Soil Erosion, Climate Change and Global Food Security: Challenges and Strategies. Part 6.

This is the sixth 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

 
15. Biochar, and its potential contribution to improving soil quality and carbon capture.

Bioochar is the specific term applied to charcoal when it is used as a soil-amending agent60. In 2001 a paper was published61 which refers to a farmer in Acutuba (an ancient settlement in the Amazonas state of northwestern Brazil), who had grown crops on terra preta soils for 40 years without needing to add any fertilizer. Astonishing as this seems, these "dark earth" soils possess a remarkable vitality and fertility, and it is speculated that along the Rio Negra the large populations described by Francisco de Orellana, in the Chronicles of his 1542 quest to find the mythic city of El Dorado, were sustained by terra preta de indio - Portuguese for "Indian black earth" (Figure 9). The Amazonian soils are notoriously poor in quality, despite the lush forest that grows on them, and in contrast the terra preta is a legacy of the Amazonian civilizations that lived there in the past. There has been much speculation as to the origins of terra preta soil, in particular whether it was deliberately created to improve the fertility of the region, or whether it was an accident of nature or serendipitous to the way of life among the Amazonian tribes. What seems clear is that the essential component of the soil is a type of charcoal, which may have been formed either by a kind of composting process or by burning biomass which became added to the soil, either deliberately or by chance. The present consensus of opinion is that the soils were formed deliberately by local farmers, who knew well the causes of its quality. It is thought that in soil, biochar is able to bind the essential nutrients N, P and K, and impede dramatically the rate at which they are washed away by the continual rains. Minute pores are formed in the charcoal over time which can hold more nutrients on its larger surface area and possibly act as "condominiums" for microorganisms to grow-in which increases their density in the soil. There is little hard evidence for the latter hypothesis, although the abundance of bacteria in soil and also in methane-producing biodigesters is certainly increased by the presence of biochar62.

The idea is to create "terra preta nova", or artificial terra preta by deliberately adding charcoal to soil with the aim of recreating the properties of natural Amazonian terra preta. The whole scheme would necessitate building large-scale biochar factories, and sacrificing enormous areas of land on which to grow the precursor biomass. This should, however, be compared with potential complementary methods for regenerative agriculture which depend far less on added N,P and K, through growing year-round cover crops, and forest gardens, which once established are largely self-sustaining. As already noted, the Rodale Institute concluded that 40% of all human carbon emissions could be sequestered if their methods were implemented over the world’s entire acreage of arable land53.

[Fig 9]

The world emission of carbon dioxide now amounts to an annual 31.6 billion tonnes, which contains 8.6 billion tonnes of carbon. Therefore, to reduce this by 40% by instead growing biomass and converting it to biochar, would require the creation of some 3.4 billion tonnes of biochar, year upon year, and is clearly a considerable undertaking. Some insight into the scale of the current interest and activities in the field of biochar may be gleaned from inspection of the International Biochar Initiative (IBI) website [http://www.biochar-international.org/] and the work of the U.K. Biochar Research Centre at Edinburgh University [http://www.biochar.org.uk/], which has produced the DEFRA Report “An Assessment of the Benefits and Issues Associated with the Application of Biochar to Soil.” [http://randd.defra.gov.uk/Document.aspx?Document=SP0576_9141_FRP.pdf], in which it is concluded that 10% of Britain’s carbon emissions could be sequestered in the form of biochar. There is also a series of conferences held under the auspices of the U.S. Biochar Initiative (USBI), [http://www.2012.biochar.us.com]. The Amazonian Indians created their terra preta soils over many years, to their fully self-generating glory, and described the soil as physically "growing", which may suggest an accretion process involving fungi and other microbiota.

