Nitrogen use efficiency, an indicator that describes how much fertilizer reaches a harvested crop, has decreased by 22% since 1961, according to new findings by an international group of researchers who compared and averaged global data sets.
Excess nitrogen from fertilizer and manure pollutes water and air, eats away ozone in the atmosphere, and harms plants and animals. Excess nitrogen can also react to become nitrous oxide, a greenhouse gas that is 300 times more potent than carbon dioxide.
Significant disagreements remain about the exact value of nitrogen use efficiency, but current estimates are used by governments and in international negotiations to regulate agricultural pollution.
“If we don’t deal with our nitrogen challenge, then dealing with pretty much any other environmental or human health challenge becomes significantly harder,” David Kanter, an environmental scientist at New York University and vice-chair of the International Nitrogen Initiative, told New Scientist in May. Sri Lanka and the United Nations Environment Programme called for countries to halve nitrogen waste by 2030 in the Colombo Declaration.
Whereas the global average shows a decline, nitrogen fertilizing has become more efficient in developed economies thanks to technologies and regulations, and new results out last month from the University of Minnesota as well as field trials by the International Fertilizer Development Center are just two examples of ongoing research to limit nitrogen pollution without jeopardizing yield.
Too Much of a Good Thing
Nitrogen is an essential nutrient for plant growth: It is a vital aspect of amino acids for proteins, chlorophyll for photosynthesis, DNA, and adenosine triphosphate, a compound that releases energy.
Chemist Fritz Haber invented an industrial process to create nitrogen fertilizer in 1918, and the practice spread. Since the 1960s, nitrogen inputs on crops have quadrupled. In 2008, food production from nitrogen fed half the world’s population.
Yet nitrogen applied to crops often ends up elsewhere. Fertilizer placed away from a plant’s roots means that some nitrogen gets washed away or converts into a gas before the plant can use it. Fertilizer applied at an inopportune moment in a plant’s growth cycle goes to waste. At a certain point, adding more fertilizer won’t boost yield: There’s a limit to how much a plant can produce based on nitrogen alone.
“One of the things that is evident in nitrogen management, generally, is that there seems to be a tendency to avoid the risk of too low an application rate,” said Tony Vyn, an agronomist at Purdue University.
In many parts of the world, cheap subsidized fertilizer is critical for producing enough food. But gone unchecked, subsidies incentivize farmers to apply more than they need. And according to plant scientist Rajiv Khosla at Colorado State University, who studies precision agriculture, farmers struggle to apply just the right amount of fertilizer probably 90% of the time.
The 90% Efficiency Goal
According to an average of 13 global databases from 10 data sources, in 2010, 161 teragrams of nitrogen were applied to agricultural crops, but only 73 teragrams of nitrogen made it to the harvested crop. A total of 86 teragrams of nitrogen was wasted, perhaps ending up in the water, air, or soil. The new research was published in the journal Nature Food in July.
Globally, nitrogen use efficiency is 46%, but the ratio should be much closer to 100%, said environmental scientist Xin Zhang at the University of Maryland, who led the latest study. The crops with the lowest nitrogen efficiency are fruits and vegetables, at around 14%, said Zhang. In contrast, soybeans, which are natural nitrogen fixers, have a high efficiency of 80%.
The European Union Nitrogen Expert Panel recommended a nitrogen use efficiency of around 90% as an upper limit. The EU has reduced nitrogen waste over the past several decades, though progress has stagnated.
The United States has similarly cut losses by improving management and technology. For instance, even though the amount of nitrogen fertilizer per acre applied to cornfields was stable from 1980 to 2010 in the United States, the average crop grain yields increased by 60% in that period, said Vyn. Those gains can be hidden in broad-stroke indices like global nitrogen use efficiency.
“The most urgent places will be in China and India because they are two of the top five fertilizer users around the world,” Zhang said. China set a target for a zero increase in fertilizer use in 2015, which showed promising early results.
New research from the University of Minnesota using machine learning–based metamodels suggested that fertilizer amount can be decreased without hurting the bottom line.
Just a 10% decrease in nitrogen fertilizer led to only a 0.6% yield reduction and cut nitrous oxide emissions and nitrogen leaching. “Our analysis revealed hot spots where excessive nitrogen fertilizer can be cut without yield penalty,” said bioproducts and biosystems engineer Zhenong Jin at the University of Minnesota.
Applying fertilizer right at the source could help too: A technique developed by the International Fertilizer Development Center achieved an efficiency as high as 80% in field studies around the world using urea deep placement. The method buries cheap nitrogen fertilizer into the soil, which feeds nitrogen directly into a plant and reduces losses.
Meanwhile, Vyn said researchers must focus on sharpening scientific tools to measure nitrogen capture. The differences in nitrogen inputs in the databases analyzed by the latest study were as high as 33% between the median values and the outliers.
“The nitrogen surplus story is sometimes too easily captured in a simple argument of nitrogen in and nitrogen off the field,” Vyn said. “It’s more complex.” His research aims to improve nitrogen recovery efficiency by understanding plant genotypes and management factors.
One of Zhang’s next research steps is to refine the quantification of nitrogen levels in a crop, which is currently based on simplistic measurements. “There has been some scattered evidence that as we’re increasing the yield, the nitrogen content is actually declining. And that also has a lot of implications in terms of our calculated efficiency,” Zhang said.
—Jenessa Duncombe (@jrdscience), Staff Writer
Duncombe, J. (2021), Index suggests that half of nitrogen applied to crops is lost, Eos, 102, https://doi.org/10.1029/2021EO162300. Published on 23 August 2021.
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