A team of scientists has shown that rice paddies in two of China's most important grain producing regions release soil nitrogen in strikingly different ways, and that these differences can be predicted using fast, low cost laboratory tests. The findings could help farmers tailor fertilizer doses to local soils, avoiding waste while maintaining high yields.
"Our work shows that farmers in different rice regions are not starting from the same nitrogen baseline in their soils, even when their yields look similar," said lead author Siyuan Cai of the Institute of Soil Science, Chinese Academy of Sciences. "By using simple, rapid soil tests, we can estimate how much nitrogen the soil will supply over a season and adjust fertilizer rates accordingly, which is good for both farm profits and the environment".
The study focused on paddy soils in the Yangtze River Delta in eastern China and in Northeast China, two regions that produce roughly comparable amounts of rice but use very different nitrogen fertilizer rates. Over the past three decades, farmers in the Yangtze River Delta have applied on average about 279 kilograms of nitrogen per hectare each year, compared with about 159 kilograms per hectare in Northeast China, yet their rice yields have remained similar.
To understand how soil nitrogen supply differs between these two rice systems, the team collected 36 paddy soil samples from the Yangtze River Delta and 24 samples from Northeast China. They then ran a long term anaerobic incubation experiment, keeping soils waterlogged at 30 degrees Celsius for 112 days to mimic flooded field conditions and track how much organic nitrogen was converted into plant available ammonium over time.
The researchers also ran a series of shorter incubations lasting from a few days to two weeks, and combined these with rapid chemical extractions using mild solutions and hot water to pull out easily decomposed nitrogen compounds. By comparing the quick tests with the 112 day benchmark, they asked how soon and how simply long term nitrogen mineralization could be predicted in each region.
Despite having lower total nitrogen and organic carbon than the black paddy soils of Northeast China, the gleyed paddy soils of the Yangtze River Delta released more mineral nitrogen over the 112 day incubation. On average, the long term nitrogen mineralization potential in the Yangtze River Delta soils was about 29 percent higher than in the Northeast China soils, and the fraction of soil organic nitrogen that was mineralized was about 63 percent higher.
Most of that nitrogen was released early: about 85 percent of the total mineralized nitrogen in Yangtze River Delta soils and about 65 percent in Northeast China soils appeared within the first two weeks of incubation, indicating that a large share of the mineralizable nitrogen is in relatively labile, easily decomposed forms. The team found that a seven day incubation was enough to represent long term mineralization in Yangtze River Delta soils, whereas Northeast China soils required about 14 days to reach a similar level of predictive power, reflecting differences in organic matter quality and protection by soil minerals.
The rapid extraction tests also behaved differently by region. In the Yangtze River Delta, ultraviolet absorbing dissolved organic nitrogen measured at 260 nanometers in a mild bicarbonate extract was strongly linked to long term mineralization potential, while in Northeast China, hot water extractable ammonium nitrogen was the most informative indicator. Across both regions, soil pH emerged as a decisive control, explaining up to about 59 percent of the variation in long term mineralization in the Yangtze River Delta and a smaller but still significant share in Northeast China.
These regional contrasts help explain why similar rice yields are achieved with much lower fertilizer inputs in Northeast China than in the Yangtze River Delta. Previous monitoring has shown that nitrogen use efficiency in Northeast China is higher, meaning more of the applied fertilizer ends up in the crop rather than being lost to water or the atmosphere.
By combining soil properties, short term incubation data and rapid extraction indices in region specific statistical models, the researchers could explain more than 80 percent of the variation in long term nitrogen mineralization in both regions under controlled conditions. Such models could be built into decision tools that estimate how much nitrogen the soil will supply during the growing season and recommend lower, site specific fertilizer rates without sacrificing yield.
"Instead of treating all rice paddies the same, our results argue for locally calibrated nitrogen tests and fertilizer guidelines," said corresponding author Xu Zhao. "If we know how quickly a given soil releases nitrogen, we can cut back fertilizer in high supply areas and avoid under fertilizing in low supply areas, helping farmers meet production goals while reducing pollution".
The authors note that their experiments were conducted at constant temperature and moisture in the lab, without added fertilizer, and that field conditions are far more variable. They call for future studies that test the models under real world climate, water management and fertilization regimes, and that incorporate temperature sensitivity and moisture fluctuations into nitrogen release predictions.
Even so, the work provides a scientific basis for more precise nitrogen management in two of China's flagship rice regions, and illustrates how understanding the "hidden" nitrogen dynamics of soil can support both food security and environmental protection.
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Journal Reference: Cai S, Liu Y, Chen Y, Liu X, Xu L, et al. 2026. Nitrogen mineralization characteristics, drivers, and prediction in paddy soils of representative regions in the Yangtze River Delta and Northeast China. Nitrogen Cycling 2: e002 doi: 10.48130/nc-0025-0014
https://www.maxapress.com/article/doi/10.48130/nc-0025-0014
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