Optimizing nitrogen fertilizer management to improve yield, N use efficiency and profit of spring maize in coastal saline soil

Objectives We tested the availability of optimizing N fertilizer management based on in-season root N management (IRNM), to improve the efficiency of maize production and reduce the environmental risks caused by N fertilizers in coastal saline soil.

Methods The field positioning trail with split plot design was carried out in Dongying City, Shandong Province from 2020 to 2022, and Zhengdan 958 was used as the test variety. The main treatment was N rate, namely: no N input control (CK), optimized N application rate (Opt) based on IRNM, and 70% and 130% of the optimized N application (Opt70%, Opt130%), and farmers’ N application practice (FNP). The sub-treatment was three N fertilizers, namely: ammonium sulfate (AS), calcium ammonium nitrate (CAN), and urea (Urea). The aboveground biomass was measured at the six-leaf stage (V6), tasseling stage (VT) and mature stage (R6) of spring maize, and the grain yield, grain and straw N content were determined at harvest. Before sowing and after harvesting maize, 0−90 cm soil depth samples were taken to analyze the inorganic N content. The apparent N balance, agronomic efficiency of N fertilizer (AE_N), partial factor productivity of N fertilizer (PFP_N), and economic benefits of N application in spring maize planting system were calculated.

Results Opt treatment was recorded 8.5%, 4.1% and 12.7% higher grain yield than Opt70% in 2020, 2021 and 2022 (P<0.05). Compred with Opt130%, Opt treatment significantly increased grain yield in 2020, and maintained similar grain yield and biomass in 2021 and 2022, thereby increased PFP_N by 33.3%−50.9%, 2.6%−21.4%, 15.1%−23.2%, respectively (P<0.05). Compared with FNP, Opt treatment decreased the total N application rate by 30.7%−46.4% while maintained similar N uptake, biomass and yield, thus enhanced the average PFP_N significantly by 90.6%−115%, 38.9%−49.8%, 52.4%−60.7%, the total income by 22.8%. Under Opt treatment, applying the three nitrogen fertilizers harvested similar grain yield in 2020 and 2021, but applying CAN harvested significantly higher yield than urea, and similar yield with AS in 2022. CAN under Opt increased the PFP_N by 14.3%−17.2% and 9.8%−14.6% higher than AS and Urea, increased AE_N by 8.9%−18.9% higher than AS, reduced apparent N loss by 10.3% and 11.0% compared with AS and Urea, respectively. However, the three-year average N application efficiency of CAN was 18.5% lower than that of AS treatment.

Conclusions Optimized N application based on IRNM could precisely meet the large N demand at six-leaf and tasseling stage of maize, thereby maintain maize yield and biomass in low fertilization rate, improve N use efficiency, reduce soil residual N_min and apparent N loss. Under optimized N application rate, ammoniam sulfate should be selected as the highest benefit of maize production and relatively cheap price. Targeting maize yield of 10 t/hm², the application of 150−194 kg/hm² ammonium sulfate in IRNM technology is recommended for the synergistic increase of spring maize yield, N use efficiency and economic benefit in the coastal saline soil region.