1488. A Hidden “Paradox” in Intensive Tropical Rice Farming

Across tropical Asia, intensive lowland rice farming—supported by large inputs of chemical fertilizers and two to three harvests per year—plays a crucial role in meeting rising food demand. However, concerns have long existed about how such systems affect soil health over the long term.

To address this issue, researchers from the Japan International Research Center for Agricultural Sciences (JIRCAS), the International Rice Research Institute (IRRI), and the Philippine Rice Research Institute (PhilRice) conducted a detailed analysis of soils from long-term experimental rice fields in the Philippines. These fields have been continuously cultivated for more than 30 years under intensive management.

Organic matter increases, but usable nitrogen declines
Soil organic matter is widely regarded as the foundation of soil fertility because it supports nutrient cycling and crop production. As expected, the study found that total soil carbon and nitrogen gradually increased over several decades in flooded paddy fields, where low oxygen conditions slow down the decomposition of organic matter.
Surprisingly, however, the amount of nitrogen available for rice uptake steadily decreased over time. In other words, despite the accumulation of organic matter in the soil, the nitrogen that rice plants actually need became less accessible. This finding reveals a clear mismatch between soil organic matter storage and soil fertility.

Phosphorus accumulation and serious potassium depletion
The study also examined changes in soil nutrient availability. Phosphorus tended to accumulate in the soil when fertilizer inputs exceeded crop uptake. Importantly, the researchers found that phosphorus levels could be reduced by adjusting fertilizer management, suggesting that phosphorus can be controlled relatively easily through appropriate practices.
In contrast, potassium depletion emerged as a major concern. Continuous high-yield rice cultivation removed large amounts of potassium from the soil, causing potassium levels to fall below critical deficiency thresholds. This indicates that the soil’s natural ability to supply potassium is no longer sufficient to sustain intensive rice production.

Implications for sustainable rice production
These findings demonstrate that simply continuing fertilizer application is not enough to maintain intensive tropical rice systems over the long term. To secure the future of rice production, more integrated management strategies are required.
Potential approaches include introducing temporary non-flooded periods to restore the soil’s capacity to supply nitrogen, making more strategic use of phosphorus already stored in the soil, and strengthening potassium fertilization to prevent further depletion. By managing water and nutrients together, it may be possible to overcome this soil fertility paradox and ensure the sustainability of rice farming in the tropics.

Original Article:
Tomohiro Nishigaki, Miwa Arai, Takanori Okamoto, Olivyn Angeles, Wilfredo B. Collado, Kazuki Saito
Long-term soil carbon accumulation and nutrient depletion under intensive chemical fertilization in tropical rice systems
Field Crops Research, Volume 342, 2026
https://doi.org/10.1016/j.fcr.2026.110490

Contributor: NISHIGAKI Tomohiro (Crop, Livestock and Environment Division)