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.