Promotion of nitrogen accumulation through enhanced soil-water interface activity in rice-aquatic animal co-culture systems: a review

Rice-aquatic animal co-culture systems, as a representative ecological agriculture model, substantially reshape nitrogen migration, transformation, and retention through interactions among rice plants, floodwater, and soil interfaces. Although numerous studies have reported improved nitrogen-use efficiency and reduced environmental losses, the micro-scale regulatory mechanisms have not yet reached a unified understanding, and many key influencing factors remain insufficiently integrated. This review synthesizes current understanding of nitrogen cycling in such systems from physical, chemical, and biological perspectives. Physically, bioturbation by fish, shrimp, and crabs thins the diffusive boundary layer and enhances solute exchange, facilitating oxygen and nitrogen transport across the soil-water interface. Oxygen released from rice roots further forms micro-oxic zones that intersect with reduced patches created by animal disturbance, generating dynamic redox gradients. Chemically, periodic shifts in redox potential and dissolved organic carbon regulate the spatial dominance and rates of nitrification, denitrification, anaerobic ammonium oxidation, and nitrogen fixation. Abundant carbon inputs strengthen electron flow, enabling more complete nitrogen reduction and lowering N2O emissions. Biologically, ammonia-oxidizing, denitrifying, anammox, and nitrogen-fixing microorganisms establish complementary metabolic networks that drive efficient cycling among ammonium, nitrite, nitrate, and organic nitrogen, supporting sustained internal nitrogen regeneration. Enhanced interface activity ultimately increases nitrogen accumulation in rice plants, sediments, and microbial biomass, while reducing volatilization, leaching, and greenhouse gas emissions. Comparative studies indicate that optimizing water levels, animal density, and nitrogen inputs can enhance nitrogen-use efficiency by 20%–40% and improve ecosystem stability. This review highlights the pivotal role of interface processes in nitrogen cycling and provides scientific guidance for advancing efficient, low-emission, and sustainable rice-based agroecosystems.