Effect of the Conservation Reserve Program (CRP)

on Soil Carbon

By

Jay D. AtwoodSteven R. Potter

Jimmy R. WilliamsM. Lee Norfleet

22 March 2005

Atwood and Norfleet are with USDA, NRCS, Resource Inventory and Assessment Division.Potter and Williams are with The Texas A&M University System, Texas Agricultural Experiment Station, Blackland Research and Extension Center, Temple TX

Presented at the “Third USDA Symposium on Greenhouse Gases and Carbon Sequestration in Agriculture and Forestry”, March 21-24, 2005, in Baltimore, Maryland

* 18,446 total NRI CRP points

Figure 2. Regions defined for analysis of 1997 NRI CRP.(only the 8 – digit Hydrologic Units with analyzed CRP points are shaded)

WestNorthern Great PlainsSouthern Great PlainsUpper MidwestSouth CentralNortheastSoutheast

Figure 3. Domain of the 1997 NRI CRP analysis.

*Acres of practices exceed enrollment since each enrolled acre may have multiple practices.

0102030405060708090

100110120

Northeast NorthernGreat Plains

SouthCentral

Southeast SouthernGreat Plains

UpperMidwest

West NationalTotal

% o

f N

RI

CR

P a

cres

Contracted Practice Acres

Modeled for Crop

Modeled for CRP

Matched Successful Simulations

Figure 4. Acreage of crops modeled.

0

1,000

2,000

3,000

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5,000

6,000

Win

ter W

heat

- Fall

ow

Win

ter W

heat

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Whe

at - F

allow

Soybe

anCor

n

Cropl

and P

astu

re

Alfalfa

Hay

Sorgh

um

Cotto

n

Spring

Whe

at

Barley

Oats

Grass H

ay

Perman

ent P

astu

re

Corn

Silag

e

Potato

Peanu

t

Figure 5. CRP cover by type and region

0

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Northeast NorthernGreat Plains

SouthCentral

Southeast SouthernGreat Plains

UpperMidwest

West NationalTotal

Percent of areaIntroduced Grass

Native Grass

Trees (with grass)

Wildlife Habitat

Figure 6. Definition of Model Representative Land Units and Crop and CRP simulations for the 1997 NRI CRP points.

NRI CRP Points

Define Prior Crop simulations,with acreage weights, based on NRI crop history

If not corn, wheat or pasture

If corn, wheat or pasture

Determine acreage shares for corn, pasture, and wheat types

Conduct Crop scenario model simulationsfor 3 tillage types for each crop

Crop type 1

Croptype n

Calculate weighted total/average results for Crop Scenario with acreage weights and shares

Define CRP simulations for 4 cover types withacreage weights based on county level CRP enrollment data, e.g., 75% Introduced Grasses 15% Native Grasses 7% Trees 3% Wildlife habitat

Conduct CRP scenario model simulations for each CRP cover type

Calculate weighted total/average results for CRPScenario using acreage weights and shares

Compare Scenarios

Cluster points by region, state, climate, soil, and type of structural conservation practices

Figure 7. Counts and average acreage representation of model land units and simulations by region.

0

1

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3

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5

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9

Crop CRP

Upper Midwest

Northern Great Plains

South Central

Southern Great Plains

West

Southeast

Northeast

National

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Count Acres (10s)

Upper Midwest

Northern Great Plains

South Central

Southern Great Plains

West

Southeast

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National

RepresentativeModelLand Units

Simulations per model land unit

Figure 8. Key analytical assumptions

1. If land had not been enrolled in CRP, the former crop mix would have continued, but tillage,

conservation practice, and nutrient management would have evolved like non-enrolled land.

2. Procedures were applied to set initial soil carbon levels at levels consistent with having a

history of cultivation

3. Simulations made with the EPIC model (version with Century model type carbon accounting)

4. Five sets of simulations, each with different stochastically generated weather, were averaged

5. Continuous mono-cropping except for wheat – fallow rotations.

6. Wind erosion calibrated to 1997 NRI levels via lower limit on soil surface moisture

7. Net field level soil loss estimates with Modified Theoretical Small Watershed (MUST) version

of USLE were on average 50% the magnitude of the NRI USLE soil losses

Figure 9. Overview of soil C and N processes in the EPIC model.