When trees and other forms of biomass are grown, CO2 is absorbed via photosynthesis. If the biomass is then pyrolysed (heated to cause chemical decomposition) there remains a solid carbonaceous residue which can improve the growing properties of some soils, if it is integrated into the top layers. Improving the fertility of soils might also reduce the amount of chemical fertilizers that need to be applied to soil to obtain adequate crop-yields, and result in some curbing of our demand upon a declining world resource of phosphate rock15. In truth, we will have to redesign the way we live, not only to rely far less on personal transportation, but to recycle nitrogenous and phosphate components from human and animal waste2,15, in order to grow food, let alone grow an additional crop for biochar. Ideally those two practices can be integrated, so for example, the proverbial chaff from wheat might be converted into a stable form of carbon-rich material that persists in soil for thousands of years, or at least hundreds, actually drawing-down carbon from the atmosphere and cooling the earth. To be effective, the biochar production process must produce more energy overall than it consumes. When biochar is tilled into soil, it can improve its fertility, crop yield, fertilizer requirements and water-retention abilities.

Thus, many pressing issues are addressed in a single action, in respect to global warming, phosphate and water shortages, and the difficulty in growing enough food to feed the burgeoning world population and alleviating poverty in the developing (non-legacy) world. Put in such terms, biochar begins to sound little short of a miracle. As noted, humans are emitting around 8.6 billion tonnes of carbon into the atmosphere annually from burning fossil fuels, and so that amount must be absorbed in addition to remediating the levels of CO2 that are already present. In rough numbers, it would be a reasonable aim to deplete the amount of CO2 in the atmosphere to pre-industrial levels, or say a drop from 400 to 300 parts per million (ppm), or 100 ppm. The mass of the atmosphere is 5.3 x 1015 tonnes (less than one millionth the total mass of the Earth), and thus we need to remove:

(44/30) x (12/44) x 100 x 10-6 x 5.3 x 1015 = 2.12 x 1011 tonnes, or 212 billion tonnes – also expressed as Gigatonnes (Gt) – of carbon. In this sum, 30 is assumed to be the average molecular mass of an "air" molecule, 12 is the atomic mass of carbon and 44 the molecular mass of CO2. Over a 38 year period (so that we have accomplished our task by 2050, the year when all governmental carbon emissions targets are to be met), we thus need to remove 212 + (38 x 8.6) = 539 Gt of carbon, which works out to 14.2 Gt per year.

If we assume a mean crop-mass of 30 tonnes per hectare per year, of which we may deduce around 40% is carbon, based on a carbohydrate formula of C6H12O6, this amounts to 0.4 x 30 = 12 tonnes of carbon per hectare per year. Hence we would need 14.2 x 109/12 ha = 1.18 x 107 km2, i.e. around 12 million square kilometres of land on which to grow it. This can be compared with 150 million km2 for the total land mass of the earth, of which around 15 million km2 is arable and around another 30 million km2 is pasture land. There are swathes of existing forest (including rainforests) but to clear them would be extremely foolish, since they are principal carbon-sinks, although growing trees (e.g. sycamore, etc.) as part of a managed sustainable programme (harvesting them at regular intervals) might make a substantial contribution to the total carbon-capture volume. Not all arable crops can be converted to biochar, but manure, etc. might be, from the animals and humans that eat them. Probably, to achieve the aim of capturing 539 Gt of carbon over 38 years would require working close to the limits of the planet's growing capacity, and a concomitantly vast investment in engineering, along with policy, commercial, social and all other aspects, in an integrated programme.

Like many other postulated sustainable technologies, biochar too may fail the crucial "Scale Test" in the final feasibility analysis. The International Biochar Initiative (IBI) aims to have 1 Gt/year of carbon being drawn from the atmosphere by 2050. In which case, let us assume that the target is 40 years away, and that there is currently (on a Gt scale) about zero biochar being produced currently. Even if we assume a linear growth in the technology [i.e. the "wedge" obtained by drawing a straight line on a piece of graph paper from 0 - 1 Gt (on the y-axis) up to 40 years (on the x-axis)] only 20 Gt of carbon is accounted for, or a reduction of about 10 ppm, which is relatively minor, and the attendant biomass production and processing would nonetheless be simply colossal when viewed en masse. That said, if that level is achieved by, and sustained beyond 2050, 1/10th of all carbon (10%) captured per year is significant, and the quantity could represent a higher proportion of total emissions if fossil fuel burning has by then been significantly curbed, either deliberately or because we have less of them available. The main benefit of biochar is likely to be in terms of improving soil quality, if it is employed as a soil-amending agent, and thereby reduces demand on water and nutrients to grow crops.

