Soil Erosion, Climate Change and Global Food Security: Challenges and Strategies. Part 1.

This is the first 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


Abstract.

An overview is presented of the determined degree of global land degradation (principally occurring through soil erosion), with some consideration of its possible impact on global food security. Most determinations of the extent of land degradation (e.g. GLASOD) have been made on the basis of “expert judgement” and perceptions, as opposed to direct measurements of this multifactorial phenomenon. More recently, remote sensing measurements have been made which indicate that while some regions of the Earth are “browning” others are “greening”. The latter effect is thought to be due to fertilization of the growth of biomass by increasing levels of atmospheric CO2, and indeed the total amount of global biomass was observed to increase by 3.8% during the years 1981─2003. Nonetheless, 24% of the Earth’s surface had occasioned some degree of degradation in the same time period. It appears that while long-term trends in NDVI (normalized difference vegetation index) derivatives are only broad indicators of land degradation, taken as a proxy, the NDVI/NPP (net primary productivity) 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. The severity of land degradation through soil erosion, and an according catastrophic threat to the survival of humanity may in part have been overstated, although the rising human population will impose inexorable demands for what the soil can provide. However, the present system of industrialized agriculture would not be possible without plentiful provisions of cheap crude oil and natural gas to supply fuels, pesticides, herbicides and fertilizers. It is only on the basis of these inputs that it has been possible for the human population to rise above 7 billion. Hence, if the cheap oil and gas supply fails, global agriculture fails too, with obvious consequences. Accordingly, on grounds of stabilising the climate, preserving the environment, and ensuring the robustness of the global food supply, maintaining and building good soil, in particular improving its SOM content and hence its structure, is highly desirable. Those regions of the world that are significantly degraded are unlikely to support a massive population increase (e.g. Africa, whose population is predicted to grow from its present 1.1 billion to 4.2 billion by 2100), in which case a die-off or mass migrations might be expected, if population control is not included explicitly in future plans to achieve food security.


Keywords.

Soil organic matter, global warming, climate change, global food security, land degradation, soil degradation, soil erosion, population.


1. Introduction.

There are particular instances in history, where publication of a book has endowed a critical shift in human thinking as its legacy. Man and Nature; Or Physical Geography as Modified by Human Action is one such example, written by George Perkins Marsh. Published1 in 1864, this is probably the first time that the effects of human actions on the environment were documented, hence auguring-in what we now think of as the conservation movement. It was Marsh’s thesis that ancient Mediterranean civilizations collapsed as a result of land degradation: deforestation caused soils to become eroded, so declining in their productivity. Marsh observed that the same trends were occurring in the United States, as he describes in the following words:

“With the disappearance of the forest, all is changed... The face of the earth is no longer a sponge, but a dust heap, and the floods which the waters of the sky pour over it hurry swiftly along the slopes, carrying in suspension vast quantities of earthly particles which increase the abrading power and mechanical force of the current, and augmented by the sand and gravel of falling banks, fill the beds of the streams, divert them into new channels and obstruct their outlets...

From these causes, there is constant degradation of the uplands, and a consequent elevation of the beds of the water-courses and of lakes by the deposition of the mineral and vegetable matter carried down by the waters...

The washing of soil from the mountains leaves bare ridges of sterile rock, and the rich organic mould which covered them, now swept down into the dank low-grounds, promotes a luxuriance of aquatic vegetation that breeds fever and more insidious forms of mortal disease by its decay, and thus the earth is rendered no longer fit for the habitation of man.”

