Abstract High planting density is essential to increasing maize grain yield. However, single plants suffer from insufficient light under high planting density. Ammonium (NH4+) assimilation consumes less energy converted from radiation than nitrate (NO3−). It is hypothesized that a mixed NO3−/NH4+ supply is more important to improving plant growth and population productivity under high vs. low planting density. Maize plants were grown under hydroponic conditions at two planting densities (low density: 208 plants m−2 and high density: 667 plants m−2) and three nitrogen forms (nitrate only, 75/25NO3−/NH4+ and ammonium only). A significant interaction effect was found between planting density and N form on plant biomass. Compared to nitrate only, 75/25NO3−/NH4+ increased per-plant biomass by 44% under low density, but by 81% under high density. Treatment with 75/25NO3−/NH4+ increased plant ATP, photosynthetic rate, and carbon amount per plant by 31, 7, and 44% under low density, respectively, but by 51, 23, and 95% under high density. Accordingly, carbon level per plant under 75/25NO3−/NH4+ was improved, which increased leaf area, specific leaf weight and total root length, especially for high planting density, increased by 57, 17 and 63%, respectively. Furthermore, under low density, 75/25NO3−/NH4+ increased nitrogen uptake rate, while under high density, 75/25NO3−/NH4+ increased nitrogen, phosphorus, copper and iron uptake rates. By increasing energy use efficiency, an optimum NO3−/NH4+ ratio can improve plant growth and nutrient uptake efficiency, especially under high planting density. In summary, an appropriate supply of NH4+ in addition to nitrate can greatly improve plant growth and promote population productivity of maize under high planting density, and therefore a mixed N form is recommended for high-yielding maize management in the field.

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