Effects of rhizosphere soil nitrification on the compensatory growth of Italian ryegrass (Lolium multiflorum Lam) based on root-produced cytokinin

Compensatory growth of forage grass can determine its optimal biomass, which is beneficial for increasing its production. The effect of rhizosphere soil nitrification on Italian ryegrass (Lolium multiflorum Lam) regrowth was investigated on the basis of root-produced cytokinin concentration in leaves to reveal the forage grass compensatory growth mechanism. The nitrification inhibitor 3,4-dimethylpyrazole phosphate was added to inhibit soil nitrification. Without soil nitrification being inhibited, compared with no clipping one defoliation cycle increased the leaf photosynthetic rate by 43.82%–53.79%, increased the leaf cytokinin content by 33.48%–34.73%, and increased the cytokinin transport from roots to leaves by 28.88%–39.47%. Nitrification inhibitor decreased soil nitrification rates by 23.33%–84.17% in the rhizosphere and by 42.71%–68.29% in the bulk soil during regrowth. In the rhizosphere micro-environment, nitrification increased soil nitrate concentration that played an important role in the transport of cytokinin from roots to leaves during regrowth, increasing the leaf cytokinin concentration. However, bulk soil nitrification rate and nitrate content had little influence on leaf cytokinin concentration during regrowth. An increase in leaf cytokinin improved the photosynthesis and the regrowth. Without soil nitrification being inhibited, total biomass at the end of regrowth period was 1.09 times higher in plants that were defoliated once than in non-clipped plants. However, soil nitrification inhibitor and two defoliation cycles decreased Italian ryegrass regrowth. Super compensatory growth occurred in the one defoliation cycle Italian ryegrasses without adding soil nitrification inhibitor. Compensatory growth occurred in once-defoliated Italian ryegrass with adding soil nitrification inhibitor and in twice-defoliated Italian ryegrass without adding soil nitrification inhibitor. In conclusion, rhizosphere soil nitrification is the key factor that regulates the compensatory growth of Italian ryegrass.