amounts are in equilibrium.
Wheat was chosen as the evaluation crop and a 10-year average wheat yield (1990-2000)
was determined using agricultural statistics data. An intensive tillage system, a reduced
tillage system and a no-till system were evaluated.

A rate of 40gC/m2 /yr increase was used to determine soil organic carbon gains from
converting cropland to grassland. Historic census data was used to determine cropland
acreage totals for Oklahoma (18,125,321 acres in 1920) and 16,016,071 acres in 1950).

The results show that there were 50,884,400 tons of organic carbon gained from
conversion of cropland to grass since 1950.

There was a net loss of 62,752,791 tons of organic carbon lost to agriculture.

Impacts of Management Changes on Cropland:

Comparisons of various tillage systems used in Oklahoma were made using cropland
figures from the Conservation Technology Information Center. As would be expected,
moving from intensive tillage to forms of reduced tillage can increase the amount of
carbon stored in the soil.

The maximum gains of carbon sequestration on cropland would result from converting all
cropland to a no-till system or placing it into the Conservation Reserve Program.
Maintaining the cropland in permanent vegetation, either grass or trees, or in a no-till
program seems to sequester about the same amount of carbon. Either of these options
would provide a 1,903,919 tons/year of soil organic carbon increase.




                                                2
Phase II of the Assessment Report
The completion of Phase I of the Assessment Report has identified several research
concerns/needs that should be looked at in conjunction with completing Phase II, or that
could support/enhance Phase II in the future.

    1. Soil carbon in relation to range, pasture and forestland. This could include tying
       soil carbon potential to Ecological Site Descriptions. A base line could be
       established and effects of management decisions could be evaluated.

    2. Updating soil surveys to digital format for better spatial analysis (include nutrient
       analysis?).

    3. Change in soil carbon in relation to cropping systems (rotations), crop production,
       and management levels.

    4. Effects of man made water bodies on sequestrated carbon (acres of water in state,
       drought impact of water bodies?)

    5. Regionalization of cropping systems (expected carbon sequestrated).

    6. Field techniques/measurements (Los Alamos research).

Phase II would involve assessing the impacts of management on soil carbon in all 77
counties in the state. This would require additional research and inputs for soils and
crop/tillage systems within each county. When this part is completed, it is hoped that it
can show an increase in the potential for agricultural soils to sequester carbon, largely
through an increase in the adoption of conservation practices. Using results of this study,
would allow local managers, working with conservation planners, the ability to estimate
rates of soil carbon change (carbon sequestration) depending on the types of management
decisions that are implemented.




                                                 3
Legislative Background:
The Oklahoma State Legislature passed the Oklahoma Carbon Sequestration
Enhancement Act (Section 3-4-101 of Title 27A), and Governor Frank Keating signed
the Act into law on April 16, 2001.

Representative Clay Pope, Representative James Covey, Representative Jack Bonny and
Senator Bruce Price cosponsored the Act..

The Oklahoma Legislature in passing the Act found that:
   1. Increasing levels of carbon dioxide in the atmosphere have led to a growing
      interest in national and international forums for implementing measures to slow
      and reverse the buildup of such atmospheric constituents. These measures may
      include, but are not limited to, the establishment of systems for trading carbon
      dioxide credits, or adoption of practices, technologies, or other measures, which
      decrease the concentration of carbon dioxide in the atmosphere.

    2. Improved agricultural practices, soil and vegetation including trees, conservation
       practices, revegetation including reforestation activities and other methods of
       stewardship of soil and vegetation resources throughout the state have great
       potential to increase carbon sequestration and help offset the impact of carbon
       dioxide emissions on carbon dioxide concentrations in the atmosphere.

    3. It is in the interest of the public that the Oklahoma Conservation Commission
       document and quantify carbon sequestration associated with improved
       agricultural practices, soil and vegetation including trees, conservation practices,
       revegetation including reforestation activities, rangeland, and other agricultural
       and nonagricultural lands occurring on cropland in this state.

The Oklahoma Carbon Sequestration Enhancement Act called for an Advisory
Committee to be formed and specified membership, with members to be appointed by the
Governor. The Legislature amended the law in May 2002 to modify and add members to
the Carbon Sequestration Advisory Committee. The amendment also authorized the
Oklahoma Conservation Commission to establish and administer the carbon sequestration
certification program. The Commission was authorized to develop, in consultation with
the Department of Environmental Quality and with the advice of the Carbon
Sequestration Committee, rules including, but not limited to, uniform standards and
criteria for the certification of existing or potential carbon sinks located in the state. The
Commission was also directed to develop application requirements, and adopt site
certification conditions for each carbon sink for which an application is submitted.

