Root exudate analogues increase soil CO2 emission, iron concentration, and acidity in mangroves

Mangroves are carbon dense ecosystems. Their root exudates could remobilise buried soil organic matter in the form of CO2 emission, notably by stimulating organic matter decay indirectly via an exudate sugar-driven and microbially mediated pathway, or directly by the breakage of organo-mineral bonds. Here, we used a manipulative laboratory incubation to test the effect of root exudate type on CO2 emission in two contrasting mangrove soils: a peat soil with mostly particulate organic matter (Dumbea, New Caledonia, France) and a mineral soil dominated by organo-mineral associations (Can Gio, Vietnam). Using a custom-made 20 cm long needle with a side-port near the tip,we spiked two exudates types, oxalic acid and glucose, into the mineral and organic mangrove soils. The soil CO2 emission was quantified with a gas analyser over time. Iron and pH were mapped at high spatial resolution using two-dimensional Diffusive Equilibrium Thin-films (2D-DET) gels. The root exudate inputs significantly increased the CO2 emission in both mangroves (by an order of magnitude; p< 0.01). The organic rich and mineral mangrove soil CO2 emission responded similarly to both root exudate types. There was no difference in soil CO2 emission between glucose and oxalic acid treatment. Oxalic acid reduced the soil pH consistently across the vertical soil profile in the mineral mangrove soil, while in the peat soil there was a sharp pH decrease in the few top millimetres of soil. For both soil types, the iron concentration was multiplied by an order of magnitude under oxalic acid treatment with a peak in the soil surface, and was slightly increased under glucose treatment. Our results reveal that root exudation could be a major driver of carbon, pH, and iron dynamics in mangrove soils. These findings highlight the importance of understanding root-soil interaction to constrain mangrove carbon budgets.