A high proportion of natural ecosystems has already been converted to human-dominated use, such as cropland. There is a range of estimates of the amount of land under agricultural use. The Millennium Ecosystem Assessment found that 24% of the Earth’s land surface was under ‘cultivated systems’ (Millennium Ecosystem Assessment 2005), but Foley et al. (2005) report that 40% of the land surface was under cropland and pasture, an area similar to that covered by forest. The following section considers the potential for managing carbon in temperate and tropical agriculture and in plantation forestry.
There is a good understanding of the best ways of storing carbon in agricultural systems and practices to increase storage can be implemented now. To accelerate this, incentives to promote carbon sequestration in cropland could be considered, but would need to be carefully monitored and include life-cycle level analysis when assessing the real carbon cost of various practices. At the local scale there could be incentives for carbon storing agricultural practices and education regarding the best land management strategies to increase carbon storage.
Agricultural systems in the temperate zone tend to occupy fertile soils that would have formerly supported temperate grassland or forest. Land clearance for croplands and pasture has greatly reduced above ground carbon stocks from their original state and soil carbon stocks are also often depleted as tillage disrupts the soil, opening it to decomposer organisms and generating aerobic conditions that stimulate respiration and release of carbon dioxide. There is large potential for increased carbon storage in such systems. For example, recent estimates indicate that the full application of straw return to Chinese croplands could sequester around 5% of the carbon dioxide emission from fossil fuel combustion in China in 1990 (Lu et al. 2008).
Carbon losses in agricultural systems can be reduced in many ways, such as through conservation tillage, crop rotation, adoption of appropriate cropping systems, integrated nutrient management using compost and manure, mulching, integrated weed and pest management, and improved grazing (Lal 2008). Optimum management, that is management which best conserves carbon while sustaining food production, will depend on the specific characteristics of the agricultural system in question. Land management policy may therefore be best deployed at a local level. What is clear is that increased stocks of carbon in agricultural systems can represent a win-win situation as high levels of soil organic carbon improve nutrient and water use efficiency, reduce nutrient loss and subsequently increase crop production. Better infiltration and water retention in high organic carbon soils also increases water infiltration, reduces runoff and erosion and helps to avoid drought damage, thus contributing to the sustainability of food production.
Another option is to increase food production on some existing agricultural lands through highly targeted fertilizer and pesticide use, so-called ‘precision agriculture,’ while leaving other areas to return to natural vegetation. Cropland area in the developed world is already declining and may continue to decline in the future (Balmford et al. 2005), potentially freeing up land area that may be used to sequester carbon. Recent evidence shows that carbon gains have occurred in agricultural land abandoned after the collapse of the Soviet Union (soil gains of 0.47 t C per hectare per year, Vuichard et al. 2009). This is also known to be true of abandoned lands in Europe and North America as it is in the early stages of succession and forest development that carbon sink strength is strongest.
Biochar: A Panacea?
Biochar is a new and poorly understood technology and it is likely that its effectiveness as a carbon storing strategy will depend heavily upon economic and environmental factors. Research is still at a preliminary stage and large-scale biochar deployment is inadvisable until these uncertainties are resolved.
Biochar is an emerging technology in which organic materials are reduced by pyrolosis at temperatures of 350–500ºC, producing energy and a carbon rich charcoal that is returned to the soil as a stable form of soil carbon. Research to date has indicated that biochar may have the potential to sequester significant amounts of carbon, while providing benefits to soil fertility and nutrient retention (Lehmann et al. 2006)
There is great potential to restore carbon in tropical agricultural soils through management practices that, in the right circumstances, can also increase productivity. Agroforestry can offer particularly large carbon gains, although it can increase water demand. Agricultural carbon sequestration policies will need to be tailored to particular circumstances to allow farmers to benefit.
Many agricultural areas in the tropics have suffered severe depletion of their soil carbon stocks. Some soils in tropical agricultural systems are estimated to have lost as much as 20 to 80 tonnes of carbon per ha, most of which has been released into the atmosphere (Lal 2004a). Soil erosion, tillage and burning or removal of crop residues and livestock products reduce soil carbon levels and over time the soils have become degraded, often resulting in land abandonment.
