Effect of the seagrass Zostera capricorni on sediment microbial processes

The effect of the seagrass Zostera capricorni on sediment microbial process es was studied in a tank experiment, where vegetated and unvegetated contro l sediments were incubated in 10 and 50% of incident light. Leaf and root-r hizome biomass, shoot density, and leaf productivity were significantly hig her when plants were incubated in 50 % than in 10 % of incident Light. Nitr ogen fixation, sulphate reduction, and urea turnover in the Z. capricorni v egetated sediment were higher in the 50% than in the 10% light treatment an d higher in the vegetated than in the unvegetated sediment. The stimulation of microbial processes in the Z, capricorni vegetated sediment took place in the rhizosphere, where nitrogen fixation and sulphate reduction in parti cular were stimulated. The sediment studies were supplemented by measuremen ts of nitrogen fixation, sulphate reduction, and urea turnover by microorga nisms associated with the roots and rhizomes of Z. capricorni. The rates of nitrogen fixation and sulphate reduction associated with root-rhizomes wer e up to 40- and 7-fold higher, respectively, than the highest respective se diment rates, whereas the root-rhizome associated urea turnover was lower t han sediment rates. Nitrogen fixation and sulphate reduction associated wit h root-rhizomes could account for up to 39 and 4%, respectively, of the dep th-integrated sediment rates. Nitrogen fixed by microorganisms associated w ith root-rhizomes could supply up to 65 % of the nitrogen needed for plant growth. Further, it was estimated that 8 to 18% of the carbon fixed by Z. c apricorni was released to the sediment by the roots and rhizomes. Urea turn over was suggested to be an important intermediate in the gross production of ammonium, and a low net production of ammonium indicated rapid internal nitrogen cycling within the sediment.