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The nitrogen cycle Nitrogen is a critical component of plants. It is a structural component of chlorophyll, nucleic acids (DNA, RNA) and proteins. While abundant in air, nitrogen in the atmosphere cannot be used directly by either plants or animals and must be converted into a usable state. Rainfall often contains substantial quantities of nitrogen in the form of ammonium (NH4+) and nitrate (NO3-). On contact with the soil, both ammonium and nitrate enter the soil solution easily and are absorbed by plant roots. The microbial decomposition of organic matter results in the mineralisation of organic nitrogen to release the ion NH4+ to the soil. Depending on temperature, moisture, the level of soil aeration and presence of some plant species, NH4+ is oxidised to NO3- , both of which are readily available to plants. Nitrogen may also occur in the soil as the result of mineral weathering, animal urine and through the application of mineral fertilisers (see page 11). Certain types of bacteria (e.g. Rhizobium) can convert atmospheric nitrogen (N2) to ammonia (NH3) through a symbiotic relationship with the root nodules of leguminous plants such as clover (Trifolium) or soybean (Glycine max). This process is known as nitrogen fixing. Plants convert the ammonia to nitrogen oxides and amino acids that form proteins and other molecules. In return, the plant provides sugars to the bacteria. In order to fix nitrogen, plants such as legumes maintain an oxygen-free (anaerobic) environment near their roots so that the bacteria can exist. Soil pH, organic matter levels and the availability of trace elements such as copper can influence the distribution and activity of these specialised bacteria. In natural ecosystems, plant growth is relatively slow and the annual uptake of nitrogen is comparatively low (e.g. 30 kg N ha-1). Cultivated crops are much more demanding with nitrogen uptakes that are several orders of magnitude greater (e.g. 500 kg N ha-1). In these cases, the natural nitrogen cycle is unable to maintain optimum growth and artificial inputs must be added to the soil. Harvesting is a critical process as organic matter that would have normally decomposed on the soil surface is often physically removed from the field for processing. This means that there is an export of nitrogen and other elements from the soil to the marketplace. Corrections of N shortages through the addition of mineral fertilisers result in an increase in vegetative growth, much higher protein levels and greater yields of grain and fruit. However, excess amounts of N, above what can be used by plants, are often flushed out of the soil to accumulate in water bodies. Under the right conditions, nitrogen-driven bacterial growth can deplete the oxygen in nearby water bodies to the point that fish and other aquatic organisms die. This process is known as eutrophication. Volatilisation loss Urea CO(NH2)2 Ammonia NH3 Ammonium NH4 Immobilisation Denitrification Nitrate NO3 Leaching loss The principle pathways and transformations of the nitrogen cycle. Nitrogen is important to all life. Nitrogen in the atmosphere or in the soil can go through many complex chemical and biological changes, be combined into living and non-living material, and return back to the soil or air in a continuing cycle. Animals and plants deposit organic nitrogen into the soil. Bacteria convert organic nitrogen to plant-usable ammonium and ammonium to plant-usable nitrates. Denitrification describes the bacterial decomposition of nitrate in the soil to atmospheric nitrogen while volatilization turns urea fertilisers and manures on the soil surface into gases that go directly to the atmosphere (see also page 11). (LJ) Nutrient depletion in Africa Several studies have highlighted significant nutrient losses from African soils [28-31]. Models estimate that on average, 660 kg N ha-1 have been lost during the past 30 years from about 200 million ha of cultivated land in 37 African countries (excluding South Africa). The FAO estimates that Africa is losing 4.4 million t N every year from cultivated land - these rates are several times higher than Africa's annual fertiliser consumption of 0.8 million t N [28, 32]. N loss is driven by cultivation on nutrient-poor soils, a breakdown of traditional soil-fertility practices and poverty in rural Africa which does not permit effective fertiliser management practices. The role of soil elements in plant growth Macronutrients Macronutrients are elements that are essential to plant growth and are needed in significant amounts. For more details, see [7a]. Potassium (K) is crucial to most plant functions including stomatal control, the maintenance of turgor pressure and charge balance during selective ion uptake across root membranes. It is also an enzyme in many biochemical reactions. Potassium is highly mobile and is easily leached from leaves to be taken up in high quantities by soil microorganisms and roots. In soil, potassium may be found in minerals such as micas and feldspars, secondary aluminium silicates (e.g. illite) and some salts. Potassium is available when attached to clay and humus colloids and easily available when in solution. Potassium dissolved in soil solution as an ion is highly leachable, although loses of potassium from runoff and erosion is not a significant problem. Calcium (Ca) is used to build cell walls in plants. It helps keep P available in the root zone by binding it with other ions. Because it is bound within cell walls, it does not leach from leaves nor circulate within the plant. Calcium deficiency leads to stunted plant growth, the curling of young leaves and death of terminal buds. Calcium can be easily leached from the soil and is largely absent in the soils of central Africa. Magnesium (Mg) is the