Monday, July 28, 2014
Soil Erosion, Climate Change and Global Food Security: Challenges and Strategies. Part 5.
This is the fifth part of a much longer article published in the journal Science Progress, and which may be found here: http://stl.publisher.ingentaconnect.com/content/stl/sciprg/2014/00000097/00000002/art00001
11. Carbon Capture by Soil.
The loss of soil organic matter (SOM) is a critical factor both in soil erosion and in the loss of soil productivity, the latter from the loss of soil (depth) per se, and a decline in the structure, level of nutrients and hence the innate fertility of the soil. Soil erosion depletes the amount of carbon stored in the soil, and poses a possible source of increased carbon emissions. As we have seen, current agricultural practices tend to hasten the erosion of soil. To increase the SOM content of soil provides an effective means for taking carbon from the atmosphere and storing it, while simultaneously the soil structure is made more stable, thus mitigating the conversion of existing soil organic carbon (SOC) to CO2 which is then vented to the atmosphere. SOM has many influences on the health of soil, since it contributes nutrients to assist the growth of plants, and makes the soil more fertile, while aiding the storage and movement of water within the soil matrix. There are estimated to be some 2,200 billion tonnes of carbon stored in the top one metre depth of the world’s soil - practically three times the atmospheric budget of the gas. Through human activities that decrease the land cover, and changes in how land is used, including deforestation, urban development and greater tillage, in combination with agricultural and forestry practices that are unsustainable, soil degradation is accelerated. A report by the United Nations Environment Programme (UNEP)50, states that 24% of global land has fallen victim to a loss of its health and productivity during the last 25 years, principally as a result of unsustainable land use, and since the 19th Century 60% of the original SOC has been lost, e.g. by clearing land for agriculture and to build cities. It is thought that >20% of forests, peatlands and grasslands may suffer a reduction in their ecosystem services and biodiversity within the next two decades, with peatlands being especially vulnerable. Over 2 billion tonnes of CO2 are released from peatlands each year, due to their being drained for other (usually agricultural) purposes, which is equivalent to about 6% of the anthropogenic burden from burning fossil fuels.
The UNEP report proposes that levels of tillage should be reduced, along with the use of crop rotation, the careful use of animal manures and restricted amounts of synthetic fertilizers. It is further proposed that there should be payments made to encourage carbon storage, flood control and water quality improvement. It is considered that a global climate deal should be made including the trade of carbon credits for soils to encourage good practice, and regulations for land use change and forestry are currently in the process of being set down as a part of the deal. UNEP has identified a "critical need" to universally determine, report and confirm changes in SOC over time. It is estimated that degrading areas represent a loss of net primary productivity (NPP) of 9.56 x 108 tonnes of carbon, i.e. about one billion tonnes relative to the mean NPP over the period 1981─20033. This is around one billion tonnes of carbon that has not been removed from the atmosphere, which is equivalent to about one fifth of the global carbon emissions for the year 1980. In terms of the carbon floor tax of £16/tonne for CO2, introduced by the British Government, this amounts to around £59/tonne of carbon, or £56 billion ($87 billion) in terms of potential costing and revenue. The cost of land degradation is at least an order of magnitude larger from the point of carbon emissions from the loss of SOC, and estimates might also be made is regard to the influence of land degradation on food and water security, drought, flood and sedimentation3. Thus there are many good reasons to rebuild SOC (SOM) in the soil.
