Given the world's growing food demands, nitrogen fertilizer use is likely to increase – but using too much fertilizer can pollute waterways and the air. A new study led by Princeton University and published in the journal Nature explains how more efficient fertilizer use can benefit food production and the environment.
According to authors, the paper is among the first to globally analyze "nitrogen use efficiency" -- a measure of the amount of nitrogen a plant takes in to grow versus what is left behind as pollution.
Looking at fertilizer and crop harvest data in regions like the United States, Western Europe and Sub-Saharan Africa, the researchers examined how policies and market conditions have influenced farmers' use of nitrogen fertilizers over the past 50 years.
Based on their findings, the researchers suggest specific nitrogen use targets based on region and crop type to meet 2050 global food-demand projections and environmental stewardship goals.
Currently, the global average for nitrogen use efficiency is approximately 0.4, meaning 40% of the total nitrogen added to cropland goes into the harvested crop while 60% is lost to the environment. To reduce environmental impacts, the researchers urge an increase in global average nitrogen use efficiency to 70% so only 30% of the total nitrogen is lost to the environment. To achieve this goal, the researchers provide examples of specific targets in 2050 for various regions that take into account differences in ecological and socioeconomic conditions.
These targets could be used in tandem with the United Nation's Sustainable Development Goals, the researchers write, which were released in September 2015.
Using economic models to predict pollution
The research team looked at nitrogen surplus and nitrogen use efficiency across 113 countries over 50 years through an economic theory that predicts that as a country develops, environmental degradation worsens until reaching a certain average income at which point pollution levels begin to decrease.
In early stages of agricultural development in most countries, farming technology does not lead to a more efficient use of nitrogen fertilizers, the researchers found. Because most farmers are profit-maximizing, how they use fertilizer is influenced by price. Therefore, market signals such as the prices of fertilizer and crops, as well as government subsidies play a very powerful role.
At later stages of development, however, technology can improve nitrogen use efficiency. In the United States, for example, crop yields have increased over the past two decades without substantial increases in fertilizer use. This was only possible by adopting technologies that increased nitrogen use efficiency, the researchers found.
These improved farming technologies included more strategic irrigation, improved seed sources, slow-release fertilizers and better online planning tools. Likewise, the United States' uptick in the growth of soybeans, a plant able to produce its own nitrogen, also plays a role.
Policies have been more effective at increasing nitrogen use efficiency in Western Europe than in the United States. For example, a 1990 directive limited farmers' use of manure and the application of fertilizers near water or on slopes to reduce water contamination. This significantly increased nitrogen use efficiency while also improving crop yields. Other EU monetary and regulatory policies designed to reduce nitrogen application on cropland were also adopted.
At the other extreme, countries like China and India have gone downhill in terms of nitrogen use efficiency, owing to heavily subsidized fertilizers, which reached $18 billion per year in China in 2010.
Because fertilizer is so cheap relative to crop products, farmers tend to use excess quantities to ensure higher crop yields. China also produces increasing amounts of fruits and vegetables, which generally lead to more nitrogen in the ground and air than cereal crops, like wheat or rye. As a result, China's nitrogen use efficiency has been steadily decreasing leaving it now among the lowest in the world with excess nitrogen leading to substantial environmental degradation.
Sub-Saharan Africa is an entirely different story altogether. In Malawi, crop yields are low mainly due to the lack of access to nitrogen fertilizer, which is often unaffordable and difficult for farmers to obtain.
In this case, fertilizer subsidies could actually help increase crop production without leading to significant environmental degradation. Providing African countries with access to better farming technologies and fertilizer could help feed their growing populations without bringing additional land under cultivation and with minimal other environmental effects.
"Our research is really asking: do you have to go all the way to environmental degradation before you see the light and turn around?" co-lead author Eric Davidson of the University of Maryland Center for Environmental Science said. "Is it possible for some developing countries, like those in Africa, to follow sustainable development pathways that don't pollute while they develop?"
While ambitious, the goals highlighted by the researchers are attainable, especially if adopted as an indicator for the U.N.'s Sustainable Development Goals.
"A lot of this comes down to understanding the sociology and economics behind farmer decision-making," Davidson said. "We need to figure out how to help farmers adopt technologies that reduce pollution while still producing their needed profit outcomes."
One way to address growing global food needs is through stronger collaborations and investments in research and human resources, the authors noted. This will facilitate knowledge sharing, creating political and market environments that help incentivize the development and implementation of more efficient agricultural management technologies.