The latter is likely to be particularly significant in parts of the world where the soil is poor, e.g. Africa and Asia. In the U.K., soil tends to be very rich - too rich sometimes - but even here, the incorporation of biochar into the soil would attenuate problems from run-off waters that contain too much phosphate and nitrate. It would appear that production of biochar and algae on a local level, as part of a programme of lower-energy living, could offer some benefits. There is also (for once) the advantage that there is a multitude of people on the planet. Hence if a community of 2000 people could catch and sequester 200 tonnes of biochar per year (100 kg/person), 7 billion of us in total could sequester almost 0.7 Gt/year (close to the IBI projection of 1 Gt/year by 2050). However, it is the curbing of energy use that really counts. Back to the village algae-pond: as a total area, we would need around 3200 km2 of ponds to fuel Britain (more of which could be turned to other purposes than personalised transport, levels of which could be curbed through relocalisation), which suggests that each village pond would need to be: 3200 km2 x 100 ha/km2/60 million x 2000 = 10.7 hectares for each 2000-person community. While this is a large number, the goal seems far more tenable when broken down in this way. The real problem is how to process the algae either by extraction of its oil (followed by transesterification) or through bulk thermal gasification. It might be simpler to just grow the algae (and other biomass), dry it and burn it as a source of thermal energy. All of the above is going to take a great deal of engineering and hence vast resources of energy, materials, construction work and time. It has also been proposed that combining biochar with compost, as a soil amendment, might prove a beneficial means to increase SOC63.

16. Soil and the “hydrological (water) cycle”.

The hydrological connection between soil and water, and the profound importance of keeping soil covered as a means both to mitigate its erosion and to enhance its ability to retain water, is masterfully illustrated by a simple lecture demonstration http://www.youtube.com/watch?v=og9cQKxlFnE. As the video shows, when water is poured onto soil contained in a plastic vessel with a spout, it simply runs-off, washing away with it the surface layers of soil (erosion). When, however, the soil is covered with a layer of powdered horse manure (basically, digested plants), the runoff is reduced by a half, and the soil is substantially protected from erosion (less is washed away with the runoff). The water soaks through the dried manure layer and down into the soil, traversing the body of it, finally draining into the empty space below (model for groundwater recharge), once the soil is saturated. Thus we see that uncovered ground is highly vulnerable to erosion of its surface soil, with rapid runoff (flooding), and groundwater (wells and streams) not being replenished (potential water shortages). The covered soil is left moist, and thus capable of nourishing plants and crops, which can be maintained in practice through management strategies, e.g. planting cover crops around the year, controlling and timing grazing, avoiding tillage and fires which destroy ground cover and leave the soil bare.

When both the covered and bare soil demonstration set-ups are dried in the sun, it is apparent that the covered soil remains moist, while the bare soil becomes completely dry. On actual industrial farms, it is necessary to pump water onto the fields, and to use dams and irrigation systems: a technology that does not solve the basic problem, but merely treats the symptoms. Moisture also evaporates faster from bare soil, which is another route to wasting the precious resource of water. In the real world situation, groundwater begins as precipitation and enters the ground by a process called infiltration64, percolating thorough the various solid strata below the water table to recharge the saturated zone and ultimately any lower lying aquifers. Water that has fallen to earth as rain or snow enters (infiltrates) the subsurface soil and rock in varying quantities depending on the nature of the ground surface. Some of the water will stay in the shallow soil layer, within which it may slowly move in both vertical and horizontal dimensions through the soil and subsurface material. Groundwater may enter a stream/river system if it finds a route into the stream bank, or it may percolate to greater depths so that aquifers become recharged. If an aquifer is sufficiently permeable that water may move relatively unrestricted through the body of it, wells can be sunk over its total area, e.g. to extract water for drinking or agricultural irrigation.

16.1 Factors affecting infiltration

•Degree of precipitation: the intensity and duration of precipitation it the most significant factor in determining how much infiltration occurs. Water that enters the ground frequently seeps into streambeds over an extended period of time, meaning that a stream can maintain its flow even after a considerable absence of rainfall and in the absence of direct runoff from recent rain or snow.

•Soil characteristics: clay soils absorb less water than sandy soils and at a lower rate, meaning more overland runoff into streams.

