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Before a good understanding of the role of soil as a sink for atmospheric CO2 can be achieved, an appreciable understanding of the short term global carbon cycle is required. In view of global climate change, the study of the global carbon cycle is extremely relevant and soil scientists have a great role to play.
Global carbon cycle
Large quantities of carbon are actively exchanged between the atmosphere and other storage pools, including the oceans, vegetation, and soils on the land surface. The exchange, or flux, of carbon between the atmosphere, oceans, and land surface is called the global carbon cycle.
In the global carbon cycle, human activities contribute a relatively small amount of carbon, primarily as carbon dioxide, or CO2. Compared to natural fluxes from the oceans and land surface, human activities contribute relatively small amounts of carbon to the atmosphere, yet despite this, analysis of the carbon cycle suggests human activities have been a major factor driving climate change over the past 50 years.
The atmosphere currently stores about 800 Gt (giga ton) of carbon and this increases with about 4.2 Gt every year. The mean residence time (MRT) of carbon in the atmosphere is six years – see the Figure 1.
The biota through photosynthesis intercepts about 120 Gt per year and returns back about 60 Gt annually, while about same amount is sequestered into the soil. The argument is that if 10% of the 120 Gt of C released from the atmosphere to the biota is either held back by the biota or sequestered in the soil, it could be able to offset the contribution of the fossil fuel (10 Gt/yr) combustion to the atmospheric concentration of CO2.
If anthro-turbation adds only a small amount of CO2 to the atmosphere each year, why is that contribution important to global climate change? The answer is that the oceans, vegetation and soils do not take up carbon released from human activities quickly enough to prevent CO2 concentrations in the atmosphere from increasing.
Humans tap the huge pool of fossil carbon for energy, and affect the global carbon cycle by transferring fossil carbon, which took millions of years to accumulate underground, into the atmosphere over a relatively short time span. Nigeria is among the highest producers of crude oil in the world and as such can not be neglected in her contribution to the increment in the world’s atmospheric CO2 pool.
For most of human history, the global carbon cycle has been roughly in balance, and the concentration of CO2 in the atmosphere has been fairly constant at approximately 300 parts per million (ppm). In early May, 2013, however, global atmospheric concentrations of CO2, as measured atop Hawaii’s Mauna Loa volcano, reached 400 ppm.
Human activities, namely the burning of fossil fuels, deforestation, and other land use activities, have significantly altered the carbon cycle. As a result, the atmosphere contains approximately 35% more CO2 today than prior to the beginning of the industrial revolution.
As the CO2 concentration grows, it increases the degree to which the atmosphere traps incoming radiation from the sun, which further warms the planet. The table below shows the concentration of the atmospheric CO2 since the year of first measurement.
Atmospheric Concentration of CO2
The global oceans and the land surface are two huge reservoirs for carbon which mitigate the increase in atmospheric CO2 concentration because they take up more carbon than they release. They are net sinks for carbon. Very little carbon is removed from the atmosphere and stored, or sequestered by deliberate action.
If the oceans, vegetation, and soils did not act as sinks, then the concentration of CO2 in the atmosphere would increase even more rapidly. Thus, identifying viable sinks for atmospheric CO2 is a high priority with the objective of sequestering it into other C pools with long residence time.
Several options for CO2 sequestration being considered are geologic, oceanic, chemical transformations and terrestrial. In contrast to the engineering techniques (e.g. geologic), C sequestration in the terrestrial ecosystem is a natural process. It is also cost effective and has numerous ancillary benefits.
Carbon storage, sources and sinks
The soils on the land surface, atmosphere, oceans and vegetation all store carbon. Geological reservoirs also store carbon in the form of fossil fuels, such as oil, gas and coal. Of these carbon pools, dissolved inorganic carbon in the ocean is the largest, followed in size by fossil carbon in geological reservoirs, and by the total amount of carbon contained in soils – see Figure 1.
