Nitrogen (N) is the most important nutrient for plant growth and therefore farming but its management is the most complex and it has the largest negative environmental impact. This negative impact is as a result of nitrate nitrogen leaching into waterways and causing eutrophication, as well as turning into nitrous oxide gas (N2O) which has 298 times the global warming potential of carbon dioxide and ammonia gas which is a significant environmental pollutant. Therefore, ensuring that the nitrogen used in agriculture grows plants and not pollution is essential for any responsible and competent farmland management strategy.
Not all N is Equal
However, not all supplies of nitrogen used in agriculture are equal in that regard. Synthetic/mineral forms of nitrogen fertiliser, such as urea, ammonium nitrate (AN), and calcium ammonium nitrate (CAN), have a number of issues. Nitrate nitrogen is the main form that leaches from soil which means that fertilisers based on nitrate nitrogen pose a direct risk of leaching loss if they are used where there is soil drainage. Urea, the dominant form of N used in NZ agriculture, while having the highest concentration of N and lowest cost per kg N delivered (which is what drives its popularity) can’t be directly used by plants so it has to be first converted to ammonium before plants can absorb it. This can result in significant N losses via ammonia gas as the urea is converted to both ammonium and ammonia at the same time, which often means that in terms of plant absorbed N, urea is less economic than AN and CAN.
However, all the mineral, i.e. soluble forms of N are higher risk than biological nitrogen. Biological nitrogen is ‘fixed’ from atmospheric N by bacteria in symbiosis with the legumes, such as peas and clover. Biological nitrogen fixation turns the fixed nitrogen into biological forms of nitrogen, i.e. amino acids, which in turn form proteins. These biological forms of N can neither leach from the soil like nitrate, nor form gases such as nitrous oxide and ammonia, so they are inherently safer for the environment.
The amount of N that can be biologically fixed is also equal to that from the bag. Figure 1. shows the range (minimum to maximum) of nitrogen fixed, and that left behind after harvest for a range of leguminous crops. This clearly shows that the potential for biological nitrogen fixation is substantial, and can easily match, and even exceed that applied, or increasingly permitted, via synthetic nitrogen fertilisers.
Figure 1. The range (minimum to maximum) of nitrogen fixed and that remaining in the soil after harvest by a range of leguminous crops.
Biological does it Better
Current scientific research is also showing how much the legumes directly share the nitrogen they fix with other plant species they are growing with, such as grass. This is through direct root-to-root contact and also mediated by mycorrhizae fungi that link plant roots together and help them get nutrients from the soil.
Putting together all the issues around synthetic N fertilisers, such as leaching and global warming, vs. the ecologically sustainable biological nitrogen fixation, which can supply as much N as bag fertilisers, while also increasing the palatability and nutritional status of pasture and further enhancing ecosystem services such as soil health, it is clear not all nitrogen is the same.
Biologically fixed N is superior ecologically and economically.