•Soil saturation: in a common sense fashion, soil already that has already been saturated from precious rainfall cannot absorb more water and so additional rainfall instead turns to surface runoff.

•Land cover: forests and cover crops make a considerable impact on levels of infiltration/ rainfall runoff and soil erosion. As shown in the lecture demonstration (above) land cover restrains the movement of runoff, increasing the likelihood of the water soaking into the ground instead. When the land surface is covered with tarmac to create car parks, roads, and building developments, this truly impermeable barrier drives all of the precipitation directly into streams. The natural infiltration behaviour of a landscape may be influenced profoundly by agriculture and tillage of the land, so that rather than percolating into the soil, it runs off into streams instead.

•Slope of the land: intuitively, the steeper the slope of an area of land, the faster water runs off it, so a smaller degree of infiltration occurs than for a comparable area of flat land.

•Evapotranspiration: some of the water tends to remain close to the land surface. Plants need this shallow groundwater to grow, and hence it is here that they set down their roots, meaning that water is moved efficiently back to the atmosphere by evapotranspiration.

16.2 Subsurface water

An unsaturated zone and a saturated zone are typically formed when water infiltrates into soil (Figure 10). The upper part of the unsaturated zone is the soil-water zone. Precipitation is encouraged into the soil zone, because it is crisscrossed by roots, openings left by decayed roots, and animal and worm burrows. Water in the soil is used by plants in life functions and leaf transpiration, but it also can evaporate directly to the atmosphere. In the unsaturated zone, the voids—that is, the spaces between grains of gravel, sand, silt, clay, and cracks within rocks—contain both air and water, and despite the large volume of water that may be present, it cannot be pumped-out via wells because it is held in place by strong capillary forces. In the underlying saturated zone, the voids between rock and soil particles are entirely filled by water. Groundwater moves but slowly through the unsaturated zone and the aquifer and so it takes a long time for deep aquifers to be recharged (Figure 11). The degree of overlying precipitation is also a fundamental factor. The shallow aquifer that underlies the High Plains of Texas and New Mexico (of which the Ogallala aquifer is a part) - a region of low rainfall - would take centuries to recharge at its present small rate of refill. However, a shallow aquifer in, for example, the coastal plain in south Georgia, USA, may be replenished almost immediately, where the rainfall is substantial.

[Figs 10 and 11]

As we see on the next section, the overpumping of aquifers to provide water beyond their natural rate of recharge is widespread, and looks set to limit food production in the Middle East and in other parts of the world. When an aquifer is overpumped, as in these cases, the water table can be lowered to the extent that wells can "go dry" and become useless. It is below the water table that the soil is saturated and may yield sufficient water to be pumped via a well to the surface. [The water table is the surface where the water pressure head is equal to the atmospheric pressure, and may be visualized simply as the "surface" of the subsurface materials that are saturated with groundwater in a given vicinity]. In some locations, the water table is close to the land surface and water can move through the aquifer very rapidly, rendering it possible to replenish the aquifers by artificial means (Figure 12). This may be done using rapid-infiltration pits, where water is spread over the land in pits, furrows, or ditches, or small dams are made in stream channels to retain and direct surface runoff, thereby allowing it to infiltrate to the aquifer. The other common means is by groundwater injection, where recharge wells are built and water is injected directly into the aquifer.

[Fig 12]

16.3 The role of forests in preserving soil and water

Forests provide barriers to soil erosion and excessive runoff. Woodlands further protect water bodies and watercourses by trapping sediments and pollutants from various up-slope activities and land use. Forests also contribute to the availability of water, since they absorb water from direct atmospheric rainfall and from the ground through their roots. Through evapotranspiration, water is re-released to the atmosphere and into the global water cycle. A contribution to the timing of water delivery is further provided since both the infiltration of soil and its capacity to store water are maintained and enhanced by the coexistence of the forest. Tropical rainforests are especially important in providing water for the plants and animals that are sheltered by their thick canopies, so helping them to survive and protecting overall biodiversity. As we have noted in the section on deforestation, the loss of forest causes increased rainwater runoff and the erosion of topsoil, while evapotranspiration from tree foliage, a critical component of the global water cycle, is lost resulting in increased drought and desertification.