The Act required the Oklahoma Conservation Commission, with assistance from the
advisory committee, to submit a report to the Legislature by December 1, 2002.
This report was to address:
    (1) The potential economic impact from utilization of a voluntary system of carbon
        dioxide emissions trading or marketing for carbon sequestration on agricultural or
        nonagricultural lands that could be used in the event carbon dioxide emissions
        regulations are adopted in the future;


                                                 4
(2) Improved agricultural practices, soil and vegetation, including trees, conservation
practices, revegetation including reforestation activities, and other methods of
stewardship of soil and vegetation resources which occur on agricultural and
nonagricultural lands and which increase stored soil carbon and /or minimize carbon
dioxide emissions associated with agricultural practices and other types of activities that
may generate carbon dioxide emissions;

(3) Methods for measuring and modeling net carbon sequestration associated with
improved agricultural practices, soil and vegetation including trees, conservation
practices, revegetation including reforestration activities and other methods of
stewardship of soil and vegetation resources on agricultural and nonagricultural lands;

 (4) Areas of scientific uncertainty with respect to quantifying and understanding
sequestration associated with improved agricultural practices, soil and vegetation
conservation practices, revegetation activities, and other methods of stewardship of soil
and vegetation resources occurring on agricultural and nonagricultural lands; and;

 (5) Recommendations of the Committee. A draft copy of this report was submitted to the
Legislature in December 2002 and the final report will be submitted the first week of
January 2003.

The law also stated that the Oklahoma Conservation Commission shall subject to
availability of appropriations and in consultation with the Carbon Sequestration Advisory
Committee, assess agricultural and nonagricultural lands in this state for past carbon
sequestration and future carbon sequestration potential.

This Phase One Study of the assessment report is complete and includes
recommendations for Phase II.




                                                 5
Carbon Sequestration Advisory Committee:

Mr. Mike Thralls, Chairman                 Mr. Wallace Olson
Oklahoma Conservation Commission           Kelley Ranch
2800 N. Lincoln Blvd                       Vinita, Ok.
Suite 160
Oklahoma City, Ok. 73105                   Mr. Wade Rousselot
                                           Gray Oaks Ranch
Ms. Terri Blackburn                        Wagoner, Ok.
Williams Company
Tulsa, Ok.                                 Dr. Phillip L. Sims
                                           USDA Agricultural Research Service
Mr. David Branecky                         Woodward, Ok.
Oklahoma Gas and Electric
Oklahoma City, Ok.                         Dr. Jim Stiegler
                                           Oklahoma State University
Mr. Darrel Dominick                        Plant and Soil Sciences
USDA Natural Resources                     Stillwater, Ok.
Conservation Service
Stillwater, Ok.                            Mr. Henry Von Tungeln
                                           Von Tunglin Farms
Mr. Mead Ferguson                          Calumet, Ok.
Rancher
Woodward, Ok.                              Mr. Henry Taliaferro
                                           Oklahoma City, Ok.
Mr. Dennis Howard
Department of Agriculture,                 Mr. Paul Lauback
Food & Forestry                            Leedey, Ok.
Oklahoma City, Ok.
                                           Mr. Melton Jordon Jr.
Mr. Charles Sloan                          Rural Electric Cooperatives Inc.
Farmer                                     Lindsay, Ok.
Vian, Ok.
                                           Mr. Mark Coleman/Mr. Steve Thompson
Mr. Gene McVey                             Department of Environmental Quality
W. B. Johnston Grain Co.                   Oklahoma City,
Enid, Ok.




                                      6
Science SubCommittee:
Darrel Dominick, Chair
Jim Stiegler, Vice Chair
Phillip Sims
Herman Mayeux
Henry Von Tungeln
Wade Rousellot
Dan Stidham
Melton Jordan, Jr.

Individuals serving as technical advisors to the Advisory Committee:

Dr. Herman Mayeux                             Mr. Jim Henley
USDA Agricultural Research Service            Natural Resources Conservation Service
El Reno, Ok.                                  Stillwater, Ok.

Mr. Kurt Atkinson                             Mr. Jack Eckroat
Oklahoma Department of Agriculture,           Plant and Soil Sciences
Food and Forestry                             Oklahoma State University
Forestry Services Division                    Stillwater, Ok.
Oklahoma City, Ok.
                                              Mr. Dan Stidham
Mr. Jim Ford                                  Oklahoma Department of Agriculture
Natural Resources Conservation Service        Food and Forestry Division
Stillwater, Ok.                               Enid, Ok.




