A recent review in Frontiers in Microbiology finds that heavy fertilizer use by rice farmers in China is limiting the ability of soil microbes to promote crop growth. The result is a greater dependency on artificial inputs, which threatens water and terrestrial ecosystems. But researchers find that integrating legumes can help mitigate these harms and help farmers use fertilizer more efficiently.
Nitrogen fixation is a natural process that converts atmospheric nitrogen into a form that plants can use. While nitrogen is vital for plant health, too much can harm ecosystems, says soil health expert Yuting Liang of the Institute of Soil Science, Chinese Academy of Sciences.
“In flooded rice paddies, excessive nitrogen can increase nitrogen losses and nitrous oxide emissions, affecting microbial processes involved in the nitrogen cycle,” Liang tells Food Tank.
Liang notes that in contrast to conventional fertilizers, biological nitrogen fixation “supports a slower and more efficient nitrogen cycle.” But if the soil is already rich in nitrogen from fertilizers, microbes will respond by shutting the process down. “Many nitrogen-fixing microorganisms tend to reduce their activity because the need to fix atmospheric nitrogen becomes much lower,” she explains.
This leads to a paradoxical feedback loop, the study explains. The more fertilizer a farmer adds to secure a harvest, the less nitrogen the soil makes on its own, and the more fertilizer the next crop needs. Instead of helping farmers cut back on fertilizer, it encourages overuse.
“It is so connected to many problems while it is essential for food production,” Wim de Vries, a soil scientist and professor at Wageningen University, tells Food Tank.
De Vries argues the first solution is to apply less fertilizer. He points to what he calls the four-R principle: the right amount, right type, right time, and right place. “The right rate is extremely important in regions like China where they over-fertilize,” he says. According to the U.N. Food and Agriculture Organization, China is the largest producer and consumer of nitrogen fertilizers and remains one of the largest rice producers globally.
Soil scientists like Mark Peoples of the Commonwealth Scientific and Industrial Research Organization say crop diversification can help reduce reliance on artificial fertilizers.
“A diverse system is likely to be more resilient and have better environmental outcomes than a landscape dominated by highly specialized, frequent cereal cropping that is heavily reliant upon fertilizer,” Peoples tells Food Tank. He notes that legume cover crops can increase soil nitrogen availability for cereal crops like rice, suggesting that increased global and regional demand for legumes can support sustainable farming.
Liang agrees, stating, “In regions where conditions are suitable, rice-legume rotations or green manure systems can further strengthen natural nitrogen cycling and increase microbial diversity.” By planting legumes during the dry season and rice during the wet season, farms retain nitrogen-fixing microbes which support soil nutrient availability.
But to encourage farmers to plant more legumes, the demand must exist, says Adrian Leip, co-chair of the U.N. Economic Commission for Europe’s Expert Panel on Nitrogen and Food. “It’s often not the farmers who have the power to change, at least not alone, because they are constrained by contracts and value chains,” he tells Food Tank. He notes that farmers need these market signals to transition to more sustainable practices, including crop rotation.
But dietary trends seem to be moving in the opposite direction, Peoples says. He sees that more eaters are seeking animal protein over pulses and legumes while governments subsidize cereal production and fertilizers.
Leip and De Vries call for a systemic approach to change this. “It’s dietary change, it’s food waste reduction, it’s circular agriculture, and it’s improved practices,” De Vries tells Food Tank. “These are the four. If you want to reduce the problem, you need to look at all four of them. And never only one is the answer.”
Leip also underscores that these solutions must work for producers: “This should be done in a way that is not harmful for the farming community. They are keeping the land intact; if they can, they would like to make it in a way that is sustainable… [while] conserving biodiversity.”
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Photo courtesy of Ivan Bandura, Unsplash
