Abstract:

To study the effects of soybean-maize rotation on soil nitrogen-fixing bacteria, using maize continuous cropping as control (MMM) and nif-H gene as indicator, we adopted high-throughput sequencing technology to analyze the community structure and diversity of soil nitrogen-fixing bacteria under two-year soybean-maize rotation cycle (MSM) and three-year soybean-maize-maize rotation cycle (SMM). The results showed that soil organic matter, total nitrogen, alkali-hydrolyzed nitrogen, ammonium nitrogen and nitrate nitrogen in 0-20 and 20-40 cm soil layers increased significantly under soybean-maize rotation. Chao1 index and ACE index of nitrogen-fixing bacteria in 0-20 and 20-40 cm soil layers under rotation were higher than those under control, and the indexes under MSM were significantly higher than those under SMM. Shannon index and Simpson index under rotation in 0-20 cm soil layer were significantly higher than those under control (MMM); and the indexes under MSM in 20-40 cm soil layer were significantly higher than those under MMM. At the phylum level, the relative abundance of Proteobacteria in 0-20 cm and 20-40 cm soil layers under MSM and SMM were higher than that under MMM. At the genus level, the relative abundance of Bradyrhizobium, Frankia and Leptotrichia in 0-20 cm soil layer under the two rotation types were significantly higher. The relative abundance of Bradyrhizobium, Pseudoacidibacter, Geobacillus and Nitrogen-fixing Vibrio in 20-40 cm soil layer were significantly higher. In 0-20 cm and 20-40 cm soil layers, Bradyrhizobium, Methylocystis and Nitrogen-fixing Vibrio under MSM were higher than those under SMM. Variance analysis showed that the abundance indexes of nitrogen-fixing bacteria (Chao1 index and ACE index) were significantly and positively correlated with the content of organic matter, alkali-hydrolyzed nitrogen and nitrate nitrogen, and the abundance of Rhizobium was extremely significantly and positively correlated with the content of organic matter, total nitrogen and ammonium nitrogen. In summary, higher soil nutrients in soybean-maize rotation were important factors for increasing the abundance of soil nitrogen-fixing bacteria and changing community structure, and the abundance of nitrogen-fixing bacteria in soybean-maize two-year cycle was relatively high. Soybean-maize rotation can increase the beneficial microbial groups with high nitrogen-fixing efficiency and multi-functions in soil.

Key words: soybean-maize rotation, rotation cycle, nitrogen-fixing bacteria, microbial community structure, nif-H gene, diversity