Superhydrophobic Sand Mulch and Date Palm Biochar Dramatically Boost Growth of Moringa oleifera in Sandy Soil: Insights into Evapotranspiration Budgeting and Metabolomic Profiling

In response to the challenge of nutrient-deficient sandy soils and water scarcity due to excessive evaporative water loss in arid regions, we developed and tested two complementary soil amendment technologies: Superhydrophobic sand (SHS) mulch and an enriched date palm biochar. In a greenhouse pot experiment, we investigated the stand-alone and synergistic effects of SHS mulch (10 mm-thickness) and biochar (2% w/w) on Moringa oleifera plants under normal (N, 100% field capacity) and reduced (R, 50% of N) irrigation scenarios. Under N and R, SHS mulch reduced evaporation by 71% and 64%, respectively; while SHS+biochar reduced evaporation by 61% and 47%, respectively, in comparison with the control (p < 0.05). Total transpiration significantly increased in SHS plants by 311% and 385% under N and R, respectively. Compared with the control, transpiration increased in biochar-amended plants by 103% and 110%; whereas, its combination with SHS increased transpiration by 288% and 301% under N and R, respectively (p < 0.05). Irrespective of the irrigation regimes, we found superior effects of SHS, biochar, and their combination on plant height (62-140%), trunk diameter (52-91%), leaf area index (57-145%), leaf chlorophyll content index (11-19%), stomatal conductance (51-175%), as well as shoot (390-1271%) and root (52-142%) biomass, in comparison with the controls. Metabolomics analysis showed significantly higher relative abundance of amino acids, sugars, fatty acids, and organic acids in the leaves of control plants relative to other treatments, as a response to water or nutrient stress induced by excessive water loss through evaporation. Next, we found a higher concentration of D-Mannose, D-Fructose, glucose, and malic acid in plants grown with SHS or biochar treatment under N and R irrigation, attributed to increased water/nutrient-use efficiency and carbon assimilation because of higher photosynthesis rates than in the control plants. Our results show that, our complementary technologies could address the challenge of water loss via evaporation from soil and maximize soil nutrient retention for improving plant growth in arid regions. This could underscore the success and sustainability of irrigated agriculture and greening efforts in arid lands.