Effect of combined nitrogen and phosphorus fertilization on summer maize yield and soil fertility in coastal saline-alkali land

Soil salinization limits food production and land use efficiency. Proper nitrogen and phosphate fertilizer application can improve saline soils, but the optimal ratio under saline-alkali conditions is unclear. This experiment set different nitrogen fertilizer application rates (60 kg/hm2, 120 kg/hm2) and phosphate fertilizer application rates (40 kg/hm2, 80 kg/hm2, 120 kg/hm2). The results showed that various nitrogen and phosphate fertilizer treatments had significant effects on grain yield, biomass yield, and soil nutrients. The T3 treatment (nitrogen fertilizer: 120 kg/hm², phosphate fertilizer: 120 kg/hm²) resulted in the highest yields, with the average grain yield and biomass yield of maize over three growing seasons being 6.70 × 10 ³ kg/hm² and 17.53 × 10 ³ kg/hm², respectively. For the three-year average, the alkaline hydrolyzable nitrogen content (228.70 mg/kg), nitrogen uptake (199.75 mg/kg) and average phosphorus uptake (239.89 mg/kg) was highest under the T3 treatment. The yield of summer maize is directly influenced by factors such as alkaline hydrolysable nitrogen, available phosphorus, readily available potassium in the soil, nitrogen uptake, phosphorus uptake, potassium uptake, soil moisture content at 0–20 cm and 20–40 cm depths. Among these, phosphorus uptake (Standardized Path Coefficient (SPC) = 0.65) and potassium uptake (SPC=0.57) have the greatest impact on the increase in grain yield (GY). The optimal application rates for maize production in saline-alkali soil are 120 kg/hm² for nitrogen fertilizer and 120 kg/hm² for phosphate fertilizer. These results can provide a theoretical basis for fertilizer management in maize production on saline-alkali land.