About 50% of the carbon produced by photosynthesis of the green plants is returned to the atmosphere as CO2 through plant respiration. The remaining 50% is the carbon assimilated and incorporated in leaves, stems, roots, and other parts of plants which is deposited on or within the soil. There, the organic substances are ingested and metabolized by a very diverse biotic community primarily by bacteria and fungi followed by an array of mesofauna and macrofauna. The ultimate product of the organic matter decay in the soil is a complex of relatively stable substances called humus. Humus in general accounts for 60 to 80% of total organic matter present in the soils. The remaining portion consists of recent organic debris of partially decomposed litter, dead roots, and the waste products of soil fauna.
From the beginning of the Industrial Revolution in the late 1800s, the agricultural expansion, clearing of forests, and particularly the burning of fossil fuels led to significant increase in the CO2 content in the atmosphere, from about 270 ppm to more than 380 ppm. Concurrently, there has been an increase in the content of other so called greenhouse gases, such as methane CH4 and nitrous oxide NO. The impact, so far, appears to be a rise in the average global temperature of more than 0.6 ⁰C. This warming trend is expected to increase significantly in the coming decades unless decisive measures are taken to control it.
Carbon Exchange in the Terrestrial System
The soils of the globe with the biota they support are the major organic carbon absorber, depositories, and releasers. Soils contain approximately a total of 1,700- Gt (billion metric tons) of carbon to a depth of 1-m and as much as 2,400 -Gt to a depth of 2-m. Soil biota (plants and animals) contain an estimated additional 560-Gt. This is compared to 750- Gt of carbon in the atmosphere. Therefore, the amount of organic carbon in the soils is more than four times that in the terrestrial biota and more than three times that in the atmosphere.
The amount of organic carbon in soils is variable depending on the balance between the inputs and outputs. The inputs are due to CO2 absorption from the atmosphere by photosynthesis and its incorporation into soils by the residue of plants and animals. The outputs are due to the decomposition of soil organic compounds which releases greenhouse gases, CO2 under aerobic conditions, CH4 under anaerobic conditions, and nitrous oxide NO under certain conditions of organic matter decay. NO is considered another strong greenhouse gas.
The soil- organic matter content in most cases is less than 5% by mass of the soil material and is generally concentrated in the upper 20-40-cm of topsoil. However, that content varies widely from less than 1% by mass in some Aridisols to 50% or more in waterlogged organic soils such as Histosols. In addition to soil content of organic carbon, some soils of arid and semiarid regions also contain large quantities of inorganic carbon in the forms of Ca and Mg carbonates. These carbon reserves are estimated to total 695-748-Gt, and not as large as the organic carbon. Soil inorganic carbon tends to dissolve in acidic soil solutions and is subject to leaching accompanied by CO2 release into the atmosphere.
Table 1. Estimated mass of carbon in soils of the world. (Source: USDA, after Hillel and Rosenzweig, 2009).
Soil order Area Organic C
103 km2 Gt
Alfisols 13,15 9 90.8
Andisols 975 29.8
Aridisols 15,464 54.1
Entisols 23,432 232.0
Gelisols 11,869 237.5
Histosols 1,526 312.1
Inceptisols 19,854 323.6Mollisols 9,161 120.0
Oxisols 9,811 99.1
Spodosols 4,596 67.1
Ultisols 10,550 98.1
Vertisols 3,160 18.3
Other soils 7,110 17.1
TOTALS 130,667 1,699.6
Soils with a high organic matter content, known as organic soils, form where prolonged saturation with water causes O2 deficiency which in turn inhibits oxidation and promotes the accumulation of partially decomposed organic matter known as peat. Such waterlogged areas are variously known as bogs, fens, marshes, swamps, or more commonly called wetlands. These soils emit carbon as CH4 gas but at a much lower rate than would be the emission rate of CO2 gas if the soils were well aerated. When cultivated for agricultural use, such soils are generally drained, and the consequent aeration accelerates oxidation of peat and spurs CO2 emission. Cultivated peat soils may lose as much as 20-Mg (mega gram) carbon/ha/yr in tropical and subtropical climates and about half that in temperate climates. These soils tend to shrink and subside unevenly and even can catch fire and burn beyond control.
