Root exudation: meassuring a missing component of carbon flux estimates in mangroves

Mangroves are among Earth's most carbon-dense ecosystems, yet belowground carbon cycling remains poorly understood compared to aboveground processes. Root exudation, the release of labile organic compounds from live roots, represents a critical pathway for transferring plant-derived carbon to soils. However, exudation has never been quantified in situ in mangroves due to technical challenges. Here, we developed and applied a sealed-cuvette system to quantify root exudation across two dominant species (Avicennia alba and Rhizophora apiculata) and contrasting wet–dry seasons in a deltaic mangrove (Can Gio, Vietnam). Mean root exudation rates were 0.135 ± 0.035 mg C·g⁻¹·h⁻¹ for Avicennia and 0.078 ± 0.017 mg C·g⁻¹·h⁻¹ for Rhizophora, with seasonal rates (pooled across both species) of 0.060 ± 0.013 mg C·g⁻¹·h⁻¹ for the wet season and 0.103 ± 0.031 mg C·g⁻¹·h⁻¹ for the dry season. Gamma GLMs testing for effects of species and season revealed no statistically significant differences in exudation rates (species: p = 0.093; season: p = 0.16), though substantial individual-level variation was observed within each group. Mangrove root exudation rates were comparable to global averages reported across terrestrial ecosystems (~0.058 mg C g⁻¹ h⁻¹), indicating similar root-level carbon release despite contrasting environmental conditions. When multiplied by mangroves' extensive fine-root biomass, and scaled to hectare and annual timescales, preliminary estimates suggest the exudation flux may represent a non-negligible and previously unaccounted-for component of mangrove carbon budgets.