Soil Carbon Sequestration through Desertification Control

Desertification, a natural process, is exacerbated by anthropogenic activities. It reduces soil productivity, jeopardizes food security, impairs environment quality, accelerates global warming and exacerbates global security risks. Degradation of soil and vegetation aggravates the emission of greenhouse gases into the atmosphere. The biophysical processes of soil and vegetation degradation are closely interlinked by social, economic and political factors which govern land ethics and are prone to the tragedy of the commons. The annual rate of global emissions has increased from 1.3%/yr in the 1990s to 3.3%/yr for 2000-2006, with dire consequences to global warming and environment quality. While global emissions must be reduced by 2030 through a range of options, by 100 billion tonnes (Pg) to stabilize atmospheric abundance at 650 ppm of CO2 eq., and by 250 Pg at 490-540 ppm of CO2 eq., there exists an opportunity to utilize C sink capacity of the degraded and desertified terrestrial biosphere. Desertification control and restoration of degraded soils and ecosystems has a potential to sequester 0.9-1.9 Pg C/yr. However, conversion to a restorative land use and adoption of recommended management practices remain a challenge for the resource-poor farmers and small land holders of the regions prone to desertification. Thus, creating another income stream through the trading of C credits may provide the much needed financial support to invest in land restoration. Introduction Anthropogenic emissions between 1850 and 2006 are estimated at ~ 330 Pg C due to fossil fuel combustion and cement manufacture, and an additional 158 Pg from land use change and soil cultivation. Furthermore, the emissions due to fossil fuel combustion have increased from 7.0 Pg/yr in 2000 to 8.4 Pg/yr in 2006 (Canadell et al., 2007). The average growth rate of emissions due to fossil fuel combustion and cement manufacture have increased from 1.3%/yr in the 1990s to 3.3%/yr for 2000-2006. Consequently, the atmospheric concentration of CO2 in 2006 of 381 ppm is the highest since several million years (Canadell et al., 2007). Total anthropogenic emissions are estimated at 7.0 Pg C/yr for 1970-1999, 8.0 Pg C/yr for 1990-1999, and 9.1 Pg C/yr for 2000-2006. For the same periods, the atmosphere has absorbed 3.1 Pg/yr, 3.2 Pg/yr and 4.1 Pg/yr, respectively. The capacity of the natural sinks (e.g., land, ocean) was 56.3% for 1970-1999, 60.0% for 1990-1999 and 54.9% for 2000-2006 (Table 1). The capacity of land sink alone for the same period was 28.1%, 27.2% and 24.2%, respectively. Thus, the capacity of land sink has progressively declined, probably due to an increase in the extent and severity of desertification and degradation of world soils and ecosystems. Table 1. Recent trends in the global carbon budget (Recalculated from Canadell et al., 2007). Parameter 1970-1999 1990-1999 2000-2006 I. Sources (Pg C/yr) Fossil fuel combustion 5.6 6.5 7.6 Land use change 1.5 1.6 1.5 Total 7.1 8.1 9.1 II. Sinks (Pg C/yr) Atmosphere 3.1 3.2 4.1 Ocean 2.0 2.2 2.2 Land 2.0 2.7 2.8 Total 7.1 8.1 9.1 Capacity of All Natural Sink (%) 56.3 60.0 54.9 Capacity of Land Sink (%) 28.2 27.2 24.2 Desertification and the degradation of soil and vegetation in drylands impacts about 1 billion people worldwide in more than 100 countries (Doolittle, 1997). It leads to a decline in soil and environment quality and perpetuates the food deficit (Lu, 2001). Of the 6.31 billion hectares (Bha) of the world’s dryland area, 5.17 Bha or 69% is presumably desertified to some degree through degradation of either vegetation, soil or both (UNEP, 1991; 1992). Of the desertified land area, 1.016 Bha is desertified cropland and rangeland and 2.57 Bha of degraded vegetation in the rangeland. In comparison, Oldeman and Van Lynden (1998) estimated that the area affected by desertification is 1.137 Bha. Of this, 0.489 Bha is severely/extremely desertified. Similar to the estimates of land area affected by desertification, those of the rate of desertification also vary widely. The annual rate of desertification is estimated at 5.83 million hectare (Mha) or 0.132%/yr of the world’s drylands (UNEP, 1991; Mainguet, 1991). Even if these statistics are nearly correct, it is hard to understand the reasons for a nearly lack of concrete action by the global community. Some consider that the process of desertification is set in motion by the “tragedy of the global commons” (Hardin, 1968). It is the overgrazing of the common rangelands and indiscriminate deforestation of the common woodlots which lead to the mining of soil fertility, depletion of the soil carbon pool, and over-exploitation of the groundwater leading to denudation of the vegetation cover, acceleration of soil erosion by water and wind, and increase in the frequency and intensity of dust storms. As Aristotle said, “What is common to the greatest number gets the least amount of care. Men pay most attention to what is their own: they care less of what is common”. Indeed, desertification is a biophysical process of aridization of a region, but it is driven by a strong interaction between natural resources (e.g., soil, vegetation, water, climate) with socio-economic and political factors. The principal cause of desertification, land misuse and soil mismanagement leads to a progressive decline Soils, Society & Global Change 113 