Optimizing phosphate fertilizer rate and sources for enhancing yield and grain zinc of maize under saline conditions

Crops grown on coastal saline-alkali soils in China often receive excessive phosphorus (P) fertilization to overcome low P availability. However, it often leads to reduced grain zinc (Zn) concentration-a critical micronutrient for human and animal health. The interactive effects of P application rates and fertilizer sources on maize (Zea mays L.) yield and grain Zn accumulation under saline stress remain unclear. This study aims to identify optimal P fertilizer rates and sources to reduce the P-Zn antagonism while sustaining high yield. A three-year field experiment was conducted to test five P rates (0, 13, 26, 39, and 78 kg P ha^-1) and three P fertilizer sources including single superphosphate (SSP), monoammonium phosphate (MAP), and ammonium polyphosphate (APP). Measurements included maize yield, grain and straw concentrations of P and micronutrients, soil Olsen-P, DTPA-Zn. Linear-plateau regression and structural equation modeling were used to identify optimal P management. Results showed that a yield plateau of 9.14 Mg ha^-1 was attained at an optimal P fertilizer rate of 24.1 kg P ha^-1, which closely matched the treatment level of 26 kg P ha^-1. At this rate, the three-year average yield was highest with SSP, intermediate with APP, and lowest with MAP. Compared to no P supply, increasing P application rates significantly reduced grain Zn concentrations by 13.1-26.4% and straw Zn concentrations by 27.9-47.2%, even though it increased Zn harvest index. Compared to APP, SSP and MAP resulted in 8.7% and 9.7% of increase in grain Zn concentration, respectively. Structural equation modeling revealed that aboveground biomass, grain yield, soil Olsen-P, and soil pH negatively affected grain Zn concentration. At the yield plateau, grain and straw Zn concentrations were 14.1 and 9.5 mg kg^-1, respectively, with soil DTPA-Zn around 0.81 mg kg^-1. These results suggest that applying SSP at 26 kg P ha^-1 significantly improved maize yield compared with MAP, while also maintaining higher grain Zn concentration compared to APP. The optimizing P rates and sources and rates can partially mitigate P-Zn antagonism while sustaining high maize yields in the coastal saline soil.