Since the ‘Green Revolution’ of the 20th Century, the application of artificial nitrogen fertilisers throughout the world has become commonplace. Their use increases yields thus creating the opportunity for farmers to maximise profit. However despite the fact that 70% of the nitrogen applied is wasted, application levels remain high in order to optimise yields. This fertiliser wastage is a major cause of environmental concern, therefore any activity to increase the nitrogen uptake within crops could reduce the need for high application rates. This conversion is called nitrogen use efficiency.
Nitrogen Use Efficiency is in essence the dry mass productivity of a plant per unit nitrogen taken up. There are therefore two key considerations in its calculation: the ability of the plant to take up nitrogen and the efficiency with which nitrogen is used. These can be driven by either genetic or environmental factors.
Research into wheat production has shown that genetic effects are relatively small in comparison to environmental factors affecting NUE (Barraclough et al., 2010) (Godfrey et al., 2010). These environmental factors include agronomic practices based upon climate or geography. As available nitrogen increases so NUE decreases, yet the improved yields from increased available nitrogen prevent farmers from reducing their applications. Nevertheless if it were possible to maintain yields whilst improving NUE from reduced application levels this would result in economic and environmental benefits for all stakeholders.
There is little that can be done to affect the temperature, rainfall or soil type in which a crop is grown, particularly when global climate changes increase the level of uncertainty. However use of predictive modelling can assist in helping farmers to manage the risks that they face and to adopt different agronomic practices accordingly. Indeed predictive modelling may also help farmers to make decisions about either the variety or type of crops that they grow so that they may better be able to optimise yields.
A number of alternative agronomic practices have been researched to improve NUE, which collectively can be termed as precision nitrogen management. This includes:
- Site-specific nitrogen management of crops using sensor based technology to assess soil and crop nitrogen content, airborne imaging, red edge and infra-red photography, hyperspectral imaging and various other diagnostic tools and models can help farmers to manage fertiliser applications only where they are needed in the crop. A comparison of these systems against common agricultural practices for a wheat crop showed an improvement in NUE of up to 368% and potential fertiliser reductions between 10-80% (Diacono, Rubino and Montemurro, 2013).
- Timing of fertiliser application has been shown to be highly important in affecting NUE. The risk of nitrogen loss has also been shown to be greater when applied in autumn than in spring regardless of tillage system for corn crops (Vetsch and Randall, 2004). Rapid early season nitrogen uptake is also beneficial for the plant to establish its root development system which has the effect of increasing tolerance to stress and improve nitrogen uptake later in the season.
- Management of crop rotations – particularly to include legumes where it has been shown that higher wheat yields are achieved after a legume crop, due to the symbiotic relationship with the nitrogen fixing rhizobium bacteria (Hirel et al., 2011). The legume crop could be either a legume green manure or crop such as peas.
- Animal sources of nitrogen from grazing the land contribute to improve soil organic matter and build microbiological activity. Such efforts to improve the soil health enhance mineral fertiliser uptake and improve the water holding capacity of the soil (Fageria and Baligar, 2005). Since nitrogen uptake is limited by water availability this can only serve to improve NUE particularly in climates with low rainfall.
- Research has been carried out into the effects of arbuscular mycorrhizal fungi on nitrogen use efficiency. Although these fungi have been shown to improve nutrient uptake by plants (Adesemoye and Kloepper, 2009), evidence suggests that the effects on nitrogen uptake are stronger where available nitrogen is low (George, Marschner and Jakobsen, 1995).
Breeding varieties that are better able to take up nitrogen from the soil and use it to make better grain (with higher %N content) will also improve NUE. In cereal crops multiple studies have been undertaken to show the genetic basis of NUE, however the complexities of the trait as well as its interaction with the environment have yielded varying results (Garnett et al., 2015). Some examples of genetic emphasis and breeding focus are listed below:
- Exploration of whole plant physiological studies combined with associated underlying plant metabolic pathways can be used to understand, research and manipulate different steps of nitrogen uptake, assimilation and recycling to the final deposition in the seed (Hirel et al., 2007). However although some efforts have been made to improve NUE in crops, the exact regulatory pathways remain elusive and therefore the inclusion of NUE in crop breeding programmes is very limited (Li, Hu and Chu, 2017).
- High rooting densities to allow more efficient N uptake from soils and may facilitate nitrogen capture from subsoils which may hold nitrate from previous cropping cycles (Thorup-Kristensen, 2002).
- The development of ‘stay-green’ crop types which relate to the delayed foliar senescence has been shown to improve nitrogen uptake and therefore the overall NUE in crops such as rapeseed (Bouchet et al., 2016) and wheat (Kipp, Mistele and Schmidhalter, 2014). Furthermore the ‘stay-green’ trait has been shown as a valuable trait for improving crop stress and therefore hold a valuable key to resilience to potential future climate change (Thomas and Ougham, 2014).
Breeding for NUE alone is highly complex however the inclusion of NUE in genetic strategies for future varieties and cultivars is important. Despite this some businesses have bred commercial NUE varieties for example Arcadia Biosciences markets an NUE rice which (it claims) has 30% more nitrogen uptake than other rice varieties (Arcadia Biosciences, 2010).
Although there is a clear case both environmentally and commercially to improve nitrogen use efficiency in crops, unfortunately until there are economic consequences which will encourage an improvement in agricultural practices there is little incentive for farmers to reduce their artificial fertiliser use. Whilst there are clear practical opportunities to improve nitrogen use efficiency, it may be necessary to combine this with the development and proper application of environmental policies (Lassaletta et al., 2014) in order to develop a clear improvement strategy in order to reduce the use artificial fertilisers whilst achieving global food security.