Increasing Italian ryegrass (Lolium multiflorum Lam.) regrowth via inoculation with an ammonia-oxidizing bacterial strain

This study aimed to investigate the effect of a soil ammonia-oxidizing bacterial (AOB) strain on Italian ryegrass (Lolium multiflorum Lam.) regrowth. Potted Italian ryegrass was used as test material. We isolated and screened an AOB strain from the soil for use in inoculation of other soils to increase nitrification. The sequence of the two isolated and screened AOB strains (S2_8_1 and S2_7_25) were 100% similar to that of Ensifer sesbaniae and Acinetobacter pittii. The nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) was also added to the soil to inhibit nitrification. The experimental design involving the inoculation of the soils with the AOB strain included seven treatments: (1) regrowth without any additives, (2) regrowth with blank enrichment medium added to the soil, (3) regrowth with S2_8_1 strain inoculation, (4) regrowth with S2_7_25 strain inoculation, (5) regrowth with DMPP added to the soil, (6) regrowth with DMPP added to the soil and with S2_8_1 strain inoculation, and (7) regrowth with DMPP added to the soil and with S2_7_25 strain inoculation. Compared with regrowth treatment without any additives, the regrowth treatment with S2_8_1 and S2_7_25 inoculations significantly increased leaf biomass by 23.5–48.6%, the leaf net photosynthetic rate by 27.9–48.5%, the rhizosphere soil nitrification rate by 43.0–144.2%, leaf cytokinin concentrations by 23.7–24.9%, and xylem sap cytokinin concentrations by 32.5–43.2%. The increase of nitrate nitrogen content in rhizosphere soil induced by S2_8_1 and S2_7_25 promoted the transfers of cytokinin from roots to leaves, resulting in the increase of cytokinin concentration in leaves. This increase in leaf cytokinin concentrations improved Italian ryegrass regrowth. However, the S2_8_1 and S2_7_25-induced increases in the nonrhizosphere soil nitrification rate were not closely related to Italian ryegrass regrowth. The tested AOB strains have considerable potential to increase Italian ryegrass regrowth.