Fish in rice paddies may boost soil phosphorus without fertilizer

By AI, Created 2:05 PM UTC, May 25, 2026, /AGP/ – A new study from South China Agricultural University says rice-fish coculture can raise plant-available phosphorus in flooded paddy soils without added fertilizer. The findings point to a low-input farming approach that could cut costs and reduce nutrient pollution in phosphorus-poor regions.

Why it matters: - Phosphorus is essential for crop growth, but much of it in flooded paddy soils is locked in forms plants cannot use. - Rice-fish coculture could help farmers improve soil fertility without adding more mineral fertilizer. - The approach may lower production costs and reduce environmental pollution tied to fertilizer use.

What happened: - Researchers from South China Agricultural University studied rice-fish coculture and its effects on phosphorus cycling in paddy soils. - The study was published in Pedosphere in June 2025. - The paper is identified by DOI 10.1016/j.pedsph.2024.06.002. - The researchers used a controlled microcosm experiment with four treatments: rice monoculture, rice-fish coculture, and each system with phosphorus addition.

The details: - The fish-inclusive system without added phosphorus increased soil phosphorus availability compared with conventional monoculture. - Fish presence raised phosphatase activity in the soil. - Fish altered soil pH. - Fish promoted microbial biomass turnover. - Those changes helped convert less accessible phosphorus into plant-available forms. - The study found stronger activity and diversity among phoD-harboring bacteria, which encode alkaline phosphatases involved in phosphorus mineralization. - Keystone genera including Vibrio and Geomonas were strongly linked with soil phosphorus dynamics. - Structural equation modeling showed that labile phosphorus was driven mainly by moderately labile phosphorus and microbial processes, not by external fertilizer input. - The researchers linked the effect to fish bioturbation and nutrient-rich excreta, which appear to stimulate beneficial microbial shifts and nutrient mobilization.

Between the lines: - The findings suggest rice-fish coculture is doing more than sharing land between two food systems. - Fish appear to act as a biological catalyst for soil microbes that unlock phosphorus already in the soil. - That makes the method a potential fit for regions where fertilizer is costly, scarce or causing environmental damage. - The results also point to a broader strategy of managing soil biology, not just adding nutrients.

What’s next: - The study suggests future work could focus on optimizing soil management by stimulating phoD-harboring bacteria. - Researchers see potential for scaling rice-fish coculture in Asia and other regions facing phosphorus scarcity. - Broader adoption could support climate-resilient agriculture and more efficient nutrient use. - Prof. Jiaen Zhang said the findings open new opportunities to reduce fertilizer dependency while supporting crop productivity in phosphorus-deficient regions.

The bottom line: - Rice-fish coculture may be a practical, nature-based way to improve phosphorus availability in paddies while reducing reliance on chemical fertilizer.