Maize Seedlings Prefer NO3- Over NH4+ Independent of pH Changes

Maize (Zea mays L.) N uptake affects N use efficiency (NUE), which is affected by the N form for uptake. However, maize N preference for NH4+ or NO3- is unclear. A field experiment and a (15) N tracing study was conducted with various soils differing in pH plant with maize. Maize N (NO3- and NH4+) uptake rate significantly varied with soil pH, but NO3- uptake rate (U-NO3) was much higher than NH4+ uptake rate (U-NH4) irrespectively of the pH, i.e., maize generally preferred NO3- uptake over NH4+ at the seedling stage. The generally higher U-NH4 and U-NO3 in acidic compared to alkaline soils in the laboratory study were consistent with the maize yield in the field experiment results. The positive relationship between U-NO3 and U-NH4 (P < 0.01) indicated a synergistic effect between NH4+ and NO3- uptake. The longer NH4+ residence time could explain the higher NH4+ and total N uptake rates in acidic soil than alkaline soil. With maize planted, the rate of autotrophic nitrification (O-NH4) reduced between 30 and 75% compared to unplanted soils supporting N uptake via a prolonged NH4+ residence time. The rate of heterotrophic nitrification (O-Nrec) was stimulated by maize plants, accounting for up to 45% of total NO3- production in acidic soils. The role of O-Nrec was illustrated by a positive correlation of O-Nrec/(O-NH4 + O-Nrec) with U-NO3 (P < 0.01). Maize seedling preferred NO3- over NH4+ irrespectively of pH. Moreover, maize can regulate soil N supply through interactions between plant and soil N transformations to meet their N acquisition.

Automated summary

The study found that the uptake rate of different forms of nitrogen by maize is affected by soil pH, and maize generally prefers NO3- over NH4+. The longer residence time of NH4+ in acidic soil can explain the higher NH4+ and total N uptake rates. Maize planting can reduce the rate of autotrophic nitrification by prolonging NH4+ residence time, while the rate of heterotrophic nitrification is stimulated by maize plants.