in soil quality, a breakdown of structural properties, an increase in susceptibility to crusting and compaction, an increase in losses of water by surface run-off and evaporation, acceleration of soil erosion by water and wind, depletion of plant nutrients and soil organic matter (SOM), and the overall decrease in the net primary productivity (NPP). One of the consequences of the process of desertification is the loss of soil carbon (C) pool, some of which is emitted to the atmosphere as carbon dioxide (CO2) (Lal, 2001; 2003). The soil C pool, comprising soil organic C (SOC) and soil inorganic C (SIC), is highly dynamic and can be a source of atmospheric CO2 under the conditions of inappropriate land use and soil mismanagement (Coutput > Cinput). Therefore, desertified soils have a much lower SOC pool than the undegraded soils under the protective cover of the natural vegetation. Thus, restoration of degraded/desertified soils can reverse the process and lead to an increase in the soil C pool along with replenishment of soil fertility and an increase in activity and species diversity of soil fauna and flora (Lal, 2001). Comparing the 1990s with 2000-2006, the emission growth rate of CO2 has increased from 1.3% to 3.3%/yr because of the growth in the world economy and its C intensity (Canadell et al., 2007). There is also a long-term increase in the airborne fraction of the CO2 emissions, an indication of the decline in the efficiency of the natural CO2 sink of land and ocean (see Table 1). With the current and future threat of global warming, there is a strong interest in developing cost-effective and efficient strategies of sequestering atmospheric CO2 with ancillary benefits. The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) has estimated that the cumulative emission reduction needed between 2000 and 2030 is 100 Pg to stabilize the atmospheric abundance of CO2 at 650 ppm of CO2-eq, and 250 Pg to stabilize at 490-540 ppm CO2-eq (Bohannon, 2007). The strategies identified to reduce these emissions include the following: cuts in greenhouse gases other than CO2, emission capture and storage, energy conservation and efficiency, renewable energy including nuclear and biofuels, and C sequestration in terrestrial sinks including the terrestrial biosphere. Therefore, the objective of this paper is to describe factors affecting desertification and assess the potential of desertification control and restoring degraded soils and ecosystems to sequester C and mitigate global warming. 1. Global Warming and Desertification There is a close link between global warming and desertification. The process of desertification is likely to be exacerbated by the current and projected global warming. The historical evolution of human awareness about the impact of the atmosphere on the earth’s climate has been vividly explained by Weart (2003), and is briefly summarized here. The process of global warming has been recognized since 1783, when a volcanic fissure in Iceland erupted, pouring out several km of lava in the atmosphere. A peculiar haze dimmed the sunlight over Western Europe for months. During his visit to Europe in 1783, Benjamin Franklin noticed the unusual cold that summer and speculated that it might have been caused by the volcanic fog (Weart, 2003). The energy budget of Earth’s atmosphere was first estimated by Joseph Fourier in the 1830s. He argued that the sun’s radiation is balanced by the invisible infrared radiation emitted by the earth, and that the earth’s atmosphere somehow keeps some of the radiation in. John Tyndall (1862) observed that some of the trace gases in the atmosphere (CO2, CH4, H2O vapour) were opaque to infrared radiation while O2 and N2 were not (Weart, 2003). Tyndall argued that these trace gases in the atmosphere have the same effect as does the glass in the greenhouse, and thus named these as “greenhouse gases”. John Tyndall argued that the atmosphere acts as a barrier to outgoing radiation and raises temperature at the Earth’s surface (Tyndall,1863). Swante Arrhenius (1896) and his colleague Arvid Högbom (1897) estimated that doubling the CO2 concentration in the atmosphere would raise the earth’s temperature by 5-6 °C (Weart, 2003). It was Högbom who estimated that the temperature in the Arctic regions would rise about 8-9°C if the carbonic acid increased 2.5-3 times. Alfred Wallace, in his book Man’s Place in the Universe made two important observations: (i) the atmosphere (areal ocean) allows sun rays to pass but its constituents (water vapour and carbonic acid) intercept and absorb the sun’s radiation; and (2) an increase in Earth’s temperature may cause some extremely violent and intense storms to uproot even the largest of trees (Wallace,1903). The second observation is extremely relevant to Hurricane Katrina that hit New Orleans, USA in 2005 and Cyclone Sidr that hit Bangladesh in 2007. In his lecture to the Royal Metropolitan Society, Guy Stewart Callender (1938) hypothesized that carbon dioxide produced by industry is responsible for global warming. Time Magazine carried a lead article entitled the “Warmer World” in its 2 January 1939 issue which stated that “Gaffers who claim that winters were harder when they were boys were quite right” (Time, 1939: 27). The theme reappeared in the 26 March 2006 issue of Time Magazine with a lead article entitled “The Tipping Point” which stated that “Polar ice caps are