Soil is the fragile, living skin of the Earth. Consensus of opinion is that land degradation is a major threat to the environment and a potentially significant limitation to the likely success of feeding a human species that is becoming not only more populous (9.6 billion by 2050), but more affluent. The details, however, are complex, disputed and multifarious. In particular, estimates of the extent of soil erosion (by both water and wind), thought to be a principal cause of land degradation, and the provision of a global view of the situation are debatable quantities, since in reality, the process varies according to region, areal dimension and timescale. At issue too is determining the degree to which land is degraded and an exact relation of this to the loss of its agricultural productivity, since there are often insufficient hard data from which firm conclusions can be drawn. At one extreme, it has been proposed that around one third of the world’s available arable land has fallen victim to soil erosion in a 40 year period, and that globally soil is “lost” at a mean rate of 17 tonnes per hectare per year (t/ha/year). Others argue that the latter is an extravagant claim, being based on data derived from a limited number of measurements made in a small region of central Belgium, and cannot be taken to represent an “average rate of soil loss for Europe”, let alone for the wider world. A detailed compilation, based on data from a large number of measurements taken across the European continent, suggests that a more realistic value for the rate of soil erosion is nearer 1─5 t/ha/year, which is nonetheless greater than the natural rate of formation of new soil. It should be noted that “soil erosion” is a technical term and refers to soil that is moved around over an area, but which is not necessarily removed from it and deposited elsewhere. In addition, soil which is lost may be replaced by soil that has been removed from some other location, and so the net loss might be mitigated. Extreme events (e.g. floods, tornadoes) will move far more soil in a short period than is implied in quoting units such as t/ha/year, which is merely an average rate of loss over a far longer timescale (usually of several years or more).

The use (or misuse) of the Universal Soil Loss Equation (USLE) and modified versions of it, is also the subject of some debate regarding just how valid its predictions are of the rate of soil loss at the behest of particular influences that are ascribed by its parameters. The results seem to vary according to exactly where and under what circumstances the USLE is being applied, and in many instances it works very well but in others it fares less adequately. Determinations of the amount of soil being removed are often made on the basis of the amount of sediment that is deposited from some ascribed source. Discrepancies may be identified, however, since not all of the soil that is removed may end up at a single destination (e.g. a river delta), but some of it can become deposited/trapped in additional locations. That the presence of soil organic matter (SOM) is essential for the fertility of a soil has been known since the dawn of agriculture, and that this is impaired by the loss of SOM. The term soil organic carbon (SOC) refers to the elemental carbon content of a soil: this is distributed in different pools. Many of the world’s soils are depleted in SOC, some of them severely, a situation that is exacerbated by industrialized agricultural practices, and restoring the SOC, ideally through the products of photosynthesis, yields many separately identifiable but interconnected benefits. Thus, by increasing the SOC, the structure of the soil is improved, which increases its holding capacity for water, and allows better drainage, hence there is more groundwater and less flooding, while droughts are mitigated. The agricultural productivity of the soil is heightened, enlarging crop yields, and soil erosion is attenuated, especially if the ground is also covered by mulching or with cover crops. Degraded lands may be restored in their production through increasing their SOC content, giving better soil and water quality. The improved soil structure leads to a better retention of water and nutrients, and smaller inputs onto the farm of oil-based fuels, fertilizers, pesticides and herbicides, while biodiversity is enhanced. The term “biodiversity” does not only refer to what is visible above the ground, but also to the root systems of plants, and their accompanying fungal rhizosphere which is an essential part of the soil food web. The importance of rebuilding the soil food web – the ecosystem of microbes, and larger creatures that dwell in soil – is central to maintaining food production in perpetuity, i.e. achieving a system that is truly sustainable.

Of potentially great environmental significance is the prospect that climate change might be mitigated through the removal of atmospheric carbon, taken up by plants through photosynthesis, which is then stored in the soil. Afforestation/ reforestation is considered a key action in storing carbon in biomass (trees) and in soils. Sound management practices offer the potential to mitigate and adapt to climate change, and it is the latter that threatens to increase the potential for soil erosion, diminished soil quality, lower agricultural productivity, with expectedly adverse impacts on food security and global sustainability. Hence this provides one of the more severe tests that might be imposed on humans during the remainder of the 21st century: not only must we mitigate climate change but accept the reality of it and adapt our behaviour and practices to best effect. Relevant management practices are those pertaining to carbon, nitrogen, manure, in low-input systems (also known as sustainable agriculture) and grazing land. Management choices over conservation practices such as no-till, conservation agriculture, and returning crop residue (rubble) to the field to improve nutrient recycling can influence positively carbon sequestration and assist the mitigation of and our adaptation to climate change. Additionally, management of grasslands, restoration of degraded or desertified lands, nitrogen management, to reduce greenhouse gas emissions, precision conservation management on a field and/or watershed scale, along with other management choices can also aid in this cause. Management for climate change mitigation and adaptation is essential for environmental conservation, sustainability of cropping systems, improving the quality of soil and water, and ensuring food security.

