Croplands are often considered carbon neutral, ie the carbon dioxide (CO2) taken up during growth is released back to the atmosphere once the plant material has been consumed or used for combustion.
This statement is true to a certain degree. However, it does not factor in greenhouse gas (GHG) emissions from soil or GHG emissions beyond the field boundary. In this article, we discuss the potential for arable farming to reduce GHG emissions.
Low carbon, high potential
Arable soils have the lowest soil organic carbon (SOC) concentrations compared to other land types due to high levels of disturbance and periods of fallow.
However, this property itself presents a unique opportunity, as long-term tillage soils can act as larger sinks for potential SOC storage.
Conventional v conservation tillage
Soil cultivation increases the breakdown of soil aggregates and crop harvesting reduces the amount of organic carbon retained on the field.
Although some CO2 is released during ploughing, the big losses are caused by the fact that protected carbon is released and can be broken down to CO2 by soil microbes.
Conservation tillage describes a broad category of ploughless cultivation practices (eg minimum, reduced, non-inversion, no-till).
The primary goal of these less intensive systems is to maintain soil structure and preserve SOC stocks. This is done in two ways.
Greater aggregate stability has been observed under conservation tillage management compared to ploughing
Firstly, a fraction of crop residue is retained on the surface after harvest.
Preventing the immediate turnover of residue allows organic matter to build up in the topsoil. Secondly, decreasing the turnover of older SOC in the subsoil may lead to increases in the proportion of SOC held in aggregates.
Greater aggregate stability has been observed under conservation tillage management compared to ploughing, due to greater biological activity and the protection provided by residues retained on the soil surface.
Currently, conservation tillage is practiced on 157m hectares (ha) (11% of cropland area) worldwide.
Strip tillage, for example, performs the soil cultivation and seed drilling simultaneously, cutting sowing times by 50%
In Ireland, approximately 300,000ha of land is used for crop production. A growing number of larger farms are switching from ploughing to conservation or reduced tillage due to the lower establishment costs and faster sowing times.
In a 2009 Teagasc study, minimum tillage was shown to reduce fuel costs by 45% compared to plough-based systems (€82/ha vs €148/ha).
Strip tillage, for example, performs the soil cultivation and seed drilling simultaneously, cutting sowing times by 50%.
While many reduced systems are used, some are still cultivating quite deeply; if that is the case, they are unlikely to differ from ploughing in terms of the net SOC impact.
The big question remains as to whether reduced tillage is an effective technique for mitigating CO2 emissions on Irish farms.
The problem
Numerous studies point out that conservation or no-tillage simply redistributes the carbon nearer to the soil surface.
This topsoil enrichment and subsoil depletion results in no net increase in the SOC stock. In soils, the accumulation of organic carbon typically takes 25-100 years; accrual rates tend to be initially rapid and slow down as time progresses.
This means soils with lower SOC concentrations have a greater potential for SOC accumulation than those with high initial SOC concentrations.
If farmers revert to cultivation, any gains in topsoil SOC will be lost
Most of the carbon sequestered in no-tillage systems is in easily decomposable (labile) forms. If farmers revert to cultivation, any gains in topsoil SOC will be lost.
On top of this, reduced or no-tillage systems may also increase emissions of nitrous oxide (N2O), a powerful GHG with a global warming potential 265 times that of CO2 on a 100-year basis.
Nitrous oxide is a by-product of microbes metabolising ammonium and nitrate in the soil, ie from organic and mineral fertiliser. Emissions are intensified in wetter soils and at hotter temperatures.
Furthermore, soils that are less disturbed during cultivation will tend to hold more water and are more prone to greater N2O losses.
Research and reduced tillage
A number of experiments were conducted at Teagasc to evaluate the effect of reduced tillage on soil carbon and GHG emissions.
Soil carbon was measured over a 10-year period and was found to be higher in the top 20cm compared to conventional ploughing but similar when the entire plough depth (35cm) was taken into account.
However, the proportion of protected carbon was higher, as were earthworm numbers.
In terms of N2O, research in spring barley and winter oilseed rape fields, it was found that emissions were generally quite low.
