Synergistic Effects of Salinization and Artificial Root Exudates on Soil Phosphatase Activity in Coastal Soil

Abstract

Coastal soil salinization from rising seawater levels has adverse impacts on soil function, seed germination, and plant growth. Root exudates play a key role in supporting microbial activity, nutrient cycling, and plant health, yet little is known about the combined effects of salinization and the addition of artificial root exudates (AREs) on soils. We investigated the impacts of sodium chloride (NaCl) and AREs, alone and in combination, on soil microbial function and the growth and survival of Chamaecyparis thyoides saplings. In a three-by-three fully factorial matrix, the combined NaCl and AREs treatment increased soil phosphatase activity more than the additive effects of the individual treatments, suggesting a synergistic effect on phosphatase activity. Sequencing revealed that salinity shaped bacterial community composition more than AREs, and fungal communities assembled more stochastically than bacteria. Soil respiration increased rapidly when treated with a high concentration of AREs, but this spike was delayed in the presence of high NaCl concentrations, suggesting a stressed microbial community. Treatments containing the higher NaCl concentration were lethal to C. thyoides saplings, with a 78% mortality rate within 1 week at soil salinities of ~1 dS/m. The results align with the stress gradient hypothesis, as the addition of AREs increased soil phosphatase activity more in the newly stressed, salinized environment than in unsalinized soil. These data highlight the important role of coastal vegetation showing that root exudates can partially offset salinity stress by enhancing soil microbial functions, while also defining salinity thresholds beyond which cedar sapling survival is unlikely.