• C and N dynamics interact directly with soil moisture, temperature, erosion, tillage, soil density, leaching, and translocation functions of EPIC;

• Equations describe the role of soil texture in stabilization of soil organic matter;

• C and N compounds are allocated into five compartments in terms of turnover time:

– Metabolic litter, with a time span of months;– Structural litter, with a time span of months to years;– Biomass (active), with a time span of months;– Slow humus, with a time span of 20 to 50 years;– Passive humus, with a time span of 400 to 2000 years;

• C and N can be lost through leaching or in gaseous form to the atmosphere;

• There are four key differences between the equations of EPIC and Century (Izaurralde et al., 2001) :

– EPIC’s leaching equations move organic matter from surface litter to subsurface layers;– Temperature and water controls affecting transformation rates are calculated with equations already

in EPIC;– The surface litter fraction in EPIC has a slow, but not passive compartment; and– Lignin concentration in EPIC is modeled as a sigmoidal function of plant age.

Figure 10. Initialization of Soil Carbon (%) Starting Point

Issue – Many of the pedon samples in the soil survey database appear to be from non-cultivated conditions; soil carbon is high relative to cultivated conditions.

Soils in the soil survey data base were screened for “AP” layer, indicating history of cultivation. Those with an “AP” layer and having less than 10% organic matter (5.7% organic carbon i.e., mineral soils only), were used to fit the following equation:

Y = aX -bX

where Y = soil organic carbon (%)

X = depth (in cm)

The equation was initially fit at the hydrologic group level within each of the 10 USDA Farm Production regions. The regions were subsequently combined into four groups.

The equation was used to set the soil organic carbon by layer in all soils for the study except for the “Organic” and “other” texture groups which were left at soil survey levels.

Figure 11. Hydrologic Group Effect of Formula to Set Initial Soil Carbon (%)

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1.0

1.2

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1.6

A B C D National

Layer 1 Survey

Layer 1 Formula

Layer 2 Survey

Layer 2 Formula

A – low runoff potential, high infiltration rateB – moderate infiltrationC – slow infiltrationD – high runoff potential, very slow infiltration rate

Figure 12. Regional Effect of Formula to set Initial Soil Carbon (%)

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2.2

Northeast NorthernGreatPlains

SouthCentral

South East SouthernGreatPlains

UpperMidwest

West National

Layer 1 Survey

Layer 1 Formula

Layer 2 Survey

Layer 2 Formula

Figure 13. Water Induced Soil Loss Estimation

USLE = C*P*R*K*(LS)where C is the crop management factor (range of 0 to 1)

P is the conservation practice supporting factor (range of 0 to 1)R is the rainfall factorK is the soil erodibility factor(LS) is the factor based on slope length (L) and slope (S)

*EPIC calculates USLE daily, with daily C and R The NRI uses long run average annual C and R, prediction of long term annual average soil erosion

MUST replaces R with R = 2.5*(Q*qp)0.5

where Q is runoff volume in mmis a function of daily rainfall, a retention parameter, and soil water content- retention parameter depends on soil, land use, management, and slope

qp is the peak runoff rate in mm per hour

is a function of infiltration characteristics, rainfall intensity, and watershed area* MUST was theoretically derived from sediment concentration data bases MUST is predicted for every storm event; individual storm events are summed for the year

**Except for a few comparison tables and charts, all results based on MUST.

Figure 14. Crop water erosion rates by region, period, and scenario.

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9

Northeast NorthernGreatPlains

SouthCentral

Southeast SouthernGreatPlains

UpperMidwest

West NationalTotal

Crop years 1-10

Crop years 11-20

Crop years 21-30

CRP years 1-10

CRP years 11-20

CRP years 21-30

Figure 15. Crop wind erosion rates by region, period, and scenario.

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Northeast NorthernGreat Plains

SouthCentral

Southeast SouthernGreat Plains

UpperMidwest

West NationalTotal

Crop years 1-10

Crop years 11-20

Crop years 21-30

CRP years 1-10

CRP years 11-20

CRP years 21-30

Figure 16. Regional soil carbon storage benefit due to CRP.

-0.2

-0.1

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Years 1 - 10 Years 11 - 20 Years 21 - 30

ton

s/ac

re/y

ear

Southeast

Northeast

Southern Great Plains

Northern Great Plains

West

South Central

Upper Midwest

National

Figure 17. Year end soil carbon by region for non-forage crops and CRP.

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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Years

( ton

s/ac

re)

Upper Midwest-CRPUpper Midwest-cropNortheast-CRPNortheast-cropWest-CRPWest-cropNorthern Great Plains-CRPNorthern Great Plains-cropSoutheast-CRPSoutheast-cropSouth Central-CRPSouth Central-cropSouthern Great Plains-CRPSouthern Great Plains-cropNational - cropNational - CRP

Figure 18. Difference in annual soil C change ((CRPt-CRPt-1)-(cropt-cropt-1))*.

