Nitrogen Source Preference in Maize at Seedling Stage Is Mainly Dependent on Growth Medium pH

Abstract

To improve crop nitrogen recovery efficiency (NRE), plants must be supplied with their preferred form of nitrogen (N). However, whether pH affects crop N-form preference remains unclear. Here, we aimed to explore how maize (Zea mays L.) preference for NH4+ and NO3− is affected by pH and to determine the critical pH controlling this preference. Maize plants were grown with NH4+ or NO3− in different soils (pH 4.32–8.14) and nutrient solutions (pH 4.00–8.00). After harvest, plant dry weights, N content, N uptake, NRE, soil pH, and exchangeable aluminum (Al) were measured. Compared with the effect of NO3−, NH4+ decreased maize dry weight, N uptake, and NRE by 28–94% at soil pHs of 4.32 and 4.36 and a solution pH of 4.00, whereas it increased these parameters by 10–88% at soil pHs of 6.52–8.02 and solution pHs of 7.00 and 8.00. NO3− increased soil pH and decreased soil exchangeable Al content at soil pHs of 4.32–6.68. Critical soil and solution pHs for changing plant growth and N uptake preference for NH4+ vs. NO3− ranged from 5.08 to 5.40 and from 5.50 to 6.59, respectively. In conclusion, the preference of maize seedling growth and N uptake for NH4+ vs. NO3− mainly depends on the pH of the growth medium, and maize seedlings generally prefer NO3− in strongly acid soils but NH4+ in neutral to alkaline soils.