Optimized Planting Density and Nitrogen Rate Increased Grain Yield and Water-Nitrogen Use Efficiency of Two Maize Cultivars under Mulched Drip Fertigation by Improving Population Photosynthesis and Grain-Filling Characteristics

The characteristics of photosynthesis and grain filling play a significant role in determining maize (Zea mays L.) yield. Planting density and nitrogen (N) rate are two factors affecting the growth, physiology, and grain yield of maize. The coupling effects of planting density and N rate on individual and population photosynthetic rates, grain-filling characteristics, grain yield, water use efficiency (WUE), and N partial factor productivity (NPFP) of two maize cultivars (QS51 and ZD958) under mulched drip fertigation in northwest China were investigated. Three planting densities (D1: 80,000 plants ha(-1), D2: 100,000 plants ha(-1), and D3: 120,000 plants ha(-1)) and three N rates (N0: 0 kg ha(-1), N180: 180 kg ha(-1), and N240: 240 kg ha(-1)) were designed. The results showed that the population photosynthetic rate, grain yield, WUE, and NPFP were significantly affected by planting density and N rate for both QS51 and ZD958, and their interaction had a significant effect on grain yield, WUE, and NPFP. Nitrogen application significantly improved grain-filling rates compared with N0, but there was no significant difference between N240 and N180. The D2N180 treatment obtained the maximum grain yield (15,693 kg ha(-1) for QS51 and 17,644 kg ha(-1) for ZD958), WUE (3.42 kg kg(-1) for QS51 and 3.05 kg kg(-1) for ZD958), and NPFP (98.37 kg kg(-1) for QS51 and 83.93 kg kg(-1) for ZD958). It was concluded that the optimized planting density and N rate improved grain yield and water-nitrogen use efficiency of QS51 and ZD958 by increasing population photosynthetic rate, grain-filling rate, and grain weight. This study enhanced our understanding of how optimized planting density and N rate maintained the sustainable maize production under mulched drip fertigation in northwest China.