Changes in soil bacterial communities with increasing distance from maize roots affected by ammonium and nitrate additions.

• 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]