The information in the blog below comes from a paper published last year which examines the different management options that are available to farmers to improve nitrogen use efficiency within farm production systems. To read the full paper please click here.
A common fact that we are presented with, which frames this debate is the need to feed more people with more food using less resources. Nitrogen plays a big part in this, as nitrogen is a key component of crop growth and increased yield, which will be needed to feed more people. In the past, increased food production has been made possible by the production and use of commercial Nitrogen fertiliser (with associated increases in emissions).
In order to address the current need for sustainable intensification, nitrogen use efficiency is a key component. How we increase crop productivity at the same time as protecting natural resources and the environment must be intrinsically linked with managing nutrients more efficiently and minimising losses.
Improving Nitrogen use by crops
When crops are grown under laboratory conditions, the amount of nitrogen which is taken up by the plant ranges from 45-65%, while on-farm the plant nitrogen uptake (as a percentage of applied nitrogen) is often below 40%. This demonstrates that there are opportunities to alter management practices to more efficiently use Nitrogen inputs to reduce N losses that affect direct and indirect nitrous oxide emissions.
The issue is not as simple as just reducing the amount of N fertiliser that is applied, as this would jeopardise sustainable food production.
The grand challenge is how to improve Nitrogen use efficiency that leads to reduced nitrous oxide emissions while also achieving greater N effectiveness in crop and livestock production (i.e. more food output per unit of N input).
Other challenges exist with improving efficiency in that there is no single management change that can bring about both increased crop productivity and reduced nitrous oxide emissions equally well across different soil and climatic conditions.
Another compounding issue is that direct nitrous oxide losses are equivalent on average to about 1% of the nitrogen applied (so not a massive economic loss). To gain greater interest and be more likely to achieve significant reduction in both direct and indirect nitrous oxide emissions, it will be necessary to focus more broadly on practices which lead simultaneously to greater nitrogen use efficiency and effectiveness.
Nitrogen losses that occur through volatilisation of ammonia (from spreading slurry and manure on warm days, or application of urea), and nitrate run off (when Nitrogen is applied and the crop is not actively taking it up), leaching and drainage pathways may receive more attention as these losses represent a greater economic loss to the farmer than direct nitrous oxide emissions.
Research shows that ‘mismatched timing of Nitrogen availability with crop need is probably the single greatest contributor to excess N loss in annual cropping systems.’ There is a global initiative that details the ‘4R Nutrient Stewardship initiative, which is based on the principle of using the right nutrient source, at the right rate, right time and in the right place to achieve the basic economic, social and environmental elements of sustainability.
It is also important to remember that adapting management to mitigate losses of nitrous oxide must be balanced with other considerations.One of my commonly used adages is that we work within complex biological systems, and as such it is important to not simply ‘swap pollutants’ between leaching and gaseous losses.
Use of precision farming technology
An increasing number of farmers and crop advisors around the world have access to GPS resources and GIS. This along with advances in technologies may make it increasingly possible to better match N rates and times of application with are sensitive to in season crop N demands.
The use of Nitrogen sensing technology aims to better match crop nitrogen needs with in-season sensitivity that leads to improved Nitrogen use efficiency, and greater farm profitability.
American studies have shown that using Nitrogen sensors across 16 trial sites could potentially save farmers 10-50kg of nitrogen per hectare on maize.
The uptake of these technologies by farmers is now well underway, indeed this paper states that the Yara N-Sensor was being used on more than 1.2 million hectares of the total 104 million cropland hectares in the EU-27.
Not synchronising nitrogen applications with crop needs has been cited as potentially the single greatest contributor to excess nitrogen loss in annual cropping systems. However achieving synchrony in nitrogen supply may not always reduce cumulative nitrous oxide emissions, instead, management practices that influence the rate of nitrification and soil nitrite accumulation, may be most likely to reduce nitrous oxide emissions.
Winter cover crops help provide soil cover and thus minimise erosion, help to build organic matter levels (and soil carbon), and help in the capture and retention of excess inorganic nitrogen. However there are some studies which suggest that in some soils cover crops stimulate and increase nitrous oxide emissions because of the release of carbon and nitrogen from crop residues (I will be writing another blog on this subject in a couple of weeks). Site variables including soil type, irrigation, variety and mix of cover crop, soil structural stability and organic matter Nitrogen mineralisation will all impact on the level of nitrous oxide that is emitted.
Management of livestock
One of the most promising ways for many livestock growers to enhance nitrogen use efficiency is to more optimally manage the protein content of the diet. In ruminants, the bulk of excreted Nitrogen is in the urine, while in pigs it is present in urine and faeces. Nutritionists can aim to make adjustments in crude protein levels in the diet to match animal nutritional requirements and significantly reduce ammonia and nitrous oxide emissions. Taking full account of manure nitrogen content, and maintaining optimum stocking rates, as well as the inclusion of clover within pastures can potentially raise the whole farm N use efficiency from 30% to nearer 65% which reduces Nitrogen losses and improves farm profits.
Management of manure
Incorporating manures greatly reduces ammonia emissions, with leaves a more ‘potent’ product, which is then more susceptible to losses of nitrous oxide. However when the manure is incorporated, and ammonia losses reduced, effectively a smaller amount of that manure is needed to provide the crop’s nutrient requirements and therefore the potential for nitrous oxide production is reduced.
This trade-off is often seen, and reducing losses of ammonia is important (since these losses indirectly emit nitrous oxide).
Improving nutrient use efficiency in livestock production systems will require site specific and targeted combinations of genetic improvements, feed planning and rationing for utilisation, and improved storage, handling and application of manure.
An emphasis on increased crop or animal outputs per unit of Nitrogen input (more efficient use of inputs) will help nitrous oxide emissions and also safeguard natural resources and improve profits. As with all agricultural issues though, we can’t look at nitrogen in isolation, research, policies and advice need to consider how to improve other essential nutrients, water and production as they all impact on nitrogen use efficiency.