The most significant Greenhouse gas (GHG) emissions from arable cropping in the UK are associated with the use of artificial nitrogen fertilisers. The other significant operation is cultivations (frequency, intensity, and depth) and the effect that has on Soil Organic Matter and subsequent GHG emissions.
Summary of emissions:
For arable cropping in general in the UK, the breakdown of GHG emissions is:
- 60 – 70% of all GHGs are related to artificial nitrogen (fertilizer) production and application
- 20% are related to fuel use and field operations
- 10 – 15% from P and K fertilizers, organic manures and liming
- 10% from sown seeds (emissions associated with its growing, processing etc)
- 1% from crop protection chemicals
Clearly understanding how nitrogen fertilizers contribute such a proportion of the total GHG emissions and the opportunities for reducing these is where to focus first.
There have been a number of studies, both in-field and through modelling techniques looking at the effect of reducing or cutting out all nitrogen (N) fertilizer applications.
Just reducing or removing N fertilizers from a conventional system will result in a corresponding reduction of yield and a number of studies have looked at the most N 'efficient' application rate for balancing the greatest yield of crop per gram of GHG emitted. For winter wheat this would be (depending on a host of normal farm variables such as soil type, previous cropping, winter rainfall and so on) around 150 kg N/ha which is well below the current commercially optimum rate of 190 - 210 kg N/ha that is the average over farms in the UK.
Not using any N fertilizers at all is a core part of an organic crop production system. This approach reduces GHG emissions on a farm basis, but because of the lower yields achieved and because organic farming relies on building up fertility through the use of leguminous crops (to fix nitrogen from the air) which are then ploughed in (with consequent release of GHGs). Therefore the carbon emissions from a ton of organic wheat compared to a ton of conventionally produced wheat is not so clearly different.
It has also been argued that since organic cropping requires a fertility building component (25 – 70%) in the rotation that is not producing any edible grains or oilseeds for humans, the lower intensity of the organic system requires extra land to be farmed when compared to conventional farming to produce the same overall quantity of product and thus any benefit from not using N fertilizers is reduced. There are equally robust arguments that challenge the current 'need' to produce large amounts of grains and oilseeds which are then fed to livestock rather than humans with the consequent overall reduction in available food and all the GHG emissions associated with livestock production.
Understanding how N fertilizer production and use gives rise to such large GHG emissions is important to understand the opportunities to reduce those emissions.