Although the differences are marginal, the current research suggests that reduced tillage systems will induce higher N2O nitrogen losses from soils
Another study (2008-2010) observed higher cumulative N2O emissions in spring barley under reduced tillage when a cover crop (mustard) was sown during the autumn/winter months.
When modelled, however, the additional CO2 uptake by the crop offset the increased emissions of N2O.
A three-year study on winter wheat (2009-2011) found that N2O emissions were more strongly determined by inter-annual weather variability rather than variability attributed to crop establishment system.
Although the differences are marginal, the current research suggests that reduced tillage systems will induce higher N2O nitrogen losses from soils.
Carbon loss and storage
It has become evident that the extended fallow (or intercropping) period is the major pathway of carbon loss from tillage systems. There is no uptake of CO2 by plants and ploughing accelerates CO2 losses from soil.
One solution is to sow a cover crop. This can enhance CO2 uptake via photosynthesis, where the net loss of carbon would have been larger in the absence of plants.
A 2010 study by Teagasc examined field-scale CO2 exchange for spring barley sown under conventional and reduced tillage systems over two seasons
Additionally, research over European croplands indicates that sowing winter crops can increase net photosynthetic CO2 uptake and reduce soil CO2 losses.
Winter crop varieties (eg barley, wheat, oats, oilseed rape) sown in early autumn also provide a CO2 sink in arable fields that limit CO2 losses.
Volunteer growth represents yet another CO2 sink. A 2010 study by Teagasc examined field-scale CO2 exchange for spring barley sown under conventional and reduced tillage systems over two seasons.
Overall CO2 exchange was unaffected by tillage regime but the growth of volunteer barley seedlings increased net carbon uptake by 0.12t C/ha in the reduced tillage plots during the fallow period.
Building SOC stocks
Two further management practices are aimed at increasing SOC in arable systems.
Straw incorporation increases the input of organic matter into soil.
Long-term addition of straw has been observed to increase SOC by 7-17% in the top 15cm of soil, but no change from the 15-60cm depth.
Research in an Irish winter wheat field estimated that reduced tillage + straw retention can mitigate 0.18-1.0t C/ha/year.
Inputs of manure will also enhance SOC stocks, resulting in annual sequestration rates of 0.33t C/ha
However, retaining straw on site is an expensive abatement measure due to the high selling price of straw at €35/t and its low nitrogen content.
Inputs of manure will also enhance SOC stocks, resulting in annual sequestration rates of 0.33t C/ha.
Importing manure, however, does not result in a net increase of SOC. This effect is the result of two processes.
Firstly, animal digestion will lower the initial carbon content of the silage/grain.
Secondly, the resulting manure represents carbon that is redistributed from one farm to another, in contrast to straw incorporation of the previous crop.
Impact of land-use change
– arable to grassland
If our croplands are sources of CO2, what effect will land use change have on carbon storage?
Researchers at UCD have reported some recovery of SOC in tillage lands where grassland/leys were included in the rotation, offering a potential mechanism for SOC storage.
European grasslands can sequester 0.25-1.75t C/ha/year with temperate moist climates exhibiting the greatest potential.
Irish grasslands have a sink potential of 0.5t C/ha/year, while a 2004 study found that the C sequestration rate of two dairy farms in southwest Ireland was close to 2.0t C/ha/year.
Grass has higher SOC, but ...
The EPA’s report of Ireland’s National GHG inventory states that agriculture accounts for 33% of Ireland’s greenhouse gas emissions.
The vast majority of this is associated with methane produced by cows and from slurry storage as well as manure and fertiliser application to grassland.
The average GHG emissions from their tillage farms is 2t CO2 equivalent (CO2Eq) per hectare compared with over 8t CO2Eq/ha from a dairy farm
The carbon (C) footprint of crops (GHG emissions per kg product reported in CO2 equivalents) is substantially lower than for livestock-sourced products, particularly meat products.
According to the Teagasc National Farm Survey Sustainability report, the average GHG emissions from their tillage farms is 2t CO2 equivalent (CO2Eq) per hectare compared with over 8t CO2Eq/ha from a dairy farm.
And an estimated 75% of the losses from those tillage farms came from the livestock on those specific farms.
