Kathleen Knight
The Use of Cover Crops and Conservational Tillage for Carbon Sequestration
Atmospheric 𝐶𝑂2 levels are at the highest in human history approaching 400 ppm. Since
the industrial revolution levels have been increasing and toady acceleration rates have
accelerated to 2.11 ppm per year, a rate that is 100 times faster than the increase when the last ice
age ended (𝐶𝑂2 Trend, 2014). The majority of this increase can be attributed to fossil fuel
combustion, land use change, and soil cultivation. The last two practices have also resulted in
soil carbon loss and further climate change, with the carbon being lost to the atmosphere as 𝐶𝑂2.
Soils are an important resource that hold a large percentage of the Earth’s carbon, with a total
carbon pool that is four times that of the biotic pool, and three times the atmospheric pool. With
some soil carbon losses of one-half to two-thirds the original soil carbon pool (Lal, 2004). To
reduce 𝐶𝑂2 losses and net emissions, recommended management practices on agricultural soils
include: the practice of low or no tillage, reduced fallow period, and cover crops.
Between the industrial revolution and the start of the new millennium, global carbon
emission are estimated at 270±30 Pg (Pg = petagram = 1015 g = 1 billion tons) due to fossil fuel
combustion and 136±55 Pg due to land use changes and soil cultivation. While the depletion of
the soil organic pool has contributed an additional 78±12 Pg of carbon to the atmosphere (Lal,
2004). Soil and agricultural practices account for about 1/3 of emissions, while instead these
recommended management practices can turn the soil into a carbon sink that will reduce
atmospheric levels, while improving soil quality.
Conventional agricultural practices cause a greater 𝐶𝑂2 flux to the atmosphere. These
practices involve intensive tillage that primarily is used to incorporate left over plant residue into
the soil through the use of a plow, leaving very little plant residue on the soil surface. Tillage is
also used to break up soil clods for better seed germination, and sometimes during crop growth
to keep weeds down. This is a problem because it mixes the plant residue into the soil, disrupts
macroaggreates, increases aeration, and stimulates microbial breakdown of soil organic matter
and accelerates soil organic matter decomposition. Since the organic matter is brought closer to
the soil microbes, with better conditions, it increases the biological oxidation of soil carbon
which is lost to the atmosphere as 𝐶𝑂2. Then if cover crops aren’t used, fields are left barren
after cultivation until it is time to plant again. Combined, barren fields and tillage greatly reduce
ground cover and make the soil very susceptible to erosion, resulting in a loss of soil carbon to
the atmosphere (Amado et al., 2006; Peregrina et al., 2014).
Conservation tillage, on the other hand, is a system in which at least 30% of crop residue
is left in the field, and can be 70% or higher for no-till systems. It is important for soil
conservation because it helps to reduce erosion by reducing soil temperature fluxes, building up
organic matter, and conserving soil moisture. The accumulation of crop residues at the surface
results in an enrichment of soil organic matter in the surface layers, which causes an increased
abundance of microorganisms. Decreased disruption also allows for more growth resulting in
more fungi, bacteria, arbuscular mycorrhizal fungi, and actinobacteria (Mathew et al., 2012).
Fig. 1: Effects of conventional vs. conservation tillage on soil nutrients, water, and biota.
(House & Parmelee, 1985)
Reicosky found that the 𝐶𝑂2 flux to the atmosphere, or soil respiration, is greatest after
tilling. These levels decreased after 24 hours, but sometimes remained high, presumably based
on higher carbon concentrations and root decomposition rates. Then long-term rates decreased
greatly, going on to lose more carbon to the atmosphere if further machinery and tillage was used
during the season. Between a chisel and moldboard plow, there wasn’t significant differences,
but they did find that fluxes can vary based on crops that are grown, see figure 2 (Reicosky et al.,
1997).
