Effects of row spacing and maize variety on abundance and function of denitrifying bacterial flora in maize rhizosphere

Denitrification is a key pathway of nitrogen loss in the rhizosphere. It is unknown whether changes in variety and row spacing of crop have any impacts on denitrification in the rhizosphere. Thus this study compared the differences in abundance and function of denitrifiers in the rhizosphere of different maize varieties and row spacings. The aim of the study was to provide scientific basis for reducing rhizosphere denitrification losses and improving nitrogen use efficiency. Based on data from two independent field experiments, the study investigated the effects of five maize varieties ('Xundan 20', 'Annong 8', 'Zhengdan 958', 'Pinyu 18' and 'Longping 206') and four row spacings (20 cm, 30 cm, 40 cm and 50 cm) on denitrification capacity, abundance of denitrifiers, N₂O/(N₂O+N₂) ratio and soil respiration in the rhizosphere using biochemical and molecular microbiological methods. Denitrification capacities in the rhizospheres of 'Xundan 20', 'Annong 8' and 'Zhengdan 958' were significantly lower than those in the rhizosphere of the other maize varieties. With decreasing row spacing, denitrification capacity in rhizosphere soil obviously increased. The abundances of nirS-denitrifiers in the rhizospheres of 'Longping 206' and 'Pinyu 18' were significantly higher than those in the rhizospheres of the other maize varieties. The abundances of nirK-and nosZ-denitrifiers in the rhizospheres of 'Xundan 20' and 'Annong 8' were the highest. In all the row spacings, 20 cm had the highest abundances of nirS-and nirK-denitrifiers. As for nosZ-denitrifiers, the most abundance was found in the 40 cm row spacing treatment. The change of variety had a certain impact on N₂O/(N₂O+N₂) ratios (e.g. 'Annong 8' had the lowest ratio), but the effect of row spacing was no significant. Correlation analysis showed there was a significantly positive correlation between denitrification capacity and soil respiration or the abundance of nirS-denitrifiers. But not significant correlation was found between denitrification capacity and the abundances of nosZ or nirK-denitrifiers. This suggested that differences in the abundances of nirS-denitrifiers and organic carbon in the rhizosphere were the dominant factors causing the changes in denitrification capacity. Changes in variety and row spacing could affect denitrification of maize rhizosphere as well. The screening of varieties with low denitrification loss and the regulation of denitrification processes in the rhizosphere were effective ways of reducing rhizosphere denitrification loss and improving crop nitrogen use efficiency.