According to some estimates, some 2─3 billion tonnes of carbon per annum might be stored in soils for the next 50 years. This should be compared with a current anthropogenic carbon emission level of ca 9 billion tonnes/year, of which around 50% is absorbed by existing terrestrial sinks, and hence around half the remainder could be drawn from the atmosphere and stored in soils, should prevailing emissions continue. However, it is practically certain that they will not, because our use of the fossil fuels (gas, oil and coal) will fall dramatically during this century, meaning that a potential draw-down of CO2 from the atmosphere is possible, perhaps by 50 ppm. While it is derived from biomass, biochar (the name specifically applied to charcoal when it is employed as a soil amending agent) requires energy to run the pyrolysis units that produce it. Biochar is most effective in improving the quality of poor soils, in which it seems to increase nutrient and water retention, and stimulates the growth of microbes in the soil, including mycorrhizal fungi, which both improves the soil structure and fertility and aids the decomposition (or immobilization) of pollutant materials as a bioremediation strategy. Permaculture may offer a host of advantages, including the production of food using reduced inputs of fuels, water and fertilizers, and an absence of pesticides and herbicides, while simultaneously building SOC. Though more readily applicable on smaller areas than are employed in most contemporary farming, such an approach taken by billions of individuals could prove of great significance in ensuring future food security and community resilience. It would be an oversight too, should “seed saving” not be mentioned in a general discussion of future food production. A consideration of these interrelated matters provides the essential substance of this review.

2. Soil.

Civilizations, throughout history, have thrived or fallen according to the goodness of their soils2, and our ability to feed ourselves and our animals depends on a sufficient access to high quality, fertile soil. Jethro Tull (1674-1741) introduced an improved seed-drill that enabled an efficient and consistent planting of seeds, such that the latter were used less wastefully. It was Tull, however, who conceived the erroneous belief that weed-seeds were introduced from manure, and that fields should be heavily ploughed in order to pulverize the soil and release nutrients from it. Guided by this line of thinking, in the 20th century, farmers ploughed fields well beyond the degree necessary to control weeds, and by a combination of such over-ploughing and drought, the Dust Bowls were created in the prairie regions of the Central United States and Canada. The social consequences of the latter were famously dramatised by John Steinbeck in his 1939 novel, The Grapes of Wrath.

Soil2 is made up of layers (soil horizons) which mainly consist of minerals that differ from their primary materials in texture, structure, colour, porosity, consistency, reactivity (pH, redox behaviour), and in chemical, biological and other physical characteristics. Soil is the final result of the consequences, in combination, of climate (temperature, precipitation), relief (slope), organisms (flora and fauna), primary materials (original minerals), and timescale. The material we know as soil (Figure 1) consists of rock particles that have been altered by chemical and mechanical processes, including weathering (disintegration) and accompanying mechanisms of erosion (movement). Soil forms a porous structure and may be envisaged as a three-state system (Figure 2): solid(s) (minerals – clay, silt and sand), liquid (water), and gas (air). The density of most soils lies in the range 1─2 g/cm³.

[Figs 1 and 2]

A good quality soil contains (by volume) 45% minerals, 25% water, 25% air, and 5% of organic material. In a given soil, the mineral and organic components are considered to be constant, while the percentages of water and air may vary, such that the increase in one is balanced by the reduction in the other, i.e. air may be driven out by water, or water be replaced by air as the soil drains. The simple mineral mixture of sand, silt, and clay will evolve, as time passes, into a soil profile that contains two or more horizons, which differ in certain properties, as indicated above. The depth of the horizons can vary considerably from one to another and the boundaries between them are rarely sharply defined. Since the pore space of soil contains both gases and water, the aeration of the soil influences the health of the flora and fauna it contains, but also the emission of greenhouse gases. The colour of soil depends principally on the minerals it contains. Many of the colours are owed to the presence of various iron minerals (Figure 3), and the development and distribution of colours in a soil profile are a consequence of chemical and biological weathering of the primary minerals present, particularly through redox reactions.

