The danger here in WANA is that unwise agriculture could release it back into the air.  This is especially so of the steppe, the great plains used for livestock grazing that cover vast tracts of land from Mongolia, through Central Asia, to the Middle East. This is fragile land. In hot, dry environments, carbon sequestration is much lower than in the cold, wet lands of the North (the best example of carbon sequestration is a peat bog or a coal seam--both composed of ancient biomass that was originally created from carbon in the atmosphere). At Maragha, the research station in the steppe where ICARDA works with Syria's Steppe Directorate, the soil layer containing carbon is just 2 cm deep. Unwise cereal cultivation on this land had caused loss of natural shrubs leading to  soil erosion, and overgrazing has not helped (see The battle for the steppe in Caravan No. 3). Such carbon as had been sequestered into that soil and vegetation will now be released back into the atmosphere as CO₂ because of rapid oxidation.
        This happens quickly. Since cultivation of the North American prairie started around 1900, no less than 60% of the soil carbon content has been lost. A graphic illustration of how this process happens was the horrific Dust Bowl of the 1930s.  The biomass that contains the carbon was also holding the soil together. Anyone who thinks that soil erosion is a subject solely for scientists should read John Steinbeck's The Grapes of Wrath to understand the scale of the tragedy. The land blew away.
        Are we to conclude, from all this, that agriculture in fragile environments--for example, WANA--reduces carbon sequestration, and automatically contributes to global warming?
        No.

lobal warming has become one of the chief preoccupations of popular journalism.  Emission of greenhouse gases, we are told, has led to the greenhouse effect--a heating-up of the Earth's climate that will change the ecology of every part of the world and  reduce the polar ice-caps, and may put the Statue of Liberty under water. Every coastal city will be at risk; the sea will lap around the streets of New York, Bombay and London, to name just three.

        The destruction of the Amazonian rainforests for unsustainable agriculture has, we are told, hastened this process by robbing the world of its lungs, so that "bad" gases hold sway over "good". So agriculture is one of the villains of the piece?
        Well, yes, in that instance it could be. But it is more complicated than that. The great majority of greenhouse-gas emissions come from industry and motor vehicles. Emission of such gases has global impact irrespective of their geographical origin. North America is the biggest offender, followed by first Eastern and then Western Europe. And, arguably, appropriate and well-planned agriculture in fragile environments can slow the process.
        First, let us define the problem. The world is, without question, getting warmer. The temperature has always fluctuated, but what is happening is something more.  Between 1900 and 1950, there was a 0.4°C increase; then, from 1990 to 1996, there was an increase of 0.2°C--a dramatic acceleration. The 10 warmest years in the last 130 have all occurred in the last 15 years. The Intergovernmental Panel on Climate Change (IPCC) stated recently that "the balance of evidence suggests there is a discernible human influence on global climate."
        The gases causing this (the "greenhouse gases") are: methane (CH₄), emitted by ruminants (including sheep and cattle) and termites; nitrous oxide (N₂0), from wetlands and fertilizer use; and carbon dioxide (C0₂), from the burning of fossil fuels (motor cars included!), from burning of straw, from slash-and-burn practices and from deforestation. All these are potentially relevant to agriculture. Methane we can forget for our purposes. Although it is emitted by livestock, the amount produced on the vast feedlots of North America dwarfs any produced in the Middle East, despite our spiralling flock sizes. Nitrous oxide is produced by agriculture; it originates as nitrate fertilizer put into the soil, and if the soil is poorly aereated, the soil bacteria will cause it to be emitted as N₂0 instead of as oxygen. But this happens more in flooded land such as rice paddies, which are not a major feature in the West Asia and North Africa (WANA) region where ICARDA mainly works. We will return briefly to nitrogen later, but the main one of the trio we are interested in is carbon dioxide.
        It is definitely linked to global warming. Geological surveys, and investigation of ancient air-pockets in the polar ice-cap, indicate that it was present in the air at the rate of around 300 parts per million (ppm). In 1900 it was about 290 ppm. But in 1996 it was 360 ppm and is expected to rise at the rate of 2-3 ppm over the next 50-100 years.
        As we have said, most of the carbon dioxide warming up our planet is emitted by the industrial world. The role of agriculture here is not that it is producing C0₂; rather, it is that its introduction into areas that were permanent forest prevents CO₂  being harmlessly absorbed and retained in the vegetation. Most of the carbon is not in the air; it's in the sea and in the ground, and that is where it should stay. It is pulled into the land by plants, to be converted into plant matter--that is, biomass.
        But deforestation of the rainforests destroys the existing biomass, replacing it with agricultural biomass that doesn't last, because the laterite below cannot support it for long. Writing in Nature in 1994, Canadian and Venezuelan scientists reported that continuous agriculture without supplementary fertilization was viable for 65 years on temperate prairie, but just three years on Amazonian soil--after which it had no potential for agriculture at all.
        Carbon sequestration will then cease altogether on that piece of land. Just what effect this could have is illustrated by the amount of carbon held in the soil on ICARDA's experiment station in Syria. Held in the top 20 cm, it is 16 tons per hectare. And that is a low figure, for millenia of agriculture have left the carbon level quite low in the Fertile Crescent.