This is due to the fact that arable systems are more nutrient-use efficient and do not produce methane. The carbon footprint of wheat and barley is 0.3-0.4kg CO2Eq/kg grain, compared to 13-19kg CO2Eq/ kg meat (beef or lamb).
Are there other greenhouse gas issues?
Nitrous oxide from N fertiliser application is the other main source of GHG in tillage systems. Tightening up farm N balances will reduce losses to air and water. Measures to do this include:
Optimise pH: The application of lime as a soil conditioner and specifically to neutralise soil acidity and raise pH to an agronomic optimum level confers many benefits in terms of (a) crop production, (b) soil nutrient availability, (c) fertiliser efficiency and (d) crop productivity.Fertiliser distribution: Ensure that fertiliser spreaders are properly calibrated and serviced to ensure even fertiliser distribution. Settings will be determined by the particle size, density and flow rate of the fertiliser.Cover crops: Planting these will reduce nutrient leaching and subsequent soil incorporation will improve soil fertility.Protected urea: Substitute straight urea with a protected urea product. This will reduce ammonia volatilisation and improve nitrogen-use efficiency. Also match fertiliser application rate to crop demand. Switching to reduced tillage regimes alone is unlikely to reduce soil CO2 emissions or enhance SOC sequestration in the long term.
However, as outlined by researchers at Rothamsted, “Reduced tillage has a larger role to play as a strategy for contributing to global food security and the protection of soils, and thus to climate adaptation…”
Reduced tillage practices have been shown to have positive impacts on a plethora of soil properties, such as soil aggregate stabilisation, pore connectivity, soil water storage, hydraulic conductivity, cation exchange, earthworms and microbial communities.
However, these benefits may be offset if fields require higher inputs of herbicides and pesticides. All of the above factors need to be considered by growers who contemplate switching their cultivation technique.
For CO2 mitigation in croplands, increased sowing of winter and/or cover crops can reduce CO2 emissions during the fallow period.
However, more research is required to examine the potential contribution of grassland/leys as short-term carbon reservoirs in crop rotations.
Croplands are often considered carbon neutral, ie the carbon dioxide (CO2) taken up during growth is released back to the atmosphere once the plant material has been consumed or used for combustion.
This statement is true to a certain degree. However, it does not factor in greenhouse gas (GHG) emissions from soil or GHG emissions beyond the field boundary. In this article, we discuss the potential for arable farming to reduce GHG emissions.
Low carbon, high potential
Arable soils have the lowest soil organic carbon (SOC) concentrations compared to other land types due to high levels of disturbance and periods of fallow.
However, this property itself presents a unique opportunity, as long-term tillage soils can act as larger sinks for potential SOC storage.
Conventional v conservation tillage
Soil cultivation increases the breakdown of soil aggregates and crop harvesting reduces the amount of organic carbon retained on the field.
Although some CO2 is released during ploughing, the big losses are caused by the fact that protected carbon is released and can be broken down to CO2 by soil microbes.
Conservation tillage describes a broad category of ploughless cultivation practices (eg minimum, reduced, non-inversion, no-till).
The primary goal of these less intensive systems is to maintain soil structure and preserve SOC stocks. This is done in two ways.
Greater aggregate stability has been observed under conservation tillage management compared to ploughing
Firstly, a fraction of crop residue is retained on the surface after harvest.
Preventing the immediate turnover of residue allows organic matter to build up in the topsoil. Secondly, decreasing the turnover of older SOC in the subsoil may lead to increases in the proportion of SOC held in aggregates.
Greater aggregate stability has been observed under conservation tillage management compared to ploughing, due to greater biological activity and the protection provided by residues retained on the soil surface.
Currently, conservation tillage is practiced on 157m hectares (ha) (11% of cropland area) worldwide.
Strip tillage, for example, performs the soil cultivation and seed drilling simultaneously, cutting sowing times by 50%
In Ireland, approximately 300,000ha of land is used for crop production. A growing number of larger farms are switching from ploughing to conservation or reduced tillage due to the lower establishment costs and faster sowing times.
