'Preferential' ammonium uptake by rice does not always turn into higher N recovery of fertilizer sources under water-saving irrigation

Ammonium (NH4+) has been reported as the preferred mineral N source (a preference for NH4+ uptake over NO3-) for rice (Oryza sativa L.), the predominant supply of NH4+ has been hypothesized as a means of increasing the N recovery efficiency (NRE). However, the contribution of NH4+ preference to NRE is unclear under different irrigation regimes. To evaluate the NRE of rice as affected by varied N supply forms, rice (cv. 'Suijing 18') was grown in a pot experiment with N-15-NH4+ or N-15-NO3- applied to flooded and water-saving irrigation regimes. At harvest, plant biomass, N accumulation, and the fate of N-15-labeled N in the plant-soil system were assessed. Ammonium-N application enhanced rice shoot growth compared with nitrate-N under both irrigation regimes. This enhancement by NH4+ was further promoted under water-saving irrigation. In contrast, nitrate-N enhanced rice root growth, more fertilizer-derived N residues in the soil compared with those of NH4+ treated, regardless of the irrigation regimes. Ammonium enhanced the NRE of rice compared with NO3- treated under flooded irrigation. However, the so-called 'preference' for NH4+ by rice did not always turn into high N recovery efficiency, and there was no difference in NRE of rice between varied N supply forms under water-saving irrigation. Even if NH4+ is 'preferred' by the rice, the N cycle in the paddy soil is complex and dynamic, especially under water -saving irrigation which resulted in a rapid conversion of the applied (NH4+)-N-15 to NO(3)(-)and hence less opportu-nities for rice to take up the applied (NH4+)-N-15. Therefore, the nitrogen preference of plants is not the only critical factor in selecting a proper nitrogen fertilizer source for rice, it is better to take the match of plant N preference, N fertilizer type, and irrigation regime into consideration to optimize irrigation and fertilization management and increase the NRE of plants.

Automated summary

This study evaluated the impact of different irrigation regimes and nitrogen supply forms on the nitrogen recovery efficiency (NRE) of rice. It was found that rice preferred ammonium (NH4+) over nitrate (NO3-), but this preference did not always result in high NRE, especially under water-saving irrigation. Therefore, when selecting a nitrogen fertilizer source for rice, factors such as the plant's nitrogen preference, fertilizer type, and irrigation regime should be taken into consideration.