In many natural environments nitrogen is the primary factor in limiting plant growth (provided there is sufficient water supply and an absence of pests and disease). It is an essential macronutrient for plant metabolic pathways, protein synthesis and is a major component of chlorophyll. Although nitrogen accounts for 78% air, plants can only utilise reduced sources of this element such as the inorganic forms of ammonium (NH4–) and nitrate (NO3–) salts from soil. In order to become available for uptake, plants rely on a process of nitrogen fixation which is carried out naturally by fixation microorganisms. Some nitrogen fixing bacteria symbiotically associate with particular plant groups such as rhizobium bacteria and legumes. Despite this only about 2% of soil nitrogen is naturally found in inorganic form.
As with any business, farmers seek to maximise profit and typically in cropping systems this equates to optimising yield. In wheat systems, high yields of high quality grain (i.e. high % nitrogen content) requires high input and uptake of nitrogen (Barraclough et al., 2010). Whilst grain and fertiliser price ratios are favourable, farmers are driven to apply high levels of nitrogen above those that are most efficient for crop stability in an effort to maximise potential yields and returns.
The process of nitrogen fixation is energy intensive since it requires the breaking of the triple covalent bond of N2 and as in nature, the industrial manufacture of nitrogen fertilisers requires significant amounts of energy to break the bonds of N2 molecules. Conventional agriculture has relied upon the Haber Bosch process as a critical source of synthetic nitrogen fertiliser which catalysed the “Green Revolution”. As a result the use of synthetic fertiliser is an essential element in delivering global food security.
Nitrogen use efficiency on a global basis for cereal products is approximately 33% (Raun and Johnson, 1999) and between 20-30% for rice (Peng, Tang and Zou, 2009). The nitrogen that is not taken up by plants is lost through leaching, volatilisation or denitrification. Rainfall or irrigation will cause nitrate to leach from the soil into water systems. If excessive nitrate leaches in this manner it can lead to eutrophication, algal blooms and hypoxia (Canfield, Glazer and Falkowski, 2010). Volatilisation occurs when ammonium salts convert to ammonia gas which is lost to the atmosphere. Denitrification occurs in anaerobic conditions in saturated soils, when microorganisms convert nitrate back to gaseous forms of nitrogen including nitrous oxide which is a powerful greenhouse gas and cause of ozone depletion. The use of nitrogen fertilisers has been a major contributing factor in the increase of atmospheric nitrous oxide by 20% over the past century (Thomson et al., 2012).
As a result of the intensive energy requirement during manufacture and damaging environmental impacts of nitrogen fertilisers, as well as the economic cost throughout cropping supply chains it has become essential to increase the nitrogen use efficiency of crops in order to reduce the use of artificial fertiliser and minimise the wastage currently demonstrated across the world’s most important crops. Such action is essential to ensure a more sustainable agriculture.