Years

*Only non-forage crops included.

-1.0

-0.5

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(ton

s/ac

re)

NortheastSoutheastUpper MidwestNorthern Great PlainsSouthern Great PlainsSouth CentralWestNational

Figure 19. CRP affect on soil C storage by region, period, and tillage type(non-forage crops only).

-0.2-0.10.00.10.20.30.40.50.60.7

0.80.91.01.11.21.31.41.51.61.7

Conv. Mulch NoTill Conv. Mulch NoTill Conv. Mulch NoTill

y 1 - 10 y 1 - 10 y 1 - 10 y 11 - 20 y 11 - 20 y 11 - 20 y 20 - 30 y 20 - 30 y 20 - 30

ton

s/a

cre/

yea

r

Northeast

Southeast

South Central

S. Great Plains

Upper Midwest

N. Great Plains

West

National

Figure 20. Percent of area losing and gaining > 5% soil C by region and period.

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Crop Loss>5% Crop Stable Crop Gain >5% CRP Loss >5% CRP stable CRP Gain > 5%

Per

cen

t o

f A

cres

Southeast

Northeast

South Central

Upper Midwest

Southern Great Plains

Northern Great Plains

West

National Total

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Crop Loss>5% Crop Stable Crop Gain >5% CRP Loss >5% CRP stable CRP Gain > 5%

Per

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f A

cres

Years 1-10

Years 11-20

Years 21-30

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Crop Loss>5% Crop Stable Crop Gain >5% CRP Loss >5% CRP Stable CRP Gain > 5%

Per

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cres

Figure 21. Average (non-acreage weighted) CRP C sequestration rates by soil texture group and period.

-0.3-0.2

-0.1

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0.9

1.0t/y/a

Years 1-10

Years 11-20

Years 21-30

Truncated from 6.5, 5.5, and 1.9

Source Spatial Scope Type tons/acre/year

Gebhart et al. (1994) Great Plains Field studies at 5 sites 0.54

Lal et al. (1999) National Average of published studies 0.22

Paustian et al. (2001) Great Plains - grass cover simulation model 0.30

" Great Plains - tree cover simulation model 1.96" Great Plains - overall simulation model 0.42" Central U.S. Two studies, 19 sites -0.21 to 0.83" Illinois cited study - cropland abandoned to

grass0.45

Eve et al. (2003) U.S. - by 10 USDA Farm Production Regions

Intergovernmental Panel on Climate Change

0.04 to 1.25

Kurcharik (2003) Wisconsin 14 paired crop and CRP sites 0.11Sperow et al. (2003) National Intergovernmental Panel on Climate

Change0.15

Lewandrowski et al. (2004)

National, 13 regions Intergovernmental Panel on Climate Change

0.33 to 0.50

Lewandrowski et al. (2004)

National, 13 regions, subset of cropland converted to forest

Intergovernmental Panel on Climate Change

0.75 to 1.66

This study Regional averages, years 1 to 10 0.33 to 0.89 0.60This study Regional averages, years 11 to 20 -0.02 to 0.43 0.16This study Regional averages, years 21 to 30 -0.20 to 0.14 0.06

Figure 22. Estimated CRP Soil C Benefits

Figure 23. Effect of alternative soil erosion and initial soil C on CRP C sequestration rate estimate, national level.

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Soil Loss Reduction10.5, Soil Survey C

Soil Loss Reduction10.5, Initialized C

* Soil Loss Reduction7.5, Initialized C

Soil Loss Reduction4.7, Soil Survey C

t/a/y

years 1-10

years 11-20

years 20-30Reported

Alternative: CRP erosion benefit of 10.5 t/a/y Soil survey initial C No smoothing for wheat-fallow

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1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

Figure 24. Effect of alternative soil erosion and initial soil C on selected regional C accumulation (tons/acre).

Upper Midwest alternative CRP and Crop CRP erosion benefit of 10.5 t/a/y Soil survey initial C No smoothing for wheat-fallow

NE alternative CRP and Crop

Upper Midwest report CRP and Crop

NE report CRP and Crop

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t/a/

y

Northeast - alternative

Northeast - report

Upper Midwest - alternative

Upper Midwest - report

Figure 25. Effect of erosion and initial C on difference in annual C change ((CRPt-CRPt-1)-(cropt-cropt-1))*.

Alternative: CRP erosion benefit of 10.5 t/a/y Soil survey initial C No smoothing for wheat-fallow