Effects of narrow plant spacing on root distribution and physiological nitrogen use efficiency in summer maize

The objective of this study was to understand the effects of plant spacing on grain yield and root competition in summer maize (Zea mays L.). Maize cultivar Denghai 661 was planted in rectangular tanks (0.54 m × 0.27 m × 1.00 m) under 27 cm (normal) and 6 cm (narrow) plant spacing and treated with zero and 7.5 g nitrogen (N) per plant. Compared to normal plant spacing, narrow plant spacing generated less root biomass in the 0–20 cm zone under both N rates, slight reductions of dry root weight in the 20–40 cm and 40–70 cm zones at the mid-grain filling stage, and slight variation of dry root weights in the 70–100 cm zone during the whole growth period. Narrow plant spacing decreased root reductive activity in all root zones, especially at the grain-filling stage. Grain yield and above-ground biomass were 5.0% and 8.4% lower in the narrow plant spacing than with normal plant spacing, although narrow plant spacing significantly increased N harvest index and N use efficiency in both grain yield and biomass, and higher N translocation rates from vegetative organs. These results indicate that the reductive activity of maize roots in all soil layers and dry weights of shallow roots were significantly decreased under narrow plant spacing conditions, resulting in lower root biomass and yield reduction at maturity. Therefore, a moderately dense sowing is a basis for high yield in summer maize.