Effects of rhizosphere wettability on microbial biomass, enzyme activities and localization

Recent studies proved that the rhizosphere of some plant species turn water repellant upon drying. The effects of rhizosphere water repellency on microbial biomass and enzyme activity are not known. We hypothesized that rhizosphere water repellency limits microbial biomass and enzyme activities and increasing rhizosphere wettability by application of surfactants raises enzyme activities, especially during repeated drying/rewetting cycles. Maize plants were grown in rhizoboxes and subjected to six drying/rewetting cycles for eight weeks. Half of the plants were irrigated with water and the other half with surfactant solution. After six drying/rewetting cycles, we measured: i) enzyme activities and distribution using zymography; ii) microbial biomass carbon; and iii) shoot and root biomass. Application of a selected surfactant: i) increased the β-glucosidase and phosphatase activities by 5.3 and 2.9 times, respectively, in the regions close to the roots (0–0.5 mm distance from the root surface); ii) enlarged the area with high enzyme activity by 1.46-fold for β-glucosidase and by 1.2-fold for phosphatase; iii) increased microbial biomass carbon by 1.57-fold; and iv) increased root biomass by 1.24-fold. The enhanced microbial activities upon surfactant application were explained by the improvement of the rhizosphere wettability upon surfactant application. The higher wettability maintains the stability of microbial habitats and enhanced enzyme activities in the rhizosphere during repeated drying/wetting cycles. These modifications have the great potential to improve plant performance under water deficit condition.