As land under tropical agriculture occupies a wide range of soil types and climates, the capacity for carbon sequestration can differ considerably. In hot and dry areas where soil has been degraded, implementation can restore carbon and prevent further losses. In humid climates the potential for carbon sequestration can reach one tonne per ha. According to some estimates, degraded soils represent half of the world’s carbon sequestration potential (Lal 2004a).
One management practice with a high potential for carbon sequestration in tropical areas is agroforestry. In agroforestry systems, food production is combined with tree planting. Because of the trees, agroforestry systems store more carbon as plant biomass and have a higher potential for soil carbon sequestration than conventional agricultural systems (Nair et al. 2009). Biodiversity benefits may also be realised. Average carbon storage by agroforestry practices is estimated at around 10 tonnes per ha in semi-arid regions, 20 tonnes per ha in sub-humid and 50 tonnes per ha in humid regions, with sequestration rates of smallholder agroforestry systems in the tropics being around 1.5–3.5 tonnes of carbon per ha per year (Montagnini and Nair 2004). In addition, agroforestry systems can reduce the pressure on natural forests thereby having indirectly a positive effect on carbon storage in the latter (Montagnini and Nair 2004).
However, as with conventional agricultural systems, sustainable management practices also need to be adopted in agroforestry systems to ensure carbon sequestration and sustainable water use.
In some systems, interference interactions between crop species and trees planted as part of agroforestry measures may have a negative impact on crop yields (Garcia-Barrios 2003). In these circumstances, compromise solutions may be best, aiming to store reasonable rather than maximum amounts of carbon while still ensuring profitability from crops (Verchot et al. 2005).
Nevertheless, the creation of biochar plantations should be approached with great caution. While the use of biochar could be realised in a number of ways including shifting cultivation, charcoal production and the recycling of agricultural wastes (Lehmann et al. 2006) the most likely large-scale source of biochar production is from the burning of biofuels. To be justified as a carbon storage strategy, the amount sequestered must exceed that produced in moving it between its site of production, burning and application. In the case of crop residues it must be ensured that biochar addition provides a similar carbon gain to the simple return of these materials at the site of production. The impacts of large-scale biochar production on biodiversity and long-term agricultural sustainability (e.g. nutrient depletion) are unknown.
Timber forestry can be adapted to increase the amount of carbon held in plantations.
Approximately 4% of the global forest area is represented by plantations (FAO 2006). They supply a substantial proportion of the demand for timber products. Plantations can sequester significant amounts of carbon and are generally considered to be carbon sinks, unless they replace natural forests, which are usually richer in carbon. The largest potential carbon gains for plantations are on marginal agricultural land and degraded soils (Lal 2004b). However, in some cases plantations deplete soil carbon stocks and careful management is therefore necessary. By increasing the rotation period for cutting and implementing site improvement strategies, soil carbon stocks can be replenished and more carbon sequestered by the vegetation. The use of mixed stands instead of monocultures sees beneficial effects on biodiversity and reduces the occurrence of pests whilst enhancing timber production and carbon sequestration (Jandl et al. 2007).
There may be other trade-offs too. Tree plantations can support groundwater recharge and upwelling but may also considerably reduce stream flow and salinise and acidify some soils, thus leading to negative effects on water quantity and quality, as well as soil quality (Jackson et al. 2005). Negative impacts on groundwater supplies and river flows from afforestation are particularly prevalent in the dry tropics (Bates et al. 2008).
SUMMARY – HUMAN DOMINATED ECOSYSTEMS
It is clear that much land needs to be kept for agricultural use but it is also possible that the area required for food production will stabilise in the future. The largest readily achievable gains in carbon storage are in agricultural systems where the technical potential for carbon mitigation is significant, estimated at around 0.6 Gt of carbon dioxide equivalent per year by 2030 (Smith et al. 2008).
In the agricultural sector, if best management practices were widely adopted, it is estimated that 5.5–6 Gt of CO2e can be sequestered per year by 2030, which is comparable to emissions from that sector. About 90% of this potential could be achieved through carbon sink enhancement (Smith et al. 2007a) and about 10% from emission reductions. The majority of the potential (70%) can be realised in developing countries (Smith et al. 2007b). The largest mitigation potential lies in cropland management, grazing land management and the restoration of cultivated organic soils and degraded lands.