Objectives We assessed the changes of nitrogen absorption and soil microbial diversity in rhizosphere soil of tomato intercropped with potato onion to understand the biological mechanism of improving nitrogen nutrition of tomato.

Methods The experiment consisted of two parts: field trials and pot cultivation. The treatments in the field experiment include tomato monocropping, potato onion (Nong'an) and tomato intercropping, and potato onion (Wuchang) and tomato intercropping. We determined the growth index, nitrogen uptake and yield of tomatoes to elucidate the effects of intercropping on tomato nutrient uptake, distribution and yield. The pot experiment included 4 treatments: monoculture of tomato, potato onion (Wuchang) monocropping, potato onion and tomato intercropping, and no seedling (control). The contents of ammonium and nitrate nitrogen and soil enzyme activity in rhizosphere soil of the potted tomato were measured. We used the Miseq high-throughput sequencing technology of soil bacteria and fungi diversity to clarify the effects of intercropping on the rhizosphere soil nutrients.

Results 1) The height of tomato plant increased significantly, and the yield of tomato increased significantly by 8.49%–16.92% after intercropping. 2) The two intercropping treatments showed similar results. After 60 days of intercropping, the dry weight of tomato increased significantly, the distribution index of dry matter to roots decreased, while the distribution index to shoots increased. The distribution index to stems and leaves decreased, but the distribution index to fruit increased. Intercropping for 90 days significantly increased nitrogen distribution to roots and stems, but decreased nitrogen distribution to leaves. 3) After 37 days of intercropping, the ammonium nitrogen content in the rhizosphere soil of tomato increased significantly, while the nitrate nitrogen content decreased significantly. Intercropping changed the ratio of ammonium and nitrate nitrogen in soil and promoted the nitrogen absorption of tomato. The activities of urease and dehydrogenase in tomato rhizosphere soil were significantly higher than those of monoculture and no seedling treatments, and the dehydrogenase of intercropping potato onion rhizosphere soil was significantly higher than that of monoculture and no seedling treatment, and other differences were not significant. 4) The fungal and bacterial community structure in rhizosphere soil of tomato intercropping with potato onion was changed. Intercropping with potato onion increased the abundances of Actinobacteria, Bacteroidetes, Flavobacteria, Chlorobia, Chloroflexi, Anaerolineae, Deinococci, Gemmatimonadetes, Clostridia, Mollicutes, Planctomycetacia, Alphaproteobacteria and Opitutae. Moreover, the relative abundances of Pseudomonas, Rhizobium, Bacillus and Sphingobium increased in the rhizosphere of tomato in the intercropping system at the genus level. For fungi, the relative abundances of Penicillium, Aspergillus and Chaetomium increased, but decreased the abundance of Eurotium and Thielavia in the rhizosphere of tomato in the intercropping system.

Conclusions Intercropping two kinds of potato onion increased the plant height and dry weight of tomato, as well as the nitrogen uptake and yield. Intercropping reduces the distribution of nitrogen nutrients to leaves, while increasing the distribution of nitrogen nutrients to roots, flowers and fruits, and improves nitrogen utilization. At the same time, by changing the structure of bacterial and fungal flora in the rhizosphere soil of tomato, the soil nitrogen effectiveness of intercropping potato onions significantly improved, which promoted the nitrogen absorption of tomato and laid the nutrient foundation for the improvement of tomato yield.