Split injection with BNI breaks the trade-off between NH3 and greenhouse gas emissions in rice-wheat rotations

Farmers have long faced a difficult choice when feeding their crops. To grow enough food for a growing world, they must apply nitrogen fertilizer, but the way they apply it often creates a cascade of environmental problems. When farmers spread dry fertilizer on the surface of their fields, a large portion of that nitrogen escapes into the air as ammonia gas, contributing to smog and acid rain. If they try to fix this by burying the fertilizer deep in the soil to keep it from escaping, they often trigger a different problem: the buried nitrogen can transform into nitrous oxide, a potent greenhouse gas that traps heat far more effectively than carbon dioxide. For decades, scientists have struggled to find a method that stops both the ammonia from rising and the greenhouse gases from forming without sacrificing the crop yield.

In the rice-wheat fields of China's Taihu region, a team of researchers has tested a new approach that appears to solve this dilemma. They combined two specific techniques: injecting liquid fertilizer deep into the soil in two separate doses, and adding a natural substance that slows down the chemical breakdown of nitrogen. The results, observed over two full growing cycles, show that this combination not only cuts emissions of ammonia and greenhouse gases but also increases the amount of grain harvested. The study suggests that by changing where and how nitrogen is delivered to the plant, farmers can protect the air and the climate while still feeding the population.

The experiment took place in a region known for its intensive agriculture, where rice is grown in the summer and wheat in the winter. The researchers set up a series of test plots to compare different ways of managing nitrogen. One group received the traditional treatment: dry urea fertilizer spread across the surface of the field in three separate applications. Another group received a reduced amount of this same surface fertilizer. A third group received the reduced amount but with a biological nitrification inhibitor added, a substance derived from sorghum roots that naturally slows the conversion of ammonium into nitrate. The final group, which became the focus of the breakthrough, received a water-soluble liquid fertilizer injected ten centimeters deep into the soil. This injection happened twice: once at the start of the season and again later, timed to match the crop's growing needs. This deep injection group was also tested with and without the biological inhibitor.

The results revealed a stark contrast between the methods. The traditional surface spreading of fertilizer led to massive losses of ammonia. In the rice season, the surface treatment released over forty-six kilograms of nitrogen per hectare as ammonia gas. In the wheat season, the loss was nearly fourteen kilograms per hectare. This loss not only pollutes the air but represents wasted money for the farmer. When the researchers added the biological inhibitor to the surface fertilizer, the problem got worse. The inhibitor successfully slowed the breakdown of nitrogen, but because the fertilizer was sitting on the surface, the nitrogen stayed there longer in a form that could easily escape as gas. In the wheat season, this combination actually increased ammonia loss by sixty-seven percent compared to the reduced fertilizer alone.

The deep injection method, however, changed the outcome entirely. By pushing the liquid fertilizer ten centimeters underground, the researchers effectively trapped the nitrogen where the plant roots could reach it. In the plots where fertilizer was injected without the inhibitor, ammonia losses dropped to less than one kilogram per hectare, a level statistically indistinguishable from the control plots that received no fertilizer at all. The nitrogen was simply not available to escape into the air because it was buried deep in the soil, away from the atmosphere.

This physical separation of the nitrogen from the air also solved the greenhouse gas problem. The study measured emissions of nitrous oxide and methane, the two main greenhouse gases from agriculture. The traditional surface method produced the highest levels of these gases. The deep injection method, even without the inhibitor, significantly reduced nitrous oxide emissions by nearly sixty percent in rice and forty-three percent in wheat. When the researchers added the biological inhibitor to the deep injection method, the reduction was even more dramatic. In the rice season, nitrous oxide emissions fell by seventy-one percent compared to the traditional method. In the wheat season, the reduction was fifty-four percent. The methane emissions, which are primarily produced in the flooded rice fields, also dropped by about thirty-four percent in rice and forty-seven percent in wheat when using the combined deep injection and inhibitor method.

The mechanism behind this success lies in how the nitrogen moves and changes in the soil. When fertilizer is spread on the surface, it sits in the top layer where oxygen is plentiful, allowing bacteria to quickly convert it into nitrate, which then escapes as nitrous oxide or washes away. When fertilizer is injected deep, it stays in a zone with less oxygen, where it remains as ammonium, a form that plants prefer. The biological inhibitor further slows down the bacteria that would otherwise turn this ammonium into nitrate. Because the nitrogen is held deep in the soil and kept in the ammonium form, it is not available to be converted into the gases that harm the climate. The study confirmed this by measuring the soil directly, showing that the ammonium remained concentrated near the injection site while the nitrate levels stayed low.

Beyond the environmental benefits, the new method improved the harvest. The plots using split deep injection with the biological inhibitor produced the highest yields of all. The rice harvest increased by nearly six percent compared to the traditional method, and the wheat harvest jumped by almost twenty percent. This was achieved even though the researchers applied less total nitrogen to these plots. The traditional method used three hundred kilograms of nitrogen per hectare for rice, while the new method used only one hundred and eighty. The efficiency of the new method meant that more of the fertilizer ended up in the grain rather than being lost to the air or soil. The economic analysis showed that despite the higher cost of the liquid fertilizer and the specialized equipment needed to inject it, the savings from reduced environmental damage and the increased crop value made the new method more profitable overall.

The study highlights a critical lesson about agricultural technology: the placement of a solution matters as much as the solution itself. The biological inhibitor, when used on the surface, made the ammonia problem worse. But when the same inhibitor was used with deep injection, it became part of a highly effective system. The deep placement prevented the ammonia from escaping, allowing the inhibitor to do its job of slowing down nitrate formation without the penalty of increased air pollution. This combination of deep injection and biological inhibition offers a way to break the trade-off that has plagued farmers for years. It demonstrates that it is possible to reduce nitrogen pollution and greenhouse gas emissions simultaneously while increasing food production, provided the fertilizer is delivered directly to where the plant needs it. The researchers note that for this method to become widespread, better machinery for injecting liquid fertilizer on a large scale will be needed, but the potential for a cleaner, more productive agriculture is now clearly within reach.