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.

Soil Erosion, Climate Change and Global Food Security: Challenges and Strategies. Part 4.

This is the fourth 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


9. Establishing a relationship between land degradation, soil productivity and crop yields.

The productivity of some lands has fallen by 50% as a result of soil erosion and desertification, and the according reduction in crop yields in Africa lies in the range 2─40%, with a mean loss of 8.2% for the continent overall11. The loss of productivity in South Asia has been reckoned at 36 million tons of cereal equivalent with a value of $5.4 billion as a result of water erosion and $1.8 billion from wind erosion11. It is a vexed matter to make a definite connection between the extent and processes of soil erosion and declining crop yields, since the latter may result from various influences. In some cases, the crop yields do not fall markedly, and may even increase for a time, despite the soil being eroded, e.g. if a compensatory increase is made in fertilizer inputs. Crop yields may be impaired43 by an excessive removal of nutrients from the soil, which are not replenished; the impact of pests and diseases; weed infestations; and the greater frequency of drought as a consequence of climate change. Other factors – which may be associated with soil erosion – can also be culpable for a reduction in crop yields, e.g. a restriction in the possible rooting depth, e.g. when the soil depth becomes limited, and the roots touch the bedrock or a clay layer; a reduction in the water capacity of the soil; a decline in SOM (SOC) content; an increasing salinity or sodicity of soil; other changes in the chemical composition of the soil: e.g. the presence of aluminium or heavy metal cations, or a reduction in soil pH (acidification) in general. All of the above, in one way or another, are connected to some type of soil degradation, the most common being soil erosion by water43.

It can be said that practically any adverse environmental change is likely to lead to soil erosion and a decrease in biomass yield, such is the inextricable complexity of the underlying components of these phenomena. To invoke a spectrum of impact, we may at one extreme consider the conversion of dryland savannah to continuous cropping (the practice of growing the same crop in the same space year after year) of soya beans (soybeans). As a result of this change, the combined influence of loss of vegetation cover and soil disturbance will aggravate and accelerate soil erosion. Although the crop (a legume) will contribute some nitrogen to the soil, and some organic matter, a tipping point will ensue eventually when production is impaired, as a result of a thinning of the topsoil, colloid loss, and a reduction in the water-retaining capacity of the soil. However, the input of resources (e.g. fertilizers, irrigation) and technological means can allow production to be continued unabated. At the other end of this spectrum are the “badlands” – a result of the mistreatment of semiarid ground, where serious soil erosion has occurred, with gullies, rills, pipes and other related aspects – which are completely lacking in vegetation. As far as apportioning blame for the loss of vegetation to soil erosion is concerned, both extremes are really “chicken and egg” situations: erosion must result in a reduced soil quality, which impairs plant production and reproduction - allowing that this might be masked by technology and other inputs – but at the same time, a loss of vegetative cover provokes soil erosion. It is rare, however, that a landscape becomes entirely barren, because soil that is “lost” by erosion is transported to other regions, bearing nutrients, organic matter and water. Such “bestowals” are prevalent particularly in South Asia, where “sediment harvesting” is possible, e.g. the nullah plugs in India. Hence, the “cause and effect” paradigm of soil erosion and crop productivity should be treated with caution, since an adverse effect on one location may transfer an advantage to somewhere else43.

Some confidence is justified in connecting soil erosion and crop yields, primarily on the basis of experimental runoff plots, where measured soil losses are related to both current and future yields, though not exclusively to the underlying mechanisms of soil erosion. As a general trend, plots of crop yield versus cumulative soil depletion (t ha-1) reveal curvilinear, inverse-exponential type relationships, i.e. the yield drops as the soil gets thinner. Hence, there is an initially sharp loss of productivity, followed by stages in which the impact is successively less. While alternative behaviour has been identified, the overall message is that it is comparatively easy to bring back slightly degraded land into economic use, and that the net returns are always better if the yield has not fallen to under 50%. In contrast, when land has been severely degraded, to bring it back into useful (or even economic) production is a tremendously difficult task. Some soils (Figure 7) are far more resilient than others, e.g. a Nitosol (clay-rich, and common on basic rocks in highlands) which may be orders of magnitude more resistant to productivity loss than an extremely sensitive soil, such as a Ferralsol or Acrisol (common in arid, humid tropical rainforests), especially under good levels of management43. However, it has been pointed out that the results reported from studies of the dependency of crop productivity on the degree of soil erosion, are inconsistent in respect to both the magnitude of the response and indeed the shape of the response curve. Accordingly, an analysis was undertaken44 to determine whether general patterns, or key features with either a physical or methodological origin, in terms of the measurement of soil loss, might be identified.