In a 2009 Teagasc study, minimum tillage was shown to reduce fuel costs by 45% compared to plough-based systems (€82/ha vs €148/ha).
Strip tillage, for example, performs the soil cultivation and seed drilling simultaneously, cutting sowing times by 50%.
While many reduced systems are used, some are still cultivating quite deeply; if that is the case, they are unlikely to differ from ploughing in terms of the net SOC impact.
The big question remains as to whether reduced tillage is an effective technique for mitigating CO2 emissions on Irish farms.
The problem
Numerous studies point out that conservation or no-tillage simply redistributes the carbon nearer to the soil surface.
This topsoil enrichment and subsoil depletion results in no net increase in the SOC stock. In soils, the accumulation of organic carbon typically takes 25-100 years; accrual rates tend to be initially rapid and slow down as time progresses.
This means soils with lower SOC concentrations have a greater potential for SOC accumulation than those with high initial SOC concentrations.
If farmers revert to cultivation, any gains in topsoil SOC will be lost
Most of the carbon sequestered in no-tillage systems is in easily decomposable (labile) forms. If farmers revert to cultivation, any gains in topsoil SOC will be lost.
On top of this, reduced or no-tillage systems may also increase emissions of nitrous oxide (N2O), a powerful GHG with a global warming potential 265 times that of CO2 on a 100-year basis.
Nitrous oxide is a by-product of microbes metabolising ammonium and nitrate in the soil, ie from organic and mineral fertiliser. Emissions are intensified in wetter soils and at hotter temperatures.
Furthermore, soils that are less disturbed during cultivation will tend to hold more water and are more prone to greater N2O losses.
Research and reduced tillage
A number of experiments were conducted at Teagasc to evaluate the effect of reduced tillage on soil carbon and GHG emissions.
Soil carbon was measured over a 10-year period and was found to be higher in the top 20cm compared to conventional ploughing but similar when the entire plough depth (35cm) was taken into account.
However, the proportion of protected carbon was higher, as were earthworm numbers.
In terms of N2O, research in spring barley and winter oilseed rape fields, it was found that emissions were generally quite low.
Although the differences are marginal, the current research suggests that reduced tillage systems will induce higher N2O nitrogen losses from soils
Another study (2008-2010) observed higher cumulative N2O emissions in spring barley under reduced tillage when a cover crop (mustard) was sown during the autumn/winter months.
When modelled, however, the additional CO2 uptake by the crop offset the increased emissions of N2O.
A three-year study on winter wheat (2009-2011) found that N2O emissions were more strongly determined by inter-annual weather variability rather than variability attributed to crop establishment system.
Although the differences are marginal, the current research suggests that reduced tillage systems will induce higher N2O nitrogen losses from soils.
Carbon loss and storage
It has become evident that the extended fallow (or intercropping) period is the major pathway of carbon loss from tillage systems. There is no uptake of CO2 by plants and ploughing accelerates CO2 losses from soil.
One solution is to sow a cover crop. This can enhance CO2 uptake via photosynthesis, where the net loss of carbon would have been larger in the absence of plants.
A 2010 study by Teagasc examined field-scale CO2 exchange for spring barley sown under conventional and reduced tillage systems over two seasons
Additionally, research over European croplands indicates that sowing winter crops can increase net photosynthetic CO2 uptake and reduce soil CO2 losses.
Winter crop varieties (eg barley, wheat, oats, oilseed rape) sown in early autumn also provide a CO2 sink in arable fields that limit CO2 losses.
Volunteer growth represents yet another CO2 sink. A 2010 study by Teagasc examined field-scale CO2 exchange for spring barley sown under conventional and reduced tillage systems over two seasons.
Overall CO2 exchange was unaffected by tillage regime but the growth of volunteer barley seedlings increased net carbon uptake by 0.12t C/ha in the reduced tillage plots during the fallow period.
Building SOC stocks
Two further management practices are aimed at increasing SOC in arable systems.
Straw incorporation increases the input of organic matter into soil.
Long-term addition of straw has been observed to increase SOC by 7-17% in the top 15cm of soil, but no change from the 15-60cm depth.
Research in an Irish winter wheat field estimated that reduced tillage + straw retention can mitigate 0.18-1.0t C/ha/year.
