Root activity and rhizospheric bacteria in response to nitrogen management in rice (Oryza sativa L.)

Although one-time application of a single polymer-coated urea (PCU) can minimize the N loss in paddy field and improve nitrogen use efficiency (NUE) in relative to rapid-release urea (RU), it shows a uncertainty in increasing rice yield. There is a proposal that one-time application of PCU mixed with RU can synergistically increase grain yield and NUE. However, few studies focus on the response of roots and rhizospheric bacteria to the nitrogen (N) management. We investigated the issue based on a two-year field experiment using an indica-japonica hybrid variety Yongyou 2640, with four N managements including N omission (0 N), split application of RU (Control), one-time application of 100 % PCU (T1) and one-time application of 70 % PCU + 30 % RU (T2). Results showed that, compared with those in the control, grain yield and total number of spikelets were significantly increased in the T2 treatment while they were comparable in the T1 treatment. Both T1 and T2 remarkably increased N recovery efficiency (RE_N). During booting, the highest α diversity in rhizospheric bacterial communities was observed in the T2 treatment, followed by T1 and control. Among the root morph-physiological traits, the redundancy analysis (RDA) highlighted the significant contribution of root oxidation activity (ROA) to bacterial communities. Additionally, the linear discriminant analysis effect size (LEfSe) identified nine specific genera taxa in the T2 treatment. The abundances of these genera, particularly the Nitrospira, highly correlated with ROA, root H⁺-ATPase activity, organic acid content, microbial biomass carbon and nitrogen (MBC and MBN) contents, MBC-to-MBN ratio, and the N accumulation during booting (NABT). These traits exhibited notable advantages in the T2 treatment, which contributed significantly to the grain yield, RE_N, and the total number of spikelets. Interestingly, the nitrate-N content was most significantly increased in T2, followed by T1 and control, rather than the ammonium-N content, which was also highly correlated with the abundance of Nitrospira. In conclusion, a combination of PCU with RU could coordinate root activity and bacterial communities, especially the ROA and Nitrospira, and facilitate the consumption or cycling of nitrate-N while mitigating the risk of its mobility, leading to a remarkably increase in NABT, and consequently, synergistically increase grain yield and NUE.