Shoot K/Na Ratio is the Key Factor for Vermicompost and Arbuscular Mycorrhizal Fungi to Improve Maize Biomass in Saline-Alkali Soil

To investigate the relationships and relative contributions of soil physical and chemical properties, and plant hormones and antioxidant enzymes to maize biomass production with the application of vermicompost and arbuscular mycorrhizal fungi (AMF) in saline-alkali soil. A greenhouse experiment was conducted in a randomized complete block design with maize grown with factorial combinations of plus and minus vermicompost and AMF additions. With the application of vermicompost and AMF, soil macroaggregates increased by 14 to 48%, salt concentration decreased by 12 to 34%, available phosphorus increased by 15 to 59%, and ammonium-N concentration increased by 26 to 40%. In response the shoot K/Na ratio increased by 43 to 261%, with consequent increases in plant nitrogen and phosphorus concentrations, and biomass. The improvements in salt concentration and nutrient availability were paralleled by shoot indole-3-acetic acid concentration increasing by 20 to 28% and shoot catalase activity decreasing by 12 to 48%, which facilitated the increase in nutrient uptake and biomass. The increased biomass was mostly attributed to shoot K/Na ratio and catalase activity, by 54.4% and 9.7%, respectively. The synergistic effects of soil physical and chemical amelioration, and plant endogenously physiological regulation with vermicompost and AMF application effectively improved maize biomass in saline-alkali soil, with shoot K/Na being the key driver of biomass enhancement, and this mechanism merits consideration as an important target to improve plant salinity tolerance and biomass production under salt stress.