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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. [ABSTRACT FROM AUTHOR]

Aims: Matching a crop's nitrogen (N) preference with the specific form of N is critical to enhance N recovery efficiency (NRE). Many studies show that maize (Zea mays L.) has a preference for NH4+ uptake over NO3−, but the contribution of this preference to NRE is unclear. This study evaluated the utilization of ammonium (NH4+) and nitrate (NO3−) by maize and its relationship with NRE. Methods: Maize (cv. 'Zhengdan 958') seedlings were grown in a soil culture experiment with 15NH4+ or 15NO3− applied to sterilized or non-sterilized soils, and in a hydroponic experiment supplied with NH4+ or NO3−. We recorded maize root and shoot growth and N uptake, and the distribution of 15N-labeled N in the soil–plant system. Results: Ammonium application enhanced maize root and shoot growth and N uptake compared with those of NO3−-treated plants in both culture systems. This enhancement by NH4+ was further promoted in sterilized soil. More applied N remained in soil supplied with NO3− than with NH4+ after seedling harvest. The NRE of NH4+ in maize was significantly higher than that of NO3−, while a similar proportion of applied N was lost with NH4+ or NO3−supply. Conclusions: Ammonium supply enhances maize growth and NRE compared with NO3− under the present experimental condition, which may be attributable to the superior capability of maize to utilize NH4+. [ABSTRACT FROM AUTHOR]

• Ammonium and nitrate reduce only maize rhizospheric bacterial diversity. • Rhizospheric bacterial community varies more significantly than bulk soil. • Maize roots recruit N-form specific bacteria according to soil pH. It is well known that the interactive effect of nitrogen (N) fertilizer and plant root would cause distinct microbial communities and associations between root surface and bulk soil. However, little is known about the continuous changes of these microbial parameters along an increasing distance from roots under different N forms. Ammonium (NH 4 +) and nitrate (NO 3 −) are two major inorganic nitrogen (N) forms available for plant growth, and may have different effects on both plant growth and soil microbiome. Here, a rhizobox containing a root zone (root growing area) and various soil compartments (0–0.5 cm, 0.5–1 cm, 1–2 cm, 2–4 cm, and 4–9 cm from the root zone) was designed to investigate the spatial distribution of soil chemical properties, bacterial diversity, and community structure along a distance from maize (Zea mays L.) roots with the addition of 15N-labeled NH 4 + or NO 3 − in an acidic red soil. Addition of NH 4 + or NO 3 − reduced soil bacterial diversity only in the maize root zone. The structures of soil bacterial communities differed more significantly with addition of NH 4 + or NO 3 − in the root zone and 0.5 cm away from the root zone compared with other soil compartments. Soil pH was the major driver of bacterial community assembly during plant uptake of N. Maize roots recruited acidophilic bacteria (e.g. Acidibacter , Burkholderia, and Catenulispora) under NH 4 + treatment, and recruited bacteria that prefer higher pH (e.g. Sphingomonas , Sphingobium , Azospirillum, and Novosphingobium) under NO 3 − treatment. We concluded that the spatial patterns of soil bacterial communities at different distances from the maize root zone were influenced by the combined effects of the N form and rhizosphere, mainly as a result of changes in soil pH during the uptake of NH 4 + and NO 3 − by maize roots. [ABSTRACT FROM AUTHOR]