It was concluded that the experimental methodology employed has an overwhelming influence on the apparent magnitude of the soil loss: i.e. from a comparative-plot method, an average loss in crop productivity of 4.3% per 10 cm of soil loss was obtained; in contrast, results based on the transect method gave a 10.9% average; while those from measurements of desurfacing averaged at 26.6%. Although there was no identifiable effect of physical variables (water deficit, physical root hindrance, nutrient deficit) on the magnitude of the response curve, nor of the particular experimental method on its shape, it was found that water deficit and physical root hindrance caused convex curves, while nutrient deficit gave rise to linear or concave curves. As a general rule, it is only when the regressor (meaning an imbalance between the crop demand and soil supply) is nutrient deficit and the experimental method is desurfacing, that the curve is concave. As an explanation for this, we may note that with a single act of desurfacing (artificially removing soil), it is the upper topsoil that is removed, and because the concentration of nutrients tends to increase toward the surface, this has a more pronounced impact on yields than when subsequent, lower lying soil layers are removed. The timescales over which natural erosion occurs are sufficiently long that the nutrients are continuously redistributed throughout the soil, and so comparable quantities of nutrients are removed layer by layer. It is also the case that where nutrient deficits are obviated by the application of fertilizer, the response curves tend to be of a convex form. This suggests that further erosion is likely to cause reductions in crop productivity of increasing severity.

[Fig 7]

When comparing results for soil loss experiments, the exact experimental methodology should be taken into account, since it is clear that practically all the variation in data from otherwise comparable settings can be explained in terms of the different types of measurement employed, e.g. the results from desurfacing measurements were as an average six-fold greater than those derived using the comparative plot approach. It appears reasonable that the more sensible estimates of soil loss are obtained from comparative plot experiments, while the other methods may lead to gross overstatements about the adversity of intensive mechanised (industrialised) agriculture on the effect of soil erosion on crop yields. The aforementioned differences in the shape of the response curve according to different physical influences, might be accounted for in terms of the behaviour of the regressor in the soil profile as a nexus with the crop response to the regressor. For example, because the concentration of nutrients decreases, but in a nonlinear fashion, as a function of soil depth, successive removal of soil has an attenuating effect on crop yields. In contrast, the availability of water decreases more or less linearly with soil depth, but how well crops grow is increasingly impaired as the availability of water is reduced. When roots extend such that they come into contact with the restricting layer (e.g. bedrock, or clay), growth is hindered, as intuition might suggest, but since the density of roots tends to increase towards the surface, the effect of soil loss is more pronounced when shallower (top)soils are present.

In the case of gradual, rather than accelerated erosion, and the soil is well nourished by effective management practices, the only significant regressors are physical hindrance and water deficit. The indicated reduction in crop yields of 4% per 10 cm of soil lost implies that those regions that are subject to moderate erosion rates (ca 10 t ha-1 year-1 – i.e. of the order of 1 mm depth of soil lost per year) will suffer an average decline in crop yield of 0.4% each decade. The latter figure might appear marginal, yet for regions in which the soil erosion is more severe, the reduction in productivity might be worsened by an order of magnitude or greater. Increasing restriction of root growth is an allying consequence of compounding loss of soil depth, and accords with a progressive fall in crop yields. It is those soils with growth restricting layers, such as clayey subsoils, pans or bedrock to which particular attention should be paid. In terms of land management, and even in regions where soil erosion has not, as yet, resulted in particular declines in crop yields, it is likely that the steady loss of soil will experience increasingly marked losses in the future. As a warning, the generally convex shape of the crop-yield/erosion response curves indicate that particular care should be given to those soils that are already eroded, but are currently still productive. The heavy application of fertilizer is all that maintains the yields of some highly degraded lands, which are likely to plummet when the soil loss exceeds a certain degree. No one should be deceived by crop productivity alone, and the underlying condition of the soil and land must be taken into account, and monitored closely.