        On the steppe, it may be better to avoid cultivation. But responsible grazing will do no harm. And, in areas where settled agriculture is sustainable, the right sort of cropping can increase carbon sequestration. On the ICARDA experiment station, scientists have been running long-term rotation trials since 1984. Recent tests showed that, where medics, which are feed legumes, had been introduced into the rotation, carbon content had increased from 1% to 1.3%. The reason is that legumes have a large below-ground biomass. Thus more plant residues are retained in the soil. All legumes are good for this (though some less so than others--with lentil, for example, the root system is pulled out at harvest). The effect is multiplied, because the cereals grown in rotation with legumes give better yield; this means more below-ground biological activity, and an accumulation of nutrients, especially N. Carbon sequestration goes up.
       As for wind erosion, this won't happen with proper agronomic practices, provided the soil is suitable for agriculture in the first place. Minimum tillage is essential. Anything which raises yield sustainably is also helpful. This includes nitrogen. Fertilizer is not necessarily bad. The issue is how it is managed. The example of nitrous oxide being released into the atmosphere applied only to flooded fields. If the soil can breathe, it won't happen.
        There is a further aspect--aggregation. This is the degree to which the soil holds together in larger rather than smaller particles. Larger are better; Dr Zuhair Masri of Syria's Soil Directorate, who has just completed a PhD thesis under the supervision of the first author, showed that increased carbon content following incorporation of legumes in the rotation increased soil aggregation and thus water ingress, as there is space between the particles, making the soil more porous. This will obviously make the land more productive and will lead to greater water-use efficiency; and to higher yield, producing more biomass and increaing carbon sequestration.
        This has encouraging implications. If the process of carbon enrichment in the soil could be accelerated and high levels  maintained, the soil will be much improved; the danger of soil erosion will be reduced, and so will the levels of one "greenhouse gas" in the atmosphere.
       These observations give cause for great optimism. ICARDA has been working for years on the replacement of fallow and/or continuous cereal with feed legume/cereal rotation. The purpose was to break the pest/disease cycle, raise yields and provide an alternative source of feed for sheep, bringing net benefits to the farmer (see Feed for the future in Caravan No. 1). Now it seems that these rotations are actually improving the physical and biological properties of the soil.
        From this work, we have drawn three conclusions:
• Improved soil organic matter--e.g. carbon--is critical for semi-arid areas.
• A build-up of this matter is compatible with intensive farming in the dry areas.
• Intensive agricultural production and the need to prevent global warming are also quite compatible.
        Or, in other words, protect the environment--grow more food! ICARDA will be pursuing a number of related themes in its research from now on. It will look, for example, at tillage, seasonal variations, stubble residue decay (an important part of the WANA farming system--see Stubble--a burning issue), the impact of sewage sludge on soil organic matter, and the biomass carbon/nitrogen relationship. But one thing now seems certain. With very careful management, fragile environments can--up to a point--be farmed intensively. And doing so should actually slow down global warming.

Dr John Ryan is Soil Scientist, ICARDA. Mike Robbins is Science Writer/Editor, ICARDA.