improving the use of nitrogen in agriculture

Over recent years the use of fertilisers containing nitrogen has increased with demand for food.  Using these fertilisers adds nitrogen to the soil allowing a greater yield of crop to be grown.  They are used in both arable and livestock farming, spread or sprayed not fields for cattle to graze on.  In a previous post I discussed the problems relating to increased use of these fertilisers, namely nitrous oxide emissions, an important greenhouse gas.  These nitrous oxide emissions are resultant of both mineral fertilisers containing nitrogen and organic fertilisers such as animal manure.  A major issue arisen recently is that these fertilisers are not efficiently used.  In this post, articles suggesting various mitigation strategies to improve the efficiency of fertiliser use and consequently reduce nitrous oxide emissions will be discussed.  

Articles by Monteny et al (2005) and Pautisan et al (2004) discuss various strategies for reducing emissions describing and explaining how they work, as summarised bellow:

• Type of fertiliser - This is important as there are many different types of fertiliser and some such as nitrogen based fertilisers result in greater emissions than others, for example ammonium.  
• The use of slow release fertilisers - These have been formulated to attempt to coincide nitrogen release with plant growth.  Here the fertiliser has been coated meaning the release of nitrates is much slower and much more controlled.  This reduces nitrous oxide emissions as only one application of fertiliser is necessary and there is a much smaller pool of nitrogen in the soil, restricting loses.  
• Addition of nitrification inhibitor - this can be added to fertiliser, examples being Nitrapyrin and Dicyandiamide.  These inhibitors aim to delay the transformation of Nitrogen into nitrous oxide helping to match the timing of the supply with crop demand.  
• Land drainage - It is commonly thought that there is a relationship between nitrous oxide emissions and water filled pore space.  When water filled pore space is above 70%, it results in significant nitrous oxide emissions.  Therefore improving the soil’s physical conditions, for example reducing soil wetness through draining, will reduce emissions.  Wet, compact soil conditions lead to anaerobic conditions, enhancing denitrification.  
• Conducting soil Nitrogen tests - This is where soil is tested to discover how much fertiliser is actually required to achieve the desired crop yield.  It is a common problem that soil is over fertilised and too much nitrogen is added to the soil, meaning that the plants only use their required amount and the excess is lost, either as nitrous oxide or through leaching.  Therefore by measuring how much is in the soil and knowing how much is needed for the crop it is possible to add sufficient fertiliser to grow the crop rather than adding an excess amount.  Mcswiney and Robertson (2005) undertook a study in South West Michigan USA over three years to show that nitrous oxide emissions increase mainly in response to additions of nitrogen to crops that exceeded their needs.  They added different amounts of various fertilisers to nine fields and measure nitrous oxide fluxes, available nitrogen in the soil and grain yields.  They found that nitrous oxide fluxes could be reduced by using less fertiliser, whilst having no effect on crop yield.   
• Improving the timing of fertiliser addition - Often fertiliser is not added when the plants are at their full capacity to absorb it, consequently leading to losses through leaching and as nitrous oxide.  Chambers et al (2000) state that in order to reduce losses fertiliser should not be applied between autumn and early winter.  
• Cover crops - these can also reduce nitrous oxide emissions from soil as they can catch any residual nitrogen left in the soil instead of it being left bare, where it would otherwise be emitted as nitrous oxide or leached.  

An article by Zhu and Chen (2002) looks at the success that some of these strategies have had in China.  Through undertaking filed micro plot experiments with nitrogen based fertiliser they were able to demonstrate that nitrogen recovery in rice plants when fertiliser is added to the crop in its early growing stage, is in the range of 22-52%.  Whereas this can be increased to 55-69% when added at the vigorous growth stage, reducing nitrogen loss.  Another possible strategy discussed in this article is deep placement.  This is where fertiliser is placed deep into the soil, rather than just sprayed on the surface.  This again was successful in reducing nitrogen loses, in Fengqiu, Henan Province, nitrogen loss through ammonia volorisation was between 20-48% when urea was surface broadcast.  This was reduced to 11-18% when placed deep into the soil.  

There seem to be many positives regarding these strategies to reduce nitrous oxide emissions, many are easy to implement, are cost effective and are shown to be successful.  Most are related to improving the efficiency in using fertilisers, improving timing and reducing wastage though excess application.  Using control released fertilisers or nitrification inhibitors have the slight disadvantage in being more expensive, but in the long run less fertiliser is used and consequently the cost of labour to implement them is reduced which is attractive to farmers.  However a disadvantage is apparent that nitrous oxide fluxes are reliant on environment, experimentation is required on the local scale to develop an optimal nitrogen management scheme, which requires longterm investment and research.