by Dr Anna Podgórska, Plant Biologist
Much worse than your personal carbon footprint: The nitrogen footprint of industrial agriculture
In a perfect world, plants would be able to grow under steady environmental conditions, have sufficient water and microelement supply, avoid the dangers of herbivores and pathogens, and receive just the right amount of light. Looking more specifically at a plant’s chemical ability to flourish, one of the most important contributors to its growth is the element Nitrogen. Plants require this mineral element in the greatest amount (2% of plant dry matter) among absorbed macronutrients. Nitrogen is incorporated into the structure of proteins, DNA, chlorophyll and all kinds of essential molecules, so no wonder it is considered a building block of an organism.
Nitrogen based fertilizers as a stepping stone to the plant’s perfect world?
Modern agricultural technology uses a few different tricks in attempting to create a plant’s perfect world – adjusting the environment that plants are grown in, using greenhouses, artificial irrigation, chemical pest control, artificial illumination, and, of course: fertilizers, mostly based on different forms of nitrogen. Given the fact that our world is covered in an atmosphere containing mostly nitrogen (the harmless nitrogen gas N2 is, with almost 80% , the main component of the air) – you might wonder why agriculture even has the need to use nitrogen derivatives. This is simply due to the fact that, unfortunately, the air’s nitrogen gas is not accessible for use by organisms. Plants are dependent on inorganic nitrogen forms supplied through the soil. So modern agriculture resorts to various derivatives of nitrogen – as a result of these interventions, a remarkable boost in crop quality and yield has been achieved over the recent decades.
However, all these measures come at the cost of excessive resource use and require large amounts of energy, water and fossil fuels, making agriculture the main global consumer of fresh water and the major emitter of CO2. But this is only half of the story. While it is commonly known that our excessive emission of carbon dioxide leads to an increase in the planet’s average temperature, which leads to devastating changes in the climate – the damage done by derivatives of nitrogen that are used in industrial fertilizers are less known.
Let’s have a closer look at nitrogen derivatives commonly used in modern agricultural fertilizers
One of the derivatives is nitrous gas (N2O) – commonly known as ‘laughing gas’. You might know this gas as an anesthetic used in medicine. (Just a side remark: short term exposure to laughing gas is not a problem, but long-term exposure would be dangerous for humans as nitrogen gas, when carrying oxygen, can act as a potent oxidizer). Especially at high temperatures, like on hot summer days, it can be classified as toxic since it can oxidize all kinds of biomolecules. At even higher temperatures it is used as an oxidizer in rocket propellants or engines.
Nitrous – alias “laughing” gas – is actually not that funny for our environment
The environmental effects that ‘laughing gases’ have are actually not that funny: Nitrous oxide is a particularly potent greenhouse gas as it is over 300 times (!) more effective at trapping heat in the atmosphere than carbon dioxide. This gas damages the ozone layer. Overall human-caused N2O emissions have increased by 30% over the past four decades. And what is important to mention here: Half of the N2O is generated by agriculture through the use of fertilizers. No wonder that industrial agriculture is a significant contributor to climate change. Additionally, nitrogen oxide causes an increase in particulate matter and acid rain, which definitely aren’t nice side effects.
And there is yet another harmful sibling of Nitrogen: Nitrate
Another main nitrogen derivative used in agricultural fertilizers is nitrate (NO3-). Unfortunately, nitrate used in agriculture doesn’t fare much better than nitrous gas: Plants which absorb high rates of NO3 after the harvest (Anna, is it after? Or before harvesting?) may be big and look nice and green, but are not really healthy. Additionally, consuming NO3- through food may be dangerous for humans. It can cause various issues like methemoglobinemia (blue baby syndrome), hypertrophic changes in the thyroid, and the endogenous formation of N-nitroso compounds, which are potent carcinogens.
But probably the most harmful effect of the use of nitrate-based fertilizers is, that most of its NO3- is washed out of the soil solution or evaporates into the atmosphere in the form of potent NOx-greenhouse gases. NO3- leaching is a major reason for water pollution, which affects eutrophication of all water reservoirs. The content of NO3- in drinking water is a major danger for human health, even in well developed countries and areas like the Münsterland, where despite good filtration systems, the ground water still carries high levels of NO3- .
To Summarize: Agribusiness as usual is not an option
In sum, the use of nitrogen derivatives in agriculture is not just an ecological problem affecting the biodiversity, but also decreases the availability of drinking water for us. The nitrogen pollution problem is increasing worldwide – in recent years this issue was recognized, and a European Nitrogen Assessment was formed to raise awareness. They came to the agreement that nitrogen containing agricultural runoff is not only contaminating our land, water, and food, but also the air. So, we actually should not only care about our Co2 footprint, but also about our nitrogen footprint. But is there a way to get out of this vicious circle?
Research in plant biology for a more sustainable use of fertilizers
As we have learned, the ways that nitrogen fertilizers (both inorganic and organic) are used in agriculture is far beyond optimal and actually gives rise to many problems.
Let’s go back to where we started: We said that in a plant’s perfect world, they would be able to grow in the most perfect of conditions. However, perfect growing conditions are usually not the case. But just like we put on clothes to protect us from cold, plants have evolved mechanisms that enable them to cope with unfavorable conditions.Those built-in mechanisms allow the plant to change its own physiology. And this is where research in plant biology, such as the work of Dr Anna Podgorska, WiRe Fellow in 2021/22, may help us implement a more sustainable agricultural use of fertilizers. To put it more specifically: Research like Anna’s will help us find a well-dosed and targeted use of fertilizers that are as unharmful as possible. Anna’s research currently revolves around the question: Is there an alternative for the use of nitrate?
Ammonium – the model sibling of nitrate and nitrous gas?
Anna’s specific starting point is the analysis of the effects of ammonium on the metabolism of plants. Ammonium – NH4+ – is yet another nitrogen derivative that is used as a fertilizer. From an economic standpoint, things look positive: the invention of the Haber–Bosch process, where atmospheric nitrogen (N2) converts to ammonia (NH3), enabled cheap production of NH4+ -based fertilizers. Ammonium as a nitrogen source for plants is particularly attractive when compared with nitrate, as it is less susceptible to leaching from the soil solution. Also, from abiological point of view, ammonium seems to be the better nitrogen source for plants because it does not need to be reduced in order to be assimilated into metabolism. So, you might wonder, why don’t we use ammonium as the only nitrogen base in agriculture?
One of the most intriguing phenomena in plant physiology is that despite ammonium being more efficient energetically, most plants cultured on ammonium as the sole nitrogen source exhibit serious growth inhibition. In research, this is commonly referred to as ‘ammonium toxicity syndrome’. To date, the mechanisms underpinning the ammonium toxicity in plants have not been resolved. Anna’s research focuses on understanding mechanisms that affect plant development under ammonium nutrition. If we improve nitrogen use efficiency and aid the targeted breeding of plants that can handle ammonium fertilization efficiently, we will be able to reduce our industrial agriculture’s nitrogen footprint. This in turn will help us reduce global warming and keep our planet as liveable as it currently is.
Understanding plants’ physiology and metabolism will help us save our climate
To put it in a nutshell, plants’ potential of adapting to various environmental conditions is insufficiently understood, especially with regard to crop breeding. This is where Anna’s research comes in: Understanding the plants’ mechanisms of environmental monitoring and acclimation is of fundamental importance if we are to transition to a sustainable agriculture that will also be fundamental to a climate friendly bioeconomy.