Oxygen loss to the rhizosphere of submerged plants will similarly vary with plant morphology, but a significant loss of oxygen to the sediment from roots of wetland and submerged plants is inevitable (Armstrong et al., 1994). Meristems in root tips must be supplied with sufficient oxygen to support mitosis and efficient energy utilization (Armstrong, 1979; Crawford and Braendle, 1996). To ensure sufficient oxygen supply along the length of roots to the root apex, the radial loss from root surfaces of seagrasses and other aquatic plants seems to decline substantially with increasing distance to root tips (Fig. 6; Armstrong, 1971; Connell et al., 1999; Armstrong et al., 2000; McDonald et al., 2002). However, it is likely that the radial loss of oxygen from root surfaces to the rhizosphere is vital to protect root tissues by oxidizing reduced phytotoxins such as Mn2+, Fe2+ and sulfide (Mendelssohn and Postek, 1982; Armstrong et al., 1996; Lee et al., 1999; Marba et al., Chapter 6, section III.E).
The proportion of oxygen lost to the sediment is difficult to estimate precisely. A comparison of oxygen release from roots of different submerged aquatic macrophytes have documented the high variability among species ranging from about 1% to 100% of total oxygen release in the light (Sand-Jensen et al., 1982). Caffrey and Kemp (1991) found that about 10% of the oxygen produced by photosynthesis in Z. marina was released by below-ground tissues, but these estimates could be too low because measurements were conducted with the roots and rhizomes in non-reducing media and therefore with less steep concentration gradients between plants and media than are likely to occur in nature. Also, oxygen release from roots, expressed as a proportion of photosynthetic oxygen evolution, is a rather confusing expression, since it implies that all oxygen released originates from plant photosynthesis, which is not the case. The oxygen released to the sediment in the light is produced by leaf photosynthesis, but oxygen lost to the sediment in darkness originates from the water column.
There is no doubt that the oxygen released from roots to rhizospheres of submerged macrophytes can
contribute significantly to aerobic mineralization of organic matter within the sediments (Sand-Jensen et al., 1982). For the tropical seagrass, Cymodocea rotundata, the estimated amount of oxygen released to the sediment was about the same magnitude as oxygen transported from the water column to surface sediments (Pedersen et al., 1998, 1999). Accordingly, the oxygen loss to sediments has important implications for the degradation of organic matter but potentially also for other redox processes such as sulphide reoxidation (Lee and Dunton, 2000) and coupled nitrification-denitrification (Caffrey and Kemp, 1992). By leaking oxygen from the roots at different rates during light and dark periods, the rhizosphere immediately around roots of aquatic plants experiences fluctuating aerobic and anaerobic conditions which may promote denitrification (Christensen and Sorensen, 1986; Caffrey and Kemp, 1990; Caffrey and Kemp, 1992; Flindt, 1994). However, in situ observations of coupled nitrification-denitrification in beds of Zostera marina and Z. noltii have not demonstrated higher rates than in bare sediments (Rysgaard et al., 1996; Risgaard et al., 1998), so there is a need for more detailed analysis of the complex interactions between plant oxygen release and sediment processes (see also Marba et al., Chapter 6).
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