Nitrogen use efficiency and yield components of rice as influenced by reducing nitrogen application under salinity conditions in a pot experiment

Salinity and overuse of nitrogen (N) fertilizers are two major environmental concerns for rice cultivation that make improving nitrogen use efficiency (NUE) and yield under saline conditions indispensable. However, little is known about NUE during the reproductive stage or rice yield under both salt stress and low N input. This study evaluated the effects of reduced nitrogen on NUE, yield and its components for rice grown in saline soil. A pot experiment was conducted with four N and three NaCl levels using four rice cultivars. High salinity nullified the effects of N on the rice growth and grain yield (GY). By increasing N levels, N efficiency for GY was 15.09, 6.82, and 1.94 under non-saline, moderate, and high salinity levels, respectively. Under moderate salinity, most measured parameters including shoot dry weight (SDW), shoot N content at active tillering (SNC), effective panicle number (EPN) and GY increased significantly with N levels, but to a lesser extent than those of plants grown under non-saline conditions. Regardless of salinity level, increasing N levels significantly reduced agronomical NUE (agNUE) at active tillering, but neither at heading nor at maturity. Under saline conditions, agNUE was strongly and positively correlated with absorption NUE (0.90 and 0.97, p < 0.001) and with a lesser extent to physiological NUE (0.67 and 0.62, p < 0.05) under moderate and severe salinity, respectively during active tillering. Therefore, the improvement in N uptake during active tillering should be investigated as a way to enhance the NUE, EPN, and GY of rice grown in saline conditions.