Monday, July 28, 2014
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.
Saturday, July 26, 2014
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.
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.
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.
Wednesday, July 23, 2014
5. Climate change.
As noted later, the effects of climate change must be considered in the context of soil erosion and land degradation: in particular a more vigorous hydrological cycle is anticipated as a result of rising global temperatures, with much harder rainfall in some regions19. Hence, in the absence of mitigating measures, the future rate of soil erosion can be expected to increase on the global scale. There are a number of prevailing factors at play, the most significant being the erosive force of rainfall; however, we must also consider the following: (a) the changing moisture regime might alter patterns of biomass growth, and affect the canopy layer; (b) the latter may amend both the plant residue decomposition rates (since the underlying processes are driven by temperature and by moisture and are connected with the activity of soil microbes) in addition to the biomass production rates; (c) in consequence of varying rainfall and evapotranspiration rates - which changes infiltration and runoff ratios - the soil moisture content might be affected; (d) a fall in the SOM content may weaken the structure of some soils, rendering them more susceptible to erosion, and the amount of runoff could be increased as a result of surface sealing and crusting; (e) non-erosive winter snowfall might turn into erosive rainfall as the winter temperatures increase; (f) when permafrosts melt, a previously non-erodible soil state can be converted to a highly erodible form; and (g) land use changes, e.g. to grow more cereals, as the global climate and local weather patterns change, may lead to further erosion.
It has been estimated that a ca 1.7% change in soil erosion is likely for each 1% change in total precipitation resulting from climate change20. It is common practice to deal with a loss in soil fertility from erosion by applying greater quantities of artificial fertilizers, which actually compounds the problem [since the soil food web (Figure 4) dies back further2] and incurs yet more water and soil pollution, as opposed to simply giving the land sufficient time to regenerate naturally. The relationship between the mycorrhizal fungi in the soil and, for example, phosphate fertilizer can be thought of as pseudo-addictive, since the fungi act symbiotically with the roots of plants, delivering to them phosphorus (and other nutrients) drawn from the soil, in return for carbohydrate bestowed to the fungi from the plant (formed by photosynthesis). The over-application of artificial phosphorus discourages the growth of the fungi, with the result that the plants become increasingly dependent on artificial phosphorus inputs.
6. Monitoring, measuring, and modelling erosion.
To model erosion accurately is difficult, because of the complexity of the detailed processes of erosion, which involve aspects of climatology, hydrology, geology, chemistry, and physics, etc. Due to the non-linear nature of erosion models, they tend to be difficult to use numerically, and it is accordingly difficult or impossible to make predictions about large scale events on the basis of results taken from plots on much smaller areas. Erosion models are either process-based or empirically based. The former models are physically based and provide a mathematical description of the processes of detachment, transport, and deposition: by solving the equations that describe them, estimates are obtained of soil loss and sediment yields that occur from a given land surface area. The science of erosion is not sufficiently developed that some input of empirical data can be avoided. The fundamental difference between process-based and other types of erosion models is that the sediment continuity equation is used, as is discussed later. Empirical models relate management and environmental factors directly to soil loss and/or sedimentary yields on the basis of statistical methods21. A detailed review22 of process-based and empirical erosion models has been published, including a discussion of conceptual models, which are a kind of intermediary stage between the process-based and entirely empirical models. The most usual model that is employed to assess the degree of erosion and to permit an outlook toward conservation strategies is the Universal Soil Loss Equation (USLE), which remains under improvement and development.
