Regulatory effects of salt-nitrogen interaction on the growth and photosynthetic characteristics of Limonium bicolor in the Yellow River Delta.

The Yellow River Delta, as a typical coastal saline soil distribution area in northern China, also faces increasingly prominent nitrogen input problems. To investigate the effects of salt-nitrogen interaction on the growth and photosynthetic physiology of the typical halophyte Limonium bicolor in the Yellow River Delta, this study established a pot-controlled experiment with three salt gradients (no salt addition, 0.3%, and 0.6% salinity) and four nitrogen addition levels (0, 15, 30, and 45 kg·N·ha-1·yr-1). Growth parameters, photosynthetic characteristics, element contents, and soil physicochemical properties of L. bicolor were measured. The results showed that salt and nitrogen addition had significant interactive effects on plant height, biomass, net photosynthetic rate, and other indicators. Compared with the no-salt and 0 kg·N·ha-1·yr-1 treatment, the treatment combining 0.6% salinity and 45 kg·N·ha-1·yr-1 significantly increased plant height, aboveground biomass, and net photosynthetic rate. Nitrogen addition increased the net photosynthetic rate, stomatal conductance, SPAD value, and total chlorophyll content of L. bicolor in a saline environment. Salt-nitrogen interaction significantly regulated total nitrogen in aboveground and belowground parts, and total phosphorus in belowground parts, of L. bicolor. Salt significantly increased soil pH, electrical conductivity, and organic matter content. Foliar nitrogen input, accompanied by plant-soil feedback processes, altered certain soil nutrient indices, with soil total nitrogen and inorganic nitrogen contents showing significant responses; however, it should be noted that long-term high nitrogen input may drive a shift toward phosphorus limitation. In ecological restoration practices, coordinated management of nitrogen and phosphorus should be given due attention. Structural equation modeling indicated that nitrogen addition was the primary exogenous factor driving L. bicolor growth. In conclusion, moderate salt-nitrogen interaction (i.e., the 0.6% salinity and 45 kg·N·ha-1·yr-1 treatment) synergistically promoted the growth and photosynthesis of L. bicolor, although higher nitrogen addition may induce a potential nitrogen saturation-like response.