Nutrient thresholds govern carbon dioxide sink–source dynamics in seagrass ecosystems

Abstract Nutrient pollution is accelerating in coastal waters, yet its effects on carbon dioxide (CO 2 ) sink–source dynamics remain poorly constrained. Seagrass ecosystems are globally important blue carbon sinks but can shift to net CO 2 sources under excessive nutrient loading. Here, we present one of the first empirical assessments examining how dissolved inorganic nitrogen, dissolved inorganic phosphorus, and their stoichiometric balance (dissolved inorganic nitrogen/dissolved inorganic phosphorus) regulate seasonal air–sea CO 2 fluxes across tropical seagrass meadows. Using in situ measurements along a nutrient gradient in the South China Sea, we show that CO 2 fluxes respond nonlinearly to nutrient concentrations and exhibit season‐specific thresholds. Dissolved inorganic nitrogen enhances CO 2 uptake during spring and summer but promotes CO 2 emissions during autumn and winter. Dissolved inorganic phosphorus consistently reduces CO 2 influx. We identify nutrient “turning points” (maximum CO 2 uptake) and “transition points” (CO 2 sink–source reversals) that offer predictive capacity for managing carbon outcomes. Strategic nutrient management—optimizing dissolved inorganic nitrogen during growing seasons while restricting dissolved inorganic phosphorus year‐round—could enhance CO 2 uptake by up to 400% and reduce emissions by 90%. These findings reveal a previously underappreciated vulnerability of seagrass carbon sinks to nutrient pollution and provide actionable thresholds for aligning coastal nutrient management with climate mitigation goals.