Of special concern are the Gelisols of the permafrost wetlands of cold regions which are abundant in Siberia, Alaska, and parts of Canada. They contain huge amounts of undecomposed organic matter. When large areas of peat-rich permafrost are subjected to warming, they tend to thaw out and emit CH4 gas while still saturated with water. Later, when such soils are drained of excess water and aerated, aerobic decomposition will dominate and the peat will release CO2. In a warming climate, the enhanced emission of greenhouse gases from thawing permafrost is an example of a positive feedback. And the global warming due to anthropogenic greenhouse gas emissions may cause the secondary release of still greater greenhouse gases from drained peatlands and thus further exacerbate global warming.
Aside from the peatlands of cold regions, nearly 10% of global peatlands exist in the tropical lowlands and contain an estimated 70-Pg (peta gram) of carbon in the deposits as deep as 20-m. Tropical peatlands are abundant in Brunei, Indonesia, Malaysia, and Thailand as well as in parts of the Amazon Basin. Some of these deposits appear to have been destabilized by soil drainage as well as by the existence of more intense drought associated with El Niño periods. Such dry conditions may result in the spontaneous burning of peat and vegetation that may lead to rapid emissions of large quantities of CO2. When more tropical swamp forests and peat- lands are drained for agricultural use, they are likely to contribute still greater CO2 emissions to the atmosphere, particularly if El Niño events become more intense or frequent in a warming climate.
Aridisols and Histosols are two soil orders likely to be strongly affected by climate change. Histosols are organic soils with large concentrations of peat. As they tend to dry out in a warmer and drier climate, enhanced oxidation could result in accelerated release of large quantities of CO2 to the atmosphere. Aridisols cover roughly 12% of total land surface. They are especially vulnerable to soil erosion, salination, and desertification. Higher temperatures are expected to increase the intensity of evaporation and cause seasonal water shortages.
Climate change is likely to cause soil erosion via its impact on rainfall amount and intensity, vegetative cover, and patterns of land use. Wetter conditions may exacerbate the hazard of water erosion, while drier conditions may intensify wind erosion of soils. Desertification can occur when the climate becomes drier and /or the vegetative cover of an area becomes so scattered that the denuded landscape resembles a desert.
Human Factor in Soil ManagementThe soil carbon balance is greatly affected by human management, including the clearing or restoration of natural vegetation and the patterns of land use in pastoral, agricultural, industrial, and urban areas. Cultivation promotes the microbial decomposition of soil organic matter while depriving it of replenishment, especially if the cropping program involves removal of plant matter and if the soil is fallowed for considerable time periods. Organic carbon is usually lost from soils both by oxidation and by topsoil erosion. Some soils with cultivation may, over time, lose as much as one-third to two-thirds of their original organic matter content. Consequently, these soils deteriorate in quality, as their fertility declines and their structures are destabilized. Thus, such soils are important targets for mitigating the greenhouse effect by reducing and even reversing their tendency to emit greenhouse gases.
Although agricultural soils acted in the past as significant sources of atmospheric CO2 enrichment, their current carbon deficits offer an opportunity to absorb large quantities of CO2 from the atmosphere and store it as added organic matter to the soils in the coming decades. The historical loss of carbon from the agricultural soils of the globe has been estimated to total 42 to 78 –Gt. Ideally, we hope for complete restoration of that loss, i.e. a return of soils to carbon saturation of pre-agricultural state. The way of restoring soil organic matter is by minimizing soil disturbance while optimizing nutrient and water supply to maximize plant growth and retention of their residues in the soil.
In fact, soil degradation resulting from burning of vegetative cover, erosion, leaching, tillage, compaction, pollution, salination, and/or other processes reduces the capacity of soils to fully recover their original state. Even when such restoration is possible it may not be economically justifiable. The actual carbon-sink capacity of many soils (i.e. the potential restoration of their carbon content in practice) assuming full adoption of recommended strategies of soil management, may be on the order of one –half to two-thirds of the historical loss. Still, that quantity can be very significant. Only under special conditions (e.g., irrigating and intensely fertilizing high-residue vegetation or anaerobically charring organic matter so that it is highly resistant to decay and then applying it to the soil) might the organic carbon of the soil be raised above the original “virgin” levels.
The potential of soils to sequester carbon is intimately associated with the content and type of clay fraction. Sandy soils, which tend to be well aerated and have little adsorptive capacity, generally are low in organic matter content. Clayey soils, on the other hand, form relatively strong physicochemical bonds between the active surfaces of clay particles and the organic macromolecules of humus, thus humus becomes resistant to further decay. Moreover, clayey soils form water-resistant aggregates, the interiors of which restrict aeration and tend to further resist the decay of the occluded organic matter. When soil aggregates are disrupted by mechanical tillage, soil structure deteriorates and soil organic matter decomposes more rapidly.
