Rice farming with fish and shrimp cuts nitrogen loss by up to 40%, review finds

About half of the nitrogen fertilizer applied to the world's rice paddies never reaches a crop. It leaks away as ammonia gas, washes into waterways as nitrate runoff, or converts to nitrous oxide - a greenhouse gas roughly 300 times more potent than carbon dioxide over a century. Finding ways to keep more nitrogen in the plant-soil system is one of modern agriculture's central challenges.

A solution practiced for centuries in parts of China, Vietnam, and Southeast Asia may offer part of the answer. Rice-aquatic animal co-culture - growing fish, shrimp, crabs, or other aquatic organisms in flooded rice paddies alongside the crop - appears to substantially improve how efficiently nitrogen moves from fertilizer to food. A new review synthesizes the scientific literature on these systems and identifies the biological mechanisms behind their benefits, proposing that activity at the soil-water interface is the primary driver.

The numbers from the research

Properly managed rice-aquatic animal co-culture systems can increase nitrogen-use efficiency by roughly 20 to 40 percent compared to conventional rice monoculture. Greenhouse gas emissions and fertilizer requirements decline as more nitrogen is retained within plant biomass, sediment, and microbial communities rather than escaping to the atmosphere or waterways. These estimates come from the review's synthesis of multiple controlled studies, though the actual gains vary considerably depending on species combination, stocking density, and local management practices.

Three mechanisms at the soil-water boundary

The review's central argument is that the soil-water interface - the dynamic zone where rice roots, floodwater, and sediment meet - functions as an active nitrogen regulator rather than a passive boundary. Three processes drive the efficiency gains.

First, physical disturbance by aquatic animals. When fish or crabs move through a flooded paddy, their burrowing and swimming actions mix surface sediments, increase oxygen exchange between water and soil, and accelerate transport of nutrients. This bioturbation prevents stagnant zones where nitrogen tends to be lost to denitrification or volatilization, and creates conditions that favor nitrogen cycling into plant-available forms.

Second, root-zone chemistry from the rice plants themselves. Rice roots release oxygen into the surrounding sediment, creating micro-scale oxidized zones within an otherwise oxygen-limited environment. They also release organic compounds that provide energy sources for the microbial communities living around them. The resulting chemical gradients maintain a balance between oxidizing and reducing conditions that supports efficient nitrogen cycling.

Third, microbial community dynamics. The combined inputs from animal disturbance and plant roots create unusually diverse and active microbial populations. Different bacterial groups work cooperatively - some converting ammonia to nitrate, others converting nitrate back to ammonium or nitrogen gas at controlled rates. The net effect is a system that recycles nitrogen within the paddy rather than losing it.

"Our synthesis shows that the interface between soil and floodwater is not just a boundary, but a highly active regulatory zone controlling nitrogen cycling," said the study's corresponding author. "When plants, animals, and microbes interact there, they create conditions that enhance nutrient recycling and reduce losses."

A self-reinforcing system

The review describes the three mechanisms as mutually reinforcing. Animal disturbance enhances root-zone oxygen exchange; enriched root-zone chemistry supports more diverse microbes; active microbial communities process organic matter in ways that further support plant nutrition and animal feeding. The authors describe this as a "self-reinforcing loop" - a characteristic shared by other well-functioning agroecosystems.

What the review cannot tell us

Reviews synthesize existing literature but cannot fill gaps in it. The studies underlying this analysis were conducted primarily in Asian contexts, with particular emphasis on Chinese rice systems. Whether the mechanisms and efficiency gains translate at the same magnitude to rice systems in sub-Saharan Africa, South Asia, or Latin America - which have different soil types, climates, water management regimes, and available aquatic species - is not established.

The 20-to-40% nitrogen efficiency improvement figure represents a range across studies with different experimental conditions, not a single precise estimate. Stocking density, species choice, fertilizer type, and water management all influence outcomes, and the optimal combinations for different regional contexts remain an active research area. Economic analyses comparing co-culture to monoculture are also limited; the financial case for transition depends on local market prices for fish or crustaceans, which vary widely.