[Fig 3]

The soil-evolution process is most strongly influenced by the presence of water, since this medium can promote the growth of plant-life, the leaching of minerals from the soil profile, and the transportation and immobilization of various constituent components. Clay and humus are colloidal particles (< 1 micron in size) present in soil, both of which act as a repository for nutrients and moisture, and serve to buffer the variation in nature and concentration of cations and anions that are present in the soil. Thus, the contribution provided by these materials to the health and properties of soil is far in excess of what might be deduced from their relative proportion by mass of the soil. Colloids are able to solubilise, initially immobile, ions in response to changes in soil pH, and plant root behaviour. The availability of nutrients is also influenced by the soil pH. Most nutrients originate from minerals and are stored in organic material, both living (e.g. bacteria) and dead, and on colloidal particles as ions. The action of microbes on organic matter and minerals may release nutrients, render them immobile, or cause them to be lost from the soil by leaching when they are converted to soluble forms, or by their conversion to gases. Most of the available nitrogen in soils originates from the “fixation” of atmospheric nitrogen gas by bacteria. Of all the components, it is water that has the greatest influence on the formation and fertility of soil, even more so than soil organic matter (SOM)2.

2.1. Organisms

The activities of plants, animals, insects, fungi, bacteria, and humans too, all play a part in the formation of soil2. Fauna - e.g. earthworms, centipedes, beetles, etc. and microbes - mix soils by forming burrows and pores, which allow moisture and gases to diffuse through the soil matrix. As plant-roots grow in soil, channels are also created. Plants with deep taproots can penetrate the different soil layers by many metres and draw-up nutrients from considerable depths. Organic matter is contributed to the soil by plant-roots that extend near the surface, where they are quite readily decomposed. Micro-organisms, including fungi and bacteria, facilitate chemical exchanges between roots and soil and act as a reserve of nutrients. Soil-erosion may arise from the mechanical removal, by human activities, of plants that provide natural surface cover. The different soil layers may be mixed together by microorganisms, a process which stimulates soil formation, since less-extensively weathered material is mixed with more well-developed layers closer to the surface. Some soils may contain up to one million species of microbes per gram (most of these species being unclassified), making soil the most abundant ecosystem on Planet Earth. It is thought that one teaspoonful of soil may contain up to a billion organisms, which form the Soil Food Web (Figure 4).

[Fig 4]

Vegetation can prevent soil-erosion caused by excessive rain and resulting surface runoff. Plants are also able to shade soils, keeping them cooler and reducing the loss, by evaporation, of soil moisture; yet conversely, through transpiration, plants may also cause soils to lose moisture. Plants can synthesise and release chemical agents (including enzymes) – “exudates” - through their extended root-systems, which are able to decompose minerals and so improve the structure of the soil. Dead plants, fallen leaves and stems begin their decomposition on the surface, where organisms feed on them and mix the organic material into the upper soil layers; these additional organic compounds become part of the soil formation process. In addition to the essential characteristics of a particular soil – e.g. its density, depth, chemistry, pH, temperature and moisture - the precise type and quantity of vegetation that may be grown at a particular location depends on a combination of the prevailing climate, land topography, and biological factors.

2.2. Time

Soil formation is a time-dependent process that depends on the interplay of various different and interacting factors2. Soil is a continuously evolving medium, and it requires around 200─1,000 years to form a layer of fertile soil 2.5 cm (one inch) thick. Fresh material, e.g. as recently deposited from a flood, shows no trace of soil development because insufficient time has passed for the material to form a structure that may be later defined as soil. Rather, the original soil surface is buried, and the new deposit must be transformed afresh. Over a period ranging from hundreds to thousands of years, the soil will develop a profile that depends on the nature and degree of biota and climate. Soil-forming mechanisms continue to proceed, even on “stable” landscapes that may endure sometimes for millions of years. In a relentless process, some materials are deposited on the surface while others are blown or washed from the surface. At the behest of such additions, removals, and alterations, soils are always subject to new conditions. It is a combination of climate, topography and biological activity that decides if these changes are rapid or protracted.