The combined losses from the earth’s native biomass and soils caused by deforestation and cultivation during the past 300 years have been estimated to total 170-Gt of carbon, much of which has been absorbed in the oceans and some of which has accumulated in the atmosphere. In the tropics continuing land clearing for agriculture apparently results in additional emissions of CO2 into the atmosphere on the order of 1.6-Gt of carbon annually.
Taking a positive view, we may surmise that agricultural soils present a significant avenue for greenhouse gas mitigation through reduction of emissions as well as through enhancement of carbon sequestration. This can be achieved by improving the efficiency of agricultural operations (minimizing fuel- burning operations) and by promoting increased absorption of CO2 by green plants and its stable storage in the soil. The potential sequestration of carbon in global agricultural soils through improvements of management practices has been estimated to total between 600 and 900-Mt (million metric tons) per year over a period of several decades.
The recommended management practices include reforestation, agroforestry, no-till farming, planting of cover crops, and addition of soil nutrients (by fertilizers, manure, composts, and sludge). And application of soil amendments (lime to neutralize soil acidity), improved grazing, water conservation, and production of energy crops to replace fossil fuels. If used efficiently and consistently on a large scale, such practices can help to mitigate the greenhouse effect, reduce soil erosion, improve soil structure and water quality, enhance biodiversity, boost crop yields, and promote food security.
A necessary caveat is that climate, soil, and economic conditions vary widely from one location to another and from one period to another. Thus, there are no simple universal prescriptions regarding practices to manage soils to help mitigate the greenhouse effect. While the basic principles can be expressed in universal terms, their application to different sites will require certain adjustments. Overtime, practices designed to sequester carbon in soils are likely to diminish in efficacy, as the soil in each location arrives at a state of equilibrium or as its organic carbon contents attains effective saturation. In fact, there is even danger that the gains of soil carbon achieved over time (years or decades) with conservation practices may be reversed by returning even temporarily to inappropriate tillage methods or by outbreaks of fire.
However, other benefits of carbon conservation practices such as reduced energy use and the production of renewable energy (e. g., biofuels) as substitute sources for fossil fuels can continue. The important principle is that any improvement in management of soil organic matter is a worthy task in itself beyond its potential benefits in mitigating the atmospheric greenhouse effect. It can not only turn the soil from a net source to a net sink for greenhouse gases but can also boost soil productivity and reduce environmental damage caused by soil erosion.
Different feedback mechanisms are operative in the interactions between climate change and the carbon cycle. Increasing concentrations of CO2 in the atmosphere can promote greater photosynthetic rates, an effect known as CO2 fertilization. In principle, a portion of the extra photosynthetic product (plant biomass) is transferred to the soil via surface litter and the root system, and a fraction of that is stabilized as soil humus. Moreover, rising temperature tend to enhance plant growth and prolong the growing season in regions where growth is normally inhibited by cold climate. Such processes tend to moderate the effect of greenhouse gases.
On the other hand, rising temperatures may exceed optimal levels for some plants in some regions, thus restricting carbon assimilation, and also speed up decomposition of organic matter and the emission of CO2 as well as, perhaps, methane CH4 and nitrous oxide NO thus tending to exacerbate the greenhouse effect. Rising temperatures may also result in insect infestations and fungal diseases of crops. Whether positive feedbacks are likely to overtake the negative feedbacks or vice versa will depend on site-specific conditions as well as on human intervention and management of the ecosystem. Anyhow, the change in the soil temperature regime, which generally entails a change in the soil moisture regime, is certain to affect soil organic matter content and the rate of its turnover.
Agricultural Practices Affecting Soil Organic Matter ContentDepletion of soil organic matter initiates a vicious cycle of degradation, affecting food security and environmental quality, often on a regional level. Reversing that depletion via carbon sequestration can create a benign cycle of productivity gain. Enrichment of the top soil with organic matter makes it less subject to compaction, crust formation, and erosion, which in turn improves the quality of the environment downstream. It also improves soil conditions in terms of infiltration, aeration, seed germination, and plant nutrition. The agriculture sector can play a role in the mitigation of global warming in three major ways:
1. Reducing emissions of greenhouse gases by adopting such practices as no-till planting;
2. Enhanced CO2 absorption from the atmosphere by green plants through photosynthesis and storing a good portion of the carbon in the soil; and
3. Producing renewable sources of energy as biofuels from agriculturally gown biomass that can be converted to ethanol and biodiesel.
