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

Abstract

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 15N-NH4+ or 15N-NO3- applied to flooded and water-saving irrigation regimes. At harvest, plant biomass, N accumulation, and the fate of 15N-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 15NH4+ to NO3-, and hence less opportunities for rice to take up the applied 15NH4+. 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.