12. Tillage and carbon sequestration.
As we have already noted, it is widely held that no-till (no-tillage) farming leads to the sequestration of atmospheric carbon in the form of SOM, and in contrast that soil disturbance by tillage is responsible for an historic loss of SOC. No-till is practised on a mere 6% of the world's cropland overall: mostly in the U.S and Canada, Australia and South America (Brazil, Argentina and Chile). There was a media report that a survey had been carried out of no-till land in Ohio, Michigan, Indiana, Pennsylvania, Kentucky, West Virginia and Maryland by Rattan Lal and his colleagues at the Ohio State's Ohio Agricultural Research and Development Centre, where he is director of the Carbon Capture Management and Sequestration Centre. According to Lal51: "Basically, those soils that are well-drained, are silt/silt-loam in texture, that warm quickly and have some sloping characteristics prone to erosion, are excellent candidates for no-till. Clay soils or other heavy soils that drain poorly, are prone to compaction and are in areas where the ground stays cooler, may not always encourage carbon storage through no-till." Lal concludes that, at a depth of just 8 inches, in general, no-till fields will store carbon better than ploughed fields. However, at depths of 12 inches and more, the situation may be reversed. It is necessary to “know your soil”, as farmers traditionally do.
"Soil" is part of a complex interactive system, and there is no simple and single strategy for all cases. The means must be tailored to achieve the optimum outcome on whatever land is being worked. Baker et al. also emphasise the importance of the depth to which the soil is sampled in determining its SOC content52. These workers observed that in practically all cases where conservation tillage was found to sequester carbon in the soil, the soils were only sampled to a depth of 30 cm (12 inches) or less, despite the fact that crop roots frequently extend to greater depths. In those relatively few studies in which the soil was sampled to greater depths than this, no consistent accrual of carbon could be demonstrated conclusively. Rather, there were differences in the distribution of SOC, with higher concentrations in the near surface regions when conservation tillage was used, but greater concentrations in the deeper soil layers when conventional tillage methods had been used. It is thought that these contrasting outcomes may be due to tillage inducing differences in the local thermal and physical conditions that affect root growth and distribution.
At the Rodale Institute, it has been shown53 that regeneratively managed organic soils have increased their SOM by around 1% per year to a total of nearly 30%, over the 27 year duration of their study. In comparison, land farmed using industrial high-input methods has at best accrued no additional carbon, and in some cases the soil carbon content has declined over the same period. Soils that are richer in carbon tend to support plants that are more resistant to drought, pests and disease. The sequestration of carbon in soil is principally due to the presence of mycorrhizal fungi. These fungi are able to conserve organic matter by forming aggregates of it with clay and other soil minerals. In such soil-aggregates, the carbon is less vulnerable to degradation than in the form of free humus. The mycorrhizal fungi produce a highly effective natural glue-like protein, called glomalin, which stimulates a greater aggregation of soil particles. It is further found that more soil carbon is accreted using a manure-based system than in a legume-based organic system.
In the first Rodale trial plots53, carbon was captured into soil at a rate of 875 pounds of carbon/acre/year, using a crop-rotation with manure, and about 500 lbs/acre/year using legume cover crops. However, in the 1990s, it was shown that by using composted manure combined with crop rotations, organic systems can yield a carbon sequestration of up to 2,000 lbs/acre/year (2,245 kg/hectare/year). Contrastingly, fields worked with conventional tillage, and which relied on chemical fertilizers, actually lost 300 lbs/acre/year of carbon (337 kg/hectare/year). 2,000 lbs of carbon is the amount contained in (44/12) x 2,000 = 7,333 lbs of CO2, and so each acre can remove this quantity of greenhouse gas from the atmosphere, per year, by trapping it in soil in fields. (This amounts to 8,233 kg/ha/year). While it would not be easy to do entirely and in practice, we may recall the claim, mentioned earlier, that if all the 3.5 billion acres of tillable land could be so managed, 40% of all human carbon emissions could be sequestered in its soil. Roughly that amounts to 2,000 lbs/acre x 3.5 billion acres/2,200 lbs/tonne = 3.18 billion tonnes of carbon, which is 40% of the total of 8 billion tonnes of carbon emitted per year from burning fossil fuels, in agreement with the above estimate. [In metric units, 3.5 billion acres equals around 1.4 billion hectares or 14 million square kilometres (km2), and is around 10% of the Earth's land area]. The United States produces roughly one quarter of the world's carbon emissions, and has 434 million acres of tillable land. If a 2,000 lb/acre/year carbon-capture was achieved, almost 1.5 billion tonnes of CO2 would be sequestered within its soil to mitigate nearly one quarter of the entire U.S. carbon emissions from fossil fuels. Assuming an average mileage of 15,000 miles per year and 23 miles/per/gallon, this is the emissions-cutting equivalent of taking one car off the road for every two acres of land, or removing more than half the number of cars there are on the highways of the United States53.
