Optimizing the nitrogen application rate and planting density to improve dry matter yield, water productivity and N-use efficiency of forage maize in a rainfed region

Appropriate nitrogen (N) fertilization and planting density management are critical for efficient production of grain maize (Zea mays L.) and for environmental protection. However, the optimal N fertilization and planting density is still not established for forage maize that is cultivated to promote its vegetative growth and utilized for the above-ground vegetative mass. A two-year field experiment was conducted in the rainfed semiarid region of the Chinese Loess Plateau during the 2021 and 2022 growing seasons. The effects of N application rates and planting densities on the dry matter yields and the water- and N-use efficiencies of forage maize were studied. The experiment includes four N application rates (0, 90, 180, and 270 kg ha⁻¹) and three plant densities (70000, 90000, and 110000 plants ha⁻¹), covering the conventional practices of local farmers. The treatments were organized in a randomized complete block design with four replications. Averaged over the three plant densities, N application rate of 180 kg ha⁻¹ resulted in the maximum average aboveground dry matter yield (18.6 t ha⁻¹), crop N accumulation (228.5 kg ha⁻¹), dry matter water productivity (51.9 kg ha⁻¹ mm⁻¹), and dry matter precipitation productivity (62.9 kg ha⁻¹ mm⁻¹) over the two years. Moreover, increasing N application rates significantly increased the soil nitrate-N accumulation (0–200 cm) but reduced the partial factor productivity of applied N fertilizer. Across the three plant densities, the two-year average soil nitrate-N accumulation was 12.6, 32.1, and 75.7 % higher with 90, 180, and 270 kg N ha⁻¹ compared to no N treatment, respectively. The highest soil nitrate accumulation under 270 kg ha⁻¹ N application rate in 2021 (229.5 kg ha⁻¹) and in 2022 (329.7 kg ha⁻¹) may cause severe nitrate leaching loss and potential soil water contamination, driven by intensive rainfalls. Averaged over the four N rates, planting density of 110000 plants ha⁻¹ increased the crop N accumulation and PFP by 21.2 % and 15.8 % in 2021, compared to 70000 plants ha⁻¹, respectively. The interaction of N application and planting density significantly affected the aboveground dry matter yield, crop water consumption, dry matter precipitation productivity, and crop N accumulation in 2021, but the effect was non-significant in 2022. Based on these findings, application of 180 kg N ha⁻¹ and planting density of 110000 plants ha⁻¹ are suggested as an efficient management strategy for improving productivity of forage maize and soil water and N resources utilization in the arid region of the Loess Plateau and similar areas.