Conventional tillage is defined as the mechanical manipulation (pulverization, mixing, and inversion) of the plow layer that leaves ≤15% of the land surface covered with crop residues. Such tillage tends to disrupt soil structure, accelerate the decomposition of soil organic matter, and subject the bared topsoil to erosion by rain and wind. In contrast, no-till practice is defined as the avoidance of all unnecessary mechanical manipulation of the topsoil to leave it largely undisturbed and covered with surface residues throughout the time period from harvesting of the prior crop to the planting and establishment of the new crop. Such vegetative residues act as a protective mulch, which shields the soil against the direct impact of raindrops during rainy season and as well as against extreme desiccation and deflation by wind during dry periods.
The best agricultural practices are those that result in increased soil carbon and enhanced productivity due to improved soil structure and soil moisture conservation. These practices include timely and precise applications of fertilizers, use of slow-release fertilizers( to minimize leaching and volatilization), erosion prevention, shortening or elimination of fallow periods, use of high-residue cover crops, and minimizing mechanical disturbance of the soil. Such practices can protect and even restore the organic carbon content of the soil. Conversion to no-till farming has resulted in boosting carbon storage in soils at rates from 0.1 to 0.7 Mg carbon/ ha/yr. However, such positive values cannot be expected to continue indefinitely as any historically depleted soil tends to approach its prior equilibrium (C saturation) state within few decades.
The classical purpose of tillage is the eradication of weeds, the contrary practice of no-till farming may lead to greater infestation of weeds hence require increased use of herbicides. The synthesis, transport, and application of herbicides require greater consumption of fuel and results in additional emissions of greenhouse gases. Where the soils have been badly degraded in the past and their agricultural productivity severely diminished, they may be turned to perennial grassland or forestation so as to become good carbon sinks.
What is needed altogether now a new strategy for greenhouse gas-efficient farming and land management based on lowered energy consumption, greater reliance on renewable energy rather than fossil fuels, and increased carbon storage in soils. Of special importance is adoption of conservation tillage and no-tillage, which not only conserve energy but also increase soil productivity. That in turn can reduce pressure on marginal lands, stop deforestation, and maintain ecosystem function and biodiversity.
However, there are necessary caveats to consider. Some of the practices aimed at intensifying agricultural production involve greater use of energy. Such practices include irrigation, fertilization, pest and weed control, and transportation. Certain benefits of conservation farming may shrink with time. The potential for sequestration of soil organic carbon is generally finite. Soil organic carbon saturation where absorption and emission processes are in dynamic equilibrium may be achieved in several decades. Higher temperatures caused by global warming may accelerate decomposition of organic matter and inhibit carbon sequestration. The carbon balance in the soil is in a state of labile and is vulnerable to turn negative (i.e., from net absorption to net emission of atmospheric CO2) if the carbon-augmenting management is not maintained or if it is interrupted by the presence of some perturbation such as drought, flood, or fire.
Certain benefits of conservation management can continue indefinitely. Reduction of fuel use brought about by efficient farm practices, especially with adoption of zero tillage, can persist as long as that form of conservation of soil management is maintained. The same is true with the soil quality improvement, including soil fertility elevation and soil erosion control. The efficient and sustainable production of energy crops to substitute for fossil fuels can also be a continuing benefit although careful accounting is required to ensure that the energy equation of such production is indeed positive ( i.e., that the energy produced is greater than that invested in farming operations and transportation).
Good policies are needed to promote and guide carbon-efficient practices. Methods to reward carbon sequestration, however, must be based on an effective system of monitoring the results continuously since the gains achieved painfully by such practices as conservation tillage, cover crops, and residue retention can be lost rapidly by reversion to traditional tillage, residue removal or burning, and fallowing. Research is essential to develop suitable methods of monitoring by sampling or, preferably, by remote sensing. In modern precision agriculture, recognizing the soil heterogeneity in the field, fertilizers applications at well calibrated rates so as to maximize nutrient use efficiency and minimize nutrient losses, which may cause environmental pollution such as eutrophication of fresh water. Increased reliance on green manure plants such as legumes and their associated nitrogen- fixing bacteria can be very helpful practice. Moreover, the mode of soil moisture management in irrigated as well as dryland farming can significantly influence greenhouse gas absorption and emissions.
Hillel, D. and C.Rosenzweig. 2009. Soil and carbon climate change: Carbon exchange in the terrestrial domain and the role of agriculture. CSA News, Amer. Soc. Agron., Crop.Sci.Soc.Amer. and Soil Sci.Soc.Amer. 54: 4-11.
By: Mohammed Sa’id Berigari, Ph.D., Senior Soil and Environmental Scientist-USA