pH-responsive/sustained release nitrogen fertilizer hydrogel improves yield, nitrogen metabolism, and nitrogen use efficiency of rice under alternative wetting and moderate drying irrigation

Rice is challenged by inefficient nitrogen (N) fertilizer and water inputs. However, pH-responsive N fertilizers (pH-RNFs) could improve rice yield and N use-efficiency under water-saving irrigation. Thus, a pot experiment was conducted without N fertilizer (0 mg N/kg, ZNF), with 175 mg/kg mineral N fertilizer (MNF), and with 175 mg/kg pH-responsive N fertilizer (pH-RNF) by using three water regimes: alternate wetting/critical drying (AWCD, 70-60% saturation, 5 cm flooded), alternate wetting/severe drying (AWSD, 80-70% saturation, 5 cm flooded), and alternate wetting/minor drying (AWMD, 100-90% saturation, 5 cm flooded). Under the same alternate wetting/drying regime, the growth traits, yield, and N utilization of rice were found to be higher in pH-RNF than in MNF. The growth attributes and yield and the N efficient use of rice were greater in the AWMD regime than in the AWCD regime under the same N fertilization. The surface area, diameter, length density, oxidation ability, reductase activity, and peroxidase activity of roots were greater in AWMD x pH-RNF than in AWCD x ZNF, because the former treatment increased the total N contents in the rhizosphere, whereas the latter reduced them. In the photosynthesis process, leaf nitrate reductase (41.7 mu g/[g.h]) and leaf glutamine synthetase (1158.1 mu mol/[g.h]) were larger in AWMD x pH-RNF than in AWCD x ZNF at 22.7 mu g/(g.h) and 501.2 mu mol/(g. h), respectively, resulting in the amelioration of oxidative leaf cell damage, as revealed by reduced radical oxidative sequence release and lower malonaldehyde level (7.2 mu mol/g). Meanwhile, the optimum N nutrition enhanced the leaf mesophyll structure, cell cycle progression, and the development traits of the stem and panicle under the combination AWMD x pH-RNF. Panicle glutamine synthetase (24.35 mu mol/[g.h]) and catalase activity (13.86 mu g/min) were larger in AWMD x pH-RNF than in AWCD x ZNF at 14.77 mu mol/[g.h] and 7.59 mu g/min, respectively. The combination AWMD x pH-RNF resulted in the highest N crop removal efficiency (CREN, 49.5%), N agronomic use efficiency (AUEN, 7.97 g/g), N partial factor productivity (PFPN, 18.66 g/g), and N harvest index (HIN, 67.83%). However, the minimum values of CREN (10.3%), AUEN (1.49 g/g), PFPN (7.59 g/g), and HIN (43.48%) were observed in AWCD x MNF. This study suggests that AWMD x pH-RNF could promote root and shoot morpho-physio-biochemical traits, guaranteeing optimal overall plant growth with high yield and adequate N acquisition, and efficient N utilization for sustainable rice production.