Immediate tillage produces the greatest losses, but most of what tillage relates back to is
the fact that less tillage increases plant biomass, and plants are the main source of carbon, and a
main mechanism for carbon sequestration to soils. Plants add carbon to the soil through tissue
residue, root exudates, or symbiotic fungi. The carbon that enters as plant residue either
decomposes and returns to the atmosphere, or is eventually leached from soils after a few
decades to centuries. In general the rate of soil loss and accumulation can be from 0.1 to 10 Mg
C ℎ𝑎−1 𝑦𝑟 −1, and with today’s agricultural practices many crops have been found to have
negative contributions to soil stock. Especially, when compared to native land cover, which for a
lot of the U.S.’s agriculture land is grassland. In general, these areas of native vegetation have
been found as good carbon sinks, which are being lost with the conversion of native land to
cropland (Pinhiero et al., 2014).
No-till leaves a greater plant residue that can be especially useful in arid regions,
allowing for higher water content in the soil, and reduced evaporative soil water. It prevents the
formation of soil crusts and allows for greater water infiltration, which increases plant
productivity, and thus soil sequestration (Eshel et al., 2014). It also improves physical protection
of soil organic matter as it builds up on the soil aggregates, and allows for longer turnover times
for the matter, than those free particles.
In a study done by Amado et al., they found that more clayey soils had 159% higher total
organic carbon when compared to a nearby sandy loam. The clay rich soil also had higher
nutrients than sandy soils which resulted in increased biomass production and thus a buildup of
total organic carbon. Under conventional till systems, regardless of cropping and soil type, all
total organic carbon was less than native grass, but more pronounced with lower clay content.
While the no-till systems with intensive cropping with legumes, were able to maintain or
increase total organic carbon, when compared to native grass soils. No-till with legume increased
carbon accumulation when compared to double cropping systems (rye-maize) by 0.43 Mg
ℎ𝑎 −1 𝑦𝑟 −1 (Amado et al., 2006).
Fig. 2: Cumulative soil 𝐶𝑂2 flux 24 hours after tillage for three cropping systems and three different
crops, measured by two different devices.
(Reicosky et al., 1997)
Table 1: Selected chemical and physical properties of soils from no-till (NT) and conventional till (CT)
treatments.
(Mathew et al., 2012)
(Mathew et al., 2012)
Instead of leaving crops to fallow over the winter, or the off season, a cover crop system
should be used to inhance soil organic carbon sequestration. This is done by improving soil
quality, and thus plant growth, which results in higher biomass and higher sequestration from the
plants, while reducing losses from erosion. Soil quality is improved by increasesd organic matter,
macroporosity, mean aggregate size, soil permeability, and thus crop yield. As well as increased
microbial biomass and enzymatic activity, and it is found that those ecosystems with higher
biodiversity absorb and sequester more carbon, than those of low biodiversity. Benefits can be
further improved by deep rooted cover crops, which can deliever carbon to deeper soil horizons.
As well as leguminous crops which naturally add plant essential nitrogen to the soil, by fixing it
from the atmosphere. It is also found that a good cover crop can increase innocula of
mycorhizzal fungi in the soil, which form beneficial symbiotic relationships with plant roots to
assist with water and nutritent uptake. Mychrozzial colinazation was found to be higher in no till
plots (Dabney et al., 2001; Lal, 2004)
In a study of cover crops and their above ground biomass on carbon sequestration,
Pergina et al. found that there are much higher inputs into the soil surface (0-5cm deep) from
leguminous clover crop, than from barley. At respective rates of 1.19 and 0.47 Mg C ℎ𝑎 −1 𝑦𝑟 −1.
This difference is due to higher above ground biomass in clover crop that can be partially
attributed to its ability to fix nitrogen from the atmosphere. This ability to fix nitrogen, which is
an essential plant nutrient, comes from a symbiotic association that many leguminous crops have
with specialized bacteria Rhizobium that forms nitrogen-fixing nodules. This increase in nitrogen
allows for greater plant biomass production and benefits future crops by making about two-thirds
of the nitrogen fixed by the legume crop available for the next growing season. This is due to the
decomposition of their nitrogen-rich organic matter and incorporation into the soil through
microorganisms when the plants die. It was also recommended that for higher vegetative
development it is best to find cover crops well adapted for an area and climate in order to receive
the best sequestration (Peregrina et al., 2014 & USDA, 1998).