In a study of the effect of soil erosion on crop yields in Europe45, it was assumed that the applications of fertilizer compensate for the nutrient loss caused by soil erosion, and so this is not a significant factor in affecting crop productivity. Since it has been shown that rooting space and water availability are the major limiting factors in determining crop yields in eroding soils, the two elements were combined to form a single variable, called SWAP (soil water available to plants) which is considered to be the most important in accounting for the effect of soil erosion on crop yields. SWAP is determined as the product of soil depth and volumetric water content for that depth, integrated down to the normal rooting depth for a specific crop, which for wheat grown in Europe, is considered to be 1.2 metres. In principle, SWAP is the quantity of water (mm) held between field capacity (5 kPa) and the permanent wilting point (1,500 kPa), which for a soil in which cereals are grown is the sum of two components: water held at low suctions (5-200 kPa) and 0-1.2 m depth, and that held at higher suctions (200-1,500 kPa) and 0-0.5 m depth. The soil erosion data were obtained from the PESERA (Pan European Soil Erosion Risk Assessment) model. Thus, the authors made a prediction of the likelihood of soil erosion causing reduced crop yields over the next 100 years, and concluded that it was unlikely to be a problem in the productive ecosystems of northern Europe. In southern Europe, due to a combination of severe soil erosion from ancient times and slow soil formation rates (typical in Mediterranean climates because of low rainfall and rapid loss of SOC), the soils tend to be stony and shallow, with low wheat yields. The model predicts that, in contrast with northern Europe, in the southern regions, erosion-induced reductions of probably a few percent points are likely during the next 100 years.45 At a national level, the effect is projected to be most severe in Greece, Portugal, Spain and Italy, although it is not thought there will be any real threat to the agricultural productivity of Europe with the next 100 years.

The lifetime of a civilization, however, far exceeds one century and over the millennial timescale, the effects of such levels of erosion could integrate to become major impacts. Even if erosion were to prove of no direct threat to European agriculture, its control remains desirable due to its associated harmful environmental influences. While productivity might be maintained through larger inputs of artificial fertilizers, to compensate for losses of nutrients by soil erosion, agricultural sustainability is affected negatively: increased costs incurred through the manufacture of the fertilizer, with associated carbon emissions, and the runoff containing fertilizer, pesticide and herbicide may damage terrestrial and aquatic ecosystems, many of which are in a sensitive ecological equilibrium. Land whose productivity has declined may be abandoned, and thus soil erosion may drive changes in land-use. To explore this prospect in more detail, Lesvos, in the western region of Greece, was chosen46, since it has undergone accelerated erosion on marginal soils over the last century, and indeed there have been significant changes in land-use there. In 1886, some 3211 ha were under cereals, of which 53% had been converted to land only used for extensive grazing (rangeland) by the middle of the 20th Century. However, in neighbouring regions, cereals had returned to some extent in former rangeland, which indicates local scale changes which rendered some land better and other land worse for growing cereals, over time.

On the basis of a statistical analysis, it can be concluded that the physical features of the landscape have been critical in determining the abandonment and later reallocation of land under cereals. Thus, land with high slope gradients, high erosion rates and shallow soils tended to be removed from cereal production, while new arable land used for growing cereals tends to be in those areas were the soils are deep, the erosion rates low, and the slope gradients shallow. The logistic fit suggests an attribution of abandonment to the direct impact of erosion (25%) + the erosion/soil depth component (36%) + the direct impact of slope (39%). In regard to the reallocation of land, the direct influence of slope was much smaller (17%), but the influence of slope via the erosion/soil depth component was 80%. A possible reason for this is that farmers believed that the marginal productivity of the land that they abandoned had more to do with the shallowness of the soils than to steep slopes. It would then follow that soil depth mattered more when choosing land for reallocation than slope gradient. It is concluded that soil erosion is a significant driver in land-use change, though due to confounding effects it did not emerge as a significant independent variable in the analysis, and that the cultivation of cereals in western Lesvos will probably be abandoned within a few years.

