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where it needs to be-in the soil. Irrespective,
cover crops fix carbon in temporary, short-cycle,
storage while slowly adding carbon to the stable,
longer-term SOM pool. A thorough summary of
cover crop research indicates that cover crops impact on soil carbon is site specific. Results depend
on the amount of cover crop biomass (above and
below ground), how long the land has been cover
cropped, the soil carbon levels before implementing cover crops, soil type, cover crop species, tillage, and climate.
The impact of soil tillage on carbon sequestration is currently a point of controversy among
some researchers. Nonetheless, well-understood
principles of tillage suggest that less tillage, when
possible, is better than more tillage for maintaining a stable SOM pool. Tillage, like cover crops,
has both erosion and carbon input facets that
complicate analysis of its effect on SOM.
First, tillage disturbs topsoil and makes it
prone to erosion from wind and precipitation.
While this may remove SOM from the site, the
outcome of that carbon is not fully understood.
The primary impact of minimizing tillage is a reduction in the loss of carbon rich topsoil directly
by limiting soil disturbance. Overwhelming evidence indicates that no-till systems generally have
higher SOM levels in the top few inches of the soil.
The effect of no-till deeper in the soil profile is
not as certain. Conventional tillage does serve to
incorporate aboveground plant biomass into the
soil-one thing that no-till does not. This action
gets the carbon from aboveground into the soil
where it needs to be to become incorporated into
SOM pools. However, tillage is known to breakdown soil aggregates that contain carbon and aerates soil, which in turn, may accelerate decomposition of SOM and increase soil carbon loss.

A Look at the Numbers

There are a wide range of estimates being
reported as to how much carbon can be stored
in actively farmed croplands depending on the
practice being implemented. For instance, some
estimates for cover crops are as high as 1.1 to
1.8 metric tons of carbon per acre per year. The
Sustainable Agriculture Research and Education
(SARE) estimates cover crops have the potential
to sequester an average as high as 0.81 metric
tons of carbon per year. The standard developed
from a global meta-analysis of cover cropping
suggests a global average of 0.086 metric tons of
carbon annually per acre. Another heavily cited
research review1 suggests soil carbon gains of
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FIRSTQUARTER2021

TABLE 1

COMET-Planner Output for Two NRCS Conservation
Practices at Select Locations Across the United States.
NRCS Conservation Pratice

Growth Stage

Intensive Till to No Till or Strip Adding Non-Legume Cover Crops
Till (CPS 329) Non-Irrigated
(CPS 340) Non-Irrigated
Meteric Tons of Carbon Sequestered Per Acre Per Year

Boone County, IA 0.1701*
Cheyenne County, CO 0.0648
Johnson County, MO 0.1566
Lubbock County, TX 0.0864
Onslow County, NC 0.1107
Sharkey County, MS 0.1269

0.04-0.40 metric tons per acre per year is a more
plausible range.
Tillage numbers are even more difficult to
pinpoint, largely because soil sampling is labor
intensive-particularly at depths below two or
three feet. Most research has primarily focused
on SOM levels in the upper inches or foot and
researchers are just beginning to understand soil
C at deeper depths. Much of this disparity results
from a lack of consistent research methodology
for determining the amount of carbon in the soil.
In 2006, the Intergovernmental Panel on
Climate Change created the Guidelines for National Greenhouse Gas Inventories. Subsequently, using these guidelines, the USDA developed
methods for quantifying greenhouse gas fluxes in
the United States-including soil measurements.
The USDA-Natural Resources Conservation
Service (NRCS) took the lead on the agricultural
side by integrating data from multiple disciplines
with NRCS Conservation Practices to provide estimates of emission reductions or increases with
the implementation of different practices. Their
platform COMET-Farm and COMET-Planner
are user friendly tools that generate estimates
that different practices have on carbon emissions.
The estimates are listed in CO2 equivalents
to directly compare the impact of practices on
GHG emissions-but can be converted to sequestration estimates. COMET-Farm uses producer management information with spatially
explicit data on climate and soils from USDA
databases to model management impacts and
compare changes in management scenarios to
forecast future carbon sequestration. COMET-Planner on the other hand is a more generic
version that provides generalized estimates for,

0.0567
0.0270
0.1431
0.0351
0.0945
0.2403

as the name implies, planning purposes.
Output from the COMET-Planner tool on
the effects of tillage and cover crops on carbon sequestration is provided below. The output of the simulations is listed as CO2 equivalents, but since CO2 is 27 percent carbon,
converting the CO2 equivalents to carbon
sequestration is a simple task. For brevity and
demonstration purposes, results for a couple
of practices in multiple locations are provided
for comparison (Table 1).
The values generated illustrate the sensitivity of the values depending upon the region
in which the practice is implemented and reflects differences in climate, growing season
length, etc. These estimates indicate a range of
0.06 to 0.17 MT of carbon sequestered annually for converting from conventional tillage
to no till, and a range of 0.03 to 0.24 MT of
carbon sequestered annually for implementing cover crops. The estimates for cover crops
are more closely aligned with the lower end of
estimates mentioned previously. These values
appear to provide a realistic picture for comparison purposes.
It is evident that there is much to be examined regarding agriculture's role in sequestering
carbon, GHG reductions, and climate change.
This article does not discuss the great strides that
animal agriculture is taking to limit C loss to the
atmosphere. In addition, there are many more
important factors relating to carbon-particularly soil health and its impact on cropping resiliency in the face of an uncertain climate. Moving
forward, future discussions and analysis will no
doubt link these factors together into more coherent policy objectives.

1
Blanco-Canqui, H., T.M. Shaver, J.L. Lindquist, C.A. Shapiro, R.W. Elmore, C.A. Francis, G.W. Hergert. 2015. Cover crops
and ecosystem services: Insights from studies in temperate soils. Agron. J. 107:2449-2474.

Crop Insurance Today First Quarter 2021

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