6.1 The USLE.
The USLE was developed using a comprehensive range of data taken from erosion plot and rainfall simulator experiments: in all, over 10,000 plot-years worth of actual data, taken from 50 different locations in 24 U.S. states was used to calibrate the input parameters23. The USLE contains six factors, according to which the long-term average annual soil loss (A) is estimated, which are: the rainfall erosivity factor (R), the soil erodibility factor (K), the topographic factors (L and S) representing length and slope, and the cropping management factors (C and P). The equation has the simple linear structure:
A = RKLSCP
The unit plot concept is important in the context of the USLE, and is defined as the standard plot condition to determine the soil's erodibility, i.e. when the LS factor = 1 (slope = 9% and length = 72.6 feet) where the plot is fallow and tillage is up and down slope and no conservation practices are applied (CP = 1). Under these conditions:
K = A/R
Wischmeier et al.24 have devised a more straightforward means for the estimation of K, the soil erodibility factor, which involves the particle size of the soil, its SOM content, soil structure and profile permeability. If sufficient information is available, K can be approximated from a nomograph. By knowing the length and gradient of the slope, the LS factors can be determined from a slope effect chart. The cropping management factor (C) and conservation practices factor (P) are determined empirically from plot data, and are described in soil loss ratios [i.e. (C or P with) /(C or P without)]. Erosion is measured and analysed using e.g. the micro-erosion meter (MEM) and the traversing micro-erosion meter (TMEM). The MEM has been used successfully to measure bedrock erosion in a range of ecosystems across the globe, and is able to determine both terrestrial and oceanic erosion. There is also the Revised Universal Soil Loss Equation (RUSLE), which is an extension of the USLE, and other related variants. A highly informative and practical description of the use of the USLE can be found at: http://www.asu.edu/clas/shesc/projects/medland/documents/soilerosion.pdf
7. Validity of universal soil loss estimates.
Pimental et al. have asserted25 that globally, soil erosion rates are lowest in the U.S. and Europe, “averaging about 17 metric tons ha-1 year-1”. Boardman26 has investigated the origin of this figure and concluded that it is actually derived from an uncritical extrapolation of data taken from just 12 experimental plots in three small areas of central Belgium, reported by Bollinne in his Ph.D thesis at the University of Liege. [A range of soil loss of 10─25 t ha-1 year-1 (which “average” to ca 17 t ha-1 year-1) has been claimed from Bollinne’s work25]. Hence it is of some interest to know how much erosion is actually occurring across Europe, according to a more extensive compilation of measurements, taken over a larger and more representative area of the continent. Indeed, the available data indicate that the process is quite variable in its extent26, both in space and time, and ranges from (assuming a soil density of 1.4 g cm-3, since some data are quoted in units of m3 ha-1 year-1) < 1 t ha-1 year-1, over a 90 km2 area in southern Sweden, to ca 16 t ha-1 year-1 in northern France, but taken from just 33 small catchments. Arden-Clark and Evans quote27 that erosion rates in the U.K. are 1─20 t ha-1 year-1, while noting that those in the higher part of the range are rare and localised events, and typical rates of erosion are 1─2 t ha-1 year-1. Measurements made over a 709 km2 total area of localities in England and Wales produce a mean28 of 3.2 t ha-1 year-1.
Ryszowski29 reckoned the soil erosion rate in Poland to be 0.52 t ha-1 year-1. Hence, overall, it might appear that the average value of 17 t ha-1 year-1 quoted25 by Pimental et al. represents a considerable overestimate of the rate of soil erosion in Europe. Crosson too has taken issue with the figure30, this time as applied to soil erosion in the U.S. According to the USDA in 1989, an average of 17 t ha-1 year-1 is moved from U.S. croplands as a result of the combined effect of wind and water erosion; however, Crosson cites more recent data showing that by 1992, the rate had fallen to 13 t ha-1 year-1, and refers to a paper by Lal and Stewart31 in which it is stated that, annually, some 36 billion tons (metric tons = tonnes is meant) of soil are eroded worldwide: 10 billion tons from natural phenomena and 26 billion tons as a result of human activities. Crosson notes that Lal and Stewart cite a paper by Brown32 as a source for the 26 billion ton estimate, but this is based on erosion measurements for the United States, which it would appear are higher than those for Europe. At any rate the 36 billion ton figure is considerably less than the 75 billion metric ton value assessed elsewhere to be removed by wind and water erosion, and mostly from agricultural land.
In their response to Crosson, Pimental et al. assert33 that since the United States has about 11% of the world’s arable land (and approximately the same amount of pasture land), and an estimated total soil loss of 4.5 billion tons per year, assuming that the rest of the world suffers similar rates of soil loss, a total global soil loss of 40 billion tons per year is indicated. They further stress that the rates of soil erosion in Asia, Africa and South America are about double those in the U.S., and hence the 75 billion ton annual global loss of soil appears reasonable. They defend too, their contention that some 80% of the world’s agricultural land has been degraded, though Crosson criticised30 this figure as a more than three-fold overestimate, based on the GLASOD34 [Global Assessment of Soil Degradation] study by Oldman et al. which reports that about 1.03 billion ha of agricultural land has suffered moderate-to-strong erosion as a result of wind and water, or less than 25% of the roughly 4.5 billion ha global land-base, under crops, pasture and range. In a later paper, Trimble and Crosson point out the considerable disparity in quoted values for the annual soil losses through erosion from croplands (442 million acres = 179 million ha) across the United States, which vary from 2 billion to 6.8 billion tons, which suggests an average lost soil depth of ca 1─3 mm35. Some are of the opinion that the recent rates of soil erosion are as high as those in the 1930s, though others disagree35.