The atmosphere contains nearly 800 billion metric tons of carbon (800 GtC), which is more carbon than all of the Earth’s living vegetation contains3. Carbon contained in the oceans, vegetation, and soils on the land surface is linked to the atmosphere through natural processes such as photosynthesis and respiration. In contrast, carbon in fossil fuels is linked to the atmosphere through the extraction and combustion of fossil fuels.
There is a fairly uniform concentration of CO2 in the atmosphere although it shows minor variations – about 1% – by season, due to photosynthesis and respiration, and by latitude.
When carbon dioxide is released from fossil fuel combustion it mixes into the atmospheric carbon pool where it undergoes exchanges with the ocean and land surface – especially soil – carbon pools. Fossil fuel combustion results in relatively little difference to the average concentration of CO2 in the global atmosphere; however, emissions of CO2 in any one region affect the concentration of CO2 globally.
Constantly on daily and seasonal time cycles, the oceans, vegetation and soils exchange carbon with the atmosphere. In contrast, carbon from fossil fuel is not exchanged with the atmosphere, but is transferred in a one way direction from geologic storage, at least within the time scale of human history. As more CO2 is added to the atmosphere, its heat trapping capacity becomes greater, explaining why the amount of carbon stored in the atmospheric pool is important.
Each storage pool including oceans, soils, and vegetation is considered a sink for carbon because each pool takes up carbon from the atmosphere. Again, each storage pool is also a source of carbon for the atmosphere, because of the constant exchange or flux between the atmosphere and the storage pools.
How much carbon is exchanged?
Figure 1 shows that more than 90 GtC is exchanged each year between the atmosphere and oceans, and close to 60 GtC is exchanged between the atmosphere and the land surface annually. Human activities – primarily land use change and fossil fuel combustion – contribute about 10 GtC to the atmosphere each year.
The average net flux – which is the amount of CO2 released to the atmosphere versus the amount taken up by the oceans, soils and vegetation – would be close to zero if the human contribution of CO2 was subtracted from the global carbon cycle. Many scientists reported that for 10,000 years prior to 1750, the net flux was less than 0.1 GtC per year when averaged over decades.
Soil surface-atmosphere flux
There is an indication by many scientists that the land surface – vegetation plus soil – accumulates more carbon per year than it emits to the atmosphere. This further indicates that the land surface acts as a net sink for CO2 at present.
Soil scientists and environmentalists are extremely interested in the key management measures for increasing the amount of carbon sequestered by soils, typically through land use changes such as agricultural and/or forestry practises. Soil carbon sequestration refers to the process of restoring depleted soil carbon through recommended land use and soil management.
Tropical deforestation and fossil fuel combustion are believed to be responsible for the largest share of CO2 released to the atmosphere from land use changes and anthropogenic activities. Tropical deforestation and other land use changes released approximately 1.6 GtC per year to the atmosphere in the 1990s, and may be contributing similar amounts of carbon to the atmosphere today.
Deforestation releases more carbon than is captured by forest re-growth within some regions, yet net forest re-growth in other regions takes up sufficient carbon so the land surface acts as a global net sink of approximately 1 GtC per year.
Manage the soil to influence the carbon cycle
The main idea here is to study how carbon travels between atmosphere and biota, making use of improved practises that will encourage soil carbon sequestration.
1. Scientific challenges in short term carbon cycle: • Understand the biogeochemical mechanisms determining the carbon exchanges between the land, oceans and atmosphere • How these exchanges respond to climate change through climate-ecosystem feedbacks, which may accentuate or dampen both regional and global climate change • What are possible interventions to manage these feedbacks?
2. Role of terrestrial ecosystems in the short term carbon cycle –By being a source or sink of atmospheric gases via: • Natural and anthropogenic disturbances • N enrichment by converting N2 into reactive N • S deposition Climate change and soil carbon: • Release of CO2 by warming induced decomposition • Increase in erosion Why emphasise soil carbon sequestration?
As was explained earlier, if 10% of the 120 Gt of C released from the atmosphere to the biota is either held back by the biota or sequestered in the soil, it could offset the contribution of the fossil fuel (10 Gt/yr) combustion to the atmospheric concentration of CO2.