                                          7
Phase One of the Assessment Report Objectives:
1) Use “best technology” to determine and display historic soil organic carbon (OC)
amounts in Oklahoma soils.
        a) OSU Department of Plant and Soil Sciences Soil Carbon Study using
           the NRCS National Soil Characterization Soil Sample Database
        b) NASIS State Soil Survey database
        c) MIADS soil survey data set

2) Determine the amount of soil organic carbon loss since settlement
        a) Tillage
        b) Accelerated erosion

3) Identify and display general land use information for Oklahoma
        a) MIADS Land Use Database
        b) CTIC Crop Data

4) Develop estimates for soil organic carbon gains and losses through soil management
practices
        a) Literature search
        b) USDA NRCS Soil Conditioning Index Worksheet


Methods

Objective 1: Determine historic soil organic carbon amounts for Oklahoma.
Oklahoma State University Department of Plant and Soil Science recorded the soil
organic carbon values from National Soil Survey laboratory soil characterization database
for each layer of 150 different soil series. Many of these soil types had multiple soil
samples available for analysis. These soil types represented approximately 60 percent of
the state’s land area.

The soil carbon values were graphed to give a representative soil organic carbon curve.
An average soil bulk density (1.44g/cc) was used so the amount of soil carbon in the Acre
Furrow Slice (0-6 in.) and the entire soil could be determined. The soil samples were also
identified as either cropped or native condition.

The soil carbon values were then given to Oklahoma NRCS GIS Section and the actual
sample values were attached to the representative soil map unit in the State Soil Survey
MIADS Soil Database. These values were also used to refine the existing soil organic
matter values in the State Soil Survey NASIS database.




                                               8
For those soil types that were not represented by actual soil sample data the ‘high’
organic matter value in the Oklahoma State Soil Survey (NASIS) database was converted
to organic carbon (.58 * OM) and compared with similar soils where there was actual
organic carbon data. Some adjustments were made to make sure similar soils had similar
values.

To determine the amount of pre-settlement soil organic carbon in Oklahoma, the tons of
organic carbon in the acre furrow slice and total soil was multiplied by the acreage of
each soil map unit.

Objective 2: Determine the amount of Soil Organic Carbon Loss since settlement. To
determine the amount of soil organic carbon loss from agriculture, the weighted average
of soil organic carbon in the acre furrow slice and total soil for arable lands was
calculated. A carbon depletion of 38% was used to determine how much carbon was lost
in the plow layer. A carbon depletion of 2% (weighted average) was used for the rest of
the soil profile.

Historic census data was used to determine cropland acreage totals for Oklahoma
(18,125,321 ac. 1920 Agriculture Census- 16,016,071 ac. 1950 Agricultural census).
Cropland acreage was assumed to come from arable lands.

To determine the losses from accelerated erosion all severely and moderately eroded soil
map units in the state were identified and their acreage calculated. Moderate and severe
erosion results in a 50% to 75% loss of the original surface layer. This equates to the
complete loss of the original acre furrow slice. A 20% loss of the amount of organic
carbon in the pre-settlement acre furrow slice represents organic carbon losses due to
accelerated erosion.

Objective 3: Identify and Display general land use information for Oklahoma. The
Oklahoma NRCS MIADS Land use database was used to determine the spatial extent of
general Oklahoma land use. This data represents pre-1990 land uses. The data does not
reflect the impact of CRP. The cropland acreage figures were adjusted to reflect the most
current cropland information from the Conservation Tillage Information Center (CTIC).

Objective 4: Develop estimates for soil organic carbon gains and losses through changes
in soil management practices. Soil carbon gains or losses for croplands were determined
using the USDA-NRCS Soil Conditioning Index worksheet. This worksheet evaluates the
effect of crop residue, tillage, and soil erosion to determine if a soil is increasing in soil
organic carbon, decreasing in soil organic carbon, or soil organic carbon amounts are in
equilibrium.

Wheat was chosen as the evaluation crop. A 10-year average wheat yield (1990-2000)
was determined using agriculture statistics data. This yield (29.47 bushels. /ac.) was used
to determine residue totals for the evaluation. This yield equals 3802 pounds of both top
growth and root biomass.




