Measuring Soil Carbon Stocks
A System for Quantifying and Verifying Change in Soil Carbon Stocks due to
Changes in Management Practices on Agricultural Land
Brian McConkey, (mcconkeyb@em.agr.ca) and Wayne Lindwall (lindwallw@em.agr.ca)
Agriculture & Agri-Food Canada, June 1999
process is called carbon sequestration. Figure
Summary
1 illustrates soil carbon changes over time on
agricultural lands.
Carbon Soil Sinks:
# Remove CO2 from the atmosphere
# Encourage sustainable development Land management practices on
# Offer environmental benefits agricultural land that increase carbon
# Can be measured accurately at sequestration include reduction in tillage,
reasonable cost restoring degraded land, improving pasture
management, and reducing fallow periods. In
addition to sequestering carbon in the soil,
these soil-improving practices also increase soil
Repaying the Soil Carbon Debt productivity, enhance the quality of water
draining from agricultural land, and provide a
When land was broken from natural more hospitable environment for wildlife
forest or grassland for agriculture, a large inhabiting that agricultural land. Hence, these
amount of the native soil organic matter was practices are fundamental to a more sustainable
lost as CO2 to the atmosphere. However, if future.
land management practices are changed in ways
that increase the soil organic carbon, the Figure 2 shows an example from
reverse occurs and CO2 is effectively removed western Canada of how improved land
from the atmosphere and put into the soil. This management practices restore soil C. In this
case, land that had been conventionally
agriculture
90
80
70
Soil carbon
60
Soil C Soil C Soil C 50
Decrease Stable Increase 40
30
20
Initial Management
cultivation change 10
0
100-yr 80-yr Continuous
Effect on Conventional Conventional Native Grass
atmospheric ~ Management then 20-yr
CO2 N o -Tillage
Figure 1. Soil carbon changes due to initial Figure 2 Canadian example of
land conversion to agriculture, attainment of restoration of soil carbon over 20 years
a new equilibrium, followed by adoption of from adoption of no-tillage practices.
management practices that sequester
carbon. 1
managed with frequent tillage and fallow has a
debt or deficit of 30 Mg/ha or 35% less carbon
than adjacent land under native grass. However,
the land that had been conventionally managed
and then converted 20 years ago to no-tillage
without fallow has regained 16 Mg C/ha or
about one-half of the soil carbon debt.
Reducing Measurement Variability
When measured through strictly random
sampling, the amount of soil carbon appears
very variable. Owing to this variability, some
have argued that it will be difficult to quantify Figure 3. Careful sampling is an essential step to
and verify changes in soil carbon stocks due to reducing variability of soil carbon measurements.
changes in land management practices.
However, a team of Canadian scientists has past are analysed for soil C in a random order
developed a reliable method to minimize the along with samples from the current time. This,
variability. This method is the basis for in combination with rigorous laboratory quality
accurately verifying estimates of soil carbon (C) control procedures, eliminates the potential for
changes due to land management changes. This even minute variation in soil C assessments
method involves: across time resulting from the slight shifts in the
dry-combustion C analysis procedure itself.
• Measuring soil C changes on the same
small benchmark over time. The benchmarks System for Quantifying and
are located carefully to minimize soil variations Verifying Changes in Soil C Stocks
within the benchmark itself. Multiple soil
samples for C analysis are taken within the
benchmark. Collectively, these actions greatly Pilot Project in Canada
reduce the effects of spatial variability for
comparisons across time. To improve the soil quality, including
• Benchmarks are located in known rebuilding soil organic matter, many western
landscape positions and include upper, mid and Canadian farmers have adopted no-tillage crop
lower slopes so that the variation of soil C with production practices. A group of these no-
topography is fully accounted. tillage farmers, in cooperation with a team of
• The density of all soil samples is Canadian scientists from government and
carefully determined. Further, soil is sampled in universities, has initiated a pilot project using a
10 cm increments to well below the depth system to quantify and verify the soil C changes
where important soil C changes occur due to due to this adoption of a no-tillage system. The
agricultural management. Careful adjustment is pilot project involves the province of
made so that soil density differences over time Saskatchewan, which contains 20 million
or place do not affect soil C stock comparisons. hectares of the crop land, one-half of Canada’s
• Soils are carefully processed (including total. Figure 4 is a simplified schematic
exacting treatment of surface plant litter and representation of this soil C quantification and
subsurface large plant roots). verification system.