The notion that converting to organic farming causes the build-up of SOC has been explored recently by Gattinger et al. From a statistical analysis of 74 studies of organic farms (OFs) vs non-organic farms (NOFs), they concluded that organically farmed soils have consistently higher SOC concentrations and higher carbon stocks and sequestration rates, than their non-organic counterparts54. However, this interpretation has been called into question by Leifeld et al., who argue that the data was biased because the organic inputs to the OFs were a factor of four higher than for the NOFs55. They assert further that the claimed effect on climate change mitigation is unreasonable because the application of manure to the OFs, simply represents manure that would otherwise have been used elsewhere and so does not represent a net removal of carbon from the atmosphere to soil, but a movement of carbon from one site to another. In their response to these criticisms56, Gattinger et al. emphasised that the observed difference in external carbon inputs between OFs and NOFs can be attributed to the fact that the field comparisons were not from fertilization experiments, but from pair-wise farming system comparisons where the design and the underlying treatments reflected the particular and prevailing farming practices employed in the region where the studies were conducted. In respect to the second criticism, Gattinger et al. did in fact state in their original paper54 that “Further, the estimation of carbon sequestration alone does not equate to climate change mitigation...”, for which they gave a variety of reasons. Fundamentally, the evidence is that organic farming practices do enhance SOC stocks.
13. Enhancing, rebuilding, and regenerating soil.
It is possible to address and mitigate the phenomenon of soil erosion and indeed to enhance and rebuild soil; nonetheless, it is common that the appropriate practices are avoided because maintaining the status quo leads to immediate benefits (e.g. high crop yields). This is a shortsighted view, however, because if allowed to continue, the quality of the land will decline such that crop yields eventually must fall, even to the point where the land is abandoned. By creating a better soil-structure, along with increasing its SOM content and by impeding runoff, the soil may be rebuilt. The procedure involves biological, chemical and physical processes, but it is unlikely that a soil can be entirely restored - along with its attendant flora and fauna – that was created only over a period of hundreds or even thousands of years. In northern Thailand, farmers initially responded by adding organic matter from termite mounds to the clay-poor soils there to increase their productivity, but over the longer-term this practice could not be maintained. Workers from the International Water Management Institute (IWMI), in cooperation with Khon Kaen University and local farmers, experimented with adding the smectite clay, bentonite, to the soil, which assisted its retention of water and nutrients. By a supplement of 1,256 kg per hectare, an increase in the average yield of 73% was achieved, and the risk of crop failure on degraded sandy soils during years of drought was reduced by the addition of bentonite to them. According to a survey carried out in 2008 among 250 different farmers in northeastern Thailand, and some 3 years following the initial trials, IWMI were able to determine that the average yields were 18% higher from those lands that had been treated with bentonite, and through this practice, some farmers were able to increase their income by growing vegetables, for which a more fertile soil is needed57.
14. Land management actions for the purpose of mitigating and adapting to the effects of climate change.
As we have alluded, rising global temperatures are expected to have an impact on the future of agriculture, in terms of heavier and more violent rainfall on the soils in some regions of the world; in addition, sea level rise will affect low-lying lands particularly. An increasing rate of soil erosion, with a reduction in soil quality and agricultural productivity might therefore be anticipated. Since the food requirements of a human species must rise in proportion to the expected 30% increase in its population by 2050, the effect of climate change can only make matters of food security and global sustainability more acute. It has been proposed that the good management of soil is the single best contribution we can make to climate change mitigation and adaptation58. Both management practices on the field and off-site can play a role in this, serving to maximise the conservation of soil and water, so to increase agricultural food production per hectare of land. It is future generations who will benefit or suffer from the decisions that we make now, regarding the management of soils and crop residues, in terms of soil quality and water resources. Hence there is the need to bring rates of soil erosion, expected to rise in the wake of climate change, to a minimum rate. The introduction of conservation agriculture, growing cover crops, leaving residues to cover the soils, using crop rotations, and returning crop residue, will improve the quality of soil and curb its erosion.