                                                 9
An intensive tillage system (<15%) residue, a reduced tillage system (20-30%) residue on
the surface, and a no till system (>30%) residue were evaluated. The residue needed to
achieve equilibrium was computed. The difference between the average residue produced
and the residue needed for equilibrium represented the soil organic carbon gain or loss.
To determine soil organic carbon gains from converting cropland to grassland a rate of
40gC/m2/yr. increase was used. This converts to 357 lbs. C/ac/yr.


Findings:
Historic Data - Pre-Settlement
2,303,598,211 Tons of soil organic carbon (OC) in Oklahoma soils
548,400,560 Tons of soil organic carbon in the acre furrow slice

Soil Organic Carbon Losses
84,414,303 Tons of soil organic carbon loss to tillage in the AFS
19,180,846 Tons of soil organic carbon loss to tillage below AFS
10,042,043 Tons of soil organic carbon loss to accelerated erosion
113,637,192 Tons of soil organic carbon loss since settlement


Findings:
Soil Organic Carbon Gains since 1950’s
50,884,400 Tons organic carbon from conversion of cropland to grass

Net Loss of Oklahoma’s soil organic carbon
62,752,791 Tons of soil organic carbon lost to agriculture


Findings:
Soil Organic Carbon Sequestration Rates on Croplands

Management                       Acres           Rate           Gain             Loss
                                               Tons/ac/yr.      Tons/yr.        Tons/yr.
Intensive tillage               4,348,484       -0.176                          765,333
(No Conservation)
Intensive tillage               4,348,484       -0.099                          430,500
(Conservation)
Reduced tillage                 1,920,343       0.02            39,789
No-Till                         1,426,484       0.176          251,061
CRP                             1,023,785       0.178          182,233




Findings:


                                              10
Impacts of Management Changes on Cropland

         Management                          Acres              Rate             Gain
                                                               Tons/ac/yr.      Tons/yr.

Intensive tillage to Reduced tillage        4,348,484           .196             859,358
Intensive tillage to Intensive tillage      4,348,434           .077             334,829
(With Conservation)
Intensive tillage to No Till                4,348,484           .3515          1,528,492
Intensive tillage to CRP                    4,348,484           .3545          1,541,537
Reduced Tillage to No Till                  1,920,343           .1955            375,427
Reduced Tillage to CRP                      1,920,343           .1985            381,188


Maximum gains from conversion
1,903,919 tons/yr. if all cropland converted to No-Till or CRP

Calculations:
Average tons OC in AFS LCC* 1-4                                     13.87
Average tons OC in pedon LCC 1-4                                    59.88
Estimated OC losses in AFS over 30 yr.                              38%
Estimated OC losses in pedon over 30 yr.                             2%
Cropland acreage used to calculate tons of OC lost                  16,016,071 (1950 value)
Present Cropland acreage                                            7,871,309 (CTIC)
Difference                                                          8,144,762

Cropland to Grassland sequesters carbon at a rate of 40g/m2/yr. or .1785/T/ac/yr.
8,144,762 * .1785T/ac/yr. *35years

To compute how much soil loss is due to accelerated erosion
13.87 * .20 =2.77 tons * 3,620,056 acres of eroded soils
This uses the theory that the upper 6 in. soil eroded away with a 20% loss of carbon

Carbon sequestered from CRP 40g/m2/yr. or 357 lbs. of OC/ac/yr.
Residue conversion .37 of residue is carbon and .25 is left to soil

Intensive tillage erosion           6.1 tons/ac/yr. OKC Loam soil
Reduced Tillage erosion             3.5 tons/ac/yr. OKC Loam
No Till                            1.5 tons/ac/yr. OKC Loam

(*LCC= Land Capability Classes)




                                                11
Exhibit 1




            12
Exhibit 2 - Source - USDA Natural Resources Conservation Service, State Soil Survey MIAD database
This map illustrates the distribution of total soil organic carbon in Oklahoma. Deep, clayey, bottomland
soils contain the highest amounts while sandy or shallow and very shallow soils contain the least.

Exhibit 3 Source - USDA Natural Resources Conservation Service, State Soil Survey MIAD database.
This map illustrates the soil organic carbon amounts in the acre furrow slice (0-6 inch layer). Loamy and
clayey soils developed under grasslands typically have the highest amounts. Sandy soils and shallow soils
developed under forest canopy have the lowest amounts.




