Fig. 4

Young rice plant growing in a paddy field with flooded soil (left) and details of the biogeochemical reactions in the rhizosphere (right). Left: formation of new adventitious roots with enhanced aerenchyma spaces and tight barriers to radial O₂ loss enables deep rooting in the flooded soil resulting in rhizosphere oxidation, especially in regions near the root tip where the barriers to radial O₂ loss are rarely formed (see Fig. 3). Right: oxic zones in the interface between roots and rhizosphere facilitate aerobic mineralization with formation of CO₂ (1), CH₄ oxidation to CO₂ (2) but still with CH₄ production in the surrounding anoxic soil (3). The oxic zones also support nitrification (conversion of NH₄⁺ into NO₃⁻) (4) with some of the NO₃⁻ feeding into denitrification (5) or aerobic methanotrophy (6). The barrier to radial O₂ loss formed on the basal parts of the roots restricts inward radial diffusion of CH₄ (*) preventing plant-mediated diffusion of CH₄ (Fig. 2). Diffusion in or out of the oxic zones are indicated by stippled lines, and greenhouse gases are indicated in orange colour font. Created with BioRender.com.