Increasing nitrogen absorption and assimilation ability under mixed NO3− and NH4+ supply is a driver to promote growth of maize seedlings

Compared with sole nitrate (NO₃⁻) or sole ammonium (NH₄⁺) supply, mixed nitrogen (N) supply may promote growth of maize seedlings. Previous study suggested that mixed N supply not only increased photosynthesis rate, but also enhanced leaf growth by increasing auxin synthesis to build a large sink for C and N utilization. However, whether this process depends on N absorption is unknown. Here, maize seedlings were grown hydroponically with three N forms (NO₃⁻ only, 75/25 NO₃⁻/NH₄⁺ and NH₄⁺ only). The study results suggested that maize growth rate and N content of shoots under mixed N supply was little different to that under sole NO₃⁻ supply at 0–3 d, but was higher than under sole NO₃⁻ supply at 6–9 d. ¹⁵N influx rate under mixed N supply was greater than under sole NO₃⁻ or NH₄⁺ supply at 6–9 d, although NO₃⁻ and NH₄⁺ influx under mixed N supply were reduced compared to sole NO₃⁻ and NH₄⁺ supply, respectively. qRT-PCR determination suggested that the increased N absorption under mixed N supply may be related to the higher expression of NO₃⁻ transporters in roots, such as ZmNRT1.1A, ZmNRT1.1B, ZmNRT1.1C, ZmNRT1.2 and ZmNRT1.3, or NH₄⁺ absorption transporters, such as ZmAMT1.1A, especially the latter. Furthermore, plants had higher nitrate reductase (NR) glutamine synthase (GS) activity and amino acid content under mixed N supply than when under sole NO₃⁻ supply. The experiments with inhibitors of NR reductase and GS synthase further confirmed that N assimilation ability under mixed N supply was necessary to promote maize growth, especially for the reduction of NO₃⁻ by NR reductase. This research suggested that the increased processes of NO₃⁻ and NH₄⁺ assimilation by improving N-absorption ability of roots under mixed N supply may be the main driving force to increase maize growth.     

Root Function in Nutrient Uptake and Soil Water Effect on NO3- -N and NH4＋-N Migration

Root function in uptake of nutrients and the effect of soil water on the transfer and distribution of NO3^--N in arable soil were studied using summer maize （Zea mays L. var. Shandan 9） as a testing crop. Results showed that root growth and water supply had a significant effect on NO3^--N transfer and made NO3^--N distributed evenly from bulk soil to rhizosphere soil. Under a natural condition with irrigation, the difference of NO3^--N concentration at different distance points from a maize plant was smaller, while obvious difference of NO3^--N concentration was observed under conditions of limited root growth space without irrigation. Whether root growth space was restricted or not, the content of soil NO3^--N decreased gradually from 10 to 0 cm from the plant, being opposite to the root absorbing area in soils. When root-grown space was limited, changes of NO3^--N concentration at different distances from a plant were similar to that of water content in tendency. Results showed that NO3^--N could be transferred as solute to plant root systems with water uptake by plants. However, the transfer and distribution of NH4^--N were not influenced by root growth and soil water supply, being different to NO3^--N.