                                                     13
Exhibit 4 Source - USDA Natural Resources Conservation Service, State Soil Survey MIAD database




                                                14
Methods Used to Derive Organic Carbon Levels in Oklahoma Soils

The assessment of organic carbon content for Oklahoma soils was performed using a
digital soils database provided by the Natural Resources Conservation Service (NRCS),
and data from hard copies provided by the Department of Plant and Soil Sciences,
Oklahoma State University (OSU). Soil samples were collected and described in the field
(as part of the ongoing soil survey) from across different parts of Oklahoma then the
samples were sent to the NRCS National Soil Survey Center – Soil Survey Laboratory in
Lincoln, NE for a chemical analysis of organic carbon as well as other soil properties.
Organic carbon was one of many soil properties selected for characterization. This
information was then entered into a database. Soils data obtained from OSU was
characterized in the field and chemically analyzed for organic carbon as well as other soil
properties in the Soil, Water, & Forage Analytical Laboratory located on the campus of
OSU. Organic carbon values were obtained from both laboratories using the wet
combustion method; Walkley-Black modified acid-dichromate digestion, FeSO4 titration,
automatic titrator (Nelson and Sommers, 1982).

The database included soils data from Arkansas, Colorado, Kansas, Louisiana, Missouri,
New Mexico, Oklahoma, and Texas. Knowing that soil series don’t necessarily stop at
state boundaries, the database was queried for all soil series represented within an area
bound by longitudes between 93° West and 103° West and by latitudes between 32°
North and 38° North. This list was grouped by soil series name, so that no duplicated
series occurred. Data from the hard copies, obtained from OSU was then manually
entered into the database. The new compiled set of soil series was analyzed by NRCS soil
scientists to determine the soil series represented in Oklahoma and to verify the series
names were current and not obsolete.

This list of Oklahoma soil series was then linked back to the original database to query all
pedons in the database that matched the Oklahoma soil series names. Soil series names in
the multi-state database were represented by two possible series names. The first possible
soil series name was the “sanam” or the “sampled as” name and was the assumed name at
the time of field sampling. It was not changed even though more information may have
been available at a later time. The second possible series name was the “cname” or the
“current name”. This was the name given after further analysis of the pedon information
and was changed due to overriding soil characteristics. However, both names were not
always provided in the database, in those cases whichever series name was given that was
the series name used. If both columns were represented with different series names, the
“cname” or “current name” took precedence over the “sanam” or “sampled as” name. If
both names matched then that name was used. Other information obtained with each soil
series was the soil survey number (the site identifier - it consists of a 2 digit calendar year
when sampled, a 2 letter state FIPS code, a 3 digit county code, a 3 digit code indicating
sampling sequence in the year, and an optional satellite code), the pedon ID number, the
sample ID (a unique value for each horizon), the horizon nomenclature, the horizon layer,
the center of the horizon, percent organic carbon, the ecological site name, and the
ecological site index number.




                                                 15
A literature review was performed to obtain information on the factors that affect organic
carbon levels in the soil (Bernoux et al., 1998; Burke et al., 1989; Davidson and
Lefebvre, 1993; Honeycutt et al., 1990b; McDaniel and Munn 1985; Percival et al., 2000;
Parton et al., 1987; and Schimel et al., 1994). Four key contributing factors were
identified from the literature review and were as followed: climate, soil texture, soil
depth, and soil drainage. In order to make some generalized groupings and to extend the
database to series not sampled; each series was given a four-digit number.

The first digit represented a moisture regime across Oklahoma. Oklahoma was broken
into five generalized moisture regimes based on annual average precipitation from 1961-
1990 (Daly, 1998). As a general arrangement these regimes typically match up with
Major Land Resource Areas outlined by the NRCS. Typically regime 1 was represented
by the Southern High Plains, regime 2 by the Central Rolling Red Plains, regime 3 by the
Central Rolling Red Plains and part of the Cross Timbers, regime 4 by part of the Cross
Timbers, the Cherokee Prairies, the Grand Prairies, the Eastern Cross Timbers, and
regime 5 by the Ozark Highland, the Ouachita Mountains, and the Western Coastal
Plains.

The second digit represented the family particle size. Number 1 represented soils with a
clayey subsurface texture, number 2 represented soils with a loamy subsurface texture,
number 3 represented soils with a silty subsurface texture, and number 4 represented soils
with a sandy subsurface texture.

The third digit represented the soil depth. Soils characterized of having a deep to very
deep profile were given a number 1. Soils characterized of having a moderately deep
profile was give a number 2, and a soil characterized of having a shallow to very shallow
profile was given a number 3.

