Since the 19th century we know that certain microorganisms play a key role in the nitrogen cycle. They are responsible for carrying out essential steps in the process, producing different forms of nitrogen that can be used by plants. Nitrogen-based fertilisers used in agriculture accelerate this cycle with major environmental and economic consequences. Thus, an ‘intelligent’, slow and progressive distribution of nitrogen could be used to limit many of these negative effects.
Fertilisers put nitrogen into soil
Plants need nitrogen to grow, using either ammonium or nitrate (nitrogen-rich molecules) as a source. Ammonium finds its way naturally into soil at a rate of 110 million tonnes per year. This happens through deposition after lightning and the death of organic material, but mainly via the fixation of nitrogen from the atmosphere such as in the nodules of leguminous plants. Nitrate, however, is the result of conversion from ammonium by microorganisms.
Even though plants can take up both, it is the conversion of ammonium to nitrate, which has major consequences on agricultural systems. Vast quantities of nitrogen (an additional 100 million tonnes per year) added to soil in the form of fertilisers accelerates microbial activity, resulting in the overproduction of nitrate from ammonium.
This excess nitrate is responsible for the negative environmental impact because it makes the nitrogen more mobile, increasing its pollution potential by allowing it to move out of the soil in water or even into the air. Nitrate pollution in runoff water, groundwater and rivers encourages algal blooms and contaminates drinking water. It also leads to substantial increases in the emission of nitrous oxide (N2O), the third most important greenhouse gas, concentrations of which have increased by 20% since pre-industrial times. N2O is also tipped as the compound primarily responsible for depletion of stratospheric ozone in the 21st Century.
Intelligent fertilisers: a smart solution
The main effect of ‘intelligent’ fertilisers is to favour a slow, gradual release of nitrogen over weeks into soil. In doing so, the proportion that is actually used by the plants is increased, thereby increasing efficiency of the fertiliser. Hence, the activity of soil microorganisms is reduced and therefore less ammonium is converted to nitrate. The result of such is less nitrate available to leach into waterways, reducing pollution.
Another benefit of progressive-release fertilisers is to reduce hazardous N2O emissions. Studies have shown that large-scale, rapid application of ammonium fertilisers favours growth in microorganism populations capable of transforming them at high concentrations. This results in a dramatic increase in activity of certain microbial populations, particularly those which contribute the most to nitrous oxide emissions. Hence, by using a slow-release fertiliser instead, the overactivity of these microorganisms is prevented resulting in lower emissions.
These progressive-release fertilisers respond adequately to some of the problems posed by traditional fertilisers, at least. But this is not the only approach we can take. Another example is to use inhibitors to limit the growth of bacterial populations that cause the negative consequences of nitrogen transformation. The idea is not to remove them, or sterilise them, but rather it starves them, to control their activity.
This allows for both better control of the nitrogen balance in soil and ensures that crops will benefit from most of the added nitrogen fertiliser instead, making it possible to reduce the quantities added to soil. The two approaches are not exclusive, of course. Anything that makes it possible to reduce nitrogen-associated pollution in intensive agriculture is welcome.