Soils of the Crofts

70 What is nitrogen? Nitrogen is a gaseous element that forms 78% of our atmosphere. With oxygen constituting 21% and other gases making up the remaining 1% (primarily argon, carbon dioxide and very small amounts of other gasses). Why do we need it? Nitrogen is an important component of proteins, the building blocks of life. Proteins help build our bodies structure and act as enzymes. Enzymes allow chemical reactions to take place within our bodies. Nitrogen is also a major plant nutrient, required for healthy and vigorous growth, as it is a major component of chlorophyll. Chlorophyll allows plants to photosynthesise, and thus grow. Where do we get nitrogen? Although the largest source of nitrogen on Earth is held within the atmosphere, it cannot be used directly by the vast majority of plants and animals. Plants and animals must therefore gain nitrogen from other sources. Some bacteria can remove nitrogen from the atmosphere, a process called ‘nitrogen fixation’. Some plants form symbiotic relationships with nitrogen fixing bacteria. This is where each party gains an advantage working together. The plants gain a source of nitrogen and the bacteria gain access to carbohydrate, a food source, from the plants. Legumes, such as peas, beans and clover form this relationship with a bacteria called ‘Rhizobium’. Farmers and crofters often plant clover with grass, the grass benefiting from the clover ‘fixing’ nitrogen from the atmosphere. The nitrogen cycle (see diagram on page 71) The nitrogen cycle describes the way that nitrogen moves through the Earth’s ecosystem. Bacteria capable of fixing nitrogen take nitrogen from the atmosphere. Plants can then use this nitrogen to grow. Animals can then eat the plants, allowing them to grow and transferring the nitrogen from the plant to the animal. When a plant or animal dies, bacteria and fungi begin the action of decomposing the dead plants or animal tissue. They use the components of the dead tissue, the carbohydrate, proteins and minerals to grow. Some of the nitrogen present within the tissue is not absorbed by the bacteria and fungi, but released as ammonium (NH 4 + ) and nitrate (NO 3 - ). Ammonium and nitrate do not have any organic elements in their structure, so are called ‘mineral nitrogen’ compounds. The ammonium and nitrate released within the soil can be absorbed through plant roots and used for growth by the plants. When mineral forms of nitrogen, i.e. ammonium (NH 4 + ) and nitrate (NO 3 - ), are used by plants, bacteria and fungi, it is said to be ‘immobilised’. This is where mineral nitrogen, ammonium (NH 4 + ) and nitrate (NO 3 - ) is converted to organic compounds, e.g. plant, animal, bacterial tissue. Not all of the ammonium (NH 4 + ) and nitrate (NO 3- ) released by decomposition is immobilised. Some undergoes a process called ‘denitrification’. Denitrification is where some bacteria convert the ammonium (NH 4 + ) and nitrate (NO 3 - ) to gaseous nitrogen forms i.e. nitrogen (N 2 ), ammonia (NH 3 ) and nitrous oxide (N 2 O). This gaseous nitrogen can then enter the atmosphere, where it can be ‘fixed’ by other bacteria, thus starting a new cycle.