Another study is reported of the response of soil erosion and sediment export to land-use change in four agricultural regions of Europe47, which over the past decades has been driven mainly by the introduction of new technologies. Thus, through the introduction of mechanized machinery, synthetic fertilizers, herbicides, pesticides and new cultivars, an increase in land productivity of 400─500% has been achieved. As a result, intensification has been the case in those regions where these new technologies could be suitably implemented, while abandonment or de-intensification occurred (reduction in inputs) in those areas that were less suitable. From a simulation of the response of erosion to land-use change over the past 50 years, it was inferred that de-intensification of land-use in marginal agricultural areas resulted in a strong reduction in erosion and the transport of sediments to rivers. This reduction in erosion is frequently enhanced by the conversion of a type of land-use that worsens erosion to one that is less harsh, e.g. the conversion of arable land to forest, on steeper slopes. The innate soil fertility also plays a role, since it is arable land with sandy and clayey soils (suitable but less erodible) that tends to be abandoned earlier, while the more long-standing arable land is typically that on silty soils, which are suitable but erodible. The issue of soil fertility and land area is crucial, if one result of climate change is that populations may move toward more polar regions. As is clear from Figure 8, the relative land area decreases, especially toward the south, and in the direction of both poles the soil tends to be of the poorer kind.

[Fig 8]

10. Peak oil, peak gas and peak phosphorus.

The greatest adverse impact on our system of industrialised agriculture would be the loss of a cheap supply of crude oil14 (“peak oil”), and the fuels, pesticides and herbicides that are derived from it. Although there is a cornucopian counterargument that peak oil can be disregarded, on the grounds that there are vast quantities of “oil” in the earth, it ignores or confounds what the term actually means. Specifically, peak oil refers to the rate of production of crude oil, not the size of the total hydrocarbon body there may lie in the multifarious reservoirs of global geology. To use an analogy, it is the size of the tap not the tank that matters. Much of the “oil” that remains will be recovered only with a far lower energy return than conventional crude oil, and much of what is claimed may not prove worth recovering at all. The bulk of the world’s tally is present in oil shale, and is not petroleum but a solid, primordial material called kerogen, which must be “cracked” (thermally decomposed) by heating it to 500 oC, in order to produce a liquid form that resembles crude oil2,14. Unsurprisingly, this requires a high input of energy, and which is comparable to the amount of energy that would be recovered by burning the resulting “oil”. It has also been proposed that “peak phosphorus” can be expected at some time during the present century, based on various analyses of how much phosphate rock there is available and its likely recovery rates15. Since phosphate rock is mined using machinery powered by oil-refined fuels, the loss of a cheap supply of crude oil would impact on the production of this principal source of phosphorus fertilizers, in addition to its restrictive influence on running farm machinery.

It is the occurrence of peak oil, and peak natural gas (a source of hydrogen, and hence ammonia from which nitrogen fertilizers are made), with their attendant consequences, that may invalidate many predictions made about how agriculture might prevail (and all other human activities for that matter), for the next 100 years, or even the next 20 years, since we may have to grow food largely in the absence of their inputs. In which case, protecting the soil is paramount. Having continued access to phosphorus is critical, and it has been proposed that more of this element might be got from the soil48. Indeed, the amount of phosphorus in soil is by far in excess of the amount that is mobile and hence available to plant roots; the vast majority being present in the form of insoluble compounds2. Moreover, the amount of phosphorus fertilizer that is applied to crops is probably twice that actually necessary because of the tendency to ignore the longer-term effect of the residual phosphorus in soil. Due to an historical lack of application of phosphorus fertilizer in Africa, it has been estimated that it will be necessary to apply 30─50% more of it to the soils there, and probably for a period of 30─50 years, in order to regain pre-depleted levels of the element48. From another study, it was concluded that phosphorus can be used more efficiently by both improving the uptake of it (P-acquisition efficiency) and a greater productivity per unit of P taken up by plants (P-use efficiency). The growth of crop-plants that have overall lower P-concentrations, and the undertaking of further research into the associated plant genetics is stressed49.