On the basis of on-farm productivity effects, it was concluded that should the rates of erosion that prevailed in 1982 continue for the next 100 years, the crop yields (output per hectare) would be reduced only by around 2─4%, and hence increased federal funding to reduce the erosion is not justified. Most of the estimations made of soil erosion are based on models, particularly the Universal Soil Loss Equation (USLE) although this itself has been developed and “calibrated” using more than 10,000 plot-years worth of actual data, taken from 50 different locations in 24 U.S. states. Its successor, the Revised Universal Soil Loss Equation (RUSLE), has been improved using further experimental data and in two studies of measured soil loss rates, taken from over 1,700 plot-years worth of data on 205 research plots in 20 locations across the U.S., it was shown that the USLE and RUSLE actually predict rates of soil loss reasonably well, even for post-1960 conditions23. A limitation of USLE and RUSLE is that they predict the amount of soil moved on a field, which is not necessarily the same as the amount of soil physically removed from the field. To estimate the latter, the sediment delivery ratio (SDR) is determined. The SDR is given by the amount of sediment delivered from an area divided by the gross erosion of that same area. Expressed as a percentage, SDR reflects how efficient the watershed is in moving soil particles from an area where erosion is occurring, to the location at which the sediment yield is measured. The model assumes that a relatively small amount of eroded soil leaves a field or stream basin, and some sediment is presumed to be deposited by wind on the field, or along streams as alluvium.
However, it is often assumed that the USLE measures soil actually removed from the land, and the variance in SDR according to particular conditions is not taken account of. As an example, in the 1970s, the sediment delivery to streams from a 3 km2 area in Wisconsin amounted to just 8% of the USLE prediction35, with the remainder thought to be stored as colluvium (loose, unconsolidated sediments that have been deposited at the bottom of hillslopes by either rainwash, sheetwash, slow continuous downslope creep, or a combination of these processes). In contrast, in the 1930s, the sediment delivered was 123% of the USLE value, due to the then frequent effect of gullying downslope from agricultural fields. Also in the 1930s, it was common that the skies over the eastern United States were filled with enormous clouds from the Dust Bowls, which moved out over the Atlantic Ocean, due to severe wind erosion. It is thought that, more recently, the erosion process amounts more to local redistribution than wholesale loss. To predict the latter phenomenon, the Wind Erosion Equation (WEE) has been used, and as with the USLE there is a mass discontinuity problem: namely that even though soil may be eroded from one area, most of the particles are simply moved onto other fields, and so the net soil loss may be overestimated. It has been claimed that large areas of the U.S. have annual erosion rates of >25 t ha-1, and yet the sediment yields were often in the range 0.5─2.0 tonnes ha-1, including a contribution from significant erosion of the banks and stream channels35. Thus, the total sediment delivered to streams has been reckoned at 2.7─4.0 billion tons, but the actual amount measured at this destination is nearer 0.5 billion tons, with the inference that a large quantity of the sediment must be stored in the watershed. Alternatively, as Trimble and Crosson have averred35, the rate of soil removal may be far less than is apparent using the USLE, calling for more field studies and monitoring of sediment mass, a view endorsed by Nearing23.
Cerdan et al. have used erosion plot data to compile a comprehensive database with which to investigate the rates and spatial variations in sheet and rill erosion across Europe36. It was demonstrated that land use is overwhelming in its influence on erosion rates, which may be greater by an order of magnitude on conventionally tilled arable land, as compared with those surfaces that are permanently covered by vegetation. The erosion rates tend to be lower in the Mediterranean, due to the protective effect of the rocks in the stony soils there. However, because these soils are already very thin, any further loss of soil could be highly detrimental. The average rate of sheet and rill erosion was determined to be ca 1.24 t ha-1 year-1 for the entire area covered by the CORINE database (essentially the whole of Europe - both eastern and western – plus Turkey), and 3.6 t ha-1 year-1 for arable land. Evidently, there are “hot spots” for erosion, but these are masked by quoting a figure for an average erosion rate for Europe. A review22 has been published of the USLE and its family of related models, as applied to the determination of event soil loss, and runoff. It is concluded that the predictive power of the method works well in some locations but poorly in others. One problem in the use of the USLE to predict event erosion is that it was originally designed to model long-term average soil loss, and the event rainfall-runoff factor contained in it does not consider runoff explicitly.