• Stored air-dried soil samples from the
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and other land use changes involving
Large-area or perennial vegetation).
National Soil C
Stock Changes
3. Scaling Up: Soil C changes for these
situations are predicted with the soil C
Verification model. These are integrated to make large-
Soil, Weather, & area or national estimates using a
Management
Databases
Geographical Information System (GIS).
Benchmarked
Farm Fields
4. Verification: The accuracy of the soil C
model predictions are audited by comparing
the predictions with the rich set of carefully
Land-Farming
Auditing GIS System-Weather measured C changes in the benchmark
Situations situations. Further, if sufficient benchmarks
are available so that all important land-
Soil C
Model Soil C model farming system situations are represented, an
Land Parameterization independent estimate of soil C changes is
Use available by scaling up the benchmark soil C
changes directly.
Remote Basic Research/
Sensing Plot Measurements
A Closer Look at Verification Benchmarks
Figure 4 System for quantifying and
verifying changes in soil carbon stocks. In the Canadian pilot project, a network of
150 benchmarked fields were established,
System Description covering the agriculturally developed portion of
the province of Saskatchewan (see Figure 5).
The core of the system is the model of soil The benchmarked fields include every important
C dynamics. This science of soil C dynamics is combination of soil type, texture, and regional
relatively well developed and several soil C climate. The benchmarks were established just
models (e.g. CENTURY) have been used before cooperating farmers converted these
successfully to predict changes in soil C in a fields to no-tillage in 1997. On these fields,
wide range of environments. The basic system 2x5m benchmarks were located with Global
involves: Positioning System (GPS) and with a buried
electromagnetic markers. These benchmarks
1. Model Refinement: appropriate C model were carefully sampled according to exacting
parameters are derived and the soil C model protocol to minimize variability. The farmers
is thoroughly tested using a large set of soil were instructed to manage their fields normally
C research experiments and data. without regard to the benchmark (there is no
2. Define Situations: From databases of soils, visible marking of the benchmarks). Soil carbon
landform, weather, and farm management, on the benchmarks will be measured again three
important situations that result from a years after the initial soil sampling.
combination of the farming system, land, and
regional weather are identified. Remote Uncertainty of Soil C Changes Low
sensing supplements database information on
no-tillage extent. (Remote sensing will be A well-designed network of passive
more important when the system is expanded benchmarks on farm fields is a cost-effective
to include soil C changes due to changes in and powerful method of confirming that
management of pastures, farm wood lots,
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estimates of soil C stock changes are accurate. A Win-Win Option for the
Based on our Canadian pilot project, a Environment
benchmark verification system can be
implemented for a total cost less than 5 cents Sustainable development requires that we
(i.e. US $0.05) per hectare. With an leave future generations a productive soil
appropriate quantification and verification resource. In this light, the wisdom of applying
system, the uncertainty of changes in soil C soil-improving practices is unarguable. These
stocks due to changes in land management practices improve the health and fertility of the
practices will be smaller than those for soil and decrease the use of fossil fuel, fertilizer,
greenhouse gas emissions from agriculture and other inputs per unit of food grown.
included under the Kyoto protocol. Opportunities to apply soil improving land
management practices exist on farm land
throughout the world. Practices that improve
the soil clearly contribute to both environmental
and economic objectives.
Figure 5 Map of benchmarked fields used in the Canadian pilot project involving the system
to quantify and verify soil C stock changes from the adoption of no-tillage farming practices.
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