As noted, large amounts of carbon taken from the atmosphere can be sequestered by soils in the form of SOM, and this process may assist in our adaptation to climate change and extreme weather events by maintaining the land productivity. By increasing SOM and hence the water-retaining capacity of soils, the probability is greater that crops may endure more dry conditions and the planting of drought resistant varieties should be explored, which are able to increase the storage of water in a forthcoming scenario where the air temperatures and rates of evapotranspiration are greater. A higher SOM content and an associated improved soil aggregate structure might also increase the capacity of soils to drain. Crops grown on more productive soils, with a deeper soil profile, have a larger root zone (space where roots can grow) and can store more water. A more extensive root system means that greater amounts of water and nutrients can be accessed by the plants, rendering them less vulnerable to inhospitable climatic conditions. It is small farmers who tend to manage low-input systems, and hence they may herald the way to a smaller scale kind of farming, in fitting with the ideas of localisation (re-localisation) that are part of the philosophy of the growing Transition Towns movement59. Through localisation and the establishment of resilient communities, a future is envisaged where populations are removed from the threat of peak oil and climate change, by being able to provide more of their essentials, particularly food and materials, at the local level, rather than being at the behest of external supply lines, which may fail. It is sometimes said that “Britain is just three days from anarchy”, meaning that if there were to be a loss of the national oil/fuel supply, within three days the supermarket shelves would be empty and people might start looting from their neighbours to survive. Most likely, the shelves would be empty within the first day, and mayhem would swiftly ensue.
Precision (target) conservation methods are also key to practices of conservation at the level of the field or watershed, and thus it should be possible to determine those areas in the watershed that are best suited to be riparian zones or wetlands, e.g. for carbon storage in the permanent vegetation of riparian forest. The control of nitrogen compounds is an important aspect of ameliorating climate change, which may be converted to nitrous oxide in soil, and released into the atmosphere as a potent greenhouse gas. In addition N is sequestered along with C in SOM, and so the increased concentration of this material serves a further purpose. The implementation of other practices, e.g. growing cover crops and legumes (which fix N2) in the crop rotation, increases the chances that more N will be cycled by soils. Models such as CEAP and GRACEnet can be used to draw conclusions about conservation practices and aid the adaptation of agriculture to the expected consequences of climate change58.
Research is necessary to find better means for carbon sequestration in soils, for the management of nitrogen, and improved controlled release fertilizers. The possible crop-use of manure, along with its employment to generate biogas and to recover N and P nutrients by biodigestion/fermentation, are also important topics for investigation. Overall, means for the production through agriculture of food, fibres and energy, which impact less on the environment and require smaller inputs of fuel, other energy, synthetic fertilizers and water, while simultaneously preserving and rebuilding soil and conserving water, are sought. The implementation of various underpinning factors to achieve this will involve political, financial and policy decisions and practices. As the levels of SOM increase in soils, it may be necessary to apply smaller amounts of nitrogen fertilizers, particularly where legume crops are grown, and cover crops. Appropriate decisions must be made in terms of management to reduce the potential for erosion. Off-site conservation practices, including buffers, riparian zones, and wetlands, may contribute further ecosystem services, e.g. sequestering carbon and removing nitrogen from the environment. It is those decisions of management which serve us to mitigate and adapt to climate change that are crucial to conservation, rendering cropping systems sustainable, ensuring the quality of soil and of water and establishing food security.