The last digit in the four-digit series represented the soil drainage class. Soils
characterized with a well to excessively well-drained profile were given the number 1.
Soils characterized with a moderately well drained profile were given the number 2.
Soils characterized with a somewhat poorly drained profile were given the number 3, and
soils characterized with a poorly drained profile were given the number 4. An example of
this four-digit classification is illustrated by the Bethany soil series. Bethany is
represented by the four-digit number sequence of 3111. This sequence of numbers is
interpretive of a soil located in the Central Rolling Red Prairies, having a clayey
subsurface texture, with a deep to very deep profile, that is well to excessively well
drained. Soil properties were obtained from the official soil series descriptions (Soil
Survey Division).

The database was then queried for soils with data from 0-250 cm and for pedons without
null organic carbon values. The soils were then separated in to two groups, the “cropped”
and the “noncropped” soils. To do this the assumption was made that pedons
characterized with a horizon suffix identifying a plowed layer had at sometime been a
“cropped” soil and soils lacking the suffix was grouped as a “noncropped” soil. Next all
buried horizons were discarded from each pedon. All of the pedons were then grouped by



                                               16
soil series names. Groupings of the same soil series ranged from one pedon to as many as
nine pedons per series.

Each grouped soil series was then plotted on an x-y scatter plot, with percent organic
carbon shown as a function of soil depth. Literature review supported the idea that
organic carbon has an exponential decay curve with soil depth (Hilinski, 2001; Bernoux
et al., 1998; and Elzein and Balesdent, 1995). In fact when most of the data was fitted
with an exponential decay curve relationship of the two were represented by a coefficient
of determination of values >.7.

Calculating the organic carbon content per pedon, the curve will yield the total amount of
organic carbon. To calculate the area under the curve for each series represented, an
integration of exponential decay was used (CRC). The units on the scatter plot were
converted from percent organic carbon as a function of depth to the pounds of organic
carbon as a function of pounds of soil per acre pedon. Organic carbon was converted
from a percent basis to a mass unit by dividing by 100. Converting the soil depth to a
mass per volume unit required calculating an acre area times a given depth. The given
depth was soil specific. For example each soil depth was determined from the four-digit
number sequence. Soils characterized with a deep to very deep profile were calculated to
200 cm deep. Soils characterized with a moderately deep profile were calculated to 100
cm deep, and soils characterized with a shallow to very shallow profile were calculated to
50 cm deep. Dealing with a volume, the soil density had to be calculated. While soil bulk
density generally increases with soil depth, a uniform bulk density was assumed for all of
the soil profiles. The assumed density was 1.44 g/cm3 . Multiplying the volume of soil
times the soil density yielded the mass of soil per acre pedon unit needed to integrate the
area under the curve.

Using the converted units, new scatter plots were generated and fitted with exponential
decay curves. From the curves, the slope of each line was generated as well as the y-
intercept. Using the formula in Figure1, the slope and y-intercept generated from the
exponential decay curves, and integrating the area under the curve yielded pounds of
organic carbon per acre pedon. This value was then converted to tons of organic carbon
per acre pedon. In addition to this calculation tons of organic carbon per acre furrow slice
was also calculated. This was determined by integrating the top six inches of soil for each
series.
                                                  K2

                                   Area =     ∫        -β1 x
                                                  β0 e dx
                                             K1



                                  Figure 1
A spreadsheet was prepared and submitted to the NRCS on September 1, 2002. The
spreadsheet included each soil series (mostly noncropped soils but some cropped), the
calculated tons of organic carbon per acre pedon, the calculated tons of organic carbon
per acre furrow slice, the ecological site name, and the ecological site index number.




                                                  17
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Brejda, J.J., M.J. Mausbach, J.J. Goebel, D.L. Allan, T.H. Dao, D.L. Karlen, T.B.
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Honeycutt, C.W., R.Ld. Heil, and C.V. Cole. 1990b. Climate and topographic relations
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Houghton, R.A., J.E. Hobbie, and J.M. Melillo. 1983. Changes in the carbon of
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Lal, R., R.F. Follett, J. Kimble, and C.V. Cole. 1999. Managing U.S. cropland to
         sequester carbon in soil. J. Soil Water Conserv. 54:374-381.



                                              18
McDaniel, P.A., and L.C. Munn. 1985. Effect of temperature on organic carbon-texture
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The Potential of U. S. Cropland to Sequester Carbon and Mitigate the Greenhouse Effect;
       R. Lal, J. M. Kimble, R. F. Follett, C. V. Cole.




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