melting faster than ever...More and more land is being devastated by drought...Rising waters are drowning low lying communities...By any measure, Earth is at the Tipping Point” (Kruger, 2006). The dynamic nature of Earth’s climate has influenced the evolution of human and other biota on earth (Linden, 2006). Climate change can change civilization and human history (Fagan, 2004). While the greenhouse effect is a natural process, essential to life on earth, it is the acceleration of the natural process by anthropogenic activities that leads to the so-called ‘global warming’. The rate of increase in temperature by global warming is >0.1 °C/decade, so that the ecosystems cannot adjust to this rapid change. For each 1° C increase in global temperatures, the vegetation zones may move pole-ward by 200-300 km. The global warming observed since the middle of the 20 century, estimated at an increase of earth’s mean temperature by 0.6 ± 0.2 °C and a sea level rise of about 18 cm, is attributed to an increase in atmospheric abundance of trace gases (CO2, CH4, N2O) since the Industrial Revolution occurred around 1750 (IPCC, 2001; 2007). While the principal source of anthropogenic emission is fossil fuel combustion, the impact of land use and land use change and of soil cultivation on the emission of trace gases in the atmosphere cannot be overemphasized. Ruddiman (2003; 2005) argues that a trend of increase in atmospheric concentration of CO2 began 8000 years ago, and that in the case of CH4 emission 5000 years ago, corresponding with the dawn of settled agriculture with attendant deforestation, soil cultivation and the spread of rice paddies and raising of cattle. Ruddiman (2003) estimates the pre-industrial emissions at 320 Pg primarily from terrestrial sources (biota and soil) caused by deforestation, land use change and soil cultivation. By comparison, emissions from land use change are estimated at 136 Pg between 1850 and 2000 (IPCC, 2000), and 158 Pg between 1850 and 2006 (Canadell et al., 2007). In contrast, emissions from world soils 114 Climate Change and Restoration of Degraded Land Chemical Sequestration 1. Chemical scrubbing 2. Mineralization Biotic Sequestration 1. Afforestation, Reforestation 2. Biofuel plantations Pedologic Sequestration A. Organic Carbon i. Agricultural soils ii. Degraded soils iii. Wetlands B. Secondary carbonates i. Biochar CO2 Sequestration to Reduce Atmospheric Abundance Oceanic Sequestration 1. Deep injection 2. Fe-Fertilization 3. Transfer and burial of biomass deep into the ocean Geologic Sequestration 1. Old oil wells 2. Unmineable coal mines 3. Saline aquifers 4. Stable and porous rock strata Figure 1. Sequestering atmospheric CO2 into long-lived Carbon pools (sinks). caused by ploughing and tillage are estimated at 78 ± 12 Pg (Lal, 1999). It is the depletion of the SOC pool that has created the C sink capacity in world soils because most cultivated/agricultural soils contain much less SOC pool than their potential capacity. The magnitude of depletion of SOC pool from agricultural soil is exacerbated by soil degradative processes (e.g., erosion, salinization, physical and chemical degradation). Furthermore, depletion of the SOC pool leads to declines in soil quality and the ecosystem services it provides (Lal, 2004). 2. Carbon Sequestration in World Soils and Terrestrial Ecosystems Carbon sequestration involves the capture and secure storage of atmospheric C that would otherwise be emitted or remain in the atmosphere. Carbon sequestration is important because: (i) as of yet, there are no non-carbon fuel sources as viable alternatives to fossil fuels; (ii) there is a need to stabilize atmospheric abundance of CO2; (iii) it is necessary to restore ecosystem services of degraded soils/ecosystems; and (iv) there is an urgency to advance/achieve global food security. It is in this regard that identification of an appropriate C sink (e.g., geologic, oceanic or terrestrial) is important. Choice of an appropriate sink and of a C sequestration strategy depends on numerous characteristics including the following: (i) high sink capacity; (ii) long residence time or stability of the sink; (iii) low cost of C sequestration; (iv) either a positive or minimal negative environmental impact; and (v) numerous ancillary benefits. Figure 1 outlines numerous potential C sinks and strategies to transfer atmospheric CO2 into these sinks. There are pros and cons for each strategy (Socolow, 2005; Lal, 2008). Geologic sequestration involves the capture, purification, compression and transport of industrial CO2 into stable geologic strata 2000-3000 m below the soil surface. Traditionally, injection of liquefied CO2 into old oil wells is recommended to enhance oil recovery (EOR) and old coal seams to enhance coal bed methane (CBM). Geologic sequestration also involves the injection of liquefied industrial CO2 into stable rock strata and in saline aquifers where it may be converted into stable minerals. The chemical sequestration involves chemical scrubbing of industrial CO2 and its mineralization or conversion into carbonates of calcium (Ca) and other metals. Similar to geologic sequestration, oceanic sequestration involves the injection of industrial CO2 into the ocean several metres below the surface (Lal, 2008). In addition to injection, oceanic sequestration also involves the use of iron (Fe) fertilization to Soils, Society & Global Change 115