Inputs of manure will also enhance SOC stocks, resulting in annual sequestration rates of 0.33t C/ha
However, retaining straw on site is an expensive abatement measure due to the high selling price of straw at €35/t and its low nitrogen content.
Inputs of manure will also enhance SOC stocks, resulting in annual sequestration rates of 0.33t C/ha.
Importing manure, however, does not result in a net increase of SOC. This effect is the result of two processes.
Firstly, animal digestion will lower the initial carbon content of the silage/grain.
Secondly, the resulting manure represents carbon that is redistributed from one farm to another, in contrast to straw incorporation of the previous crop.
Impact of land-use change
– arable to grassland
If our croplands are sources of CO2, what effect will land use change have on carbon storage?
Researchers at UCD have reported some recovery of SOC in tillage lands where grassland/leys were included in the rotation, offering a potential mechanism for SOC storage.
European grasslands can sequester 0.25-1.75t C/ha/year with temperate moist climates exhibiting the greatest potential.
Irish grasslands have a sink potential of 0.5t C/ha/year, while a 2004 study found that the C sequestration rate of two dairy farms in southwest Ireland was close to 2.0t C/ha/year.
Grass has higher SOC, but ...
The EPA’s report of Ireland’s National GHG inventory states that agriculture accounts for 33% of Ireland’s greenhouse gas emissions.
The vast majority of this is associated with methane produced by cows and from slurry storage as well as manure and fertiliser application to grassland.
The average GHG emissions from their tillage farms is 2t CO2 equivalent (CO2Eq) per hectare compared with over 8t CO2Eq/ha from a dairy farm
The carbon (C) footprint of crops (GHG emissions per kg product reported in CO2 equivalents) is substantially lower than for livestock-sourced products, particularly meat products.
According to the Teagasc National Farm Survey Sustainability report, the average GHG emissions from their tillage farms is 2t CO2 equivalent (CO2Eq) per hectare compared with over 8t CO2Eq/ha from a dairy farm.
And an estimated 75% of the losses from those tillage farms came from the livestock on those specific farms.
This is due to the fact that arable systems are more nutrient-use efficient and do not produce methane. The carbon footprint of wheat and barley is 0.3-0.4kg CO2Eq/kg grain, compared to 13-19kg CO2Eq/ kg meat (beef or lamb).
Are there other greenhouse gas issues?
Nitrous oxide from N fertiliser application is the other main source of GHG in tillage systems. Tightening up farm N balances will reduce losses to air and water. Measures to do this include:
Optimise pH: The application of lime as a soil conditioner and specifically to neutralise soil acidity and raise pH to an agronomic optimum level confers many benefits in terms of (a) crop production, (b) soil nutrient availability, (c) fertiliser efficiency and (d) crop productivity.Fertiliser distribution: Ensure that fertiliser spreaders are properly calibrated and serviced to ensure even fertiliser distribution. Settings will be determined by the particle size, density and flow rate of the fertiliser.Cover crops: Planting these will reduce nutrient leaching and subsequent soil incorporation will improve soil fertility.Protected urea: Substitute straight urea with a protected urea product. This will reduce ammonia volatilisation and improve nitrogen-use efficiency. Also match fertiliser application rate to crop demand. Switching to reduced tillage regimes alone is unlikely to reduce soil CO2 emissions or enhance SOC sequestration in the long term.
However, as outlined by researchers at Rothamsted, “Reduced tillage has a larger role to play as a strategy for contributing to global food security and the protection of soils, and thus to climate adaptation…”
Reduced tillage practices have been shown to have positive impacts on a plethora of soil properties, such as soil aggregate stabilisation, pore connectivity, soil water storage, hydraulic conductivity, cation exchange, earthworms and microbial communities.
However, these benefits may be offset if fields require higher inputs of herbicides and pesticides. All of the above factors need to be considered by growers who contemplate switching their cultivation technique.
For CO2 mitigation in croplands, increased sowing of winter and/or cover crops can reduce CO2 emissions during the fallow period.
However, more research is required to examine the potential contribution of grassland/leys as short-term carbon reservoirs in crop rotations.
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