Objectives In view of the problems of water evaporation and loss of water and fertilizer on soil layer in the current irrigation system in the facilities agriculture, and do not supply water and fertilizer according to the crop requirements ally, this study aimed to compare different irrigation methods, and discuss the mechanism of the increase of the yield and quality on purple leaf lettuce (Lactuca sativa L.), water use efficiency and rhizosphere microbial diversity on purple leaf lettuce by negative pressure irrigation (NPI).

Methods The pot experiments were conducted in greenhouse and employed purple leaf lettuce as test material, 3 irrigation treatments were set up: flooding irrigation (FI), drip irrigation (DI) and NPI. After harvest, the yield and quality (vitamin C, soluble sugar, anthocyanin and nitrate content) of the purple leaf lettuce, the nutrient concentration and uptake of the plant, the dynamic soil water content, water consumption and use efficiency were analyzed, and the diversity index of rhizosphere soil microbes and the structure composition of bacteria in the phylum classification were calculated.

Results The yield and quality of purple leaf lettuce under NPI could significantly increase by 68.1% and 29.0% compared to the FI and DI, respectively. NPI also increased the content of vitamin C, soluble sugar and anthocyanin, and reduced the nitrate content on purple leaf lettuce. The water consumption of the purple leaf lettuce under NPI was the least, being 23.8% and 23.8% lower than that of FI and DI, respectively. Under NPI, the concentration and uptake of nitrogen, phosphorus and potassium of purple leaf lettuce were significantly increased, which were remarkably increased by 13.0%, 14.4%, 38.4% and 90.2%, 92.6%, 135.5% compared with FI treatment, respectively. The water use efficiency was the highest with NPI treatment, which was noteworthy increased by 122.2% and 70.5% compared with FI and DI, respectively. Meanwhile, the dynamic soil water content variation from 10.3% to 11.3% for NPI, lower than that from 9.2% to 11.6% for FI and DI. Through high throughput sequencing of soil rhizosphere microbial communities, it was found that NPI has the highest microbial diversity index with the highest value of OTU, Chao1 and Shannon indices, which were 1808, 2437 and 8.48, respectively, or 15.2%, 15.7% and 3.16% higher than those of FI treatment. The relative abundance of bacteria at the phylum classification was also changed by different irrigation treatments, and the abundances of Actinobacteria, Chloroflexi, Verrucomicrobia and Planctomycetes on the bacterial phylum were increased under the NPI treatment than these of FI and DI.

Conclusions Planting purple leaf lettuce by NPI system could achieve the goal of high yield and quality and water utilization efficiency through the steady supply mechanism of soil water and fertilizer. Therefore, the NPI system significantly improved the yield and quality, water use efficiency and diversity of the rhizosphere microbial communities on purple leaf lettuce, and provided a reliable scientific basis for the sustainable development of facilities agriculture.