Figure 2 below explains a scenario where improved management practises increased soil carbon sequestration and reduced the CO2 overburden problems in the atmosphere. Assuming a soil has 100% of its carbon pool sequestered in the soil before clearing, and then as long as the land is cleared, its carbon pool progressively decreases to about 50% in about 50 years.
In some African countries, the carbon may deplete up to that amount in ten years. When cassava and other arable crops are grown on the soil, a new equilibrium is established and after this has been attained, carbon concentration will be stabilised, although it may start decreasing again because of erosion.
If a government adopts policies that will improve agronomic management systems, carbon concentration will increase again, and after about 20 years a new equilibrium called ‘Attainable Potential’ will be reached. This brings about reclamation of about 66% of what was lost.
Best management practises such as better nutrient management or better erosion management take us to the ‘Maximum Potential’. The maximum potential will equal what the original level was. There are soils in Nigeria having natural limitations such as excess water, toxicity, acidity, excess stone or phosphorous management. Overcoming these limitations can take us beyond the potential level.
Some scientists believe that the slope (∂y/∂x) of the ‘Attainable Potential’, which shows reclamation of about 66% of what has been lost, is the rate of carbon sequestration.
Negative soil carbon budget
Some practises which may create negative soil carbon budget are listed below: • Deforestation • Biomass burning • Accelerated erosion • Residual removal Positive soil carbon budget
Some practises which may create negative soil carbon budget are: • No till (NT) • Cover crop • Alley cropping • Mixed cropping
Rate of carbon sequestration
Figure 3 shows the rate of carbon sequestration in different land uses. It can be seen that the rate is highest in wetlands. In Nigeria, most wetlands are used for rice production and government initiatives called Fadama water projects are usually carried out in these areas.
Management and restoration of many of these wetlands will encourage carbon sequestration in the soils. Many scientists earlier reported that an increase in soil moisture content increases carbon sequestration.
The Conservation Reserve Program in the United States helps in sequestering about 1,000 kg of C per hectare per year. Turf which is found in areas such as lawns or golf courses has potential for sequestering carbon. Practises such as no till farming and also expanses of rangeland can sequester carbon.
Issues with ‘no till’ farming
There is confirmed evidence that nature not only gives good things, but bad things are also associated with no till farming practise. They are outlined below.
1. Constraints of adopting no till farming in developing countries There is unavailability of herbicides and pesticides. Not only are the herbicides unavailable but the majority of the farmers are resource poor and can not afford the chemicals when they are available for systemic or contact manoeuver of existing vegetation to be cleared before planting. For example, one widely used herbicide that is used in most farms in developing countries comes from England and it’s not affordable for resource poor farmers.
2. Effectiveness of no till in C sequestration and Soil Organic Matter (SOM) • Increase in surface and decrease in sub-soil C • Hidden C costs of chemicals
3. Cellulosic ethanol from crop residues in industrialised countries One of the issues related to carbon study by many soil scientists is that most of the samples for analysis are taken in the upper centimetres. Professor Rattan Lal of Ohio State University in the United States carried out research on ten different soils in Ohio, and seven of the soils followed the pattern shown in Figure 4.
During the Soil Science Society of Nigeria Conference held in Nigeria in March, 2013, Lal reported that the study of soil carbon should be done deeper in the profile. He observed that with the no till farming practise, more carbon is sequestered in the surface (0-20 cm depth) but not in the sub-surface.
A look at the Lal’s figure below shows that more carbon is sequestered in the subsurface soil when the soil is ploughed. Lal’s view is that soil scientists should encourage the study of soil carbon deep down in the profile because of changes in trends.
Temptations for the policy makers
Most policy makers are tempted when they become aware of the approximate fuel value of one Mg of crop residue. The values are shown above.
It is of essential importance that these policy makers become aware of our target to sequester 10% of the 120 Gt of C released from the atmosphere to the biota. We cannot achieve this if we continue to remove crop residues from farms because these residues carry in them some amounts of the carbon required to balance the global carbon budget.
Published: 31st May 2013 in AWE International