Peregrina et al. also touched on the fact that for best sequestration rates and data, long
term studies should be done. Their study was short term, taking place over 3 years, and found
that even after 5 years; carbon sequestration hadn’t reached a steady state. That over longer
periods of time, with similar agricultural practices, soil has a greater potential to sequester
carbon. This is particularly true for uncultivated areas or perennial crops that have more time to
sequester carbon, grow deeper roots, and give back to the soil. Well managed grassland and
forests are examples of these that have shown to serve as good carbon sinks. First uncultivated
carbon sinks of grasslands had immediate sequestration ranging from 0.11 to 3.04 Mg C ℎ𝑎 −1
𝑦𝑟 −1 with an average of 0.54 Mg C ℎ𝑎 −1 𝑦𝑟 −1 . Forests are a good example of benefits over
longer period of time with sequestration rates maximized by maintaining 20-50 year rotations
with an average of 14.1 g C 𝑚−2 (Conant et al., 2001; Paul et al., 2002; Peregrina et al., 2014).
In a study done by Franzluebbers, when tillage studies of those with and those without
cropping systems were compared, the effect of a conservative tillage system on soil organic
carbon sequestration became more apparent. Soil organic carbon with no-till was found to be two
times greater with cover cropping than without. In this case, no-tillage with cover cropping
added carbon to the soil through above and below ground crop production. It is also possible that
this limited decomposition of dried organic matter in the soil during crop growth by taking soil
water from the heterotrophic decomposing organisms to the autotrophic plants (Franzluebbers,
2005).
Table 3:
(Peregrina et al., 2014)
Table 4:
(Franzluebbers, 2005)
Based on the above research and data for the most carbon sequestration through
agricultural practices, a good recommendation would be to use a no-till and intensive cover
cropping system. Meaning that the field is never left barren, covered with the last crops plant
residue, but also quickly planted with a cover crop that is both ideal for that season as well as
area and climate. This, as well as choosing a leguminous cover crop, can improve soil quality
and increase biomass production. Especially when done long term, these practices can help
reduce soil carbon losses, while increasing carbon sequestration to the soil. All of which can
contribute to decreasing 𝐶𝑂2 levels.
Soils aren’t the complete solution to the climate change problem, but they can definitely
play a significant role as they sequester atmospheric 𝐶𝑂2, preventing future level increases. Soils
help mitigate some of the negative effects of climate change while humans work on the other
two-thirds of emissions, by working to greatly reduce emissions from fuel combustion and
researching greener energy alternatives. On top of this, soil quality improves and soil losses
decrease, allowing for a greater crop yield and better food security. It’s going to take time to
implement and encourage farmer’s to use these practices, but in the long-term and even the
short-term, the benefits will outweigh any additional costs through the change in agricultural
practices. Soil is a precious resource intertwined in ecosystems that is vital for life. So much of
life relies on the health of the soil to provide nutrients, food, filtration, habitat, water storage etc.
Taking care of the soil will in turn take care of the life in, and on top of it.
“A nation that destroys its soils destroys itself.”
-Franklin D. Roosevelt
References:
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Potential of carbon accumulation in no-till soils with intensive use and cover crops in southern
Brazil. J. Environ. Qual. 35(4): 1599.
Conant, R.T., K. Paustian, and E.T. Elliott. 2001. Grassland management and conversion into
grassland: effects on soil carbon. Ecol. Appl. 11(2): 343–355.
Dabney, S.M., J.A. Delgado, and D.W. Reeves. 2001. Using winter cover crops to improve soil and
water quality. Comm. Soil Sci. Plant Analysis 32(7-8): 1221–1250.
Eshel, G., D. Lifschitz, D.J. Bonfil, and M. Sternberg. 2014. Carbon exchange in rainfed wheat fields:
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