New Protocol Recommendations For Measuring Soil Organic Carbon Sequestration
URBANA, ILL.
Increased levels of greenhouse gases, particularly
carbon dioxide (CO2), have been associated
with the burning of fossil fuels,
deforestation, cultivation of grasslands,
drainage of the land, and land use changes.
Concerns about long-term shifts in climate patterns
have led scientists to measure soil organic
carbon (SOC) in agricultural landscapes and to
develop methods to evaluate how changes in
tillage practices affect atmospheric carbon sequestration.
University of Illinois professor of
soil science Kenneth Olson has used data collected
over a 20-year period at Dixon Springs,
Ill., to develop a new protocol for more accurately
measuring the carbon removed from the
atmosphere and subsequently sequestered in
the soil as SOC.
“Many experiments comparing no-till to conventional
tillage on similar soils have shown notill
to have higher levels of soil organic carbon,”
Olson said. “So we know in general that no-till
is often better than conventional tillage at building
or retaining more of the organic matter in
the soil, which is important to crop productivity.
However, this does not mean that no-till is necessarily
sequestering atmospheric carbon. It is
often just losing carbon at a lower rate than
conventional tillage.” This unexpected discovery
was the result of Olson’s use of a pre-treatment
SOC measurement method that compares
change in soil organic carbon over time on the
same plots using the same tillage methods.
“This protocol does not assume that soil carbon
pools are at steady state (remain the same over
time), but measures SOC at the beginning of an
experiment, at intervals during, and at the end
of the experiment,” Olson said.
“Comparison studies with one treatment as
the baseline (usually conventional tillage) or
control and other tillage such as no-till as the
experimental treatment should not be used to
determine SOC sequestration if soil samples are
only collected and tested once during or at the
end of the study,” Olson said. The comparison
method assumes the conventional tillage baseline
to be at a steady state and having the same
amount of SOC at the beginning and at the end
of the long-term study, and this may not be
true. No-till as the experiment treatment needs
to be compared to itself on the same soils over
time to determine if SOC sequestration has really
occurred.
Olson compared two decades of data from previously
eroded Grantsburg soils on 6 percent
slopes to a 30-inch depth with low SOC content
in an attempt to quantify the amount and rates
of SOC sequestration, storage, retention, or loss
as a result of a conversion from conventional
tillage to a no-till system. Olson used both the
comparison and the pre-treatment SOC measurement
methods on the same plot area. His
analysis revealed conventional tillage and no-till
plot areas had less carbon (C) at the end of the
study than at the beginning using the pre-treatment
SOC method. According to the comparison
method, no-till sequestered 4.1 tons of C
per acre for a 17 percent gain during the 20
years of the study. However, the pre-treatment
SOC method showed that the no-till plots actually
lost 3.1 tons of C per acre, a 13 percent loss
in 20 years. Thus, no SOC sequestration had
actually occurred during the Dixon Springs
study.
There were three major reasons why the comparison
study approach was the wrong method
for measuring C sequestration on the Dixon
Springs plot area. First, the conventional tillage
plots were not at steady state and actually lost
30 percent of the C in 20 years due to erosion
and SOC-rich sediment being transported off
the plots. Second, when the no-till and conventional
tillage plots were sampled only once, it
was not possible to determine the rate of change
over time. Last, the effect of tillage equipment
breaking down the soil aggregates increased the
carbon available to microbial decomposition
and the release of C to the atmosphere as CO₂.
“Field experiments must be designed to more
carefully measure, monitor, and assess internal
and external inputs,” Olson said. “The amount
of SOC loss from soil storage during the time of
the experiment needs to be subtracted from
SOC gains to determine the change in net SOC
storage. Further, soil laboratory and field methods
for quantifying SOC concentration must be
refined to reduce under- and over-estimation
bias.”
Olson also recommends that the definition of
SOC sequestration include a reference to the
land unit. “Soil organic carbon sequestration is
currently defined as the process of transferring
CO₂ from the atmosphere into the soil through
plants, plant residues, and other organic solids
that are stored or retained as part of the soil organic
matter (humus). The retention time of sequestered
carbon in the soil (terrestrial pool)
can range from short-term (not immediately released
back to the atmosphere) to long-term
(millennia) storage,” Olson said. The SOC sequestration
process should increase net SOC
storage during and at the end of a study to
above the previous pre-treatment baseline levels
and result in a net reduction in the atmospheric
CO₂ levels. I believe that the phrase ‘of a land
unit’ needs to be added to the definition to add
clarity and to exclude the loading or adding of
organic C derived naturally or artificially from
external sources,” Olson suggested.
Olson concluded by saying that carbon not directly
from the atmosphere and from outside
the land unit should not be counted as sequestered
SOC. The definition of SOC sequestration
as defined with borders would mean any
C already in storage and transported or redistributed
to the plot area or field would have to
be accounted for and does not qualify as sequestered
SOC.
“Any manure from outside the plot area or
SOC-rich sediments transported and deposited
from adjacent upland are just redistributed or
transported C and not really sequestered SOC,”
Olson said. “That C was already in storage and
may in fact be released back to the atmosphere
if applied to the plot. For example, decomposing
manure loaded on a land unit increases the return
of CO₂ to the atmosphere and does not result
in a depletion of atmospheric CO₂ , which is
the real goal. Because we often lack the ability
to directly measure the total change in the atmospheric
CO₂ as a result of C loading on a plot
or field, we indirectly estimate it by measuring
the change in amount of SOC being stored in
the land unit.
“These proposed protocols are necessary to
move the science forward and to attempt to address
future predicted climate trends,” Olson
said. “The amount of SOC sequestered as a result
of alternative agricultural systems such as
no-till and its effects on net SOC storage
changes in the soil over time and the SOC released
to the water and atmospheric pools need
to be measured or calculated.”
Olson said that any future Cap and Trade program
will require SOC sequestration protocols
to be established. The method of measurement
is critical if SOC sequestration is to be verified.
“If landowners are to truly sequester SOC, they
must be able to prove that net carbon gains
have occurred over time in their fields and that
the increased SOC remains permanently stored
in their soil,” Olson said.
“Soil organic carbon sequestration, storage,
retention and loss in U.S. croplands: Issues
paper for protocol development” is published
online and will appear in the March 2013 issue
of Geoderma. Δ