Root effects on the spatial and temporal dynamics of oxygen in sand-based laboratory-scale constructed biofilters

It is now well known that roots introduce oxygen into the soil environment through radial oxygen loss. The oxygen dynamics surrounding roots in periodically flushed environments, however, remains unstudied. We investigated the impact of roots of the macrophyte Carex appressa (Cyperaceae), on the small (rhizosphere) scale spatiotemporal dynamics of sediment oxygen consumption in a periodically flushed soil mimicking natural percolation events. Oxygen dynamics around the roots of C. appressa were studied using a planar optode installed in a rhizobox containing sand. A sand-based culture medium was used to simulate conditions in constructed biofiltration wetlands. The use of planar optodes allowed the generation of two dimensional images of sediment oxygen dynamics, that were used to quantify the patterns and kinetics of oxygen consumption. In addition to greatly increasing the spatial heterogeneity of oxygen in the substrate, the area immediately surrounding the roots became sites of both enhanced oxygen consumption, likely due to increased microbial activity associated with the input of carbon-rich rhizodeposits, and radial oxygen loss. This study highlights the profound impact of roots upon sub-surface oxygen dynamics in the rhizosphere.