The effect of seagrasses on nitrification and deni-trification processes has been under intense investigation (Table 3), in particular in meadows affected by eutrophication, where it is essential to know the potential for nitrogen removal through denitrifica-tion (Borum and Sand-Jensen, 1996). Nitrification is controlled primarily by O2 and nitrogen as ammonium supply, while denitrification is controlled by nitrate and organic carbon supply. Often these two processes are closely coupled in time and space through nitrification-denitrification. The results on rates of nitrification and denitrification coupled to nitrification in rhizosphere sediments are inconsistent, since some workers (Iizumi et al., 1980; Caf-frey and Kemp, 1990; Shieh and Yang, 1997) report high rates, whereas other recent studies using the 15N-isotope pairing technique, report low rates of nitrification and denitrification (Risgaard-Petersen et al., 1998; Rysgaard et al., 1996; Ottosen et al., 1999, Welsh et al., 2000). High rates are associated with release of oxygen from the roots, whereas studies where low rates are measured indicate a strong competition between nitrifying bacteria and seagrass roots for ammonium, which eventually decreases the coupled nitrification-denitrification (Welsh et al.,
2000). These findings suggest that the effect of sea-grasses on the removal of nitrogen from the marine ecosystem is species-dependent, where some species increase the removal and others reduce the removal compared to unvegetated sites. Due to the large seasonal variation in modifying factors, such as the oxidation of the sediments and the nutrient uptake by the plants, a large seasonal variation in the importance of denitrification can be expected. So far, seasonal studies have focused on Zostera spp., where the denitrification was low (Risgaard-Pedersen and Ottosen, 2000), whereas larger species, with more below-ground biomass, remain to be examined. Also diel changes in pore water ammonium pools and denitrification rates have been reported in H. beaudetti meadows, with high dawn ammonium pools and denitrification rates declining during the day, indicating that nitrogen assimilation by the plant roots regulates pore water ammonium pools and thus, indirectly, rates of nitrification and denitrification in the rhizosphere (Blackburn et al., 1994). Further studies are needed to quantify the importance of diel changes for nitrogen cycling in seagrass meadows.
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