When runoff is measured or estimated reasonably accurately, the prediction may be improved, but in incorporating the direct runoff in the rainfall-runoff factor an impact on some of the other factors used in the model is incurred. Parsons et al. have attempted to predict the travel distance of different particle sizes to provide a model for the erosive impact of specific rainfall/runoff events37: this is of particular relevance to the European situation, in terms of the off-site impact of runoff and erosion. The model only considers simple cases of erosion under a spatially uniform rainfall and on slopes of uniform infiltration and gradient, but it is independent of measurement area since it rests upon the notions of entrainment and travel distances of particles, and so on sediment flux. The results resolve the “paradox” of sediment-delivery ratio, and some of the recent discussion over the validity of erosion rates made from USLE erosion plots; potentially, erosion rates can also be reconciled with the known life-spans of continents. It is concluded that some of the accepted estimates of erosion rates are fallacious, and those that are based on measurement of inter-rill erosion (short runoff plot) are likely to be much larger than reality (for entire hillsides). That noted, the significance of travel distance increases the importance of rill and gully erosion, and of the movement of fine particles to which nutrients are pollutants adhere preferentially. It is clear that far more actual experimental data must be garnered - taken on a range of scales, from small plots to hill-slopes and catchments – in order to evaluate the predictions of the model, and indeed to provide a true image of global soil erosion and land degradation.
8. Measures and realities of global soil erosion.
Results from remote sensing measurements show that the total amount of biomass over the earth’s surface has increased3,4. This may be a result of carbon dioxide fertilization, where photosynthesis and hence plant growth is stimulated by elevated levels of CO2 in the atmosphere. Globally, the amount of biomass was measured to increase by 3.8% during the years 1981─2003. However, it was also determined that 24% of the global land area suffered some degree of degradation during the same period. Hence, there are regions of “greening”, while elsewhere “browning” has occurred. According to a recent study made by the Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO) and the Australian National University, in some of the earth’s driest regions (arid regions of Australia, North America, the Middle East and Africa) foliage has increased by 11% during the past three decades, as a consequence of CO2-fertilization38. Due to different definitions and terms, as noted earlier, there is a wide variance in the estimates of both the degree and rate of global land degradation. Most of the policy decisions over land have been made on the basis of two major sources: the extent of global desertification by Dregne and Chou39, and that of global land degradation by the International Soil Reference and Information Centre by Oldeman et al.34, termed Global Assessment of Human-induced Soil Degradation (GLASOD).
The two sets of figures are not strictly comparable, because while Dregne and Chou considered only dry areas they also included the status of vegetation on the rangeland. The terminology also differs between the two studies: Dregne and Chou used the terms38 slight, moderate, severe and very severe to denote the degree of degradation, while Oldeman et al. used the terms34 light, moderate, strong and extreme, and hence the degree of correspondence between the levels of designation in the two studies is uncertain. Oldeman et al. tried to separate natural degradation from that which had been caused by human activities. Eswaran and Reich40 attempted to determine the vulnerability of land to degradation and desertification, but only considered arid, semi-arid, and sub-humid regions, according to the definition of UNEP; their estimates of water erosion also include humid areas. According to GLASOD34, of the different erosion mechanisms, it is water erosion that is the most important, and afflicts some 1,094 million ha (56%) of the total area that is impacted upon by human-induced soil degradation. Globally, wind erosion affects 548 million ha (38%) of the terrain that is degraded. Chemical soil deterioration affects 239 million ha (12%) of the total, and physical soil deterioration occurs over 83 million ha (4%). The most important subtype of displacement of soil material is through the influence of water or wind. An area of 920 million ha is affected by water erosion (365 million ha in Asia, and 205 million ha in Africa), and 454 million ha by wind erosion. The principal chemical deterioration of soils involves the loss of nutrients and this affects 135 million ha worldwide, of which 68 million is in South America. Salinization follows next in order of its impact, and afflicts some 76 million ha globally, of which 53 million ha is in Asia. An area of 22 million ha is affected by pollution, of which 9 million ha is located in Europe. The most significant subtype of physical soil deterioration is compaction, and occurs over an area of 68 million ha, of which 33 million ha is in Europe, and 8 million ha is in Africa.
