Belowground growth is the primary driver of radial oxygen loss from Typha angustifolia to sediment

Summary Wetland plants must supply oxygen to their rhizomes and roots to maintain aerobic respiration despite soil anoxia. Excess oxygen can diffuse into the environment, strongly affecting rhizosphere biogeochemistry. This radial oxygen loss (ROL) has often been observed to increase in the daytime, perhaps supported by photosynthetic oxygen production and stomatal opening. However, dynamics in rhizosphere oxygen pools do not derive exclusively from diel cycles. Using oxygen‐sensitive planar optodes (and simultaneous leaf gas exchange measurements), we compared the timing and magnitude of variation in rhizosphere oxygen pools due to diel cycles vs root and rhizome development in the widespread aquatic macrophyte Typha angustifolia . Diel cycles drove only slight variation in oxygen pool sizes ( c. 5% oxygen saturation). Strongly oxygenated regions – regularly 25–30% oxygen saturation – appeared as roots and rhizomes grew, and then disappeared after several days of maturation. Expansive plumes of oxygen also developed upon the emergence of lateral roots and persisted for several weeks. These results indicate that root and rhizome development are dominant determinants of ROL in T. angustifolia . Seasonal and environmental controls over timing of belowground development likely have strong implications for rhizosphere biogeochemistry ranging from microbe‐controlled decomposition to redox‐sensitive transformation of metals in natural and managed systems.