Interactive Effects of Planting Density and Nitrogen Rate on Maize Root Architecture, Soil Nitrogen Cycling, and Water and Nitrogen Use Efficiencies

Optimizing planting density and nitrogen (N) application is essential for sustaining maize yield while reducing water use and N inputs. However, their interactive effects on belowground root development and the soil N environment remain poorly understood. A two-year field experiment was conducted using the maize hybrid Xianyu 335. The experiment included three N application rates, namely 198, 264, and 330 kg·ha−1, designated as N1, N2, and N3, respectively, Among them, 264 kg·ha−1 represents the conventional N application rate practiced by local farmers in the Hexi Corridor, and three planting densities, namely 75,000, 90,000, and 105,000 plants·ha−1, designated as D1, D2, and D3, respectively. The study systematically evaluated the interactive effects of planting density and N rate on maize root architecture, soil N pools, key enzymes involved in soil N transformation, and water and N use efficiencies. Planting density and N rate significantly affected root length, root volume, root surface area, soil ammonium N, nitrate N, total N stocks, and the activities of nitrate reductase (NR), glutamine synthetase (GS), and glutamate synthase (GOGAT). Their interactive effects varied markedly between growth stages and years. Root traits and NR activity generally increased from N1 to N2 but declined or plateaued at N3, with the highest values observed under N2. By contrast, GS and GOGAT activities increased continuously with increasing N application and showed no clear inhibitory response at the highest N rate. Root traits, soil N-transforming enzyme activities, water use efficiency (WUE), and nitrogen use efficiency (NUE) showed different responses to increasing planting density. WUE increased with increasing planting density, although the magnitude of increase decreased from D2 to D3, whereas NUE showed a unimodal response, with D2 exhibiting the most favorable performance. The effects of density–N interactions became more pronounced during the middle and late growth stages. A comprehensive assessment of root morphology, soil N pools, N-transforming enzyme activities, WUE, and NUE identified D2N2 as the optimal treatment combination. The D2N2 treatment produced a mean grain yield of 14.1 t·ha−1, with WUE of 0.86–12.22 kg m−3 and NUE of 31.04–61.93 kg kg−1. D2N2 promoted favorable root morphological development and enhanced N-transforming enzyme activities in the bulk soil adjacent to maize roots. It also maintained relatively high WUE and NUE and may help reduce the potential risk of nitrate leaching from cropland. Overall, a planting density of 90,000 plants·ha−1 combined with a N application rate of 264 kg·ha−1 effectively coordinated root growth with soil N transformation, balanced water and nutrient use, and supported the combined goals of high yield, water conservation, and reduced N input. These findings provide a scientific basis for developing resource-efficient and environmentally sustainable maize production systems in irrigated regions.