GLASOD categorises four degrees of soil degradation. “Light” refers to a somewhat reduced productivity of the terrain, but which is manageable in local farming systems, applies to 38% of all degraded soils (749 million ha). “Moderate” requires improvements which are often more than can be achieved by local farmers in developing nations, and accounts for 46% of the Earth’s degraded soils. Thus 910 million ha of the Earth’s surface has a greatly reduced productivity: >340 million ha of these moderately degraded lands are in Asia and >190 million ha in Africa. There are 296 million ha globally of “strongly degraded” soils (124 million ha in Africa, and 108 million ha in Asia), and it is these that it is not possible to reclaim at the farm level and which may therefore be regarded as lost land. These terrains can only be recovered through major engineering work and/or international assistance. Finally, soils that are “extremely degraded” are regarded as irreclaimable and beyond restoration, amounting to a global total of 9 million ha (>5 million ha in Africa).
GLASOD is not without its critics41, and indeed its authors were well aware of, and the first to indicate, its limitations: principally that it was based on the perceptions of experts, rather than being a direct measure of land degradation. More recently3,4, methods of remote sensing have been applied to determine the extent of global land degradation: LADA (Land Degradation Assessment in Drylands). These aim to determine the degree and trends of land degradation in drylands, degradation hotspots and bright spots (both actual and potential), using changes in net primary productivity (NPP) as a proxy measure of land degradation. [Net primary productivity (NPP) is defined as the net flux of carbon from the atmosphere into green plants per unit time. NPP refers to a rate process, i.e. the amount of vegetable matter produced (net primary production) per day, week, or year]. The most heavily degraded regions are identified to be Africa: south of the equator (13% of the global degrading area and 18% of lost global net NPP), South-East Asia (6% of the degrading area and 14% of lost NPP), south China (5% of the degrading area and 5% of lost NPP), north-central Australia and the western slopes of the Great Dividing Range (5% of the degrading area and 4% of lost NPP), the Pampas (3.5% of degrading area and 3% of lost NPP) and swaths of the high-latitude forest belt in Siberia and North America, directly affecting the livelihoods of the 1.5 billion people who live there. The results indicate that 24% of the total global land surface has suffered degradation during the past quarter century, and may be compared with the 15% of the world’s soil (not land) being degraded, according to the GLASOD study.
Much of the degradation identified by GLASOD33 does not overlap with the areas newly highlighted by LADA, demonstrating that land degradation is both cumulative and global. The authors stress that land-use changes which reduce NDVI [remotely sensed Normalised Difference Vegetative Index (Figure 6)], e.g. from forest to cropland of lower biological productivity, or an increase in grazing pressure, may or may not be accompanied by soil erosion, salinity or other symptoms of land degradation that are of concern to soil scientists. They note further that while long-term trends in NDVI derivatives are only broad indicators of land degradation, taken as a proxy, the NDVI/NPP trend is able to yield a benchmark that is globally consistent and to illuminate regions in which biologically significant changes are occurring. Thus attention may be directed to where investigation and action at the ground level is required, i.e. to potential “hot spots” of land degradation and/or erosion.
Montgomery42 has made a global compilation of studies which confirms the long held contention that the erosion rates from conventionally ploughed soils are 1─2 orders of magnitude greater than the background rates of soil production, of erosion under native vegetation, and long-term geological erosion. He concludes that on a global basis, hill-slope soil production and erosion evolve to balance geologic and climate forcing, whereas agriculture based on conventional ploughing increases the rates of erosion to unsustainable levels. At a rate close to 1 mm/year of soil loss (amounting to around 14 t ha-1 year-1), net erosion rates in conventionally ploughed fields can erode a typical hill-slope profile on a timescale of major civilizations, whereas no-till methods of farming cause rates of erosion that are nearer to those of natural creation rates of soil, and hence might set the cornerstone of a system of sustainable agriculture.