central atom of the chlorophyll molecule and is an important enzyme. It is very mobile in plants. Magnesium deficiency in plants causes yellowing between leaf veins. Low soil pH decreases the availability of magnesium to plants. Phosphorus (P) is crucial to many plant functions, a key component of most fertilisers, and often lacking in non-fertilised soils. Phosphorus forms the backbone of DNA and RNA molecules and regulates cell division, root development and protein formation (see adjacent text). Phosphorus is responsible for crop yield increases. The benefits of nitrogen (N) are described in the adjacent text. Micronutrients The phosphorus cycle Micronutrients are elements that are essential to plant growth but are required in very low concentrations (< 100 µg/g). They are generally metabolically active in plants as important enzymes. Phosphorus is another vital plant nutrient that forms the backbone of DNA and RNA molecules, forms cell membranes and regulates cell division, root development and protein formation. The future of phosphorus? Phosphorus deficiency can occur in areas of high rainfall, on acid, clayey or poor calcareous soils. Symptoms include poor growth and leaves that turn blue/green but not yellow. Due to the movement of phosphorus in plants, the oldest leaves are affected first. Fruits are small and taste acidic. Due to the essential nature of phosphorus to living organisms, the low solubility of natural phosphorus-containing compounds and the slow natural cycle of phosphorus, the agricultural industry is heavily reliant on fertilisers containing concentrated phosphoric acids (H3PO4). About 50% of the global phosphorus reserves are in North Africa and the Middle East. Large deposits of phosphate-bearing apatite exist in China, Russia, Morocco and the USA. Iron (Fe) primarily originates from chemical weathering of minerals and is not absorbed by plants in appreciable quantities; the amount found in plants is several orders of magnitude lower than the amount in the surrounding mineral soil. Iron serves as an electron carrier in enzymes. It also plays a role in nitrogen fixation and chlorophyll formation. Its movements in soil are due to chemical processes rather than an association with organic matter or uptake by plants. The presence of iron oxide gives a reddish tint to soil horizons. Recent reports have suggested that production of phosphorus may have peaked, leading to the possibility of global shortages by 2040. However, some scientists now believe that a "phosphorus peak" will occur in 30 years and that reserves will be depleted in the next 50 to 100 years. [33] Manganese (Mn) is critical to many plant functions, including photosynthesis, respiration, and nitrogen metabolism. Manganese is generally plentiful in acid soil and may reach toxic levels if pH is below 6.5. It generally leaches out of acidic soils. Due to its high reactivity, inorganic phosphorus is never found as a free element, and geologically occurs as phosphate rocks (PO43-). In natural ecosystems, phosphorus is released by the decomposition of organic matter as compounds known as orthophosphates (e.g. H2PO4-, HPO4-). These are rapidly adsorbed by soil particles or immobilised by phosphorus-consuming bacteria (e.g. Aspergillus). The pool of phosphorus that is readily available to plants is found in solution or loosely bound on to soil particles. Because of low concentrations of P in soil solutions and the competition from soil microorganisms, many plants have developed a symbiotic relationships with mycorrhiza – a type of fungus – which in essence extend the root network and provide an improved pathway for the rapid transfer of phosphorus. Zinc (Zn) is a key component of growth control hormones in plants and is used in protein synthesis. Almost half of the world’s cereal crops are deficient in zinc, leading to poor yields while zinc deficiency is the 5th leading risk factor for disease in developing countries. Zinc in soils is tightly bound to magnesium. Copper (Cu) is especially plentiful in acidic, sandy soils and is an important enzyme activator found mostly in the chloroplasts of leaves. The highly weathered, iron-rich tropical soils of Africa tend to be deficient in plant-available phosphorus. The low pH together with high levels of iron and aluminium oxides, tend to imobilse phosphorus onto soil particles thus denying its availability to plants. In such situations, large quantities of phosphorous fertiliser must be added to the soil to make a difference in crop yields. Lime-rich soils also imobilise phosphorus. Toxic elements Pollutants are contaminants that have been introduced into the natural environment and cause instability, disorder, harm or discomfort to the ecosystem. Artificially high levels of all elements can have harmful or toxic effects. Aluminium (Al) is not used in significant amounts by plants. In soils, it immobilises phosphorus and generally increases the acidity and concentration of cations. As for most elements, aluminium becomes toxic to some plants above 1 ppm and to most plants above 15 ppm. As in the case of nitrogen, it is estimated that around 0.5 million tonnes of phosphorus is lost every year from cultivated soil in Africa - double Africa's annual P consumption [22]. Photograph taken through mycorrhiza (the faint, thin around the roots of a plant Such symbiotic relations can phosphorus from soils. (PDI) a microscope of filaments) growing (brighter features). help plants extract Lead (Pb) binds with organic matter in the soil and accumulates in certain organic tissues of plants. In high enough concentrations, it can cause brain damage in humans. Other elements that are toxic to plants include arsenic (As), cadmium (Cd), sodium (Na) and even iron (if concentrations are high enough). Introduction | Soil Atlas of Africa 33