The Root Tip of Submerged Plants: An Efficient Engine for Carbon Mineralization

The cycling of iron (Fe) and carbon in the rhizosphere of submerged plants and the associated impact on organic carbon (OC) sequestration are poorly understood. We detected the spatiotemporal distribution of CO2 using a planar optode in the rhizosphere of a common submerged plant as an indicator of OC mineralization. We found that the rhizosphere was a hot spot of CO2 and that the CO2 concentration decreased rapidly from the root tip zone (maximum of 19.68 ± 0.60 matm) to the root base zone (maximum of 12.10 ± 0.29 matm), with the trend of change being the opposite of that of O2. The Fe plaques in the root tip zone had the highest concentration of amorphous Fe. However, the concentration of Fe-bound OC was not significantly different among the different root parts. Because the relative abundances of ferrobacteria decreased in the order tip (8.45%) > base (4.05%) ≈ bulk sediment (3.05%), the enrichment of CO2 in the rhizosphere was attributed to dissimilatory Fe(III) reduction, O2-induced microbial respiration, and root respiration. Our conclusion was that the root tip of submerged plants is an efficient engine for Fe oxidation–carbon sequestration and Fe reduction–carbon mineralization processes, which may affect the stability of sediment carbon pools.